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Institute on LakeSuperior Geology

45th Annual MeetingRamada Inn

Marquette, Michigan

May 4-8, 1999

Sponsored byNorthern Michigan University

and Michigan Technological University

Proceedings Volume 45:Program and Abstracts

Robert S. Regis and Theodore J. Bornhorst, editors

Institute on ~ a k e

45th Annual Meeting Rama d a I mi

Maquette, Michigan

May 4-8, 1999 Sponsored by

Northern Michigan University

and Michigan Technological University

45TH ANNUAL MEETINGINSTITUTE ON LAKE SUPERIOR GEOLOGY

Volume 45 consists of

Pan 1: Program and AbstractsPan 2: Field Trip Guidebook

Reference to material in this volume should follow the example below:Ams, D. and Hoim, D., 1999, Characterization and timing constraints of post-Penokean

meso-scale structures in the Watersmeet and Republic gneiss domes of northernMichigan (abst.): Institute on Lake Superior Geology Proceedings, 45th AnnualMeeting, Marquette, ML v.45, part I, p.2.

Volume 45 Published and distributed byInstitute on Lake Superior Geology

Mark Jirsa, Secretary-Treasurer, I.L.S.G.Minneosta Geological Survey

2642 University AvenueSt. Paul, MN USA 55114-1057

(61 2)-627-4780email: jirsaOOl @tc.umn.edu

ILSG website http://www.geo.mtu.edu/great lakes/ilsg/

ISSN 1042-9964

All volumes are available for photocopying costs fromMichigan Technological University Library Archives

Volume 45 consists of

Partl: Program and Abstracts Part 2: Field Trip Guidebook

Reference to material in this volume should follow the example below: Arns, D. and Holm, D., 1999, Characterization and timing constraints of post-Penokean

meso-scale structures in the Watersmeet and Republic gneiss domes of northern Michigan (abst.): Institute on Lake Superior Geology Proceedings, 45th Annual Meeting, Marquette, MI, v. 45, pan 1, p. 2.

Volume 45 Published and distributed by Institute on Lake Superior Geology

Mark Jirsa, Secretary-Treasurer, I.L.S.G. Minneosta Geological Survey

2642 University Avenue St. Paul, MN USA 551 14-1057

(6 12)-627-4780 email: jirsaOOl @tc.umn.edu

ILSG website http://www.eeo.mtu.edu/ereat lakes/Us$

ISSN 1042-9964

All volumes are available for photocopying costs from Michigan Technological University Library Archives

INSTITUTE ONLAKE SUPERIOR

GEOLOGY

PROCEEDINGS

Volume 45

Part 1: PROGRAM AND ABSTRACTS

Editors: Robert S. Regis and Theodore J. Bornhorst

INSTITUTE ON LAKE SUPERIOR

GEOLOGY

PROCEEDINGS

Volume 45

Part 1 : PROGRAM AND ABSTRACTS *~ ~ ~ .- .> . . .. . . . .. -. . ~, . . . . . .~ , , 1

! ; !

I


CONTENTS

PROCEEDINGS VOLUME 45PART 1 - PROGRAM AND ABSTRACTS


Institutes on Lake Superior Geology to 1955-99

Constitution of the Institute on Lake Superior Geology ii

By-Laws of the Institute on Lake Superior Geology iii

Goldich Medal Guidelines iv

Goldich Medal Committee v

Past Goldich Medalists vi

1999 Goldich Medal Recipient vi

Citation for 1999 Goldich Medal Recipient vii

Student Paper Awards Viii

Student Paper Awards Committee Viff

Student Travel Award ix

Student Travel Award Application Form ix

Board of Directors x

Local Committees x

Banquet Speaker xi

Report of the Chair of the 44th Annual Institute xii

Program xv

Abstracts

CONTENTS

PROCEEDINGS VOLUME 45 PART 1 - PROGRAM AND ABSTRACTS

Editors: Robert S. Regis and Theodore J. Bomhorst

Institutes on Lake Superior Geology to 1955-99 ......................................... Constitution of the Institute on Lake Superior Geology ......................................

BY-Laws of the Institute on Lake Superior Geology .......................................................... iii

Goldich Medal Guidelines ................................................................................................. iv

Goldich Medal Committee .................................................................. ................. v

Past Goldich Medalists ......................................................... ................ vi

1999 Goldich Medal Recipient ............................. ....................... VI

. . Citation for 1999 Goldich Medal Recipient ............. ............ vll

... ............. Student Paper Awards .................................................................. ........ vlll

... Student Paper Awards Committee ................................................................................... ~ 1 1 1

Student Travel Award ......................................................................................... ........ IX.

Student Travel Award Application Form ............................................................ . ix

Board of Directors ............................................................................................................... x

Local Committees ............................................................................................................... x

Banquet Speaker ................................................................................................................. xi

Report of the Chair of the 44th Annual Institute ............................................................... xii

Program ............................................................................................................................ xv

Abstracts ....................................................................................................................... 1

INSTITUTE ON LAKE SUPERIOR GEOLOGY

INSTITUTE NUMBER DATE PLACE CHAIRS

1 1955 Minneapolis, Minnesota C.E. Dutton2 1956 Houghton. Michigan AK. Snelgrove3 1957 East Lansing, Michigan B.T. Sandeflir4 1958 Duluth, Minnesota R.W. Marsden5 1959 Minneapolis, Minnesota E.N. Cameron & R.A. Hoppin6 1960 Madison, Wisconsin E.N. Cameron7 1961 Port Arthur, Ontario E.G. Pye8 1962 Houghton. Michigan A.K. Snelgrove9 1963 Duluth, Minnesota H. Lepp10 1964 Ishpeming, Michigan A.T. Broderick11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg12 1966 Sault Ste. Marie. Michigan R.W. White13 1967 East Lansing. Michigan W.J. Hinze14 1968 Superior. Wisconsin A.B. Dickas15 1969 Oshkosh, Wisconsin G.L. LaBerge16 1970 Thunder Bay. Ontario M.W. Bartley & E. Mercy17 1971 Duluth, Minnesota D.M. Davidson18 1972 Houghton. Michigan J. Kafliokoski19 1973 Madison, Wisconsin M.E. Ostrom20 1974 Sault Ste. Marie, Ontario P.E. Giblin2] 1975 Marquette. Michigan J.D. Hughes22 1976 St. Paul. Minnesota M. Walton23 1977 Thunder Bay. Ontario M.M. Kehlenbeck24 1978 Milwaukee. Wisconsin C. Mursky25 1979 Duluth. Minnesota D.M. Davidson26 1980 Eau Claire. Wisconsin P.E. Myers27 1981 East Lansing. Michigan W.C. Cambray28 t982 International Falls, Minnesota DL. Southwick29 1983 Houghton. Michigan T.J. Bornhorst30 1984 Wausau. Wisconsin CL. LaBerge31 1985 Kenora. Ontario CE. Blackburn32 1986 Wisconsin Rapids. Wisconsin J.K. Greenberg33 1987 Wawa, Ontario E.D. Frey & R.P. Sage34 1988 Marquette, Michigan J. S. Klasner35 1989 Duluth, Minnesota .J.C. Green36 1990 Thunder Bay. Ontario M.M. Kehlenbeck37 1991 Eau Claire, Wisconsin P.E. Meyers38 1992 Hurley, Wisconsin A.B. Dickas39 1993 Eveleth. Minnesota DL. Southwick40 1994 Houghton, Michigan T.J. Bornhorst41 1995 Marathon, Ontario M.C. Smyk42 1996 Cable, Wisconsin L.G. Woodruff43 1997 Sudbury. Ontario R.P. Sage & W. Meyer44 1998 Minneapolis, Minnesota J.D. Miller & M.A. ursa45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis

INSTITUTE NUMBER

Minneapolis, Minnesota Houghton, Michigan East Lansing, Michigan Duluth, Minnesota Minneapolis, Minnesota Madison. Wisconsin Port Arthur. Ontario Houghton, Michigan Duluth, Minnesota Ishpeming, Michigan St. Paul, Minnesota Sault Ste. Marie, Michigan East Lansing. Michigan Superior, Wisconsin Oshkosh. Wisconsin Thunder Bay. Ontario Duluth. Minnesota Houghton. Michigan Madison. Wisconsin Sault Ste. Marie. Ontario Marquette. Michigan St. Paul, Minnesota Thunder Bay. Ontario Milwaukee. Wisconsin Duluth, Minnesota Eau Claire. Wisconsin East Lansing. Michigan International Falls. Minnesota Houghton. Michigan Wausau. Wisconsin Kenora, Ontario Wisconsin Rapids. Wisconsin Wawa, Ontario Marquette. Michigan Duluth, Minnesota Thunder Bay, Ontario Eau Claire. Wisconsin Hurley, Wisconsin Eveleth, Minnesota Houghton. Michigan Marathon. Ontario Cable, Wisconsin Sudbury. Ontario Minneapolis, Minnesota Marquette, Michigan

. . C.E. Dutton . ~--., A.K. Snelgrove B.T. Sandefur R.W. Marsden E.N. Cameron & R.A. Hoppin E.N. Cameron E.G. Pye A.K. Snelerove *,. . - H. L ~ P P A.T. Broderick P.K. Sims & R.K. Hogberg R.W. White W.J. Hinze A.B. Dickas G.L. LaBerge M.W. Bardey & E. Mercy D.M. Davidson J. Kalliokoski M.E. Osuom P.E. Giblin J.D. Hughes M. Walton M.M. Kehlenbeck G. Mursky D.M. Davidson i-'fr ' P.E. Myers W.C. Cambray D.L. Southwick T.J. Bomhorst -' -st G.L. LaBerge C.E. Blackburn J.K. Greenberg E.D. Prey & R.P. Sage J. S. Klasner J.C. Green M.M. Kehlenbeck P.E. Meyers A.B. Dickas D.L. Southwick T.J. Bomhorst M.C. Smyk L.G. Woodruff R.P.Sage&W.Meyer J.D. Miller & M.A. Jirsa T.J. Bomhorst & R.S. Regis

CONSTITUTION OF INSTITUTE ON LAKE SUPERIOR GEOLOGYArticle I Name

The name of the organization shall be the "Institute on Lake Superior Geology.

Article II ObjectivesThe objectives of this organization are:A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and

scientific data.B. To promote better understanding of the geology of the Lake Superior region.C. To plan and conduct geological field trips.

Article III StatusNo part of the income of the organization shall insure to the benefit of any member or individual.In the event of dissolution the assets of the organization shall be distributed to ______________

(some tax free organization).

(To avoid Federal and State income taxes, the organization should be not only "scientific or"educational, but also "non-profit".)

Minn. Stat. Anno. 290.01, subd. 4Minn. Stat. Anno. 290.05(9)1954 Internal Revenue Code 5.50 1(c)(3)

Article IV MembershipThe membership of the organization shall consist of persons who have registered for an annualmeeting or who have indicated interest in being a member according to membership guidelinesapproved by the board of directors and have paid any applicable dues.

Article V Meetin2sThe organization shall meet once a year, preferably during the month of April. The place andexact date of each meeting will be designated by the board of directors.

Article VI DirectorsThe board of directors shall consist of the Chair, Secretary-Treasurer, and the last three pastChair; but if the board should at any time consist of fewer than five persons, by reason ofunwillingness or inability of any of the above persons to serve as directors, the vacancies on theboard may be filled by Chair so as to bring the membership of the board up to five members.

Article VH OfficersThe officers of this organization shall be a Chair and Secretary-Treasurer.A. The Chair shall be elected each year by the board of directors, who shall give due

consideration to the wishes of any group that may be promoting the next annual meeting.His/her term of office as Chair will terminate at the close of the annual meeting over whichhe/she presides or when his/her successor shall have been appointed. He/she will then servefor a period of three years as a member of the board of directors.

B. The Secretary-Treasurer shall be elected by a majority vote of members at the annualmeeting. His/her term of office shall be four years or until his/her successor shall have beenappointed.

Article VIII AmendmentsThis constitution may be amended by a majority vote (majority of those voting) of themembership of the organization.

CONSTITUTION OF INSTITUTE ON LAKE SUPERIOR GEOLOGY

Article I

Article Il

Article IU

Nams The name of the organization shall be the "Institute on Lake Superior Geology".

Obiectlves The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and

scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips.

Sam? No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution the assets of the organization shall be distributed to (some tax free organization).

(To avoid Federal and State income taxes, the organization should be not only "scientific" or "educational, but also "non-profit".) . .

,2..

Minn. Slat. Anno. 290.01, subd. 4 Minn. S t a ~ Anno. 290.05(9) 1954 Internal Revenue Code s.501(~)(3) ,.

Article IV

Article V

Article VI

Article VH

rasmkmm The membership of the organization shall consist of persons who have registered for an annual meeting or who have indicated interest in being a member according to membership guidelines approved by the board of directors and have paid any applicable dues.

&!dlws The organization shall meet once a year, preferably during the month of April. The place and exact date of each meeting will be designated by the board of directors.

Directors The board of directors shall consist of the Chair, Secretary-Treasurer, and the last three past Chair, but if the board should at any time consist of fewer than five persons, by reason of unwillingness or inability of any of the above persons to serve as directo& the vacakies on the board may be filled by Chair so as 10 bring the membership of the board up to five members.

smcsls The officers of this organization shall be a Chair and Secretary-Treasurer. A. The Chair shallbe elected each year by the boardof directors, who shall give due

consideration to the wishes of any DUUD that may be promoting the next annual meetine. Hisher term of office as Chair $lt&nate at the cl&c of theannual meeting over which hdshe presides or when hisher successor shall have been appointed. Hdshe will then serve for a period of three years as a member of the board of directors.

B. The Secretary-Treasurer shall be elected by a majority vote of members at the annual . . meeting. ~ is /her term of office shall be fouryears or until hisher successor shall have been appointed.

Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization.

BY-LAWS OF INSTITUTE ON LAKE SUPERIOR GEOLOGYI. Duties of the Officers and Directors

A. It shall be the duty of the Annual Chair to:I. Preside at the annual meeting.2. Appoint all committees needed for the organization of the annual meeting.3. Assume complete responsibility for the organization and financing of the annual meeting

over which he/she presides.B. It shall be the duty of the Secretary-Treasurer to:

1. Keep accurate attendance records of all annual meetings.2. Keep accurate records of all meetings of, and correspondence between, the board of

directors.3. Hold all funds that may accrue as profits from annual meetings or field trips and to make

these funds available for the organization and operation of future meetings as required.C. It shall be the duty of the board of directors to plan locations of annual meetings and to advise

on the organization and financing of all meetings.

H. Duties and ExpensesI. Regular membership dues of $5.00 or lesson an annual basis shall be assessed each member

as determined by the board of directors.2. Registration fees for the annual meetings shall be determined by the Chair in consultation with

the board of directors. The registration fees can include expenses to cover operationsoutside of the annual meeting as determined by the board of directors. It is stronglyrecommended that registration fees be kept at a minimum to encourage attendance ofgraduate students.

ILL. Rules of OrderThe rules contained in Robert's Rules of Order shall govern this organization in all cases to whichthey are applicable.

LV. AmendmentsThese by-laws may be amended by a majority vote (majority of those voting) of the membershipof the organization; provided that such modifications shall not conflict with the constitution aspresently adopted or subsequently amended.

Last Amended - May, 1996

BY-LAWS OF INSTITUTE ON LAKE SUPERIOR GEOLOGY

I. Duties of the Officers and Directors A. It shall be the duty of the Annual Chair to: >,,, . ,,, , .

"9.. ! ! i 8 , - 1 . Preside at the annual meeting. - , -"-<

. . . . ..,.&;, 2. Appoint all committees needed for the organization of the annual meeting. !',vp..,. *. . ! 3. Assume complete responsibility for the organization and financine of the annual meetine

over which h&he - - - -

B. It shall be the duty of the Secretary-Treasurer to: 1. Keep accurke attendance &cords of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the board of

directors. 3. Hold all funds that may accrue as profits from annual meetings or field trips and to make

these funds available for the organization and operation of future meetings as required. It shall be the duty of the board of directors to plan locations of annual meetings and to advise

on the organization and financing of all meetings.

II. Duties and E x u e e , , s - 1. Regular membership dues of $5.00 or less on an annual basis shall be assessed each member

as determined by the board of directors. 2. Registration fees for the annual meetings shall be determined by the Chair in consultation with

the board of directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the hoard of directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of graduate students.

;z F. lQW of Order The rules contained in Robert's Rules of Order shall govern this organization in all cases to which they are applicable.

N. Anw>nflnw*nfc These by-laws may be amended by a majority vote (majority of those voting) of the membership

I ;;- of the organization; provided that such modifications shall not conflict with the constitution as . , 5 ~ i i ~ . i s presently adopted or subsequently amended. . . ,

..;. .

Last Amended - May, 1996 .'t .

GOLDICH MEDAL GUIDELINES

Preamble

The Institute on Lake Superior Geology was born on or around 1955, as documented by the fact that the 27thannual meeting was held in 1981. The Institutes are exemplary in their continuing objectives of dealing with thoseaspects of geology that are related geographically to Lake Superior; of encouraging the discussion of subjects andsponsoring field trips which will bring together geologists from academia, government surveys, and industry; and ofmaintaining an exceedingly informal but highly effective mode of operation.

During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interestin the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have madeparticularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineraldeposits.

The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the regionextending over about 50 years. Subsequent medalists and this year's recipient are listed in the table below.

Award Guidelines

I) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associatedwith a substantial interest in, and contribution to, the geology of the Lake Superior region.

2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of threemembers, one to serve for three years, one for two years, and one for one year. The member with the briefestincumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appointat each spring meeting one new member who will serve for three years. In his/her third year this member shall bethe chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSGmembership.

3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of theBoard of Directors, who will then inform the Board of the nominee.

4) The Board of Directors normally will accept the nominee of the Committee, will inform the medalistimmediately, and will have one medal engraved appropriately for presentation at the next meeting of the Institute.

5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required tosupport the continuing costs of this award.

Nomination Procedures

1) Nominations shall be taken at any time by the Goldich Medal Con-imittee. Committee members may themselvesnominate candidates. The deadline for nominations is November 1.

2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation,lists of publications. curriculum vita's, and evidence of contributions to Lake Superior geology and to the Institute.

3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributedto the understanding of Lake Superior geology.

iv

GOLDICH MEDAL GUIDELINES

Preamble

The Institute on Lake Superior Geology was born on or around 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institutes are exemplary in their continuing objectives of dealing with those aspects of geology that are related geographically to ~ake~upe r io r ; of encouraging the discussion of subjects and swnsoring field trips which will brine together eeoloeists from academia, government surveys. and industry; and of maintaining an exceedingly i n f o d buthighiyeffective mode of operation.

During the course of its existence. the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particulaily noteworthy and meritorious contributions to the understanding of Lake Superior f$eolo&' and mineral deposits.

The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medalists and this year's recipient are listed in the table below.

Award Guidelines

1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region.

2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In hidher third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership.

3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee.

4) The Board of Directors normally will accept the nominee of the Committee. will inform the medalist immediately, and will have one medal engraved appropriately for presentation at the next meeting of the Institute.

5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award.

Nomination Procedures

1) Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates. The deadline for nominations is November 1.

2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita's, and evidence of contributions to Lake Superior geology and to the Institute.

3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology.

Selection Guidelines

1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including:a) importance of relevant publications;b) promotion of discovery and utilization of natural resources;c) contributions to understanding of the natural history and environment of the region;d) generation of new ideas and concepts; ande) contributions to the training and education of geoscientists and the public.

2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institutemeetings, presentation of talks and posters, and service on Institute boards, committees, and field trips.

3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of theCornniittee members.

4) There are several points to considered by the Goldich Medal Committee:a) An attempt should be made to maintain a balance of medal recipients from each of the three estates-industry,

academia, and government.b) It must be noted that industry geoscientists are at a disadvantage in that much of their work is not published.

5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute's greatstrengths and should be nurtured by equitable recognition of excellence in both countries.

Last Amended 1997

GOLDICH MEDAL COMMITTEE 1998-99

John Klasner (1999)Western Illinois University, Macomb, Illinois

Mark Smyk (2000)Ontario Geological Survey, Thunder Bay, Ontario

Rod Johnson (2001)Rod Johnson and Associates, Negaunee, Michigan

V

1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the nublic. - -

# ... ., ,:,{ m, . P,:~:, ,y 2 : 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters. and service on Institute boards, committees, and field trips.

3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members.

- . .# ..q . * ? , '

4) There are several points to considered by the Goldich Medal Committee: ." ...., * . . . a) An attempt should be made to maintain a balance of medal recipients from each'of the three estates-industry,

academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work is not published.

5 ) Lake Superior has two sides, one the US., and the other Canada. This is undoubtedly one of the Institute's great strengths and should be nurtured by equitable recognition of excellence in both countries.

Last Amended 1997

GOLDICH MEDAL COMMITTEE 1998-99 *

John Klasner (1999) Western Illinois University, Macomb, Illinois

MarkSmyk (2000) * ' 'k Ontario Geological Survey, Thunder Bay, Ontario

Rod Johnson (2001) Rod Johnson and Associates, Negaunee, ~ i c h i ' n

GOLDICH MEDALISTS1979 Samuel £ Goldich 1991 William J. Hinze1980 not awarded 1992 William F. Cannon1981 Carl IL Dutton, Jr. 1993 Donald W. Davis1982 Ralph W. Marsden 1994 Cedric Iverson1983 Burton Boyum 1995 Gene LaBerge1984 Richard W Ojakangas 1996 David L Southwick1985 Paul K. Sims 1997 Ronald P. Sage1986 G.B. Morey 1998 Zell Petennan1987 Henry H. Halls 1999 Tsu-Ming Han1988 Walter £ White1989 Jonna Kalliokoski1990 Kenneth C. Card

1999 Goldich Medal Recipient

Tsu-Ming Han

vi

GOLDICH MEDALISTS '' ,;; :G?L

1979 Samuel S. Goldich 991 William J. Hinze 1980 not awarded 992 William F. Cannon 1981 Carl E. Dutton, Jr. 993 Donald W. Davis 1982 Ralph W. Ma 994 Cedric Iverson

995 Gene LaBerge 1983 Burton Boyu !:-

1984 Richard W. Ojakangas 996 David L. Southwick 1985 Paul K. Sims 997 Ronald P. Sage 1986 G.B. Morey 998 Zell Peterman 1987 Henry H. Halls 999 Tsu-Ming Hun 1988 Walter S. White 1989 Jorma Kalliokoski ''

1990 Kenneth C. Card

19999 Goldich Medal Recipient -..

Tsu-Ming Han

CitationTsu-Ming Han

1999 Goldich Medal Recipient

The 1999 c3oldich Medal recipient epitomizes the characteristics required to receivethis unique and prestigious award. He has devoted his professional life to solving nature'smyriad challenges in the field of geology and sharing his findings with colleagues bothverbally and through published and unpublished mineral papers.

He was born during the 1920's in the Hunan Province of China. His family stressedthe importance of education. He pursued his education during a series of difficult periodsthat included being kidnapped and held by bandits, pursued by Japanese forces, and theimposition of Nationalist control followed by the cultural upheaval caused by the ascendancyof the Communists after World War II. After graduating from Northwest University in SianProvince in 1945 and a brief stint with the Bureau of Mineral Exploration, he immigrated tothe U.S. in 1947 to complete graduate work at the University of Cincinnati and theUniversity of Minnesota. His mentors at the University of Minnesota included Drs. Goldich,Gruner, and Schwartz.

After summer employment with Clevcland-Cliffs in 1952, he accepted permanentemployment at the Ishpeming Research facility. Soon after, he met and married Joy. Theyhave three children who have also benefited professionally by achieving advancededucational degrees.

He has published numerous articles on the genesis of iron formations with emphasison textural relations during diagenesis and metamorphism.

What is not generally known is that he is an expert on all facets of the beneficiationand pelletizing process that has dominated the North American iron ore industry for half acentury. His studies have assisted in continual improvement in the pelletizing process tokeep the industry competitive.

He has a special interest in the preserved algae present in several areas of theNegaunee Iron-formation at the Empire Mine and has published his findings.

It should be noted that he delivered a paper at the first Institute meeting in 1955 andcontinues his scientific inquiry as evidenced by his current presentation at the Institute.Indeed, the only change noticeable since his formal retirement in 1992 is that he now wearstennis shoes to the office.

It is my pleasure to introduce to the Institute the 1999 recipient of the Goldich Medal,my friend and our colleague, Tsu-Ming Han.

Tom Wagonner

vii

- , . Citation : .. . .. ,

I Tsu-Ming Han 9 Goldich Medal Recipien

I The 1999 Goldich Medal recipient epitomizes the characteristics required to receive this unique and prestigious award. He has devoted his professional life to solving nature's myriad challenges in the field of geology and sharing his findings with colleagues both

r verbally and through published and unpublished mineral papers.

He was born during the 1920's in the Hunan Province of China. His family stressed the importance of education. He pursued his education during a series of difficult periods that included being kidnapped and held by bandits, pursued by Japanese forces, and the imposition of Nationalist control followed by the cultural upheaval caused by the ascendancy of the Communists after World War U. After graduating from Northwest University in Sian

Province in 1945 and a brief stint with the Bureau of Mineral Exploration, he immigrated to the U.S. in 1947 to complete graduate work at the University of Cincinnati and the University of Minnesota. His mentors at the University of Minnesota included Drs. Goldich, Gmner, and Schwartz.

;, ,. ,,, After summer employment with Cleveland-Cliffs in 1952, he accepted permanent

employment at the Ishpeming Research facility. Soon after, he met and married Joy. They have three children who,, have also beqegted, professionally b y , .achiev&g *-* advanced educational degrees. .,.~ t , , .

He has published numerous articles on the genesis of iron formations with emphasis on textural relations during diagenesis and metamorphism.

What is not generally known is that he is an expert on all facets of the beneficiation and pelletizing process that has dominated the North American iron ore industry for half a century. His studies have assisted in continual improvement in the pelletizing process to keep the industry competitive.

He has a special interest in the preserved algae present in several areas of the Negaunee Iron-formation at the Empire Mine and has published his findings.

It should be noted that he delivered a paper at the first Institute meeting in 1955 and continues his scientific inquiry as evidenced by his current presentation at the Institute. Indeed, the only change noticeable since his formal retirement in 1992 is that he now wears tennis shoes to the office.

It is my pleasure to introduce to the Institute the 1999 recipient of the Goldich Medal, my friend and our colleague, Tsu-Ming Han.

Tom Wagonner

STUI)ENT PAPER AWARDS

Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award.Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee isappointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologicexpertise. Criteria for best student paper-last modified by the Board in 1997-follow:

1) The contribution must be demonstrably the work of the student.

2) The student must present the contribution in-person.

3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separateawards for poster vs. oral presentations.

4) In cases of multiple student authors, the award will be made to the senior author, or the award will be sharedequally by all authors of the contribution.

5) The total amount of the awards is left to the discretion of the meeting Chair and Secretary-Treasurer, buttypically is in the amount of about $300 US.

6) The Secretary-Treasurer maintains, and will supply to the Conmiittee. a form for the numerical ranking ofpresentations. This form was created and modified by Student Paper Committees over several years in an effort toreduce the difficulties that may arise from selection by raters of diverse background. The "Se of the form is notrequired, but is left to the discretion of the Committee.

7) The names of award recipients shall be included as part of the annual Chair's report that appears in the nextvolume of the Institute.

1999 STUDENT PAPER COMMITTEE

Allan Blaske, STS Consultants, LTD. Lansing, MI

Laurel Woodruff, U.S. Geological Survey, St. Paul, MN

vii

STUDENT PAPER AWARDS

Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper-last modified by the Board in 1997-follow:

1) The contribution must be demonstrably the work of the student,

2) The student must present the contribution in-person.

3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations.

4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution.

5) The total amount of the awards is left to the discretion of the meeting Chair and Secretary-Treasurer, but typically is in the amount of about $300 US.

6) The Secretary-Treasurer maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The me of the form is not required, but is left to the discretion of the Committee.

7) The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute.

1999 STUDENT PAPER COMMITTEE

.~ . ~,,. . Allan Blaske, STS Consultants, LTD, Lansing, MI

. .. .. , . , . , . . . ,

Laurel Woodruff, U.S. Geological Survey, St. Paul, MN ' ;B . : . .

STUT)ENT TRAVEL AWARDThe 1986 Board of Directors established the LL.S.G. Student Travel Award to support student participation at theannual Institutes. The awards will be made from a special find set up for this purpose. This award is intended tohelp defray some of the direct travel costs to the Institute and includes a waiver of registration fees, but excludesexpenses for meals, lodging, and field trip registration. The number of awards and value are determined by theannual Chairman in consultation with the Secretary-Treasurer and will be announced at the annual banquet.

The following general criteria will be considered by the annual Chairman, who is responsible for the selection:

1) The applicants must have active resident (undergraduate or graduate) student status at the time of theInstitute, certified by the department head.

2) Students who are the senior author on either an oral or poster paper will be given favoredconsideration.

3) It is desirable for two or more students tojointly request travel assistance.

4) In general, priority will be given to those in the Institute region who are farthest away.

5) Each travel award request shall be made in writing, to the annual Chairman, with an explanation ofneed, possible author status or other significant details.

Successful applicants will receive their awards at the time of registration for the Meeting.

INSTtTUTh ON LAKE SUPERIOR GEOLOGY

Student Travel A want Application

Please print:Student Name: _______________________________ Date: _______________

Address: _______________________________________________________

Icertify that the above named person is an active resident student.

Department Head - Typed

Department Head - Signature Date

Educational Status: ___________________

Are you the Senior Author of an oral or poster paper? Yes No _____

Will any other students will be traveliag with you? How many?

Statement of Need: (If you need mole loom, please use the back of the page.)

Other Significant Details:

Please return to:

ix

STUDENT TRAVEL AWARD

I The 1986 Board of Directors established the I.L.S.G. Student Travel Award to support student participation at the annual Institutes. The awards will be made from a special fund set up for this purpose. This award is intended to

1 help defray some of the direct travel costs to the Institute and includes a waiver of registration fees, but excludes expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chairman in consultation with the Secretary-Treasurer and will be announced at the annual banquet.

The following general criteria will be considered by the annual Chairman, who is responsible for the selection:

1) The applicants must have active resident (undergraduate or graduate) student status at the time of the Institute, certified by the department head.

2) Students who are the senior author on either an oral or poster paper will be given favored consideration.

3) It is desirable for two or more students to jointly request travel assistance.

4) In general, priority will be given to those in the Institute region who are farthest away.

5 ) Each travel award request shall be made in writing, to the annual Chairman, with an explanation of need, possible author status or other significant details.

Successful applicants will receive their awards at the time of registration for the Meeting.

ON -SUPERIOR GEOLOGY

Student TravetAwardAppUcdon

Please print:

Student Name: Date:

Address: .:: v.. , c .. ;.( ..>i.;

1 certify that the above named person Is an active resident student. .,." ; ;;. . , > ' . , . . . ~ . : ,.

Department Head - Typed <: , : ... ,,

Date Department Head - Signature L

Educational Status:

re you the Senior Author ofan oral or poster paper? Yes ___ No- Will any other students will be (reveling with you? ___ How many? - statement of ~ e e d : (If you need more room, please use the back of the Page.)

Other Significant Details:

Please return to:

BOARD OF DIRECTORS

Theodore J. Bornhorst, Chair (2002)Michigan Technological University, Houghton, Michigan

James D. Miller (2001)Minnesota Geological Survey, St. Paul, Minnesota

Ronald P. Sage (2000)Ontario Geological Survey, Sudbury, Ontario

Laurel G. Woodruff (1999)U.S. Geological Survey, Mounds View, Minneapolis

Mark A. Jirsa (2000) Institute Secretary-TreasurerMinnesota Geological Survey, St. Paul, Minnesota

LOCAL COMMITTEES

GENERAL Co-CHAIRS

Robert S. RegisDepartment of Geography/Earth Science, Northern Michigan University, Marquette, Michigan 49855

Theodore J. BornhorstDepartment of Geological Engineering and Sciences, Michigan Technological University, Houghton,Michigan 49931

PROGRAM AND ABSTRACTS EDITORS

Robert S. RegisDepartment of Geography/Earth Science, Northern Michigan University, Marquette, Michigan 49855

Theodore J. BornhorstDepartment of Geological Engineering and Sciences, Michigan Technological University. Houghton,Michigan 49931

FIELD TRIP GUIDEBOOK EDITOR

Theodore J. BornhorstDepartment of Geological Engineering and Sciences, Michigan Technological University, Houghton,Michigan 49931

x

BOARD OF DIRECTORS

fheodore J. Bornhorst, Chair (2002) Michigan Technological University, Houghton, Michigan

James D. Miller (2001) Minnesota Geological Survey, St. Paul, Minnesota

Ronald P. Sage (2000) Ontario Geological Survey, Sudbury, Ontario < .

Laurel G. Woodruff (1999) US. Geological Survey, Mounds View, Minneapolis

Mark A. J i m (2000) Institute Secretary-Treasurer Minnesota Geological Survey, St. Paul, Minnesota

LOCAL COMMITTEES

Robert S. Regis Department of Gwgraphy/Earth Science, Northern Michigan University, Marquette, Michigan 49855

Theodore J. Bornhorst Department of Geological Engineering and Sciences, Michigan Technological University. Houghton, Michigan 49931

,* . PROGRAM AND ABSTRACTS EDITORS

Robert S. Regis Department of Geography/Earth Science, Northern Michigan University, Marquette, Michigan 49855

Theodore J. Bornhorst Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, Michigan 49931

Theodore J. Bornhocst Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, Michigan 49931

SESSION CHAiRS

Dave Baxter, PayDay Resources, Lansing, MI

Shawn Carison, Ashton Mining, North Vancouver, BC, Canada

Ron Graber, Cliffs Mining Services Company, Ishpeming, MI

John Green, University of Minnesota, Duluth, MN

Rod Johnson, Rod Johnson & Associates, Inc., Negaunee, MI

Gene Laberge, University of Wisconsin, Oshkosh, WI

Jim Small, Edward Kraemer & Sons, Burnsville, MI

Tom Waggoner, retired Cliffs Mining Services Company, Ishpeming, MI

1999 BANQUET SPEAKER

Nelson R. HamDepartment of Geology

St. Norbert CollegeDe Pere, Wisconsin

Ice-Flow Dynamics and Landform DevelopmentAlong the Southern Margin of the Laurentide Ice Sheet

xi

SESSION CHAIKS

Dave Baxter, PayDay Resources, Lansing, MI

Shawn Carlson, Ashton Mining, North Vancouver, BC, Canada

Ron Graber, Cliffs Mining Services Company, Ishpeming, MI

John Green, University of Minnesota, Duluth, MN

Rod Johnson, Rod Johnson & Associates, Inc., Negaunee, MI

Gene Laberge, University of Wisconsin, Oshkosh, WI

Jim Small, Edward Kraemer & Sons, Burnsville, MI

Tom Waggoner, retired Cliffs Mining Services Company, Ishpeming, MI

" a,

999 BANQUET SPEAKER , .. ,

Nelson R. Ham Department of Geology

' St. Norbert College . ,

De Pere, Wisconsin

Ice-Flow Dynamics and Landform Development Along the Southern Margin of the Laurentide Ice Sheet

REPORT ON THE 44th ANNUAL MEETING OF THEINSTITUTE ON LAKE SUPERIOR GEOLOGY

MINNEAPOLIS, MINNESOTA

The 44th Annual Meeting of the Institute on Lake Superior Geology was held inMinneapolis, Minnesota on May 6-10, 1998. The meeting was sponsored by the MinnesotaGeological Survey and was held at the Holiday Inn on the West Bank of the University ofMinnesota campus.

Proceedings Volume 44 of the meeting was published in two parts:

Pan 1: Program and Abstracts (including extended abstracts of the special overview session)

Part 2: Field Trip Guidebook1—Early Proterozoic intrusive rocks of east-central Minnesota2—Geology of the southeastern portion of the Midcontinent Rift System. eastern

Minnesota and western Wisconsin3—Glacial exotica of the Twin Cities area4—Stratigraphy and hydrogeology of the Paleozoic rocks of southeastern Minnesota5—Minnesota River Valley and vicinity, southwestern Minnesota (Precambrian and

Quaternary geology)

By most measures, the meeting was a great success. We saw a considerable jump inattendance this year to 200 participants, compared with recent average attendance on theorder of 125. Fifty-six participants were first-time attendees to the ISLG; split about equallybetween students and related fields people—that is. those involved in Quaternary, Paleozoic,environmental, and engineering geology that are not traditional topics of the Institute. Weattribute some of this increase simply to the metropolitan location of the meeting and ourfairly aggressive appeal to the local geoscience community. To attract local and morebroadly based interest, we broke somewhat with tradition by conducting a special half-daysession overviewing the geology of the Lake Superior region. To make the meeting evenmore attractive to new-comers, we offered a first-day-only registration at a reduced rate forthose interested in attending just the special overview session; however, few took advantageof the offer.

The special overview session consisted of six 30 minute talks that outlined current ideas andnew research directions on the main geologic provinces of the Lake Superior region.Presentations were given by Ken Card (Archean), Dick Ojakangas (Early Proterozoic), BillCannon (Middle Proterozoic), Tony Runkel (Paleozoic), Carrie Patterson (Quaternary), andDave Southwick (New directions in Lake Superior geology). The talks were well presentedand well received; comments from the participants were overwhelmingly favorable. Thegeneral sessions included 23 oral presentations covering not only the typical fare ofPrecambrian topics, but also included several talks related to groundwater. In addition, 24poster presentation were on display during the course of the meeting. The banquet speakerwas Dr. Bevan French of the Smithsonian Institution, who gave an excellent talk on the roleof meteorite impacts on the geologic history of earth and other planets. At the banquet, ZellPeterman of the USGS was awarded the Goldich Medal for his important geochronologic and

xii

REPORT ON THE 44th ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY

MINNEAPOLIS, MINNESOTA

The 44th Annual Meeting of the Institute on Lake Superior Geology was held in Minneapolis, Minnesota on May 6-10, 1998. The meeting was sponsored by the Minnesota Geological Survey and was held at the Holiday Inn on the West Bank of the University of Minnesota campus.

Proceedings Volume 44 of the meeting was published in two parts:

Part 1: Program and Abstracts (including extended abstracts of the special overview session)

Part 2: Field Trip Guidebook 1-Early Proterozoic intrusive rocks of east-central Minnesota 2-Geology of the southeastern portion of the Midcontinent Rift System, eastern

Minnesota and western Wisconsin 3Ã‘Glacia exotica of the Twin Cities area Mtrat igraphy andhydrogeology of the Paleozoic rocks of southeastern Minnesota 5-Minnesota River Valley and vicinity, southwestern Minnesota (Precambrian and

Quaternary geology)

By most measures, the meeting was a great success. We saw a considerable jump in attendance this year to 200 participants. compared with recent average attendance on the order of 125. Fifty-six participants were first-time attendees to the ISLG. split about equally between students and related fields people-that is, those involved in Quaternary, Paleozoic, environmental, and engineering geology that are not traditional topics of the Institute. We attribute some of this increase simply to the metropolitan location of the meeting and our fairly aggressive appeal to the local geoscience community. To attract local and more broadly based interest, we broke somewhat with tradition by conducting a special half-day session overviewing the geology of the Lake Superior region. To make the meeting even more attractive to new-comers, we offered a first-day-only registration at a reduced rate for those interested in attending just the special overview session; however, few took advantage of the offer.

The special overview session consisted of six 30 minute talks that outlined current ideas and new research directions on the main geologic provinces of the Lake Superior region. Presentations were given by Ken Card (Archean), Dick Ojakangas (Early Proterozoic), Bill Cannon (Middle Proterozoic), Tony Runkel (Paleozoic), Came Patterson (Quaternary), and Dave Southwick (New directions in Lake Superior geology). The talks were well presented and well received; comments from the participants were overwhelmingly favorable. The general sessions included 23 oral presentations covering not only the typical fare of Precambrian topics, but also included several talks related to groundwater. In addition, 24 poster presentation were on display during the course of the meeting. The banquet speaker was Dr. Bevan French of the Smithsonian Institution, who gave an excellent talk on the role of meteorite impacts on the geologic history of earth and other planets. At the banquet, Zell Petennan of the USGS was awarded the Goldich Medal for his important geochronologic and

geologic studies of Precambrian geology in the Lake Superior region. Another highlight ofthe meeting for 40 participants was a night of baseball at the Metrodome.

This meeting also broke the mold a bit by offering five field excursions that covered asimilarly wide breadth of geology as was presented in the special overview session.Although the emphasis remained with Precambrian geology, trips highlighting the Paleozoicand Quaternary geology of central and southern Minnesota were also included. Three pre-meeting field trips were run on Wednesday, May 6th; 1) the Early Proterozoic geology ofeast-central Minnesota led by Terry Boerboom, Mark Jirsa, and Daniel Holm (26participants); 2) the Keweenawan geology of the Taylor's Falls area led by Karl Wirth, BillCordua, Bill Kean, Mike Middleton, and Zach Naiman (46 participants); and 3) Glacialexotica of the Twin Cites area led by Howard Hobbs, Alan Knaeble and Gary Meyer (21participants). Two post-meeting trips included a one-day trip on the Paleozoic stratigraphy,sedimentology, and hydrogeology of southeastern Minnesota led by Tony Runkel and BobTipping (11 participants), and a two-day excursion investigating the Quaternary and Archeangeology of the Minnesota River Valley led by Carrie Patterson and David Southwick (33participants).

The meeting was also a financial success; that is, if your measure of success is to at leastbreak even (which has been the traditionally held view). We actually turned a small profit ofseveral hundred dollars. Although registration was more costly than we had hoped, the costwas unavoidable given the metropolitan location. Part of the registration we all pay isapplied to encouraging student participation in the Institute by granting awards for studenttravel and best student papers. Eight travel awards totaling $750, and two best paper awardsof $150 each were granted. Best paper awards went to Kathleen Abbott (Macalester College)for her undergraduate work on geochemistry and petrography of Keweenawan Chengwatanavolcanic rocks of Minnesota and Wisconsin, and to Dean Peterson (University of Minnesota-Duluth) for his graduate presentation on (3IS-based mineral potential analyses of Archeanrocks in Minnesota. Congratulations to you both!

The Institute's Board of Directors met on May 7th, and the following is generalized fromminutes of that meeting:

1. Accepted the Report of the Chair 43rd ILSG2. Accepted the 1997-1998 JLSG Financial Report.3. Approved Zell E. Peterman as 1998 Goldich Medal recipient.4. Modified the language for Goldich Awards regarding dates for receipt of nominations

(now November 1) and final decision (now the end of November).5. Approved the end-of-term replacement of Dan England (Eveleth Fee Office) as

Goldich Medal committee member y Rodney Johnson (Trans Superior Resources).Rod begins a 3-year term in Novemuer of 1998.

6. Changed the student travel award's title to "Eisenbrey Student Travel Awards'. Thissatisfies a request from individual and corporate contributors to the 1996 meeting thatsuch a fund be established in honor of Ned Eisenbrey, and permits the use ofinvestment income for encouraging student participation in the Institute.

7. Approved the host location—Marquette—for the 1999 ILSG meeting. Co-chairs willbe Ted Bomhorst (Michigan Technological University) and Bob Regis (NorthernMichigan University).

xiii

geologic studies of Precambrian geology in the Lake Superior region. Another highlight of the meeting for 40 participants was a night of baseball at the Metrodome.

This meeting also broke the mold a bit by offering five field excursions that covered a similarly wide breadth of gwlogy as was presented in the special overview session. Although-the emphasis remained with Precambrian geology, trips highlighting the Paleozoic and Quaternary geology of central and southern Minnesota were also included. Three pre- meeting field trips were run on Wednesday, May 6th. 1) the Early Proterozoic gwlogy of east-central Minnesota led by Terry Boerboom, Mark Jirsa, and Daniel Holm (26 participants); 2) the Keweenawan geology of the Taylor's Falls area led by Karl Wirth, Bill Cordua, Bill Kean, Mike Middleton, and Zach Nairnan (46 participants); and 3) Glacial exotica of the Twin Cites area led by Howard Hobbs, Alan Knaeble and Gary Meyer (21 participants). Two post-meeting trips included a one-day trip on the Paleozoic stratigraphy, sedimentology, and hydrogeology of southeastern Minnesota led by Tony Runkel and Bob Tipping (1 1 participants), and a two-day excursion investigating the Quaternary and Archean geology of the Minnesota River Valley led by Carrie Patterson and David Southwick (33 participants).

The meeting was also a financial success; that is, if your measure of success is to at least break even (which has been the traditionally held view). We actually turned a small profit of several hundred dollars. Although registration was more costly than we had hoped, the cost was unavoidable given the metropolitan location. Part of the registration we all pay is applied to encouraging student participation in the Institute by granting awards for student travel and best student papers. Eight travel awards totaling $750, and two best paper awards of $150 each were granted. Best paper awards went to Kathleen Abbott (Macalester College) for her undergraduate work on geochemistry and petrography of Keweenawan Chengwatana volcanic rocks of Minnesota and Wisconsin, and to Dean Peterson (University of Minnesota- Duluth) for his graduate presentation on GIs-based mineral potential analyses of Archean rocks in Minnesota. Congratulations to you both!.

The Institute's Board of Directors met on May 7th. and the following is generalized from minutes of that meeting:

1. Accepted the Report of the Chair 43rd ILSG 2. Accepted the 1997-1998 ILSG Financial Report. 3. ro roved Zell E. Peterman as 1998 Goldich Medal recipient. 4. Modified the language for Goldich Awards regarding dates for receipt of nominations

(now November 1) and final decision (now the end of November). 5. Approved the end-of-term replacement of Dan England (Eveleth Fee Office) as

Goldich Medal committee member hy Rodney Johnson (Trans Superior Resources). Rod begins a 3-year term in Novemner of 1998.

6. Changed the student travel award's title to "Eisenbrey Student Travel Awards". This satisfies a request from individual and corporate contributors to the 1996 meeting that such a fund be established in honor of Ned =senbrey, and permits the use of &l investment income for encouraging student participation in the Institute.

7. Approved the host location-Marquette~for the 1999 ILSG meeting. Co-chairs will be Ted Bomhorst (Michigan Technological University) and Bob Regis (Northern Michigan University).

x i i i

8. Discussed a potential Y2K meeting location—Thunder Bay or other access point tothe Atikoken area.

9. Discussed a potential 2001 meeting location—Sault Ste. Marie, MI.10. Other business—contact the secretary-treasurer for details

We hope that with the success of the 1998 meeting, the ILSO took an important step towardachieving the goal of expanding its reach, both in terms of the range of geological topics andthe background of participants. While we may not be able to repeat the attendance success ofthis meeting as we move to the more typically remote conference sites, future organizers andthose of us who are long-time participants must do all we can to actively promote the ]LSGto the broadest possible audience. If we had to pinpoint one reason for the success of the1998 meeting, it would be our relentless promotion of the meeting to local geoscienceorganizations, colleges, and individuals. For our part, we will encourage Twin Citiesgeoscientists to continue their involvement, now that they have been exposed to what theILSO has to offer.

We wish to thank all of the presenters, in particular the special session speakers who wroteextended abstracts that will serve as summaries of their fields for years to come. Thisinstitute was built on its tradition of fine field trips, and this year's trips maintained thattradition, thanks in large part to efforts of the leaders. Acknowledgment is also due to thesession chairs, the best student paper award committee, the local planning committee, and theLLSG board members for making our jobs much easier by doing theirs well. A special thanksto Terry Boerboom for his flawless organization of the field trips and guidebook, and to LoriDay, the meeting coordinator, for doing all that it takes. Finally we want to thank ourdirector of the Minnesota Geological Survey, David Southwick, for giving us the freeboardand staff time needed to pull the meeting together.

Jim Miller and Mark JirsaCo-chairs of the 44th Annual ILSG

xiv

8. Discussed a potential Y2K meeting location-Thunder Bay or other access point to the Atikoken area.

9. Discussed a potential 2001 meeting 1ocationÃ‘Saul Ste. Marie, MI. 10. Other busines-ontact the secretary-treasurer for details

We hope that with the success of the 1998 meeting, the ILSG took an important step toward achieving the goal of expanding its reach, both in terms of the range of geological topics and the background of participants. While we may not be able to repeat the attendance success of this meeting as we move to the more typically remote conference sites, future organizers and those of us who are long-time participants must do all we can to actively promote the ILSG to the broadest possible audience. If we had to pinpoint one reason for the success of the 1998 meeting, it would be our relentless promotion of the meeting to local geoscience organizations, colleges, and individuals. For our part, we will encourage Twin Cities geoscientists to continue their involvement, now that they have been exposed to what the ILSG has to offer.

We wish to thank all of the presenters, in particular the special session speakers who wrote extended abstracts that will serve as summaries of their fields for years to come. This institute was built on its tradition of fine field trips, and this year's trips maintained that tradition, thanks in large part to efforts of the leaders. Acknowledgment is also due to the session chairs, the best student paper award committee, the local planning committee, and the ILSG board members for making our jobs much easier by doing theirs well. A special thanks to Terry Boerboom for his flawless organization of the field trips and guidebook, and to Lori Day, the meeting coordinator, for doing all that it takes. Finally we want to thank our director of the Minnesota Geological Survey, David Southwick, for giving us the freeboard and staff time needed to pull the meeting together.

Jim Miller and Mark Jirsa Co-chairs of the 44th Annual ILSG

x i v


Program

4 5 ~ ~ ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY

Program

Program of EventsTuesday, May 4

0800-1730 Field Trip 2

Field Trip 2: Archean Ishpeming Greenstone Belt and GoldMineralization, Michigan - Leader T.J. Bornhorst, D.J. Duskin, R.C.Johnson, R.A. Mahin, T.O. Quigley, and G.W. Scott

Wednesday, May 5

0800-1730 Field Trip 1 and 2

Field Trip 1: Early Proterozoic Strata of the Marquette Iron Range,Michigan - Leader: W.F. Cannon

Field Trip 2: Archean Ishpeming Greenstone Belt and GoldMineralization, Michigan - Leaders: T.J. Bornhorst, D.J. Duskin, R.C.Johnson, R.A. Mahin, T.O. Quigley, and G.W. Scott

1700-2000 Registration - Ramada Inn, Marquette, Michigan

1900-2100 Poster Session and cash bar in Ramada Inn(Authors at posters 1930 to 2100)

xv

Program of Events Tuesday, May 4

0800-1730 Field Trip 2

Field Trip 2: Archean Ishpeming Greenstone Belt and Gold Mineralization, Michigan - Leader T.J. Bornhorst, D.J. Duskin, R.C. Johnson, R.A. Mahin, T.O. Quigley, and G.W. Scott

Wednesday, May 5 , :-3 , .;: . .. : . ,

;+?,

0800- 1730 Field Trip I and 2

Field Trip 1: Early Proterozoic Strata of the Marquette Iron Range, Michigan - Leader: W.F. Cannon

Field Trip 2: Archean Ishpeming Greenstone Belt and Gold Mineralization, Michigan - Leaders: T.J. Bornhorst, D.J. Duskin, R.C. Johnson, R.A. Mahin, T.O. Quigley, and G.W. Scott

1700-2000 Registration - Ramada Inn, Marquette, Michigan

1900-2100 Poster Session and cash bar in Ramada Inn (Authors at posters 1930 to 2100)

1

Thursday, May 6 J

All technical sessions are in Ramada Inn, Marquette, Michigan

0730-1600 Registration - Ramada Inn JTechnical Session I Session Chair: Gene LaBerge (University of Wisconsin, Oshkosh) J

Shawn Carison (Ashton Mining)

0820-0830 Opening - 45 Annual Institute on Lake Superior Geology JR. S. Regis and T.J. Bornhorst, Co-Chairs

0830-0930 Invited PresentationSarah A. Green and Judy Wells-BuddCross Margin Transport in Lake Superior

0930-1000 Invited Presentation JW. Charles Kerfoot, John A. Green, and Lawrence J. WeiderA New Approach to Historical Reconstruction: CombiningDescriptive and Experimental Paleolimnology

1000-1030 Coffee Break J1030-1050 Ojakangas, R.W.

Sedimentology of two Deep Wells in the KeweenawanMidcontinent Rift System Near Munising, Upper Peninsula,Michigan

1050-1110 Puschner, U., Schmidt, S.Th., and Bornhorst, T.J.Low Grade Metamorphism and Hydrothermal Alteration of the JUpper Keweenawan Portage Lake Volcanics, Michigan

1110-1130 Maki, J.C. and Bornhorst, T.J.The Gratiot Chalcocite Deposit, Keweenaw Peninsula, Michigan

1200-1340 Lunch Break

1200-1340 Board of Directors Meeting J

xvH

Thursday, Mav 6

All technical sessions are in Ramada Inn, Marquette, Michigan

0730-1600 Registration - Ramada Inn

Technical Session I Session Chair: Gene LaBerge (University of Wisconsin, Oshkosh) Shawn Carlson (Ashton Mining)

0820-0830 Opening - 45th Annual Institute on Lake Superior Geology R. S. Regis and T.J. Bornhorst, Co-Chairs

0830-0930 Invited Presentation Sarah A. Green and Judy Wells-Budd Cross Margin Transport in Lake Superior

. !' .,: . ' . . . . . 0930- 1000 Invited Presentation

, : " ~. W. Charles Kerfoot, John A. Green, and Lawrence J. Weider A New Approach to Historical Reconstruction: Combining Descriptive and Experimental Paleolimnology

1000- 1030 Coffee Break

. 3'.

1030-1050 Ojakangas, R.W. Sedimentology of two Deep Wells in the Keweenawan Midcontinent Rift System Near Munising, Upper Peninsula, Michigan

1050-1 110 Puschner, U., Schmidt, S.Th., and Bornhorst, T.J. Low Grade Metamorphism and Hydrothermal Alteration of the Upper Keweenawan Portage Lake Volcanics, Michigan

11 10-1 130 Maki, J.C. and Bornhorst, T.J. The Gratiot Chalcocite Deposit, Keweenaw Peninsula, Michigan

1200- 1340 Lunch Break

1200- 1340 Board of Directors Meeting

xv i i

Technical Session II Session Chairs: Rod Johnson (Rod Johnson & Associates, Inc.)Jim Small (Edward Kraemer & Sons)

1340-1400 Easton, R.M.Metamorphic Map of the Canadian Shield of Ontario, Michigan,Minnesota, and Wisconsin

1400-1420 Vitton, S.J. and Subhash, G.Dynamic and Static Strength Strength of Aggregates: An Estimate ofRate Sensitivity of Geological Materials

1420-1440 Yeo, G.M.Is the Janice Lake Unconformity a Link between the Hearne Cratonand the La Ronge - Lynn Lake Volcanic Arc?

1440-1510 Coffee Break

15 10-1530 Czeck, D.M. and Hudleston, P.J.Structural Fabric and Strain for the Seine River Conglomerates at theWabigoon-Quetico Subprovince Boundary Near Mine Centre,Ontario

1530-1550 Cannon, W.F. and Woodruff, L.G.Mercury Distribution in Bedrock, Native Copper Ore and Soils - IsleRoyale National Park, Michigan

1830-1930 Social - Cash Bar

1930-2130 Annual Banquet at Ramada Inn

xvii

Technical Session H Session Chairs: Rod Johnson (Rod Johnson & Associates, Inc.) Jim Small (Edward Kraemer & Sons)

1340-1400 Easton, R.M. Metamorphic Map of the Canadian Shield of Ontario, Michigan, Minnesota, and Wisconsin

1400-1426 itt ton, S.J. and Subhash. G. Dynamic and Static Strength Strength of Aggregates: An Estimate of Rate Sensitivity of Geological Materials , : .. 3 B , . . . . . .

.: ..' , , , i.#&: :>:: ?:'$ . A , '3 1420-1440 Yeo, G.M.

Is the Janice Lake Unconformity a Link between the Hearne Craton . . . . . . ... . . . ! I . - . : and the La Ronge - Lynn Lake Volcanic Arc?

1440- 15 10 Coffee Break

1510-1530 Czeck, D.M. and Hudleston, P.J. Structural Fabric and Strain for the Seine River Conglomerates at the Wabigoon-Quetico Subprovince Boundary Near Mine Centre, Ontario

1530-1550 Cannon, W.F. and Woodruff, L.G. Mercury Distribution in Bedrock, Native Copper Ore and Soils - Isle Royale National Park, Michigan

1830-1930 Social - Cash Bar . . . . . . . .:? . . . S t . + . . .:i"> ,. : , "' , ,. . ,. , . . . . . . . . . . . . .

xvii i

jFriday, May 7

Technical Session III Session Chair: Ron Graber (Cliffs Mining Services Co.) JTom Waggoner (Cliffs Mining Services Co.)

0830-0850 LaBerge, G.L. JRegional Patterns on the Penokean Continental Margin in theSouthern Lake Superior Region

U

0850-0910 Ottke,D.Early Descriptions of the Natural Features on the Marquette jIron Range

0910-0930 Webster, C., Cambray, W., Scott, 0., Nordstrom, P., Wilson, E.Palmer Gneiss: A Large, Low-grade Shear Zone

0930-0950 Diedrich, T.R., and Morton, P.Investigation of CaO at Thunderbird Mine, Mesabi Range: jMineral and Stratigraphic Relationships


1020-1040 Medaris, L.G., Fournelle, J., Boszhardt, R., and Broihahn, J.Chemical and Mineralogical Comparison of Baraboo, Barron,and Sioux Argillite, Metapelite, and Pipestone

1040-1100 Graber, R.G. and Strandlie, A.J.Where are the Metamorphosed Natural Orebodies of the Mesabi jRange?

1100-1120 Scott, G.W. and Lukey, H.M. JGeology of the Tilden Mine, Marquette iron range, Michigan

1120-1140 Meier, J.G.Republic Wetland Preserve

1140-1300 Lunch

U

Jxix j

Friday, May 7 r - Â ¥ f

Technical Session ffl Session Chair. Ron Graber (Cliffs Mining Services Co.) Tom Waggoner (Cliffs Mining Services Co.)

0830-0850 LaBerge, G.L. ~ e ~ i o n a l Patterns on the Peno Southern Lake Superior Regio

0850-0910 Ottke, D.

i.g,,.. , ,, , . ~ * : 1 r o n ~ a n ~ e - ,: L --.

0910-0930 Webster, C., Cambray, W., Scott, G., Nordstrom, P., Wilson, E. Palmer Gneiss: A Large, Low-grade Shear Zone

0930-0950 Diedrich, T.R., and Morton, P. ., ,

'.!, , Investigation of CaO at Thunderbird Mine, Mesabi Range: Mineral and Stratigraphic Relationships

0950- 1020 Coffee Break

1020-1040 Medaris, L.G., Foumelle, J., Boszhardt, R., and Broihahn, J. T*

Chemical and Mineralogical Comparison of Baraboo, Ban-on, and Sioux Argillite, Metapelite, and Pipestone

1040-1 100 Graber, R.G. and Strandlie, A.J. Where are the Metamorphosed Natural Orebodies of the Mesabi Range?

1 100-1 120 Scott, G.W. and Lukey, H.M. Geology of the Tilden Mine, Marquette iron range, Michigan

1120-1 140 Meier, J.G. Republic Wetland Preserve

1 140-1300 Lunch

xix

Technical Session IV Session Chair: John Green (University of Minnesota, Duluth)Dave Baxter (PayDay Resources)

1300-1320 Boerboom, T.J. And Severson, M.Geologic Map of the Western Penokean Orogen, East-CentralMinnesota

1320-1340 Daniels, D.L., Nicholson, S.W., Cannon, W.F., and Bracken, R.E.Preliminary Aeromagnetic Map of Wisconsin

1340-1400 Johnson, R.C. and Bornhorst, T.J.Ishpeming greenstone belt - Evidence for Archean tectonic evolutionof the southern edge of the Superior Province in Michigan

1400-1420 Yeo, G.M.Low-Resolution Sequence Stratigraphy of Early ProterozoicParagneisses, Wollaston Domain, Saskatchewan


1450-15 10 Bickford, M.E. and Steinhart, W.E.Distribution of the Archean to earliest Paleoproterozoic Sask cratonbeneath deformed orogenic rocks in the Trans-Hudson orogen and itstectonic implications: Evidence from common Pb and Sm-Nd isotopicdata

1510-1530 Miller, J.D. Jr.Potential for stratiform PGE mineralization in mafic layeredintrusions of the Duluth Complex

1530-1550 Peterson, D.M. and Morton, R.L.Development of Volcanogenic Massive Sulfide Deposit ExplorationTargets in Northern Minnesota from GIS Spatial Analysis ofGeological, Geochemical, and Geophysical Criteria

1550-16 10 Loughry, J.E., Johnson, Matthew M., and Cotter, J.F.PA Geochemical and Petrologic Study to Determine the Origin of theCrowduck Lake Group, Kenora, Ontario: A ProblematicMetaconglomerate

xx

Technical Session IV Session Chair: John Green (University of Minnesota, Duluth)

I Dave Baxter (PayDay Resources)

1300-1320 Boerboom, T.J. And Severson, M. Geologic Map of the Western Penokean Orogen, East-Central Minnesota :. 4 -?-#

8 1320- 1340 Daniels, D.L., Nicholson, S.W., Cannon, W.F., and Bracken, R.E. Preliminary Aeromagnetic Map of Wisconsin

1400 Johnson, R.C. and Bomhorst, T.J. Ishpeming greenstone belt - Evidence for Archean tectonic evolution of the southern edge of the Superior Province in Michigan

1420 Yeo, G.M. Low-Resolution Sequence Stratigraphy of Early Proterozoic Paragneisses, Wollaston Domain, Saskatchewan

I 1420- 1450 Coffee Break

1450-1510 Bickford, M.E. and Steinhart, W.E.

I Distribution of the Archean to earliest Paleoproterozoic Sask craton beneath deformed orogenic rocks in the Trans-Hudson orogen and its tectonic implications: Evidence from common Pb and Sm-Nd isotopic

I data

I 1510-1530 Miller, J.D. Jr. Potential for stratiform PGE mineralization in mafic layered intrusions of the Duluth Complex

I -

1530-1550 Peterson. D.M. and Morton, R.L.

I Development of Volcanogenic Massive Sulfide Deposit Exploration Targets in Northern Minnesota from GIs Spatial Analysis of Geological, Geochemical, and Geophysical Criteria

1550-1610 Loughry, J.E., Johnson, Matthew M., and Cotter, J.F.P A Geochemical and Petrologic Study to Determine the Origin of the Crowduck Lake Group, Kenora, Ontario: A Problematic Metaconglomerate

—I

JPoster Session

Posters will be up from Wednesday 1900 to Friday 1200

Authors will be at posters on Wednesday from 1930 to 2100

Ams, D. and Holm, D., Characterization and Timing Constraints of Post-PenokeanMeso-Scale Structures in the Watersmeet and Republic Gneiss Domes of NorthernMichigan

Barber-Delach, R.D. and Cannon, W.F., Re-examining Stream Sediment jGeochemical Data from the National Uranium Resource Evaluation Program(NURE), Wisconsin and Northern Michigan

Han, T.M., Megascopic Fossils and Their Possible Contribution to theDevelopment of the Silicate Unit of the Negaunee Iron Formation, Empire Mine, JMarquette Range, North Michigan

Holm, D., Romano, D., and Mancusco, C., Comparison of Mica Ar/Ar and Rb/Sr JThennochronology Results from Northern Wisconsin and Northern Michigan

lUasner, J.S., Cannon, W.F., Schulz, K.J., and LaBerge, G.L., The Iron RiverSyncline: An Allochthonous Structural Panel in the Penokean Foreland ofNorthern Michigan

Miller, J.D. Jr, and Chandler, V.W., New Geologic Map of the Central DuluthComplex

Rausch, D.E., and Wattrus, N.J., Lake Superior's Rings: Clues to the Origin of JPolygonal Fault Systems

Jj

xxi j

Poster Session

Posters will be up from Wednesday 1900 to Friday 1200

Authors will be at posters on Wednesday from 1930 to 2100

Ams, D. and Holm, D., Characterization and Timing Constraints of Post-Penokean Meso-Scale Structures in the Watersmeet and Republic Gneiss Domes of Northern Michigan

Barber-Delach, R.D. and Cannon, W.F., Re-examining Stream Sediment Geochemical Data from the National Uranium Resource Evaluation Program (NURE), Wisconsin and Northern Michigan

Han, T.M., Megascopic Fossils and Their Possible Contribution to the Development of the Silicate Unit of the Negaunee Iron Formation, Empire Mine, Marquette Range, North Michigan

Holm, D., Romano, D., and Mancusco, C., Comparison of Mica AdAr and Rb/Sr Thennochronology Results from Northern Wisconsin and Northern Michigan

Klasner, J.S., Cannon, W.F., Schulz, K.J., and LaBerge, G.L., The Iron River Syncline: An Allochthonous Structural Panel in the Penokean Foreland of Northern Michigan

Miller, J.D. Jr, and Chandler, V.W., New Geologic Map of the Central Duluth Complex

Rausch, D.E., and Wattrus, N.J., Lake Superior's Rings: Clues to the Origin of Polygo

Saturday. May 80800-1730 Field Trip 3 and 4

Field Trip 3: Tilden and Empire Mines of the Marquette Iron Range,Michigan - Leaders: G.W. Scott, P.M. Nordstrom and H.M. Lukey

Field Trip 4: Paleozoic and Glacial Geology from Au Train to GrandMarais, Michigan - Leaders: R.S. Regis and LB. Anderton

xxi

Saturday. Mav 8

0800-1730 Field Trip 3 and 4

Field Trip 3: Tilden and Empire Mines of the Marquette Iron Range, Michigan - Leaders: G.W. Scott, P.M. Nordstrom and H.M. Lukey

Field Trip 4: Paleozoic and Glacial Geology from Au Train to Grand Marais, Michigan - Leaders: R.S. Regis and J.B. Anderton

x x i i


AbstractsAbstracts

CHARACTERIZATION AND TIMING CONSTRAINTS OF POST-PENOKEANMESO-SCALE STRUCTURES IN THE WATERSMEET AND REPUBLIC GNEISSDOMES OF NORTHERN MICHIGAN

David Ams (undergraduate student) and Daniel Hoim, Department of Geology, Kent StateUniversity, Kent OH 44242

We have measured late, meso-scale, ductile and brittle structures present in Early Proterozoic gneissdomes of northern Michigan in order to assess their origin. The Watersmeet and Republic gneiss domeslie north of the region of pervasive post-Penokean 1600-1650 Ma shortening (Hoim et a!., 1998,Geology) and southeast of thick-skinned thrusting which closed the Keweenawan rift at ca. 1080-1860Ma (Cannon, 1994, Geology). The challenge then is to determine whether meso-scale brittle and ductileshear zones formed during the late stages of Early Proterozoic gneiss dome formation or if they representdeformation associated with these younger events.

Watersmeet gneiss dome.Our observations were made on the north-east side of the dome, south of its contact with the overlying

metasedimentary rocks (Hwy 45 exposures). The dominant steep east-west fabric in Archean rocks ofthe dome are cross-cut by steep ductile, semi-brittle, and brittle shear zones. The ductile and larger semi-brittle to brittle shear zones show a dominant N70-80E strike orientation and dip steeply both north andsouth (Fig. 1). Where relative motion indicators exist (offset dikes, pegmatites, etc.), the apparent senseof motion is south-side up. Small scale brittle faults and fractures tend to be oriented NS (with steep eastdips) or NW (with steep southwest dips). Slickenlines on the NW oriented set plunge gently to the NW(Fig. 1).

The coarse, steep, EW fabric of the dome interior becomes more gentle to the north toward the contactwith the metasedimentary rocks and shows a reduction in grain size due to ductile shearing. We attributethis to ductile overprinting of the steep fabric along a basement-cover shear zone. The timing of shearingduring gneiss dome formation is constrained by a 1765 Ma hornblende Ar/Ar plateau date obtained fromthe core rocks and from a concordant muscovite plateau date obtained from metasedimentary rocks justnorth of the contact (Fig. 2; see also Schneider et al., 1996, CJES).

We propose that the ductile and semi-brittle shear zones formed during the late stages of unroofing ofthe gneiss dome (at ca. 1765-1755 Ma). In our model they represent internal deformation of the footwalldome rocks as they were unroofed from beneath the metasedimentary cover rocks along a north-dippingdetachment fault at the basement-cover contact. The brittle shear zones have a completely differentorientation from the ductile and semi-brittle ones. Their orientations and subhorizontal lineations suggestthey may be related to closure of the Keweenawan rift.

Republic gneiss dome.We focused on the numerous chlorite mineralized brittle fracture zones which occur in the Republic

gneiss dome north of Republic (Fig. 3). They cross-cut the —1735 Ma post-tectonic alkali red granite(Xgaf of Sims. 1992) and must be younger than mica cooling ages within the dome (1680-1720 Ma).They have a dominant north-northwest strike and dip moderately to steeply to the west. Most of thelineations plunge gently to the northwest. Kinematic indicators (crystal fiber steps) on a few of theseshow left-lateral, reverse motion. Ductile shear zones within the dome strike dominantly northwest anddip steeply and must be older than the mica cooling ages.

The brittle fractures do not appear to be kinc:iiatically related to the older ductile shear zones alongwhich the gneiss dome block was probably uplifted. The dominant left-lateral strike-slip motion andnorth-northwest attitude indicates they might be also be deformational features associated with closure ofthe Keweenawan rift. This interpretation is consistent with prior thermochronologic evidence for a lowtemperature Keweenawan age disturbance of this region (Schneider et al., 1996, CJES).

2

CHARACTERIZATION AND TIMING CONSTRAINTS OF POST-PENOKEAN MESO-SCALE STRUCTURES IN T H E WATERSMEET AND REPUBLI~ GNEISS DOMES OF NORTHERN MICHIGAN

David Ams (undergraduate student) and Daniel Holm, Department of Geology, Kent State University, Kent OH 44242

We have measured late, meso-scale, ductile and brittle structures present in Early Proterozoic gneiss domes of northern Michigan in order to assess their origin. The Watersmeet and Republic gneiss domes lie north of the region of pervasive post-Penokean 1600-1650 Ma shortening (Holm et al., 1998, Geology) and southeast of thick-skinned thrusting which closed the Keweenawan rift at ca. 1080-1860 Ma (Cannon, 1994, Geology). The challenge then is to determine whether meso-scale brittle and ductile shear zones formed during the late stages of Early Proterozoic gneiss dome formation or if they represent defamation associated with these younger events.

Watersmeet gneiss dome. Our observations were made on the north-east side of the dome, south of its contact with the overlying

metasedimentary rocks (Hwy 45 exposures). The dominant steep east-west fabric in Archean rocks of the dome are cross-cut by steep ductile, semi-brittle, and brittle shear zones. The ductile and larger semi- brittle to brittle shear zones show a dominant N70-80E strike orientation and dip steeply both north and south (Fig. 1). Where relative motion indicators exist (offset dikes, pegmatites, etc.), the apparent sense of motion is south-side up. Small scale brittle faults and fractures tend to be oriented NS (with steep east dips) or NW (with steep southwest dips). Slickenlines on the NW oriented set plunge gently to the NW Pie. 1 1 .--o- - * -

The coarse, steep, EW fabric of the dome interior becomes more gentle to the north toward the contact with the metasedimentary rocks and shows a reduction in grain size due to ductile shearing. We attribute this to ductile overprinting of the steep fabric along a basement-cover shear zone. The timing of shearing during gneiss dome formation is constrained by a 1765 Ma hornblende ArIAr plateau date obtained from the core rocks and from a concordant muscovite plateau date obtained from metasedimentary rocks just north of the contact (Fig. 2; see also Schneider et al., 1996, CJES).

We propose thal theductile and semi-brittle shear zones Conned during the late stages of unroofing of the gneiss dome (at ca. 1765-1755 Ma). In our model thev remesent internal deformation of the footwall dome rocks as they were unroofed from beneath the metasedimentary cover rocks along a north-dipping detachment fault at the basement-cover contact. The brittle shear zones have a completely different orientation from the ductile and semi-brittle ones. Their orientations and subhorizontal lineations suggest they may be related to closure of the Keweenawan rift.

Republic gneiss dome. We focused on the numerous chlorite mineralized brittle fracture zones which occur in the Republic

gneiss dome north of Republic (Fig. 3). They cross-cut the -1735 Ma post-tectonic alkali red granite (Xgaf of Sims, 1992) and must be younger than mica cooling ages within the dome (1680-1720 Ma). They have a dominant north-northwest strike and dip moderately to steeply to the west. Most of the lineations plunge gently to the northwest. Kinematic indicators (crystal fiber steps) on a few of these show left-lateral, reverse motion. Ductile shear zones within the dome strike dominantly northwest and dip steeply and must be older than the mica coo1i.i; ages.

The brittle fractures do not appear to be kincniatically related to the older ductile shear zones along which the gneiss dome block was probably uplifted. The dominant left-lateral strike-slip motion and north-northwest attitude indicates they might be also be defonnational features associated with closure of the Keweenawan rift. This interpretation is consistent with prior thennochronologic evidence for a low temperature Keweenawan age disturbance of this region (Schneider et al., 1996, CJES).

Fig. 1. Stereonet plots of ductile, semi-brittle, and brittle shear zones withinthe Watersmeet gneiss dome.

%39ArFig. 2. Ar/Ar mineral age spectra from the Watersmeet gneiss dome region.

Tp is plateau date. Tg is total gas date.

Fig. 3. Stereonet plots of representative ductile shear zones and mineralizedbrittle fracture zones, Republic gneiss dome region.

3

Smaller britile fracture zones

2000: Amphibolite within the domeAGEMa

1500 94-MI-3-H- Hornblende

Tg=1751±l3Ma- Tp=1765±l3Ma

1000-0_

Schist outside of the dome—

UP-i OA-M- Muscovite- Tg=1760±i4Ma

Tp=1767±l7Ma

%39Ar 100 0 100

Ductile shear zones

Fig. 1. Stereonet plots of ductile, semi-brittle, and brittle shear zones within the Watersmeet gneiss dome.

2000J imnhibolite within the dome 1 -1 Schist outside of the dome 1

94-MI-3-H Hornblende T g = 1751k13Ma

UP-1 0A-M Muscovite 1

Fig. 2. Ar/Ar mineral age spectra from the Watersmeet gneiss dome region. Tp is plateau date. Tg Is total gas date.

Fig. 3. Stereonet plots of representative ductile shear zones and mineralized brittle fracture zones. Republic gneiss dome region.

Re-examining stream sediment geochemical data from the National Uranium ResourceEvaluation Program (NURE), Wisconsin and northern Michigan

Robert D. Barber-Delach, Oak Ridge Associated Universities, at U.S. Geological Survey. Mail Stop954, Reston, VA 20192W.F. Cannon, U.S. Geological Survey, Mail Stop 954, Reston, VA 20192

In the latter 1970's a stream sediment geochemical survey was conducted in parts of northernWisconsin and northern Michigan by the NURE program of the United States Department of Energy. Adata set consisting of multi-element analyses of 3185 samples was developed spanning eight I °x2°quadrangles. The data were intended for use in evaluating undiscovered uranium resources, but alsohave application for establishing regional variations in geochemical background and baselineconcentrations. The U.S. Geological Survey is ____________________________________________expanding research on regional geochemicalpatterns, and the NURE data set is being re-examined as part of that effort.

In many places the NURE data shows obviousbiases in values between adjacent quadrangles,in some cases apparent only by visualexamination of element concentration maps. Forinstance, Figure la shows significantdiscontinuities of cobalt values betweenquadrangles in the original NURE data. Biaseswere probably introduced by somewhat differentsampling, processing, and analytic techniquesbetween various quadrangles, and probably alsoby time-dependent changes in analyticalinstrument calibration that appear not to havebeen fully accounted for over the duration of theNURE program. These quadrangle-based biasesrender use of the data problematical withoutcareful assessment and correction of analyticalbiases. Here we present two approaches bywhich the NURE data set can be corrected forquadrangle biases.First, systematic biases in analytical databetween quadrangles can be normalized bydisplaying data for each quadrangle as afunction of the statistical distribution of valuesfor only that quadrangle. Figure lb shows thesame data for cobalt as shown in Figure Ia, butwith the values within each of the eightquadrangles classified separately intopercentiles of the data set from that quadrangle.The between-quadrangle bias is largely removedand natural patterns of cobalt variation are more

4

Figure I. Comparison of unadjusted (a) andnormalized (b) cobalt concentrations for use indisplaying NURE stream sediment data.

a. Unadjusted data

Re-examining stream sediment geochemical data f rom the National Uran ium Resource Evaluation P rog ram (NURE), Wisconsin and northern Michigan

Robert D. Barber-Delach, Oak Ridge Associated Universities, at U.S. Geological Survey, Mail Stop 954, Reston, VA 20192 W.F. Cannon, U.S. Geological Survey, Mail Stop 954, Reston, VA 20192

In the latter 1970's a stream sediment geochemical survey was conducted in parts of northern Wisconsin and northern Michigan by the NURE program of the United States Department of Energy. A data set consisting of multi-element analyses of 3 185 samples was developed spanning eight l0x2' quadrangles. The data were intended for use in evaluating undiscovered uranium resources, but also have application for establishing regional variations in geochemical background and baseline concen&itions. The U.S. ~ e o l o ~ i c a l Survey is expanding research on regional geochemical patterns, and the MURE data set is being re- examined as part of that effort.

In many places the NURE data shows obvious biases in values between adjacent quadrangles, in some cases apparent only by visual examination of element concentration maps. For instance, Figure l a shows significant discontinuities of cobalt values between quadrangles in the original NURE data. Biases were probably introduced by somewhat different sampling, processing, and analytic techniques between various quadrangles, and probably also by time-dependent changes in analytical instrument calibration that appear not to have been fully accounted for over the duration of the NURE program. These quadrangle-based biases render use of the data problematical without careful assessment and correction of analytical biases. Here we present two approaches by which the NURE data set can be corrected for quadrangle biases. First, systematic biases in analytical data between quadrangles can be normalized by displaying data for each quadrangle as a function of the statistical distribution of values for only that quadrangle. Figure Ib shows the same data for cobalt as shown in Figure la, but with the values within each of the eight quadrangles classified separately into percentiles of the data set from that quadrangle. The between-quadrangle bias is largely removed and natural patterns of cobalt variation are more

Figure 1. Comparison of unadjusted (a) and normalized (b) cobalt concentrations for use in displaying NURE stream sediment data.

a. Unadjusted data . ,do.-

. adom.

b. Data normalized within quadrangles

evident. This technique has the advantage of ease and simplicity in that standard desktop GIS softwarecan perform and display the results with minimal statistical manipulation of data and time required forprocessing. Also, the technique does not require acquisition of new analytical data. However, twoimportant shortcomings of the technique are: I) it presents only relative variations of elementconcentrations and does not improve the overall accuracy of the NURE data, 2) it masks true variationsthat occur at the approximate scale of a l°x2° quadrangle.

A second technique requires new analytical data. In the 1980's, the USGS became the officialarchiving agency for NIJRE data and samples, and maintains a sample repository in Denver, Coloradoin which all N'URE samples are accessible for restudy. We have re-analyzed a subset of those samplesfor the Ashland and Rice Lake l°x2° quadrangles in northern Wisconsin and compared the results of ourre-analyses to the original data reported by the NURE program. Our analyses were done using routine40-element inductively coupled plasma-atomic emission spectroscopy (ICP-AES) Figure 2. Scatterplot of magnesium concentrationtechnique supplemented by more precise (ppm) demonstrating the quad bias of original NUREsingle-element techniques for arsenic and data, with fined regression lines and correlationselenium. Analytical precision and indices (R2).accuracy were monitored by replicateanalyses of standard samples. 2.1

- —

-

Comparison of our new values to 1.8 1concentrations reported by NURE show •0 —, $bquadrangle-based biases in virtually all • . 7elements for which a significant number , 1.5

—— 000of samples exceed detection limits (about ,. 720 elements). Figure 2 shows an extreme i.case in which NURE magnesium analyses • 0are strongly and consistently quadrangle- , — 09dependent and in which both quadrangles 0.9 -vary significantly from our new analyses. j,This consistency of bias within a 0.6quadrangle allows derivation of equationsthat can adjust the original NURE data to 03

•Rice Lake Quad

correct for previous analytical biases and -

I oAshland Quadproduce a better accord with new 7analytical data. Although this technique 0 -has inherent statistical uncertainty, that 0 0.3 0.6 0.9 1,2 1.5 1.8 2.1

uncertainty is quantifiable, and the Re-Analyzed Mg Concentrationresulting element maps are significantlymore accurate than those made from theoriginal NURE data. Although this technique does require the cost and time of re-analyzing asignificant number of NURE samples, the result produces a much improved data set without the cost ofre-analyzing the entire sample set. We plan to proceed with this technique during the coming year.

5

evident. This technique has the advantage of ease and simplicity in that standard desktop GIs software can perform and display the results with minimal statistical manipulation of data and time required for processing. Also, the technique does not require acquisition of new analytical data. However, two important shortcomings of the technique are: 1) it presents only relative variations of element concentrations and does not improve the overall accuracy of the NURE data, 2) it masks true variations that occur at the approximate scale of a l0x2"quadrangle.

A second technique requires new analytical data. In the 1980's, the USGS became the official archiving agency for NURE data and samples, and maintains a sample repository in Denver, Colorado in which all NURE samples are accessible for restudy. We have re-analyzed a subset of those samples for the Ashland and Rice Lake Iox2' quadrangles in northern Wisconsin and compared the results of our re-analyses to the original data reported by the NURE program. Our analyses were done using routine 40-element inductively coupled plasma- atomic emission spectroscopy (ICP-AES) technique supplemented by more precise single-element techniques for arsenic and selenium. Analytical precision and accuracy were monitored by replicate analyses of standard samples.

Figure 2. Scatterplot of magnesium concentration (ppm) demonstrating the quad bias of original NURE 1 data, with fitted regression lines and correlation indices (R').

2.1 , ,

Comparison of our new values to concentrations reported by NURE show quadrangle-based biases in virtually all elements for which a significant number of samples exceed detection limits (about 20 elements). Figure 2 shows an extreme case in which NURE magnesium analyses are strongly and consistently quadrangle- dependent and in which both quadrangles vary significantly from our new analyses. This consistency of bias within a quadrangle allows derivation of equations that can adjust the original NURE data to correct for previous analytical biases and produce a better accord with new analytical data. Although this technique has inherent statistical uncertainty, that uncertainty is quantifiable, and the resulting element maps are significantly more accurate than those made from the

, ,

Â¥Ri Lake Quad

, OAshland Quad

L

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1

Re-Analyzed Mg Concentration

original NURE data. Although this technique does require the cost and time of re-analyzing a significant number of NURE samples, the result produces a much improved data set without the cost of re-analyzing the entire sample set. We plan to proceed with this technique during the coming year.

Distribution of the Archean to Earliest Paleoproterozoic Sask Craton Beneath DeformedOrogenic Rocks in the Trans-Hudson Orogen and its Tectonic Implications: Evidence fromCommon Pb and Sm-Nd Isotopic Data

M. E. Bickford and William E. Steinhart, ifi, Department of Earth Sciences, Heroy GeologyLaboratory, Syracuse University, Syracuse, NY 13244-1070

Late, undeformed, ca. 1.8 Ga pegmatites, leucogranitic sheets, and post-orogenic plutons in theGlennie, Hanson Lake, and La Ronge Domains, and parts of the Hearne Province, were studiedto determine whether they were derived by melting ofjuvenile orogenic crust or of Archeanlower crustal rocks equivalent to those exposed in structural windows in the Glennie and HansonLake Domains. The isotopic composition of Pb from feldspars (206Pb/2°4Pb = 14.3 - 15.2;= 15.1 - 15.25; 208Pbf°4Pb = 34.4-36.4) and Sm-Nd TDM ages of ca. 3.2 Gafor ca. 1.8 Ga post-orogenic intrusions reveal that Archean rocks are not only exposed in thewindows, but also extensively underlie parts of the Glennie and Hanson Lake Domains in whichonly juvenile Paleoproterozoic rocks are exposed. In contrast, feldspar Pb from pegmatites in theLa Ronge Domain is isotopically similar (2°6Pb/2°'Pb = 15.4- 15.6; 207Pb/2"Pb = 15.1 - 15.2;208Pb/204Pb 34.8 - 35.2) to that ofjuvenile, ca. 1.85 Ga Paleoproterozoic plutons with TDMages of ca. 1.80-2.0 Ga, indicating derivation from similar oceanic materials. Sm-Nd data fromtwo La Ronge Domain aplites, in which Sm and Nd are apparently unfractionated, yield TDM agesof 1.8-. 19 Ga. Surprisingly, feldspar Pb data (2°'Pb/2°4Pb = 15.5- 15.8; 207Pb/204Pb = 15.1 - 15.2;= 35.0 - 35.2) from pegmatites that intrude Archean rocks of the Hearne Province, inthe hinterland west of the Needle Falls Shear Zone, also indicate derivation from juvenile sources.Thus, while these data support the view that an Archean microcontinent ("Sask Craton")underlies parts of Glennie and Hanson Lake Domains, they suggest that crust beneath La RongeDomain, and also beneath at least parts of Hearne Province, is ofjuvenile mantle derivation.Alternatively, Archean crust may be present beneath the La Ronge Domain and Hearne Province,but was below the zone of melting when the ca. 1.8 Ga pegmatites were formed. Theseobservations are consistent with interpretation of northwesterly-dipping reflectors beneathHearne Province, as revealed by Canadian Lithoprobe seismic data, as representing subductedoceanic lithosphere. The presence of oceanic lithosphere between Hearne Province and GlennieDomain indicates that the Archean microcontinent beneath the Glennie Domain is probably notsubducted Hearne crust, but more likely an exotic crustal fragment.

6

Distribution of the Archean to Earliest Paleoproterozoic Sask Craton Beneath Deformed Orogenic Rocks in the Trans-Hudson Orogen and its Tectonic Implications: Evidence from Common Pb and Sm-Nd Isotopic Data

M. E. Bickford and William E. Steinhart, III, Department of Earth Sciences, Heroy Geology Laboratory, Syracuse University, Syracuse, NY 13244-1070

Late, undefonned , ca. 1.8 Ga pegmatites, leucogranitic sheets, and post-orogenic plutons in the Glennie, Hanson Lake, and La Ronge Domains, and parts of the Hearne Province, were studied to determine whether they were derived by melting ofjuvenile orogenic crust or of Archean lower crustal rocks equivalent to those exposed in structural windows in the Glennie and Hanson Lake Domains. The isotopic composition of Pb from feldspars (20Ã̂pb/- = 14.3 - 15.2; m7~b/2d4pb = 15.1 - 15.25; ̂ "Pb/'^Pb = 34.4 - 36.4) and Sm-Nd TDM ages of ca. 3.2 Ga for ca. 1.8 Ga post-orogenic intrusions reveal that Archean rocks are not only exposed in the windows, but also extensively underlie parts of the Glennie and Hanson Lake Domains in which only juvenile Paleoproterozoic rocks are exposed. In contrast, feld ar Pb from pegmatites in the La Ron e Domain is isotopically similar ('"'Pb/^'Pb = 15.4 - 15.6; b/-b = 15.1 - 15.2; 4 =% "'PW b = 34.8 - 35.2) to that ofjuvenile, ca. 1.85 Ga Paleoproterozoic plutons with TDM ages of ca. 1.80-2.0 Ga, indicating derivation from similar oceanic materials. Sm-Nd data from two La Ronge Domain aplites, in which Sm and Nd are apparently unfl-actionated, yield TDM ages of 1.8-.19Ga. Surprisingly,feldsparPbdata (""Pb/^Pb= 15.5 - 15.8;^Pb/^'Pb= 15.1 - 15.2; m'~b/-b = 35.0 - 35.2) from pegmatites that intrude Archean rocks of the Heame Province, in the hinterland west of the Needle Falls Shear Zone, also indicate derivation from juvenile sources. Thus, while these data support the view that an Archean microcontinent ("Sask Craton") underlies parts of Glennie and Hanson Lake Domains, they suggest that crust beneath La Ronge Domain, and also beneath at least parts of Heame Province, is ofjuvenile mantle derivation. Alternatively, Archean crust may be present beneath the La Ronge Domain and Hearne Province, but was below the zone of melting when the ca. 1.8 Ga pegmatites were formed. These observations are consistent with interpretation of northwesterly-dipping reflectors beneath Heme Province, as revealed by Canadian Lithoprobe seismic data, as representing subducted oceanic lithosphere. The presence of oceanic lithosphere between Heame Province and Glennie Domain indicates that the Archean microcontinent beneath the Glennie Domain is probably not subducted Heme crust, but more likely an exotic crustal fragment.

GEOLOGIC MAP OF THE WESTERN PENOKEAN OROGEN, EAST-CENTRAL MINNESOTABOERBOOM, Terrence J., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114;boerboOl @maroon.tc.umn.edu; and SEVERSON. Mark, Natural Resources Research Institute, Universityof Minnesota, Duluth, MN 55811; [email protected]

The Minnesota Geological Survey, in cooperation with the Natural Resources Research Institute, has recentlycompleted a revised geologic map of the western portion of the Penokean Orogen in east-central Minnesota.The geologic framework for the western Penokean Orogen has been previously interpreted by Southwickand others (1988) using plate-tectonic theory (see discussion below). This map builds on that base byutilizing new drilling information and better geophysical data. Although thousands of exploratory holeswere drilled in this area from the late l800s to the 1950s, nearly all focused on locating iron, manganese, orsulfur ore. As a result, the distribution and form of the major iron-formations both within and outside theiron-mining districts are fairly well established, but little is known about the areas in between. Thus, anygeologic compilation necessarily relies heavily on geophysical interpretations.

GEOLOGIC FRAMEWORK

Southwick (1988) interpreted the Penokean Orogen to consist of several structural panels that range fromsedimentary rocks filling a foreland basin, through a main fold-and-thrust belt of sedimentary and volcanicrocks, to a complex metamorphic-plutonic terrane. The major terranes, referred to as panels in the listbelow, are separated by linear, northeast-trending aeromagnetic anomalies, which are interpreted as thrustfaults that truncate stratigraphy-related aeromagnetic anomalies within the panels. The following summaryof the structural panels is condensed from Southwick and others (1988) (Fig. 1), and modified accordinglyfor this study based on new information.

1. The Animikie basin is filled with essentially unmetamorphosed sedimentary rocks (Pokegama Quartzite,Biwabik Iron Formation, and Virginia Formation). The Animikie strata generally have a shallow southdip, but along the southern basin margin, and hence this map area, are deformed into fairly tight uprightfolds. In this area of the map, these strata unconformably overlie rocks of the fold- and thrust-belt.

2. The North Range panel is dominated by weakly metamorphosed argillite, siltstone, quartzite, iron-formation, graphitic slate, graywacke, and minor limestone and chert. Volcanic and hypabyssal maficrocks are a minor component of the North Range. Structurally, the North Range is dominated by a largenorthwest-inclined, doubly plunging synclinorium. Geophysical models (Carlson, 1985) show that thefolded Trommald iron-formation continues to the east beneath rocks of the Animikie Basin. The structuraland stratigraphic attributes of North Range rocks are characteristic of those deposited in an externaltectonic zone.

3. The South Range panel also contains an abundance of slaty and argillaceous rocks, but unlike the NorthRange, it has a substantial component of mafic volcanic and hypabyssal intrusive rocks, all of whichhave undergone low-grade metamorphism. The iron-formation in the South Range resembles Algoma-type iron-formations in that it occurs as a series of Jenses closely associated with mafic volcanic rocksand graphitic slate. This panel is characterized by narrow linear aeromagnetic anomalies. The structureof the South Range is dominated by elongate east-northeast-trending overturned folds.

The South Range panel is separated from the Moose Lake-Glen Township panel to the south byanarrow zone of graphite-rich, crenulated phyllitic schist that curves from northeast to east-west. Thisgraphitic schist may have absorbed a great deal of strain during thrust faulting.

4. The Moose Lake-Glen Township panel is dominated by quartz-rich metaclastic rocks and by maficvolcanic and hypabyssal intrusive rocks, along with graphitic argillite and schist; all were subjected tolower greensehist-facies metamorphism. This panel includes the Glen Township Formation, a sulfide-rich graphitic schist grading into a sulfide iron-formation that contains pyrrhotite, pyrite, lesserchalcopyrite, and minor sphalerite. The stratigraphic section at Glen Township consists of amygdaloidalbasalt overlying the sulfidic-graphitic schist, which in turn overlies graywacke. In places the latter twoare separated by slaty iron-formation. The base of the sequence comprises quartz-rich sedimentaryrocks that typically contain up to several percent magnetite, which causes a moderate magnetic anomaly.Several prominent linear aeromagnetic anomalies apparently reflect some combination of thrust faultswith relatively minor displacement, and stratigraphic and lithologic variation. The Glen Township stratahave undergone at least two generations of folding that produce complex aeromagnetic patterns.

GEOLOGIC MAP OF THE WESTERN PENOKEAN OROGEN, EAST-CENTRAL MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114; boerb00l @maroon.tc.umn.edu; and SEVERSON, Mark, Natural Resources Research Institute, University of Minnesota, Duluth. MN 5581 1; [email protected]

The Minnesota Geological Survey, in cooperation with the Natural Resources Research Institute, has recently comnleted a revised eeoloeic man of the western nortion of the Penokean Oroeen in east-central Minnesota. The geologic framework for thewestern ~ e n o k e k Orogen has been previously interpreted by Southwick and others (1988) using plate-tectonic theory (see discussion below). This map builds on that base by utilizing new drilling information and better geophysical data. Although thousands of exploratory holes were drilled in this area from the late 1800s to the 1950s. nearly all focused on locating iron, manganese, or sulfur ore. As a result, the distribution and form of the major iron-formations both within and outside the iron-mining districts are fairly well established, but little is known about the areas in between. Thus, any geologic compilation necessarily relies heavily on geophysical interpretations.

Southwick (1988) interpreted the Penokean Orogen to consist of several structural panels that range from sedimentary rocks filling a foreland basin, through a main fold-and-thrust belt of sedimentary and volcanic rocks, to a complex metamorphic-plutonic terrane. The major terranes, referred to as panels in the list below, are separated by linear, northeast-trending aeromagnetic anomalies, which are interpreted as thrust faults that truncate stratigraphy-related a e r ~ r n a ~ e t i c a n o d i e s within the panels. The following summary of the structural panels is condensed from Southwick and others (1988) (Fig. I ) , and modified accordingly for this study based on new information.

1. The Animikie basin is filled with essentially unmetamorphosed sedimentary rocks (Pokegama Quartzite, Biwabik Iron Formation, and Virginia Formation). The Animikie strata generally have a shallow south dip, but along the southern basin margin, and hence this map area, are deformed into fairly tight upright folds. In this area of the map, these strata unconformably overlie rocks of the fold- and thrust-belt.

2. The North Range panel is dominated by weakly metamorphosed argillite, siltstone, quartzite, iron- formation, graphitic slate, graywacke, and minor limestone and chert. Volcanic and hypabyssal mafic rocks are a minor component of the North Range. Structurally, the North Range is dominated by a large northwest-inclined, doubly plunging synclinorium. Geophysical models (Carison, 1985) show that the folded Trommald iron-formation continues to the east beneath rocks of thehimikie Basin. The structural and stratigraphic attributes of North Range rocks are characteristic of those deposited in an external tectonic zone.

3. The South Range panel also contains an abundance of slaty and argillaceous rocks, but unlike the North Ranee. it has a substantial comoonent of rnafic volcanic and hvpabyssal intrusive rocks, all of which haveundergone low-grade metakorphism. The iron-formation in the South Range resembles Algoma- type iron-formations in that it occurs as a series of lenses closely associated with mafic volcanic rocks and graphitic slate. This panel is characterized by narrow linear aeromagnetic anomalies. The structure of the South Range is dominated by elongate east-northeast-trending overturned folds.

The South Range panel is separated from the Moose Lake-Glen Township panel to the south by a narrow zone of graphite-rich, crenulated phyllitic schist that curves from northeast to east-west. This graphitic schist may have absorbed a great deal of strain during thrust faulting.

4. The Moose Lake-Glen Township panel is dominated by quartz-rich metaclastic rocks and by mafic volcanic and hypabyssal intrusive rocks, along with graphitic argillite and schist; all were subjected to lower greenschist-facies metamorphism. This panel includes the Glen Township Formation, a sulfide- rich graphitic schist grading into a sulfide iron-formation that contains pyrrhotite, pyrite, lesser chalcopyrite, and minor sphalerite. The stratigraphic section at Glen Township consists of amygdaloidal basalt overlying the sulfidic-graphitic schist, which in turn overlies graywacke. In places the latter two are separated by slaty iron-formation. The base of the sequence comprises quartz-rich sedimentary rocks that typically contain up to several percent magnetite, which causes a moderate magnetic anomaly. Several prominent linear aeromagnetic anomalies apparently reflect some combination of thrust faults with relatively minor displacement, and stratigraphic and lithologic variation. The GlenTownship strata have undergone at least two generations of folding that produce complex aeromagnetic patterns.

A previously unrecognized, syn- to late-orogenic monzonitic intrusion with a well-defined circularaeromagnetic anomaly was intersected by core drilling just northwest of Mule Lacs Lake. Another corehole drilled on this anomaly intersected ten feet of a brick-red, unmetamorphosed quartz-pebbleconglomerate. This unit is geophysically invisible and its extent is thus unknown, but it may representa conglomerate that was deposited during Mesoproterozoic time in a shallow basin on the backside ofthe Midcontinent rift, and thus is correlative with Keweenawan rocks to the east.

5. The McGrath-Little Falls panel, the southern- and internal-most component of the fold- and thrust-belt,comprises granitic to tonalitic gneiss, amphibolite-facies pelitic schist, and late- to post-tectonic graniticintrusions. An uncommon lamprophyre dike that post-dates Penokean granites is composed of fresh,euhedral clinopyroxene in a groundmass of poikilitic feldspar; it can be traced across the entire map areaby its pronounced reverse aeromagnetic anomaly. Abundant small circular to elongate positive mag-netic anomalies are indicative of mafic-ultramafic plugs similar to those that have been drilled at severallocations to the south and east. Based on aeromagnetic data, these plugs post-date the lamprophyredike. The dike is conjectured to be related to the waning stages of Penokean granite magmatism.

References:Carlson, K.E., 1985, A combined analysis of gravity and magnetic anomalies in east-central Minnesota:

M.S. thesis, University of Minnesota, Minneapolis, 138 p.Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the Penokean

Orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Reportof Investigations 37, 25 p.

8

Figure 1. Regional tectonic interpretationshowing the tectonic elements discussed inthe abstract- From Southwick and others(1998). This map does not reflect changesmade on the map accompanying this poster.

Area of revised geologic map

A previously unrecognized, syn- to late-orogenic monzonitic intrusion with a well-defined circular aeromaenetic anomalv was intersected bv core drilline iust northwest of Mille Lacs Lake. Another core hole drilled on this anomaly intersected ten feet o f a brick-red, unmetamorphosed quartz-pebble conglomerate. This unit is geophysically invisible and its extent is thus unknown, but it may represent a conelomerate that was deposited durine Meso~roterozoic time in a shallow basin on the backside of the Midcontinent rift, and thus is correlative with Keweenawan rocks to the east

5. The McGrath-Little Falls panel, the southern- and internal-most component of the fold- and thrust-belt, comnrises eranitic to tonalitic eneiss. amnhibolite-facies nelitic schist. and late- to nost-tectonic eranitic intr6sions.~n uncommon la&ronhvredike that nost-dates ~enokekn =mites is comnosed of fresh. ~ -~~ - ~ ~ ~ - - ~ ~ ~ ~ ~ ~ - ~ ~ -~~~~~ r~ - r ~ ~ , ~ - ~~~-~~ - - ~ - ~~~-~~~ ~~~~ -~ ~~~~ ~ ~ ~ ~

euhedral clinopyroxene in a groundmass of poikilitic feldspar; it can be traced across the entire map area by its pronounced reverse aeromagnetic anomaly. Abundant small circular to elongate positive magnetic anomalies are indicative of mafic-ultramafic plugs similar to those that have been drilled at several locations to the south and east. Based on aerom&$etic data, these plugs post-date the lamprophyre dike. The dike is conjectured to be related to the waning stages of Penokean granite magmatism.

References: Carlson, K.E., 1985, A combined analysis of gravity and magnetic anomalies in east-central Minnesota:

MS . thesis. University of Minnesota, Minneapolis, 138 p. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1 :250,000) of the Penokean

Orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37.25 p.

Figure 1. Regional tectonic interpretation showing the tectonic elements discussed in

Mercury distribution in bedrock, native copper ore, and soils—Isle Royale National Park,Michigan

W.F. Cannon, U.S. Geological Survey, Reston, VaL.G. Woodruff, U.S. Geological Survey, St. Paul, MN

In response to recent findings of elevated mercury in fish in some inland lakes on Isle Royale, wehave begun a study of mercury concentration in natural geologic materials of the island. Initiallywe aimed to determine if elevated mercury from native copper deposits could be contributingmercury to the impacted lakes. Small native copper deposits on the island were minedsporadically in the 1800's. Those deposits, waste dumps at abandoned mines, and wastes fromsmall processing operations, do contain anomalous amounts of mercury. Our analyses havefound individual samples with as much as 14 parts per million (ppm) mercury, and stamp sandswere found to contain from 0.6 to 0.7 ppm. In contrast, typical bedrock of the island, a sequenceof volcanic basalt flows, appears to be uniformly very low in mercury. Samples we haveanalyzed all had no detectable mercury at a detection limit of 5 parts per billion (ppb).

To determine if mercury contained in mineral deposits could be a source of mercury to impactedlakes we sampled and analyzed soil and glacial deposits on which the soils formed in two testareas. Samples were taken around the Minong mine, the largest mine on the island, to determinethe extent to which anomalous metal content of the ore and surrounding altered rocks affectedthe trace element content of soils. Also, samples were taken within the drainage basins ofSargent Lake and Lake Wagejo, two of the lakes with elevated mercury in fish. No copperdeposits are known within those basins, but deposits could be concealed beneath extensiveglacial deposits, and, if present, could be a contributor of mercury. In total, samples were takenat 65 sites. At each site a sample was taken from the A soil horizon, a dark organic mineral soil,1-6 inched below the surface. A second sample was taken from a depth of about 2 feet,consisting of either reddish sandy glacial till or till slightly modified by soil-forming processes(referred to here as BC horizon).

Results show that the copper deposit at Minong mine produces a very distinct trace elementanomaly in soil, especially the copper content of the BC horizon. Mercury also is somewhatelevated in samples of the BC horizon closest to the copper deposit. Mean copper content of BCsoils in the Minong mine area is 342 ppm and that of mercury is 44 ppb. Highest values are 1160ppm copper and 110 ppb mercury. The A soil horizon near the Minong mine averages 290 ppmCu and 70 ppb Hg. In the drainage basins feeding Lake Wagejo and Sargent Lake, BC soilsaverage 60 ppm Cu, nearly five times lower than near Minong mine, and 38 ppb Hg, essentiallythe same as at Minong mine. A-horizon soils in the Sargent-Wagejo drainage average 63 ppmCu and 133 ppb Hg. The maximum value for Hg is 370 ppb, an exceptionally high value for soilin this region. For comparison, similar soil geochemical studies that we are conducting in theChequamegon National Forest in northern Wisconsin show A-horizon soils to have a mean Hgcontent of about 54 ppb with a maximum detected concentration of 260 ppb. C-horizon soils inthat area have a mean of 19 ppb Hg and a maximum of 80 ppb. With respect to soils, Isle

9

Mercury distribution in bedrock, native copper ore, and soils-Isle Royale National Park, Michigan

W.F. Cannon, U.S. Geological Survey, Reston, Va L.G. Woodruff, U.S. Geological Survey, St. Paul, MN

In response to recent findings of elevated mercury in fish in some inland lakes on Isle Royale, we have begun a study of mercury concentration in natural geologic materials of the island. Initially we aimed to determine if elevated mercury from native copper deposits could be contributing mercury to the impacted lakes. Small native copper deposits on the island were mined sporadically in the 1800's. Those deposits, waste dumps at abandoned mines, and wastes from small processing operations, do contain anomalous amounts of mercury. Our analyses have found individual samples with as much as 14 parts per million (ppm) mercury, and stamp sands were found to contain from 0.6 to 0.7 ppm. In contrast, typical bedrock of the island, a sequence of volcanic basalt flows, appears to be uniformly very low in mercury. Samples we have analyzed all had no detectable mercury at a detection limit of 5 parts per billion (ppb).

To determine if mercury contained in mineral deposits wuld be a source of mercury to impacted lakes we sampled and analyzed soil and glacial deposits on which the soils formed in two test areas. Samples wex taken around the Minong mine, the largest mine on the island, to determine the extent to which anomalous metal content of the ore and surrounding altered rocks affected the trace element content of soils. Also, samples were taken within the drainage basins of Sargent Lake and Lake Wagejo, two of the lakes with elevated mercury in fish. No copper deposits are known within those basins, but deposits wuld be concealed beneath extensive glacial deposits, and, if present, could be a contributor of mercury. In total, samples were taken at 65 sites. At each site a sample was taken from the A soil horizon, a dark organic mineral soil, 1-6 inched below the surface. A second sample was taken from a depth of about 2 feet, consisting of either reddish sandy glacial till or till slightly modified by soil-forming processes (referred to here as BC horizon).

Results show that the copper deposit at Minong mine produces a very distinct trace element anomaly in soil, especially the copper content of the BC horizon. Mercury also is somewhat elevated in samples of the BC horizon closest to the copper deposit. Mean copper content of BC soils in the Minong mine area is 342 pprn and that of mercury is 44 ppb. Highest values are 1160 pprn copper and 1 10 ppb mercury. The A soil horizon near the Minong mine averages 290 pprn Cu and 70 ppb Hg . In the drainage basins feeding Lake Wagejo and Sargent Lake, BC soils average 60 pprn Cu, nearly five times lower than near Minong mine, and 38 ppb Hg, essentially the same as at Minong mine. A-horizon soils in the Sargent-Wagejo drainage average 63 pprn Cu and 133 ppb Hg. The maximum value for Hg is 370 ppb, an exceptionally high value for soil in this region. For comparison, similar soil geochemical studies that we are conducting in the Chequamegon National Forest in northern Wisconsin show A-horizon soils to have a mean Hg content of about 54 ppb with a maximum detected concentration of 260 ppb. C-horizon soils in that area have a mean of 19 ppb Hg and a maximum of 80 ppb. With respect to soils, Isle

Royale appears to be a more Hg-rich environment than northern Wisconsin. Both A and BChorizon soils are at least twice as high on the island as in northern Wisconsin.

The results suggest that the very elevated mercury content of soils in the Sargent Lake-LakeWagejo area is not caused by concealed native copper deposits because there is little or noelevated copper content of those soils. Results from the Minong mine area show that concealeddeposits, if present, would produce readily detectable elevated copper content of the soils.Neither is there reason to conclude that local bedrock is the source for mercury because all testedsamples of the chemically very uniform bedrock have very low mercury content. Because of thegeneral lack of mercury in the most common rocks of the island and the very limited occurrenceof native copper deposits that do contain some mercury, the mercury content of the soils on IsleRoyale seems very likely to result largely from airborne deposition. The processes causing thegreat variability of mercury content of similar appearing soils over short distances that we haveobserved in our survey remain only speculative. Nevertheless, it appears certain that mercurymust be very mobile in the soil zone in order to redistribute what must be a relatively uniformamount of deposited mercury into the highly variable observed concentrations.

10

Royale appears to be a more Hg-rich environment than northern Wisconsin. Both A and BC horizon soils are at least twice as high on the island as in northern Wisconsin.

The results suggest that the very elevated mercury content of soils in the Sargent Lake-Lake Wagejo area is not caused by concealed native copper deposits because there is little or no elevated copper content of those soils. Results from the Minong mine area show that concealed deposits, if present, would produce readily detectable elevated copper content of the soils. Neither is there reason to conclude that local bedrock is the source for mercury because all tested samples of the chemically very uniform bedrock have very low mercury content. Because of the general lack of mercury in the most common rocks of the island and the very limited occurrence of native copper deposits that do contain some mercury, the mercury content of the soils on Isle Royale seems very likely to result largely from airborne deposition. The processes causing the great variability of mercury content of similar appearing soils over short distances that we have observed in our survey remain only speculative. Nevertheless, it appears certain that mercury must be very mobile in the soil zone in order to redistribute what must be a relatively uniform

10

STRUCTURAL FABRIC AND STRAIN FOR THE SEINE RIVER CONGLOMERATES AT TIlEWABI000N- QUETICO SUBPROVINCE BOUNDARY NEAR MINE CENTRE, ONTARIO.

CZECK, Dyanna M. and HTJDLESTON, Peter J., Department of Geology and Geophysics, University ofMinnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, [email protected]

INTRODUCTION

The Superior Province consists of approximately east-west trending subprovinces defined by lithologicaldifferences, metamorphic grade, and structural boundaries (Card & Ciesielski 1986). This studyconcentrates on the boundary between the Quetico metasedimentary subprovince and the Wabigoonmetavolcanic subprovince. The Seine River Metasedimentary Group, including the Seine Riverconglomerates, are distinctive rocks found within a wedge along this boundary.

Structural work along the Quetico and Wabigoon describes subvertical foliation overprintinglarge lithological folds (Poulsen 1986, Tabor & Hudieston 1991). These structures indicate a strainhistory consisting of an early recumbent nappe deformation followed by a more ubiquitous overprintingof a subvertical flattening fabric and shear zones attributed to dextral transpression. Strain is pervasivethroughout the boundary, but also localized into more discrete shear zones including two main,connecting shear zones coincident with the Rainy Lake-Seine River and Quetico Faults. These largeshear zones are interconnected with smaller shear zones forming an anastomosing pattern. The SeineRiver conglomerates are located in a wedge between these two shear zones west of their intersection.Poulsen (1986) interpreted the Seine River conglomerates to have formed syn-kinematically duringtranspression, possibly in structures similar to pull-apart basins.

-

This preliminary structural work made it clear that the Wabigoon- Quetico subprovince boundaryis not merely a discrete suture between two tectonic microplates, but is instead a complex network ofshear zones which together accommodated oblique collision. The goal of this study is to describe howthis complex shear zone geometry is reflected in spatially variable strain magnitudes and structuralfabrics.

STRUCTURAL FABRIC

Foliations in the field show a consistent regional orientation with strike of approximately 080 andsubvertical dips. This strike is consistent with the dextral transpression model. One exception to this is inthe area of Shoal Lake where the strike changes to approximately 045, creating an overall S-like geometrywith a map pattern analogous to a dextral shear sense indicator. The same shear sense is observed at asmaller scale within the conglomerate clasts that often form sigma and delta dextral shear sense indicatorson the sub-horizontal plane.

Mineral lineations in the field are predominantly defined by chlorite and amphiboles. Generaltranspression theory predicts either vertical or horizontal lineations (Fossen & Tikoff 1993). However,lineation orientations in the field vary remarkably within the foliation plane. Typically, the Seine Grouplineations are spatially similar on the scale of 0.5-1 km. Therefore, the lineation variations are not causedby small-scale-phenomena such as clast interaction, but by some regional phenomenon. Rather thanreflecting different bulk kinematic conditions along the boundary (such as partitioned triclinictranspression). the lineation variations may reflect anastomosing shear zone patterns and variable pressuregradients within the rock.

11

STRUCTURAL FABRIC AND STRAIN FOR THE SEINE RIVER CONGLOMERATES AT THE WABIGOON- QUETICO SUBPROVINCE BOUNDARY NEAR MINE CENTRE, ONTARIO.

CZECK, Dyanna M. and HUDLESTON, Peter I., Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, [email protected]

The Superior Province consists of approximately east-west trending subprovinces defined by lithological differences, metamorphic grade, and structural boundaries (Card & Ciesielski 1986). This study concentrates on the boundary between the Quetico metasedimentary subprovince and the Wabigoon metavolcanic subprovince. The Seine River Metasedimentary Group, including the Seine River conglomerates, are distinctive rocks found within a wedge along this boundary.

Structural work along the Quetico and Wabigoon describes subvertical foliation overprinting large lithological folds (Poulsen 1986, Tabor & Hudleston 1991). These structures indicate a strain history consisting of an early recumbent nappe deformation followed by a more ubiquitous overprinting of a subvertical flattening fabric and shear zones attributed to dextral transpression. Strain is pervasive throughout the boundary, but also localized into more discrete shear zones including two main, connecting shear zones coincident with the Rainy Lake-Seine River and Quetico Faults. These large shear zones are interconnected with smaller shear zones forming an anastomosing pattern. The Seine River conglomerates are located in a wedge between these two shear zones west of their intersection. Poulsen (1986) interpreted the Seine River conglomerates to have formed syn-kinematically during transpression, possibly in structures similar to pull-apan basins.

This preliminary structural work made it clear that the Wabigoon- Quetico subprovince boundary is not merely a discrete suture between two tectonic microplates, but is instead a complex network of shear zones which together accommodated oblique collision. The goal of this study is to describe how this complex shear zone geometry is reflected in spatially variable strain magnitudes and structural fabrics.

Foliations in the field show a consistent regional orientation with strike of approximately 080 and subvertical dips. This strike is consistent with the dextral transpression model. One exception to this is in the area of Shoal Lake where the strike changes to approximately 045, creating an overall S-like geometry with a map pattern analogous to a dextral shear sense indicator. The same shear sense is observed at a smaller scale within the conglomerate clasts that often form sigma and delta dextral shear sense indicators on the sub-horizontal plane.

Mineral lineations in the field are predominantly defined by chlorite and amphiboles. General transpression theory predicts either vertical or horizontal lineations (Fossen & Tikoff 1993). However, lineation orientations in the field vary remarkably within the foliation plane. Typically, the Seine Group lineations are spatially similar on the scale of 0.5-1 km. Therefore, the lineation variations are not caused by small-scalephenomena such as clast interaction, but by some regional phenomenon. Rather than reflecting different bulk kinematic conditions along the boundary (such as partitioned triclinic transpression), the lineation variations may reflect anastomosing shear zone patterns and variable pressure gradients within the rock.

Taken together, foliation and lineation match the variations in the finite strain ellipsoidorientation. The foliation pole aligns with the minimum stretching direction, and the lineations parallelthe maximum stretching direction.

STRAIN ANALYSIS

The Seine River conglomerates are useful strain recorders. Qualitative observations andquantitative strain analyses indicate heterogeneous strain distributions within the Seine Riverconglomerates. Analysis confinns a flattening style strain that could only be a result of a three-dimensional deformational phenomenon such as transpression rather than the two-dimensional strainproduced by wrench or thrust movements.

Results to date show that the longest principal axis corresponds to the mineral lineation measuredin the field and the shortest principal axis corresponds well with the foliation pole. This supports the ideathat structural fabric measured in the field may be used to represent the orientation of the finite strainellipsoid.

The variations in strain ellipsoid orientations and magnitudes could be used to argue for multipledeformation events or complex and variable kinematic boundary conditions. However, the geometry ofthe anastomosing shear zone pattern can be used to explain the variations in the orientation of the strainellipsoid and the heterogeneous strain magnitudes within the Seine Group within a simple dextraltranspressional setting.

Poles to foliation138 Measurements (equal area)

Card. IC. D. & Ciesielski, A. 1986. DNAG Subdivisions of the Superior Province of the CanadianShield. Geoscience Canada 13, 5-13.

Fossen, H. & Tikoff, B. 1993. The deformation matrix for simultaneous simple shearing, pureshearing and volume change, and its application to transpression- transtension tectonicsJournal of Structural Geology 15, 413-422.

Poulsen. K. H. 1986. Rainy Lake Wrench Zone: An example of an Archean Subprovince boundaryin Northwestern Ontario. In: Tectonic evolution of greenstone belts Technical Report (editedby de Wit, M. J. Sc Ashwal, L. D.) 86-10. Houston TX, Lunar and planetary Inst., 177-179.

Tabor. J. R. & Hudleston, P. J. 1991. Deformation at an Archean subprovince boundary, northernMinnesota. Canadian Journal of Earth Sciences 28, 292-307.

12

113 Measurements (equal area)

Taken together. foliation and lineation match the variations in the finite strain ellipsoid orientation. The foliation pole aligns with the minimum stretching direction, and the lineations parallel the maximum stretching direction.

The Seine River conglomerates are useful strain recorders. Qualitative observations and quantitative strain analyses indicate heterogeneous strain distributions within the Seine River conglomerates. Analysis confirms a flattening style strain that could only be a result of a three- dimensional deformational phenomenon such as transpression rather than the two-dimensional strain produced by wrench or thrust movements.

Results to date show that the longest principal axis corresponds to the mineral lineation measured . -

in the field and the shortest principal axis corresponds well with the foliation pole. This supports the idea that structural fabric measured in the field mav be used to reoresent the orientation of the finite strain ellipsoid.

The variations in strain ellipsoid orientations and magnitudes could be used to argue for multiple deformation events or complex and variable kinematic boundary conditions. However, the geometry of the anastomosing shear zone pattern can be used to explain the variations in the orientation of the strain ellipsoid and the heterogeneous strain magnitudes within the Seine Group within a simple dextral transpressional setting.

Mineral lineations 113 Measurements (eaual area)

Poles to foliation 138 Measurements (equal area)

Card, K. D. & Ciesielski, A. 1986. DNAG Subdivisions of the Superior Province of the Canadian Shield. Geoscience Canada 13.5-13.

Fossen, H. & Tikoff, B. 1993. The deformation matrix for simultaneous simple shearing, pure shearing and volume change, and its application to transpression- transtension tectonics. Journal o f Structural Geoloxv IS. 4 13-422. -.

Poulsen, K. H. 1986. Rainy Lake Wrench Zone: An example of an Archean Subprovince boundary in Northwestern Ontario. In: Tectonic evolution of greenstone belts Technical Report (edited by de Wit, M. 1. & Ashwal, L. D.) 86-10. Houston TX, Lunar and planetary Inst., 177-179.

Tabor, J. R. & Hudleston, P. 1. 1991. Deformation at an Archean subprovince boundary, northern Minnesota. Canadian Journal of Earth Sciences 28,292-307.

Preliminary Aeromagnetic Map of Wisconsin

David L. Daniels, Suzanne W. Nicholson, William F. Cannon, U.S. Geological Survey. MS 954 NationalCenter, Reston, VA 20192, Robert E. Bracken, U.S. Geological Survey, MS 964, DFC Box 25046,Denver, CO 80225

The Mineral Resources Program of the U.S. Geological Survey (USGS) has conductedaeromagnetic surveys in Wisconsin during the past three years to improve the magnetic coverage of thestate. State coverage was completed with a survey carried out in 1998/1999. The new aeromagneticsurvey fills the southern third of the state not covered by existing good quality surveys (see figure).Right specifications are: north-south flight lines at ½ mile (800 m) separation draped at 1000 ft (305 m)mean terrain clearance. Right lines are east-west for a part of southeast Wisconsin.

The results of the first two years of flying were shown at 44th meeting of the ILSG in 1998(Daniels and others, 1998). These data have been released since that meeting as paper contour maps(Snyder, 1998; USGS Open-File Reports 98-431 through 439). Digital data for the 1997/1998 surveyhave now been released (Open-File Report 99-28) on CD-ROM (Daniels, Nicholson, and Cannon, 1999).

The accompanying figure shows a shaded image of the aeromagnetic field for current data inWisconsin and the areas of the first 2 years data are blocked and labeled. Three aeromagnetic surveysobtained from industry sources by the Wisconsin Geological and Natural History Survey (M. Mudrey,personal communication, 1997) are shown in southern Wisconsin. The data for the new survey are stillpreliminary. Blocks 1-5 and A-D represent the areas of the new survey that will be shown and thecontour maps that will be available later in 1999.

The rationale for continued aeromagnetic surveying has been to provide higher resolution datawith which to interpret basement geology in Wisconsin. The aeromagnetic data image basementstructures in detail and give clues to the structural evolution of the Precambrian crust, covered by glacialand Paleozoic cover.

References

Daniels, D.L., Nicholson, S.W., Cannon, W.F., 1999, Aeromagnetic surveying in Wisconsin 1997-98:Digital data files: U.S. Geological Survey Open-File Report 99-28, CD-ROM.

Daniels, D.L., Snyder, S.L., Nicholson, S.W., Cannon, W.F., 1998, New aeromagnetic surveys inWisconsin by the U.S. Geological Survey: Institute on Lake Superior Geology Proceedings v. 44, part 1,p. 62-63.

Snyder, S.L., 1998, Aeromagnetic map of part of northwestern Wisconsin and adjacent areas:U.S. Geological Survey Open-File Report 98-228, Scale 1:125,000, 2 sheets.

13

Preliminary Aeromagnetic Map of Wisconsin

David L. Daniels, Suzanne W. Nicholson, William F. Cannon, US. Geological Survey, MS 954 National Center, Reston, VA 20192, Robert E. Bracken, U.S. Geological Survey, MS 964, DFC Box 25046, Denver, CO 80225

The Mineral Resources Program of the U.S. Geological Survey (USGS) has conducted aeromagnetic surveys in Wisconsin during the past three years to improve the magnetic coverage of the state. State coverage was completed with a survey carried out in 199811999. The new aeromagnetic survey fills the southern third of the state not covered by existing good quality surveys (see figure). Flight specifications are: north-south flight lines at Vi mile (800 m) separation draped at 1000 ft (305 m) mean terrain clearance. Flight lines are east-west for a part of southeast Wisconsin.

The results of the first two years of flying were shown at 44' meeting of the ILSG in 1998 (Daniels and others, 1998). These data have been released since that meeting as paper contour maps (Snyder, 1998; USGS Open-File Reports 98-431 through 439). Digital data for the 199711998 survey have now been released (Open-File Report 99-28) on CD-ROM (Daniels, Nicholson, and Cannon, 1999).

The accompanying figure shows a shaded image of the aeromagnetic field for current data in Wisconsin and the areas of the first 2 years data are blocked and labeled. Three aeromagnetic surveys obtained from industry sources by the Wisconsin Geological and Natural History Survey (M. Mudrey, personal communication, 1997) are shown in southern Wisconsin. The data for the new survey are still preliminary. Blocks 1-5 and A-D represent the areas of the new survey that will be shown and the contour maps that will be available later in 1999.

The rationale for continued aeromagnetic surveying has been to provide higher resolution data with which to interpret basement geology in Wisconsin. The aeromagnetic data image basement structures in detail and give clues to the structural evolution of the Precambrian crust, covered by glacial and Paleozoic cover.

References

Daniels, D.L., Nicholson, S.W., Cannon, W.F., 1999, ~eromahetic surveying in Wisconsin 1997-98: Digital data files: U.S. Geological Survey Open-File Report 99-28, CD-ROM.

Daniels, D.L., Snyder, S.L., Nicholson, S.W., Cannon, W.F., 1998, New aeromagnetic surveys in Wisconsin by the U.S. Geological Survey: Institute on Lake Superior Geology Proceedings v. 44, part 1, p. 62-63.

Snyder, S.L., 1998, Aeromagnetic map of part of northwestern Wisconsin and adjacent areas: U S . Geological Survey Open-File Report 98-228, Scale 1:125,000,2 sheets.

46E

45E

44E

43E

93E

Index Map of 'Wisconsin Aeromagnetic Data

91E

•

50 'O'5°20 KM

14

92E 9oE 89 SEE 87E

46E

45E

43E

92E 91E 9oE 89E ssE siE

Index Map of Wisconsin Aeromagnetic Data

936 926 916 c c t

916 90fi me. 876

0 50 100 150 200 KM

INVESTIGATION OF CaO AT THUNDERBIRD MINE, MESABI RANGE: MINERAL ANDSTRATIGRAPHIC RELATIONSHIPS

DIEDRICH, Tamara R., and MORTON, Penelope, Dept. of Geology, University of MinnesotaDuluth, Duluth, MN, 55812, [email protected] and [email protected]

Mine geologists at EVTAC's Thunderbird Mine, a taconite mine in Eveleth, Minnesota, have beentracking the variation of naturally occurring CaO in iron ore concentrate for at least nine months.Information about the CaO is helpful in formulating the partial flux pellets that EVTAC produces. TheCaO-bearing phase was unidentified, but presumed to be a carbonate. The objective of this study was toinvestigate the nature of this CaO in terms of: 1.) The identification of its phase or phases; 2.) Location ofmajor siratigraphic contributors; and 3.) The in situ textural relationship between the CaO-bearingphase(s) and the magnetite.

Manual interpretation of X-ray diffraction results identified the CaO-bearing phase in theconcentrate to be ankerite. Other constituent minerals include quartz, hematite, siderite, greenalite,stilpnomelane, talc, and goethite.

Thunderbird Mine excavates ore from the Biwabik Iron Formation, a Precambrian unit composedof several horizons of banded iron formation. From top to bottom, these horizons are Upper UpperCherty, Middle Upper Cherty, Lower Upper Cherty, Lower Slaty, Top Lower Cherty, and Bottom LowerCherty. Horizons differ in physical characteristics, such as thickness and texture, and chemicalcomposition. When collecting ore for the initial ore mix, a combination of horizons are blasted andblended to achieve a desirable magnetic iron concentration. Statistical analysis demonstrated a strongpositive relationship between the percentage of ore derived from the Top Lower Cherty and the amount ofankerite in the resulting concentrate.

Textural relationships between the ankerite and magnetite were observed and photographed.Ankerite occurs in at least four distinct habits, relative the magnetite: 1.) Rhombohedral ankerite crystalsare surrounded by much smaller, granoblastic, recrystallized quartz, and minor magnetite; 2.) Fine-grained, irregular ankerite crystals, are intergrown with magnetite of approximately the same size; 3.) Anankerite crystal can be entirely enclosed within a single or cluster of magnetite crystals; and 4.) A singleor cluster of ankerite crystals encapsulates a magnetite crystal. The iron ore at EVTAC is processed bymagnetic separation. Examining the textural relationship between these two minerals can produceinformation relevant to make the isolation of magnetite more efficient.

15

INVESTIGATION OF CaO AT THUNDERBIRD MINE, MESABI RANGE: MINERAL AND STRATIGRAPHIC RELATIONSHIPS

DIEDRICH, Tamara R., and MORTON, Penelope, Dept. of Geology, University of Minnesota Duluth, Duluth, MN, 55812, [email protected] and [email protected]

Mine oeolomsts at EVTAC's Thunderbird Mine, a taconite mine in Eveleth, Minnesota, have been tracking thevariation of naturally occurring CaO m iron ore concentrate for at least nine months. Information about the CaO is helnful in formulatino the vartial flux nelleis that EVTAC produces. The ~~ - - - - ~~~ - . CaO-bearine phase was unidentified, but presumed to be a carbonate. The objective of this study was to investigate the nature of this CaO in terms of: I .) The identification of its phase or phases; 2.) Location of major stratigraphic contributors; and 3.) The in situ textural relationship between the CaO-beanng phase(s) and the magnetite.

Manual interpretation of X-ray diffraction results identified the CaO-bearing phase in the concentrate to be ankerite. Other constituent minerals include quartz. hematite, siderite. greenalite, stilpnomelane, talc, and goethite.

Thunderbird Mine excavates ore from the Biwabik Iron Formation, a Precambrian unit composed of several horizons of banded iron formation. From top to bottom, these horizons are Upper Upper Cherty, Middle Upper Cherty, Lower Upper Cherty, Lower Slaty, Top Lower Cherty, and Bottom Lower Cherty. Horizons differ in physical characteristics, such as thickness and texture, and chemical composition. When collecting ore for the initial ore mix, a combination of horizons are blasted and blended to achieve a desirable magnetic iron concentration. Statistical analysis demonstrated a strong positive relationship between the percentage of ore derived from the Top Lower Cherty and the amount of ankerite in the resuiting ~ o n c e n ~ t e .

Textural relationshios between the ankerite and mametite were observed and photographed. Ankexite occurs in at least four distinct habits, relative the magnetite: 1.) ~hombohedrd ankerite crystals are surrounded by much smaller, granoblastic, recrystallized quartz, and minor magnetite; 2.) Fine- grained, irregular ankerite crystals, are intergrown with magnetite of approximately the same size; 3.) An ankerite crystal can be entirely enclosed within a single or cluster of magnetite crystals; and 4.) A single or cluster i f ankerite crystals &apsulates a mapetite crystal. The iron ore at EVTAC is processed by mametic senaration. Examinino the textural relationship between these two minerals can produce - - - ~ ~ ~ ~ ~ . - information relevant to make the isolation of magnetite more efficient.

METAMORPHIC MAP OF THE CANADIAN SHIELD OF ONTARIO,MICHIGAN, MINNESOTA AND WISCONSIN

R.M Easton, Ontario Geological Survey, 933 Ramsey Lake Road, Sue/bury, OntarioJ'3E 6B5, [email protected] and R. G. Berman, Geological Survey of Canada,

601 Booth Street, Ottawa, Ontario K1A 0E8

The Geological Survey of Canada's undertook a project several years ago to produce an updatedmetamorphic map for the Canadian Shield (GSC Map l475A), which would also be used to produce ametamorphic map of the world (in conjunction with the Commission for the Geologic Map of the World).At the request of the Geological Survey of Canada, Ontario was asked to compile a metamorphic map ofthe Canadian Shield in Ontario.

In addition to simply compiling metamorphic information, Ontario approached this project withthe goal of attempting to integrate metamorphic information with the tectonic synthesis produced as partof the Geology of Ontario project, with the aim of developing improved mineral exploration models. As aconsequence, the metamorphic map of Ontario was designed so as to complement the existing set of1: 1.000,000 bedrock, surficial geology, geophysical, and tectonic assemblage maps and time-space chartsreleased in 1991-92 as part of the Geology of Ontario series. A draft of this map, at roughly 1:2,500,000scale, will be presented here.

Also at the request of the Geological Survey of Canada, Ontario has undertaken to produce apreliminary metamorphic map of those portions of the Canadian Shield exposed in Wisconsin, Michigan,and Minnesota. One of the reasons for requesting the involvement of the Ontario Geological Survey inthis compilation, as opposed to other organizations, was so as to best integrate the metamorphic patternpresent in the shield in the United States with that present across the border in Ontario. In addition, fastapproaching deadlines for completion of the project factored into this decision.

Thus. the main focus of this presentation will be on presentation of a preliminary draft of themetamorphic map for the shield rocks in the areas adjoining Ontario, for the purposes of discussionregarding:

1) the style of presentation,2) the accuracy of the data presented, and3) solicitation of possible assistance in bringing this project to a conclusion.

The metamorphic map of the Shield is being produced digitally. In the case of Ontario, theTectonic Map of Ontario was used as the geological base. Key digital aspects of the project includedigitization of polygons representing different metamorphic facies (grade) and of linework (e.g. isograds,facies boundary information), into a CAD program. In addition, point-source information (e.g.assemblage data. P-T determinations) is being input into a database. The final map product will be anattributed map. released on CD-ROM. containing all the compiled information. The attributed map willaccompany a standard coloured, hard copy map, which will mainly display metamorphic faciesdistribution. This same approach will be taken in compiling the information for Wisconsin Michigan andMinnesota, except that here, the digital base is being supplied by the Geological Survey of Canada, basedon a previously released US geological compilation map of the region.

Two special issues of The Canadian Mineralogist on "Tectonometamorphic Studies in theCanadian Shield" are being produced as part of this project. The first (vol. 34, part 5, October 1997),includes results obtained from a variety of ancillary metamorphic studies related to the compilation effort.The second issue, which is in the process of being assembled, includes regional overviews related to themetamorphic map of the Canadian Shield, as well as additional, detailed metamorphic studies.

16

METAMORPHIC MAP OF THE CANADIAN SHIELD OF ONTARIO, MICHIGAN, MINNESOTA AND WISCONSIN

R.M. Easton, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbuty, Ontario P3E 6B5, [email protected] and R G. Berrnan. Geological Survey of Canada,

607 Booth Street. Ottawa. Ontario KIA OE8

The Geological Survey of Canada's undertook a project several years ago to produce an updated metamorphic map for the Canadian Shield (GSC Map 1475A), which would also be used to produce a metamorphic map of the world (in conjunction with the Commission for the Geologic Map of the World). At the request of the Geological Survey of Canada, Ontario was asked to compile a metamorphic map of the Canadian Shield in Ontario.

In addition to simply compiling metamorphic information, Ontario approached this project with the goal of attempting to integrate metamorphic information with the tectonic synthesis produced as part of the Geology of Ontario project, with the aim of developing improved mineral exploration models. As a consequence, the metamorphic map of Ontario was designed so as to complement the existing set of 1: 1.000,000 bedrock, suficial geology, geophysical, and tectonic assemblage maps and time-space charts released in 1991-92 as part of the Geology of Ontario series. A draft of this map, at roughly 1:2,500,000 scale, will be presented here.

Also at the request of the Geological Survey of Canada, Ontario has undertaken to produce a preliminary metamorphic map of those portions of the Canadian Shield exposed in Wisconsin, Michigan, and Minnesota. One of the reasons for reauestine the involvement of the Ontario Geological Survey in this compilation, as opposed to other or&izations, was so as to best integrate the met-orphic present in the shield in the United States with that oresent across the border in Ontario. In addition. fast approaching deadlines for completion of the project factored into this decision.

Thus, the main focus of this presentation will be on presentation of a preliminary draft of the metamorphic map for the shield rocks in the areas adjoining Ontario, for the purposes of discussion regarding:

1) the style of presentation, , , , :~ i . . , ,

2) the accuracy of the data presented, and 3) solicitation of possible assistance in bringing this project to a concl&on.

The metamorphic map of the Shield is being produced digitally. In the case of Ontario, the Tectonic Map of 0ntario wasused as the geologicalbase. Key digitalaspects of the project include dieitization of nolveons reoresentine different metamomhic facies (grade) and of linework (e.e. isoerads. . -- - . . - - facies boundary information), into a-CAD program. In addition, point-source information (e.g. assemblage data. P-T determinations) is beinginput into a database. The final map product will be an attributed mao. released on CD-ROM. containinc all the com~iled information. The artnbuted map will - accompany a standard coloured, hard copy map, which will mainly display metamorphic facies distribution. This same approach will be taken in compiling the information for Wisconsin Michigan and Minnesota, except that here, the digital base is being supplied by the Geological Survey of Canada, based

, . on a previously released US geological compilation map of the region.

Two special issues of The CanadianMineralogist on "~ectonom&&phic Studiesin the Canadian Shield are being produced as part ofthis project. The first (vol. 34, pan 5, October 1997). includes results obtained from a variety of ancillary metamorphic studies related to the compilation effort The second issue, which is in the process of being assembled, includes regional overviews related to the metamorphic map of the Canadian Shield, as well as additional, detailed metamorphic studies.

16

WHERE ARE THE METAMORPHOSED NATURAL OREBODIESOF THE MESABI RANGE?

RONALD 0. GRABER AND ALAN J. STRANDLJECLIFFS MINING SERVICES COMPANY, ISIIPEMING, ML 49849

([email protected])

The nearly century-old debate regarding the genesis of the hematite-goethite ironore deposits of the Mesabi Range has recently been revisited by Morey (1999). Thehistoric debate centers on the origin of the fluids responsible for the combined oxidationand leaching of the Biwabik iron-formation to form the deposits — were they upwardmoving hydrothermal fluids (hypogene) or downward moving meteoric waters(supergene)?

Morey presents an interesting conceptual model for the genesis of the ores thatutilizes continental-scale groundwater flow during the Early Proterozoic, driven by upliftin the Penokean fold-thrust belt to the south of the Mesabi Range. This model relies onthe Pokegama Quartzite, which underlies the Biwabik iron-formation, as a regionalaquifer for the movement of basinal waters to the northern margin of the Animike basin.A minimum age for ore genesis in this model is believed to be 1650 Ma when loss of thehydraulic gradient for the groundwater flow system occurred due to erosion of therecharge area. This Early Proterozoic age for ore genesis is significantly older than theJurassic-Cretaceous age that has been generally, albeit not universally, accepted over thelast 30 plus years (Sloan, 1964; Morey. 1972).

Constraints on the timing of the hematite-goethite ores are few due to the scarcityof geologic events impacting the Mesabi Range between the Early Proterozoic and theLate Cretaceous. It is on the east end of the Mesabi Range where intrusive events andcontact metamorphism associated with the Middle Proterozoic Duluth Complex provide apotential opportunity to put additional time constraints on the genesis of the ores. Hencethe title of this paper. If ore formation occurred prior to the Middle Proterozoic, theopportunity should exist to find metamorphosed equivalents of the soft hematite-goethiteores. Detailed drilling both along the iron-formation outcrop belt and downclip as part ofexploration for Cu-Ni deposits has failed to find these metamorphosed equivalents. Infact the distinct absence of enriched orebodies of any sort east of the Duluth Complex'smetamorphic aureole has long been utilized as support for a post-Middle Proterozoic agefor ore genesis. Alternatively, thinning of the Pokegama quartzite in this same generalarea is cited by Morey to explain the absence of enriched ores on the East Mesabi.

The relative abrupt absence of enriched ores in the Biwabik iron-formation occurswithin the mining operations of LTV Steel Mining Company, managed by a subsidiary ofCleveland-Cliffs, which has been active for over 40 years (Figure 1). Detailed dataacquired through both field observations and thousands of drill holes provide insight onthe controls of the ore-forming process. The following features of the geology at LTVSteel Mining Company are in conflict with an Early Proterozoic genesis of the hematite-goethite ores:

17

WHERE ARE THE METAMORPHOSED NATURAL OREBODIES OF THE MESABI RANGE?

RONALD G. GRABER AND ALAN J. STRANDLIE C U F F S MINING SERVICES COMPANY. ISHPEMING, MI. 49849

([email protected])

The nearly century-old debate regarding the genesis of the hematite-goethite iron ore deposits of the Mesabi Range has recently been revisited by Morey (1999). The historic debate centers on the origin of the fluids responsible for the combined oxidation and leaching of the Biwabik iron-formation to form the deposits - were they upward moving hydrothermal fluids (hypogene) or downward moving meteoric waters (supergene)?

Morey presents an interesting conceptual model for the genesis of the ores that utilizes continental-scale groundwater flow during the Early Proterozoic, driven by uplift in the Penokean fold-thrust belt to the south of the Mesabi Range. This model relies on the Pokegama Quartzite, which underlies the Biwabik iron-formation, as a regional aquifer for the movement of basinal waters to the northern margin of the Animike basin. A minimum age for ore genesis in this model is believed to be 1650 Ma when loss of the hydraulic gradient for the groundwater flow system occurred due to erosion of the recharge area. This Early Proterozoic age for ore genesis is significantly older than the Jurassic-Cretaceous age that has been generally, albeit not universally, accepted over the last 30 plus years (Sloan, 1964; Morey, 1972).

Constraints on the timing of the hematite-goethite ores are few due to the scarcity of geologic events impacting the Mesabi Range between the Early Proterozoic and the Late Cretaceous. It is on the east end of the Mesabi Range where intrusive events and contact metamorphism associated with the Middle Proterozoic Duluth Complex provide a potential opportunity to put additional time constraints on the genesis of the ores. Hence the title of this paper. If ore formation occurred prior to the Middle Proterozoic, the opportunity should exist to find metamorphosed equivalents of the soft hematite-goethite ores. Detailed drilling both along the iron-formation outcrop belt and downdip as part of exploration for Cu-Ni deposits has failed to find these metamorphosed equivalents. In fact the distinct absence of enriched orebodies of any sort east of the Duluth Complex's metamorphic aureole has long been utilized as support for a post-Middle Proterozoic age for ore genesis. Alternatively, thinning of the Pokegama quartzite in this same general area is cited by Morey to explain the absence of enriched ores on the East Mesabi.

The relative abrupt absence of enriched ores in the Biwabik iron-formation occurs within the mining operations of LTV Steel Mining Company, managed by a subsidiary of Cleveland-Cliffs, which has been active for over 40 years (Figure 1). Detailed data acquired through both field observations and thousands of drill holes provide insight on the controls of the ore-forming process. The following features of the geology at LTV Steel Mining Company are in conflict with an Early Proterozoic genesis of the hematite- goethite ores:

1. Due to the economic significance that metamorphism has on the processing of themagnetic taconites, considerable effort is placed on mapping the metamorphicisograds at this operation. Further refinement of the original zonation of the DuluthComplex's contact metamorphic aureole (French, 1968) has resulted. Therelationship of metamorphic grade with the eastern limit of iron-formation enrichmentappears stronger than the correlation with either the thickness or distribution of thePokegama Quartzite. This is manifested by:

a) the abrupt termination of enrichment and oxidation along the Wentworthstructure due to increasing metamorphic grade; and,

b) the presence of enrichment east of the Siphon structure which is thecontrolling structure for a dramatic decrease in Pokegama Quartzite thickness.

2. The Aurora sill, a sizeable syenitic body that intrudes the Biwabik iron-formation inthe western portion of the mining operation, appears to have contributed to thelocalization of ore formation in the St. James and the Miller-Mohawk mines. This sillhas visual similarities to granophyres associated elsewhere with the Duluth Complexand as such has long been considered Keweenawan (White. 1954). Jf ore genesis inthese mines were Early Proterozoic, how could a Keweenawan sill constrain oreformation? Likewise, no metamorphic overprint has been reported from theseorebodies. Definitive age dating though is lacking for this intrusive.

3. A large number of the enriched orebodies at the LTV Steel Mining Companyoperation are very shallow. Drilling has been conducted through the bottom of anumber of the natural ore pits successfully finding fresh, unaltered magnetic taconitereserves below. The lack of alteration between the Pokegama Quartzite and theorebodies is problematical for ascending groundwater flow models.

French, B.M. 1968, Progressive contact metamorphism of the Biwabik iron-formation,Mesabi Range, Minnesota, Minnesota Geological Survey Bulletin 45, p.103.

Morey. G.B., 1972, Mesabi Range; in P.K. Sims and G.B. Morey, eds., Geology ofMinnesota: A Centennial Volume, Minnesota Geological Survey. pp. 204-217.

Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi Range, Minnesota-Product of a continental-scale ground-water system. Econ. Geology, vol. 94, pp 133-142.

Sloan, R.E., 1964, The Cretaceous system in Minnesota: Minnesota Geological SurveyReport of Investigations 5, 64p.

White, D.A., 1954, The stratigraphy and structure of the Mesabi Range, Minnesota:Minnesota Geological Survey Bulletin 38, 92p.

18

1. Due to the economic significance that metamorphism has on the processing of the magnetic taconites, considerable effort is placed on mapping the metamorphic isograds at this operation. Further refinement of the original zonation of the Duluth Complex's contact metamorphic aureole (French, 1968) has resulted. The relationship of metamorphic grade with the eastern limit of iron-formation enrichment appears stronger than the correlation with either the thickness or distribution of the Pokegama Quartzite. This is manifested by:

a) the abrupt termination of enrichment and oxidation along the Wentworth structure due to increasing metamorphic grade; and,

b) the presence of enrichment east of the Siphon structure which is the controlling structure for a dramatic decrease in Pokegama Quartzite thickness.

2. The Aurora sill, a sizeable syenitic body that intrudes the Biwabik iron-formation in the western portion of the mining operation, appears to have contributed to the localization of ore formation in the St. James and the Miller-Mohawk mines. This sill has visual similarities to granophyres associated elsewhere with the Duluth Complex and as such has long been considered Keweenawan (White, 1954). If ore genesis in these mines were Early Proterozoic, how could a Keweenawan sill constrain ore formation? Likewise, no metamorphic overprint has been reported from these orebodies. Definitive age dating though is lacking for this intrusive. %

ri 3. A large number of the enriched orebodies at the LTV Steel Mining Company

operation are very shallow. Drilling has been conducted through the bottom of a number of the natural ore pits successfully finding fresh, unaltered magnetic taconite reserves below. The lack of alteration between the Pokegama Quartzite and the orebodies is problematical for ascending groundwater flow models.

French, B.M. 1968, Progressive contact metamorphism of the Biwabik iron-formation, Mesabi Range, Minnesota, Minnesota Geological Survey Bulletin 45, p. 103.

Morey, G.B., 1972, Mesabi Range; in P.K. Sims and G.B. Morey, eds.. Geology of Minnesota: A Centennial Volume, Minnesota Geological Survey, pp. 204-217.

Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi Range, Minnesota- Product of a continental-scale ground-water system, Econ. Geology, vol. 94, pp 133-142.

Sloan, R.E., 1964, The Cretaceous system in Minnesota: Minnesota Geological Survey Report of Investigations 5 . 6 4 ~ .

White, D.A., 1954, The stratigraphy and structure of the Mesabi Range, Minnesota: Minnesota Geological Survey Bulletin 38,92p.

Figure 1. Geologic Map of LTV Steel Mining Company

19

Figure 1. Geologic Map of LTV Steel Mining Company

Cross Margin Transport in Lake Superior

GREEN, Sarah A. and BUDD, Judith W., Dept. of Chemistry and Dept. of GeologicalEngineering and Sciences, Michigan Technological University, Houghton, MI 49931

The Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) is focusedon cross-margin transport processes along the western shore of Lake Superior'sKeweenaw Peninsula. The dominant feature of this region is the shore-parallelKeweenaw Current, which flows to the northeast and is stongest near Eagle Harbor.Water, sediment, and hydrographic data are being correlated with satellite imagery toobtain an integrated view of transport of suspended sediments, nutrients, and organicmaterial near and across the current.

Time series SeaWiFS images from May to September, 1998 show cross margin transportevents in Lake Superior. These chlorophyll and turbidity maps reveal a highly productivesouthern corridor from Duluth Harbor to the tip of the Keweenaw. Distinctive sedimentand chlorophyll plumes can be seen offshore west of the Ontonagon River and near thetip of the Keweenaw Peninsula. Movement of the plumes offshore was highly variabledepending upon the direction of prevailing winds prior to the satellite band pass.

Simultaneously acquired SeaWiPS (Sea-viewing Wide Field-of-View Sensor) andAVHRR (Advanced Very High Resolution Radiometer) images from May 23, 1998 wereused to verify cross margin transport events in Lake Superior. The AVHRR lake surfacetemperature image reveals a dramatic northward flowing offshore eddy current at the tipof the Keweenaw Peninsula. A SeaWiFS turbidity map acquired within hours of theAVHRR SST image suggests that materials were transported offshore north of EagleHarbor, providing evidence of cross- margin transport. These patterns were not present inAVHRR and SeaWiFS images obtained before or after the May 23rd event (on May 20and May 27).

Sediment patterns, in-water optical measurements, and physical models are helpingclarify transport mechanisms and pathways in the KITES study region.

20

Cross Margin Transport in Lake Superior

GREEN, Sarah A. and BUDD, Judith W., Dept. of Chemistry and Dept. of Geological Engineering and Sciences, Michigan Technological University, Houghton, MI 4993 1

The Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) is focused on cross-margin transport processes along the western shore of Lake Superior's Keweenaw Peninsula. The dominant feature of this region is the shore-parallel Keweenaw Current, which flows to the northeast and is stongest near Eagle Harbor. Water, sediment, and hydrographic data are being correlated with satellite imagery to obtain an integrated view of transport of suspended sediments, nutrients, and organic material near and across the current.

Time series SeaWiFS images from May to September, 1998 show cross margin transport events in Lake Superior. These chlorophyll and turbidity maps reveal a highly productive southern corridor from Duluth Harbor to the tip of the Keweenaw. Distinctive sediment and chlorophyll plumes can be seen offshore west of the Ontonagon River and near the tip of the Keweenaw Peninsula. Movement of the plumes offshore was highly variable depending upon the direction of prevailing winds prior to the satellite band pass.

Simultaneously acquired SeaWiFS (Sea-viewing Wide Field-of-View Sensor) and AVHRR (Advanced Very High Resolution Radiometer) images from May 23, 1998 were used to verify cross margin transport events in Lake Superior. The AVHRR lake surface temperature image reveals a dramatic northward flowing offshore eddy current at the tip of the Keweenaw Peninsula. A SeaWiFS turbidity map acquired within hours of the AVHRR SST image suggests that materials were transported offshore north of Eagle Harbor, providing evidence of cross- margin transport. These patterns were not present in AVHRR and SeaWiFS images obtained before or after the May 23rd event (on May 20 and May 27).

Sediment patterns, in-water optical measurements, and physical models are helping clarify transport mechanisms and pathways in the KITES study region.

Megascopic Fossils and Their Possible Contribution to the Development of the Silicate Unitof the Negaunee Iron-Formation, Empire Mine, Marquette Range, North Michigan

T. M. Han, Senior Research Scientist (Emeritus), Cleveland-Cliffs Inc.

Abstract

The megascopic fossils discovered in the Early Proterozoic Negaunee Iron-Formation are theworld's oldest known fossils. These fossils have been reported as eukaryotic algae similar toGrypania spiralis (Han and Runnegar 1992). Their existence may have played a key role in thedevelopment of the thin banded "silicate unit" of the Negaunee Iron-Formation at the Empire Mine.This unit is composed of magnetite-rich layers alternating with layers having a mineral assemblageof chert, siderite, ankerite, minnesotaite and stilpnomelane in various proportions.

Two fossil types are recognized. One type is fine filaments that are long and uniform in widthand coiled like "springs". The other type is short, coarse, in whole and sectional forms, andresembles "coiled garden worms". These fossils were apparently transported by direct current anddeposited at distances from their origin. Based on the relationship between the current marks andthe orientation of the fossil patches and oval-shaped coils on the outcrops measured, the directionof the paleocurrent at the time of sedimentation was from the southwest where Archean rocks aredistributed.

21

• z"&'a-".:

,.

Fossil remains resembling "coiled garden worms"

Megaswpic Fossils and Their Possible Contribution to the Development of the Silicate Unit of the Negaunee Iron-Formation, Empire Mine, Marquette Range, North Michigan

The megascopic fossils discovered in the Early Proterozoic Negaunee Iron-Formation are the world's oldest known fossils. These fossils have been reported as eukaryotic algae similar to Grypania spiralis (Han and Runnegar 1992). Their existence may have played a key role in the development of the thin banded "silicate unit" of the Negaunee Iron-Formation at the Empire Mine. This unit is wmposed of magnetite-rich layers alternating with layers having a mineral assemblage of chert, siderite, ankerite, ininnesotaite and stilpnomelane in various proportions.

Two fossil types are recognized. One type is fine filaments that are long and uniform in width and wiled like "springs". The other type is short, coarse, in whole and sectional forms, and resembles "coiled garden worms". These fossils were apparently transported by direct current and deposited at distances from their origin. Based on the relationship between the current marks and the orientation of the fossil patches and oval-shaped coils on the outcrops measured, the direction of the paleocurrent at the time of sedimentation was from the southwest where Archean rocks are distributed.

Fossil remains resembhg "coiled garden worms"

An induced oxidation study reveals that much of the magnetite was developed and enrichedthrough Fe+ + diffusion, using lath-shaped hematite as nuclei. These laths are randomly arranged,either scattered, or in irregular clusters and existed before the magnetite development.

The gangue layers are believed to be derived from a highly sfficeous gel with carbonaceous matterand other impurities. They were subsequently recrystallized and replaced by siderite, ankerite,minnesotaite and stilpnomelane during diagenesis and low-grade regional metamorphism.

According to LaBarge et al (1987), it is reasonable to assume that the heavily populatedmegascopic eukaryotic algae were probably one of the principal oxygen suppliers for precipitatingEe+ ± as Fe(OH)3. The clusters of the lath-shaped magnetite pseudorph in magnetite laininae andgranules probably represent dehydrated and crystallized Fe(OH)3 flocculi, hematite . The effect offlocculation, if any, would cause the segregation between the siliceous gel and the Ee(OH)3.Furthermore, the specific gravity of the Fe(OH)3 is significantly greater than the sificeous gel.Consequently, the interbanded structure of the lithological unit is believed to be depositedsimultaneously through processes of selective flocculation and differential settling between theFe(OH)3 and siliceous gel under a rhymatic and cyclic maimer regulated by the life and death ofthe organism, i.e. the growing season is a period of deposition while the off-season is a period ofsolution. Furthermore, the carbonaceous matter associated with the precipitates may generateFe+ + during diagenesis and metamorphism for the development of various types of hon minerals,particularly, the development and enrichment of magnetite.

References

1. Han, T. M. and Runnegar, B. (1992) Megascopic Eukaryotic Algae from the 2.1 Billion-Year-Oldd Negaunee Iron-Formation, Michigan. American Association for the Advancementof Science, Vol. 237, pp. .232-235 -

2. Han, T. M. (1978) Microstructures of Maguetite as Guides to Its Origin in Some PrecambrianIron-Formations. Fortschr. Miner. Vol. 56 pp. 105-142(1988) Origin of Magnetite in Precambrian Iron-Formations of Low Metamorphic GradeProceedings of the Seventh Quadrennial JAGOD Symposium. pp.641-656

3. Laflerge, C. L., Robbins, E. I., and Han, T. M. (1987) A Model for the Biological Precipitationof Precambrian Iron-Formations - A: Geological Evidence. Precambrian Iron Formations.Theophrastus Publication, SA. Athens, Greece. Editors: Peter Appel and Gene LaBarge.pp.69-96

22

--f-.

Magn elite and magn elite granule layers, white; Microstructrure of a magnetite layer. 250xgangue layers, dark. Magnetite, gray; hematite grayish white and gangue, black.

An induced oxidation study reveals that much of the magnetite was developed and enriched through Fe+ + diffusion, using lath-shaped hematite as nuclei. These laths are randomly arranged, either scattered, or in irregular clusters and existed before the magnetite development.

The gangue layers are believed to be derived from a highly siliceous gel with carbonaceous matter and other impurities. They were subsequently recrystallized and replaced by siderite, ankerite, minnesotaite and stilpnomelane during diagenesis and low-grade regional metamorphism.

Magnetite and magnetite granule layers, white; Micmstructrure of a magnetite layer. 250x gangue layers, dark. Magnetite, gray; hematite grayish white and gangue, black.

According to LaBarge et a1 (1989, it is reasonable to assume that the heavily populated megascopic eukaryotic algae were probably one of the principal oxygen suppliers for precipitating Fe+ + as Fe(OH)3. The clusters of the lath-shaped magnetite pseudorph in magnetite laminae and granules probably represent dehydrated and crystallized Fe(OH)3 flocculi, hematite . The effect of flocculation. if any, would cause the segregation between the siliceous gel and the Fe(Om3. Furthermore, the specific gravity of the Fe(Om3 is significantly greater than the siliceous gel. Consequently, the interbanded structure of the lithologic111 unit is believed to be deposited simultaneously through processes of selective flocculation and differential settling between the Fe(OH)3 and siliceous gel under a rhymatic and cyclic manner regulated by the life and death of the organism, i.e. the growing season is a period of deposition while the off-season is a period of solution. Furthermore, the carbonaceous matter associated with the precipitates may generate Fe+ + during diagenesis and metamorphism for the development of various types of iron minerals, particularly, the development and enrichment of magnetite.

,. ~ , . 1. Han, T. M. and Runnegar, B. (1%) Megascopic Eukaryotii &ae from the 2.1 Billion-Year-

Oldd Negaunee Iron-Formation. Michigan. American Association for the Advancement - of science, Vol. 237, pp..232-235

2. Han, T. M. (1978) Microstructures of Magnetite as Guides to Its Origin in Some Precambrian Iron-Formations. Fortschr. Miner. Vol. 56 on. 105-142 (1988) Origin of Magnetite in Precambrian Iron-Formations of Low Metamorphic Grade Proceedings of the Seventh Quadrennial IAGOD Symposium. pp.641-656

3. LaBerge, G. L., Robbins, E. I., and Han, T. M. (1987) A Model for the Biological Precipitation of Precambrian Iron-Formations - A: Geological Evidence. Precambrian Iron Formatt.ons. Theophrastus Publication, SA. Athens, Greece. Editors: Peter Appel and Gene LaBarge. pp. 69-96

COMPARISON OF MICA Ar/Ar AND Rb/Sr THERMOCHRONOLOGY RESULTSFROM NORTHERN WISCONSIN AND NORTHERN MICHIGAN

Daniel HoIm, Denise Romano, and Craig Maneuso, Dept. of Geology, Kent State University,Kent, OH 44242; Ken Foland, Dept. of Geological Sciences, The Ohio State University,Columbus, OH 43210.

Thermochronologic data from the southern Lake Superior region has historically beendominated by the Rb/Sr method on biotite (Fig. 1; Peterman and Sims. 1988, Tectonics). Wesummarize the results of 45 new mica Ar/Ar age dates from northern Wisconsin and northernMichigan (Fig. 2) and compare these with the Rb/Sr data. Because the Rb/Sr and K/Ar systemsin biotite are known to have similar closure temperatures (300°±50°C), this summary provides anexcellent opportunity to compare how susceptible these systems are to partial or completeresetting.

Northwest Wisconsin. Rb/Sr biotite dates from this region vary from 1309 to 1730 ML Incontrast, Ar/Ar mica dates fall tightly into two groups, an older —1755 Ma group and a younger—1600 Ma group. The two groups of Ar/Ar dates are separated by a sharp post-Penokeandeforinational front in the overlying Early Proterozoic quartzites. The deformationalJthermalfront here probably represents the northern limit of Mazatzal foreland deformation in thesouthern Lake Superior region.

Northern Michigan. The 1630-1650 Ma Rb/Sr and Ar/Ar chrontours, which coincide innorthwest Wisconsin, diverge eastward into northern Michigan. Given the absence of post-Penokean quartzites in this region, we suggest that the Ar/Ar ages can serve as a proxy fordetermining the northern limit of Mazatzal deformation. We suggest that the Republic areaescaped significant Mazatzal related deformation and higher-temperature metamorphic affectsbut did undergo some form of lower-temperature, possibly hydrothermal, metamorphism at—1630 Ma.

Northeast Wisconsin. A large area of <1200 Ma Rb/Sr biotite dates (the Goodman Swell)north of the Wolf River batholith has been interpreted as an uplifted flexural bulge created byrapid loading along the mid-continent rift axis to the north (dashed ellipse of Fig. 1). In contrast,mica Ar/Ar dates from the same region define a much smaller region of <1200 Ma dates (dashedellipse of Fig. 2). An isolated 1170 Ma Ar/Ar biotite date has also been obtained from basementover 100 km southwest of the swell. We interpret the <1200 Ma dates for both systems to reflectshallow intrusion-related resetting. The larger locus of anomalously young Rb/Sr dates probablyreflects the fact that the Rb/Sr system is more susceptible to lower-temperature resetting than theAr/Ar system. This interpretation also provides an explanation for the problematic Rb/Sr andAr/Ar dates that fall between 1200 and 1400 Ma. Because much of the 1470 Ma Wolf Riverbatholith and associated plutons (Wausau Syenite complex) intruded into shallow crust andcooled through 300°C by —1400 Ma (Fig. 1 and 2), it is likely that the surrounding Precambriancountry rock to the north and west was also shallow. We suggest, therefore, that the 1200-1400Ma country rock mineral dates for both systems indicate incomplete mineral resetting related toshallow Keweenawan intrusions.

23

COMPARISON O F MICA ArIAr AND RblSr THERMOCHRONOLOGY RESULTS FROM NORTHERN WISCONSIN AND NORTHERN MICHIGAN

Daniel Holm, Denise Romano, and Craig Mancuso, Dept. of Geology, Kent State University, Kent, OH 44242; Ken Poland, Dept. of Geological Sciences, The Ohio State University, Columbus, OH 43210.

Thermochronologic data from the southern Lake Superior region has historically been dominated by the RblSr method on biotite (Fig. 1; Peterman and Sims, 1988, ~&Q&J. We summarize the results of 45 new mica ArlAr age dates from northern Wisconsin and northern Michigan (Fig. 2) and compare these with the RbISr data. Because the RbISr and K/Ar systems in biotite are known to have similar closure temperatures (300Â¡Â±50Â° this summary provides an excellent opportunity to compare how susceptible these systems are to partial or complete resetting.

Northwest Wisconsin. RbISr biotite dates from this region vary from 1309 to 1730 M a In contrast, ArIAr mica dates fall tightly into two groups, an older -1755 Ma group and a younger -1600 Ma group. The two groups of Ar/Ar dates are separated by a sharp post-Penokean defonnational front in the overlying Early Proterozoic quartzites. The defonnationallthennal front here probably represents the northern limit of Mazatzal foreland deformation in the southern Lake Superior region.

Northern Michigan. The 1630-1650 Ma RbISr and ArIAr chrontours, which coincide in northwest Wisconsin, diverge eastward into northern Michigan. Given the absence of post- Penokean quartzites in this region, we suggest that the ArIAr ages can serve as a proxy for determining the northern limit of Mazatzal deformation. We suggest that the Republic area escaped significant Mazatzal related deformation and higher-temperature metamorphic affects but did undergo some form of lower-temperature, possibly hydrothermal, metamorphism at - 1 630 Ma.

Northeast Wisconsin. A large area of e l200 Ma Rb/Sr biotite dates (the Goodman Swell) north of the Wolf River batholith has been interpreted as an uplifted flexural bulge created by rapid loading along the mid-continent rift axis to the north (dashed ellipse of Fig. 1). In contrast, mica ArIAr dates from the same region define a much smaller region of c1200 Ma dates (dashed ellipse of Fig. 2). An isolated 1170 Ma ArlAr biotite date has also been obtained from basement over 100 km southwest of the swell. We interpret the c1200 Ma dates for both systems to reflect shallow intrusion-related resetting. The larger locus of anomalously young RbISr dates probably reflects the fact that the RWSr system is more susceptible to lower-temperature resetting than the AdAr system. This interpretation also provides an explanation for the problematic RbISr and AdAr dates that fall between 1200 and 1400 Ma. Because much of the 1470 Ma Wolf River batholith and associated plutons (Wausau Syenite complex) intruded into shallow crust and cooled through 300OC by -1400 Ma (Fig. 1 and 2). it is likely that the surrounding Precambrian country rock to the north and west was also shallow. We suggest, therefore, that the 1200-1400 Ma country rock mineral dates for both systems indicate incomplete mineral resetting related to shallow Keweenawan intrusions.

-1018=-—

W:.1537+4+!.&ioa.thJ.ifs8l c*rtJL:i 4. +.ffrj¼l63:.llss..

'..5%& 1l03j.j..jtmbeau quartz'te 1249, r 'tfQightly foldedrL!:.', 1516.'i'!!

.:.LLJ.:L1.J.J.:.Lth1455

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• Biotite 0 MuscovIte

< 44', % — —.-- '111

ff011 "p "'%"' 10,natIonah11ft0i,',',',','.\.'.','l 696-,' — , , '..'' - IA!

tTrrrrrrrw1576;n.:.E.:n cm067'rt1753I'r!." r.trhTrTnT.Tr:!Tr:,Tr:tT:-r-'.r' *1011605-— -

- - 1358t -. Qua_._.._j_ _,ø.__; —- - - — -- r,-n11366rrtt1456HFlambeau quarizute :1-1372t:h-htr -'--1 -

4 (tughtlyfolded) S1581 1A7

LJ.:1461 -! -

1616

415 Edge of Late ProtiPhanerozoic covei

Compiled from Romano (1999, Kent State M.S. thesis), Mancuso (1999. KentState M.S. thesis), Hoim and Lux (1998, CJES), Hoim et al. (1996, GSAA). andSchneider et al. (1996, CJES).

Fig. 1 Rb/Sr BIOTITE AGE DATA

KEWEENAWAN 4'— 4 Republic iis

- ,,____,__,__- S'S.. —

—

1686.

Peavy

IZ3Sbj1360

'us-i 620ti's1354S 17391354

1384"74// a,408 g

4/

1385 Edge of Late ProterozolPhanerozoic cover'

KEWEENAWAN

After Peterman and Sims (1988) but excluding data of Aldrich et al. (1965)Fig. 2 Ar/Ar MICA AGE DATA ___

eptI R7A

24

Fig. 1 RbISr BIOTITE AGE DATA

Compiled from Romano (1999. Kent State M.S. thesis), Mancuso (1099. Kent State M.S. thesis). Holm and Lux (1998. CJES). Holm el al. (1998. GSAA), and Schneider et al. (1996, CJES).

.

24

ISHPEMING GREENSTONE BELT - EVIDENCE FOR ARCHEAN TECTONICEVOLUTION OF THE SOUTHERN EDGE OF THE SUPERIOR PROVINCE INMICHIGAN

JOHNSON, Rodney C., Rod Johnson & Associates, Inc., Negaunee, MI 49866,[email protected]; and BORNHORST, Theodore J., Department of GeologicalEngineering and Sciences, Michigan Technological University, Houghton, MI 49931,[email protected]

The Ishpeming greenstone belt is the southernmost greenstone belt of the SuperiorProvince. The southern boundary of the Superior Province and the Ishpeming greenstonebelt is the 1,000 km long Great Lakes tectonic zone. Late Archean greenstone andgranite terrane to the north (Ishpeming greenstone belt) are juxtaposed against Early toMiddle Archean migmatite-gneiss-amphibolite and Late Archean granite terrane(southern complex). The Ishpeming greenstone belt consists of subaqueous tholeiiticbasalt to calc-alkalic rhyolite, 10-15 km in total thickness. New U-Pb zircon dataestablish the age of volcanism at 2705.8+/-1.6 Ma. (Table 1) Recent field studies definea sequence of multiple deformation and intrusive events following volcanism. Thevolcanic rocks were enveloped by a tonalite suite just before and during the north-directed thrusting and nappe formation. This was followed by development of strike-slipshear zones and the deposition of clastic sediments into pull-apart basins. A secondphase of folding produced upright folds. A trondhjemite to granite suite intruded the beltand deformed earlier fabrics. The timing of deformation is constrained with new U-Pbzircon data yielding an age of 2668.4÷2.l/-l.8 Ma for an intrusion in this suite.Intrusions of hornblendite to syenite (appinite suite) and continued movement along shearzones represent the end of deformation. An undeformed post-tectonic granite intrusionhas an age of 2585 (Sims and Peterman, 1992).

This sequence of deformation magmatic/volcanic events are readily interpreted in amodern plate tectonic context. The volcanic arc rocks are the result of north-directedsubduction. Tonalite-suite plutons intruded the arc just prior to collision of a smallcontinent from the south (southern complex) along the Great Lakes tectonic zone(suture). Collision occurred over an extended period of time with multiple deformationand magmatic events. The appinite suite is characteristic of modern collision-relatedmagmatism. The field-based sequence of events and new age dates are important data incontinuing to unravel the tectonic evolution of the southern edge of the SuperiorProvince.

References:

Sims, P.K. and Peterman, Z., 1992, Guide to the geology of the Great Lakes tectoniczone in the Marquette area, Michigan - A late Archean paleosuture: Institute onLake Superior Geology Proceedings, 38th Annual Meeting, Hurley, WI, v. 38. part 2,p. 105-135.

25

ISHPEMING GREENSTONE BELT - EVIDENCE FOR ARCHEAN TECTONIC EVOLUTION OF THE SOUTHERN EDGE OF THE SUPERIOR PROVINCE IN MICHIGAN

JOHNSON, Rodney C., Rod Johnson & Associates, Inc., Negaunee, MI 49866, [email protected]; and BORNHORST, Theodore J., Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, MI 4993 1, [email protected]

The Ishpeming greenstone belt is the southernmost greenstone belt of the Superior Province. The southern boundary of the Superior Province and the Ishpeming greenstone belt is the 1,000 k m long Great Lakes tectonic zone. Late Archean greenstone i d granite terrane to the north (Ishpeming greenstone belt) are juxtaposed against Early to Middle Archean migmatite-gneiss-amphibolite and Late Archean granite terrane (southern complex). The Ishpeming greenstone belt consists of subaqueous tholeiitic basalt to calc-alkalic rhyolite, 10-15 km in total thickness. New U-Pb zircon data establish the age of volcanism at 2705.8+/-1.6 Ma. (Table 1) Recent field studies define a sequence of multiple deformation and intrusive events following volcanism. The volcanic rocks were enveloped by a tonalite suite just before and during the north- directed thrusting and nappe formation. This was followed by development of strike-slip shear zones and the deposition of clastic sediments into pull-apart basins. A second phase of folding produced upright folds. A trondhjemite to suite intruded the belt and deformed earlier fabrics. The timing of deformation is constrained with new U-Pb zircon data yielding an age of 2668.4+2.1/-1.8 Ma for an intrusion in this suite. Intrusions of homblendite to syenite (appinite suite) and continued movement along shear zones represent the end of deformation. An undeformed post-tectonic granite intrusion has an age of 2585 (Sims and Peteman, 1992).

This sequence of deformation magmatic/volcanic events are readily interpreted in a modem plate tectonic context. The volcanic arc rocks are the result of north-directed subduction. Tonalite-suite plutons intruded the arc just prior to collision of a small continent from the south (southern complex) along the Great Lakes tectonic zone (suture). Collision occurred over an extended period of time with multiple deformation and magmatic events. The appinite suite is characteristic of modem collision-related magmatism. The field-based sequence of events and new age dates are important data in continuing to unravel the tectonic evolution of the southern edge of the Superior Province. s -

Sims, P.K. and Peterman, Z., 1992, Guide to the geology of the Great Lakes tectonic zone in the Marquette area, Michigan - A late Archean paleosuture: Institute on Lake Superior Geology Proceedings, 38"' Annual Meeting, Hurley, WI, v. 38, part 2, p. 105-135.

Table 1. Summary of deformation sequence, volcanic/magmatic activity and constraining age dates for theJshepming greenstone belt.

Event Characteristic VolcanicfMagmatic Activity Age Dates

Undeformed post-tectonic granite 2585 Ma

Kink bands with north-easterlystriking kink planes and steeplyplunging hinges

D5 Dip slip motion along shearzones.

U4 Folding associated withintrusion of plutons.

D3 Upright folding. Appinite suite

U2 Strike-slip shearing. Trondjhemite-granite suite 2668.4+2.11-1.8 Ma

U1 Recumbent folding. Tonalite suite

Extrusion of mafic and felsic 2705.8+/-1.6 Mavolcanic rocks.

26

1 Table 1. Summary of deformation sequence, volcanic/magmatic activity and constraining age dates for the Ishepming greenstone belt.

. , Vnlcanic~Maematic Activity Age Dates

Undeformed post-tectonic granite 2585 Ma

Kink bands with north-easterly striking kink planes and steeply plunging hinges

:,'',,->$

Ds Dip slip motion along shear zones.

'.f03'] D4 Folding associated with intrusion of plutons.

D3 Upright folding. Appinite suite

Di Strike-slip shearing. Tmndjhemite-granite suite 2668.4+2.1/- 1.8 Ma , ". , , . ,

Di Recumbent folding. Tonalite suite : yw; .8fel . . . ;, , : , ,. : * : b :

Extrusion of ma ic an sic 2705.8+/-1.6 Ma volcanic rocks.

A NEW APPROACH TO HISTORICAL RECONSTRUCTION: COMBININGDESCRIPTIVE AND EXPERIMENTAL PALEOLIMNOLOGY

W. Charles Kerfoot, Lake Superior Ecosystem Research Center and Department ofBiological Sciences, Michigan Technological University, Houghton, MI 49931

John A. Robbins, NOAA Great Lakes Environmental Research Laboratory, 2205Commonwealth Blvd., Ann Arbor, MI 48105

Lawrence J. Welder, Max-Planck-Institut für Limnologie, Postfach 165, D-24302 PlOn,Germany*

Here we introduce a combined experimental and descriptive approach (termed "ResurrectionEcology") to reconstructing historical perturbations, pointing out how direct tests withsediments and hatched resting eggs complement the traditional descriptive calculation ofmicrofossil fluxes. This approach is being studied in both NSFINOAA Cooperative sites, theKITES Project in Lake Superior and the EGLEE Project in Lake Michigan. Additional corestudies involve Baltic German lakes and the Caspian Sea.

In the Keweenaw Waterway, a freshwater estuary off Lake Superior, turn-of-the-centurycopper mining impacted the resident biota. Remain fluxes document that diatom, rhizopod.and Bosmina production all declined during stamp sand discharges, but recovered rapidlyafter WWLI, moving above background levels due to developing eutrophication. In additionto biogenic silica, we discovered that bromine flux holds promise as an indicator of diatomproduction and confirmed that this element is present in several genera. Fluxes of Daphniaresting eggs also increased dramatically since the 1940's, dominated by a hybrid apparentlyproduced from crosses between offshore and interior Waterway species, after channelingpromoted greater mixing of water masses.

Toxicity studies with sediments and Daphnia clones directly tested recovery of environmentsafter cessation of mining activities. The studies document that increased concentrations andfluxes of copper in the Waterway during mining discharges were toxic to invertebrates. Oncestamp sand discharges ceased, the biota recovered rapidly due to a combination of decreasedcopper cycling and organic complexation. Although sedimentation has returned to near-background conditions and surficial sediments in much of Portage Lake are no longer toxic,eutrophication and faunal exchange with Lake Superior make it unlikely that the originalzooplankton community composition will return to the Waterway system.

27

A NEW APPROACH TO HISTORICAL RECONSTRUCTION: COMBINING DESCRIPTIVE AND EXPERIMENTAL PALEOLIMNOLOGY

W. Charles Kerfoot, Lake Superior Ecosystem Research Center and Department of Biological Sciences, Michigan Technological University, Houghton, MI 4993 1

John A. Robbins, NOAA Great Lakes Environmental Research Laboratory, 2205 Commonwealth Blvd., Ann Arbor, MI 48105

Lawrence 1. Weider, Max-Planck-Institut f i r Limnologie, Postfach 165, D-24302 PlOn, Germany*

Here we introduce a combined experimental and descriptive approach (termed "Resurrection Ecology") to reconstructing historical perturbations, pointing out how direct tests with sediments and hatched resting eggs complement the traditional descriptive calculation of microfossil fluxes. This approach is being studied in both NSF/NOAA Cooperative sites, the KITES Project in Lake Superior and the EGLEE Project in Lake Michigan. Additional core studies involve Baltic German lakes and the Caspian Sea.

In the Keweenaw Waterway, a freshwater estuary off Lake Superior, turn-of-the-century copper mining impacted the resident biota. Remain fluxes document that diatom, rhizopod, and Bosmina production all declined during stamp sand discharges, but recovered rapidly after WWII, moving above background levels due to developing eutrophication. In addition to biogenic silica, we discovered that bromine flux holds promise as an indicator of diatom production and confirmed that this element is present in several genera. Fluxes of Daphnia resting eggs also increased dramatically since the 1940's. dominated by a hybrid apparently produced from crosses between offshore and interior Waterway species, after channeling promoted greater mixing of water masses.

Toxicity studies with sediments and Daphnia clones directly tested recovery of environments after cessation of mining activities. The studies document that increased concentrations and fluxes of copper in the Waterway during mining discharges were toxic to invertebrates. Once stamp sand discharges ceased, the biota recovered rapidly due to a combination of decreased copper cycling and organic complexation. Although sedimentation has returned to near- background conditions and surficial sediments in much of Portage Lake are no longer toxic, eutrophication and faunal exchange with Lake Superior make it unlikely that the original zooplankton community composition will return to the Waterway system.

THE IRON RIVER SYNCLINE: AN ALLOCHTHONOUS STRUCTURAL PANEL IN THEPENOKEAN FORELAND OF NORTHERN MICHIGAN

KLASNER, John S., Department of Geology and U. S. Geological Survey Western IllinoisUniversity, Macomb, IL 61455, [email protected]; CANNON, William F. and SCHULZ,Klaus J., U. S. Geological Survey, National Center, MS. 954, Reston, VA 20192; andLABERGE, Gene L., Department of Geology and U. S. Geological Survey, University ofWisconsin - Oshkosh, Oshkosh, WI 54901

The Iron River syncline (James and others, 1968) lies near the southern margin of thePenokean fold and thrust belt in northern Michigan. It contains sedimentary strata of the PaintRiver Group and underlying Badwater Greenstone. The syncline has a triangular map patternand is flanked by strata of the Michigamme Formation on its north and east sides and byintensely deformed Early Proterozoic strata on its southwest side. An unusually complex,multiply-folded structure within the syncline has been recognized since the detailed studies byJames and others (1965). Traditional stratigraphic interpretation has been that the strata in thesyncline are stratigraphically above the Baraga Group and that the Badwater Greenstone andPaint River Group are the youngest parts of the Marquette Range Supergroup. More recently,an allochthonous origin for the syncline has been advocated (Sims, 1996) and the exactcorrelation of units in the syncline with autochthonous units elsewhere in the foreland isuncertain. Our recent re-examination of the region lends support to the view that the synclineis an allochthon emplaced by generally northward-directed thrusting. A thrust of considerablemagnitude is interpreted to lie at the base of the Badwater Greenstone. This thrust forms thebase of the allocthon and separates complexly deformed strata above from more simplydeformed strata below. Several sets of structural observations argue for the allochthonousnature of the Iron River syncline, the principal argument being that the syncline has a foldinghistory and fold geometry more complex than surrounding, and presumably underlying rocks. Itmust therefore have acquired some aspect of its structure either as a geographically separatedterrane or during thin-skinned emplacement over the footwall rocks.

Structures in the allocthonStudies of outcrops of the Paint River Group show that at least two and probably three or moregenerations of fold axes occur in the Iron River syncline. Most folds are tight to isoclinal withsteeply-plunging axes. Similar steeply-plunging fold axes are common in fault panels within theNiagara fault complex, part of which bounds the syncline on the south, but are rare elsewherein the foreland. Bedding and axial planar foliation are variably oriented, but generally dipsteeply. Remnants of sub-horizontal foliation occur in a few places suggesting that recumbentfolds may have been significant during early stages of folding. A cross section (James andothers, 1968) from iron mine workings at the west end of the Iron River syncline nearStaumbaugh, Michigan shows a recumbent fold that is only moderately overprinted by steeply-dipping structures.The Badwater Greenstone, although largely massive, does contain structures similar to thoseshown in the Paint River Group and appears to have the same structural history. The BadwaterGreenstone is therefore considered part of the allochthon.

28

THE IRON RIVER SYNCLINE: AN ALLOCHTHONOUS STRUCTURAL PANEL IN THE PENOKEAN FORELAND OF NORTHERN MICHIGAN

KLASNER, John S., Department of Geology and U. S. Geological Survey, Western Illinois University. Macomb, IL 61455. [email protected]; CANNON, William F. and SCHULZ, Klaus J., U. S. Geological Survey, National Center, MS. 954, Reston, VA 20192; and LABERGE, Gene L., Department of Geology and U. S. Geological Survey, University of Wisconsin - Oshkosh, Oshkosh, Wl54901

The Iron River syncline (James and others, 1968) lies near the southern margin of the Penokean fold and thrust belt in northern Michigan. It contains sedimentary strata of the Paint River Group and underlying Badwater Greenstone. The syncline has a triangular map pattern and is flanked by strata of the Michigamme Formation on its north and east sides and by intensely deformed Earlv Proterozoic strata on its southwest side. An unusuallv comolex. ~~~~

multiply~folded structurewithin the syncline has been recognized since the detailed studies by James and others (1968). Traditional stratigraphic interpretation has been that the strata in the syncline are stratigraphically above the Baraga Group and that the Badwater Greenstone and Paint River Group are the youngest parts of the Marquette Range Supergroup. More recently, an allochthonous origin for the syncline has been advocated (Sims, 1996) and the exact correlation of units in the syncline with autochthonous units elsewhere in the foreland is uncertain. Our recent re-examination of the region lends support to the view that the syncline is an allochthon emplaced by generally northward-directed thrusting. A thrust of considerable magnitude is interpreted to lie at the base of the Badwater Greenstone. This thrust forms the base of the allocthon and separates complexly deformed strata above from more simply deformed strata below. Several sets of structural observations argue for the allochthonous nature of the Iron River syncline, the principal argument being that the syncline has a folding history and fold geometry more complex than surrounding, and presumably underlying rocks. It must therefore have acquired some aspect of its structure either as a geographically separated terrane or during thin-skinned emplacement over the footwall rocks.

Structures in the allocthon Studies of outcrops of the Paint River Group show that at least two and probably three or more generations of fold axes occur in the Iron River syncline. Most folds are tight to isoclinal with steeply-plunging axes. Similar steeply-plunging fold axes are common in fault panels within the Niagara fault complex, part of which bounds the syncline on the south, but are rare elsewhere in the foreland. Bedding and axial planar foliation are variably oriented, but generally dip steeply. Remnants of sub-horizontal foliation occur in a few places suggesting that recumbent folds may have been significant during early stages of folding. A cross section (James and others, 1968) from iron mine workings at the west end of the Iron River syncline near Staumbaugh, Michigan shows a recumbent fold that is only moderately overprinted by steeply- dipping structures. The Badwater Greenstone, although largely massive, does contain structures similar to those shown in the Paint River Group and appears to have the same structural history. The Badwater Greenstone is therefore considered part of the allochthon.

Structures in rocks surrounding the Iron River synclineRocks within the Niagara fault complex, south of the syncline, have northwest-striking, steeplysouth-dipping foliation and tight to isoclinal folds that are steeply plunging, like those within thesyncline. Structures in Baraga Group rocks (primarily Michigamme Formation), which bound thesyncline on the north and east, are much different and generally have west-northwest-strikingfoliation that dips moderately to steeply south, and horizontal to sub-horizontal fold axes thattrend west-northwest. Such sub-horizontal, west-trending fold axes are rare in the Iron Riversyncline. Several areas of first generation recumbent folding with sub-horizontal axial planarfoliation have been found over the past several years within the Baraga Group rocks,suggesting the formation of nappes. Most recently, small recumbent folds with sub-horizontalfoliation have been documented near Watersmeet, Michigan, northwest of the Iron Riversyncline. Together with previous observations of similar structures north and east of the IronRiver syncline, there is now widespread evidence of an early phase of recumbent folding andgeneration of low angle axial planar foliation, now variably overprinted by more uprightstructures.

InterpretationRecumbent folds documented by underground mapping in the Iron River syncline (James andothers, 1968) and remnants of flat-lying foliation found in outcrops of the Paint River Groupsuggest that both the Iron River syncline and adjacent Baraga Group had a similar earlystructural history of recumbent folding and development of low angle foliation. However,multiple generations of steeply-plunging fold axes and steeply-dipping, variably-oriented axialplanar foliations of the Iron River syncline, are distinct from the gently west-plunging fold axeswith south-dipping axial planar foliation found in the Michigamme Formation north and east ofthe syncline. Similar steeply plunging fold axes are common in very highly strained rocks alongthe Niagara Fault and seem clearly to be integral to events related to suturing of the volcanicterranes south of the fault. This suggests that the Iron River syncline was detached fromunderlying rocks during much of its folding history. The base of the allocthon is interpreted tobe a thrust fault along the base of the Badwater Greenstone, along which the allochthon hasbeen thrust northward over foreland basin turbidite deposits of the Michigamme Formation. Wesuggest that the unusual complexity of folding within the Iron River syncline is a result ofthrusting of earlier deformed rocks northward from the suture zone. During this thrusting, anearlier set of recumbent folds was intensely redeformed. In contrast, rocks in the structuralfootwall were little deformed as shown by the fact that the Iron River syncline cross-cuts thewest-trending, sub-horizontal fold axes in the adjacent Michigamme Formation.

REFERENCES

James, H. L., Dutton, C. E., Pettijohn, F. J., and Wier, K. L., 1968, Geology and Ore Deposits ofthe Iron River-Crystal Falls District, Iron county, Michigan: U. S. Geological SurveyProfessional Paper 570, 134 p.

Sims, P.K., 1996, Structure of continental margin, in Sims, P.K., and Carter, L.M.H., eds.,Archean and Proterozoic geology of the Lake Superior region, U.S.A., 1993: U.S. GeologicalSurvey Professional Paper 1556, p.44-51.

29

Structures in rocks surrounding the Iron River syncline Rocks within the Niagara fault complex, south of the syncline, have northwest-striking, steeply south-dipping foliation and tight to isoclinal folds that are steeply plunging, like those within the syncline. Structures in Baraga Group rocks (primarily Michigamme Formation), which bound the syncline on the north and east, are much different and generally have west-northwest-striking foliation that dips moderately to steeply south, and horizontal to sub-horizontal fold axes that trend west-northwest. Such sub-horizontal, west-trending fold axes are rare in the Iron River syncline. Several areas of first generation recumbent folding with sub-horizontal axial planar foliation have been found over the past several years within the Baraga Group rocks, suggesting the formation of nappes. Most recently, small recumbent folds with sub-horizontal foliation have been documented near Watersmeet, Michigan, northwest of the Iron River syncline. Together with previous observations of similar structures north and east of the Iron River syncline, there is now widespread evidence of an early phase of recumbent folding and generation of low angle axial planar foliation, now variably overprinted by more upright structures.

Interpretation Recumbent folds documented by underground mapping in the Iron River syncline (James and others, 1968) and remnants of flat-lying foliation found in outcrops of the Paint River Group suggest that both the Iron River syncline and adjacent Baraga Group had a similar early structural history of recumbent folding and development of low angle foliation. However, multiple generations of steeply-plunging fold axes and steeply-dipping, variably-oriented axial planar foliations of the Iron River syncline, are distinct from the gently west-plunging fold axes with south-dipping axial planar foliation found in the Michigamme Formation north and east of the syncline. Similar steeply plunging fold axes are common in very highly strained rocks along the Niagara Fault and seem clearly to be integral to events related to suturing of the volcanic terranes south of the fault. This suggests that the Iron River syncline was detached from underlying rocks during much of its folding history. The base of the allocthon is interpreted to be a thrust fault along the base of the Badwater Greenstone, along which the allochthon has been thrust northward over foreland basin turbidite deposits of the Michigamme Formation. We suggest that the unusual complexity of folding within the Iron River syncline is a result of thrusting of earlier deformed rocks northward from the suture zone. During this thrusting, an earlier set of recumbent folds was intensely redeformed. In contrast, rocks in the structural footwall were little deformed as shown by the fact that the Iron River syncline cross-cuts the west-trending, sub-horizontal fold axes in the adjacent Michigamme Formation.

REFERENCES

James, H. L., Dutton, C. E., Pettijohn, F. J., and Wier, K. L., 1968, Geology and Ore Deposits of the Iron River-Crystal Falls District, Iron county, Michigan: U. S. Geological Survey Professional Paper 570, 134 p.

Sims, P.K., 1996, Structure of continental margin, in Sims, P.K., and Carter, L.M.H., eds., Archean and Proterozoic geology of the Lake Superior region, U.S.A., 1993: U.S. Geological Survey Professional Paper 1556, p. 44-51.

REGIONAL PAYIERNS ON THE PENOKEAN CONTINENTAL MARGIN IN THESOUTHERN LAKE SUPERIOR REGIONGene L. LaBerge, UW Oshkosh , Oshkosh, WI 54901 and U. S. Geological Survey

Studies of outcrops and exploration drill cores during the past decade reveal severalregional patterns in the deposition and subsequent deformation and metamorphism of theEarly Proterozoic Penokean continental margin rocks in northern Wisconsin and northernMichigan. These data help constrain models of the depositional and tectonic history of theregion.

I. Regional depositional patterns.A. Thinning westward of the basal arenite of the Chocolay Group. The Mesnardand Sturgeon Formations are prominent arenite units in the Marquette andMenominee districts, respectively, and the Sunday Quartzite forms a thin basal uniton the eastern Gogebic range. However, a basal arenite is absent on the westernGogebic range, where the dolomite unit characteristic of the upper part of theChocolay Group rests directly on Archean basement. Drill core data from the ParkFalls - Mercer area of northern Wisconsin suggests that this pattern extendssouthward toward the proposed continental margin.

B. An increase in pelitic rocks southward toward the proposed continentalmargin. Drill cores from the Park Falls - Mercer area intersect abundant peliticschists, some of which contain interbedded carbonate units. Bedded sulfides arepresent locally within the pelitic rocks. The stratigraphic position of the pelitic unitshas not been ascertained, but they may be broadly correlative with the ChocolayGroup (Cannon and others, 1998).

C. A consistent pattern of lithologies within the Palms formation of the MenomineeGroup throughout the Penokean platform. The thin-bedded argillaceous lowermember and the thick-bedded orthoquartzite upper member are recognizable fromthe Marenisco area westward to Lake Namekagon. These members are alsorecognizable in drill cores from the Pine Lake Subterrane (Cannon and others,1998) from the Lake Gogebic area southwestward at least to the Butternut, WI area,suggesting a uniform platformal depositional environment.

D. Rift-related volcanic rocks (Sims and others, 1989) are associated with iron-formation of the Menoninee Group on the proposed shelf. The Emperor Volcanicsare interbedded with iron-formation on the eastern Gogebic (LaBerge and Klasner,1994), and volcanics are present within the iron-formation on the western end ofthe Gogebic (LaBerge and others, 1995). Volcanic rocks as well as carbonaceoussediments are associated with iron-formation south of the Gogebic range in a beltthat extends from near Lake Gogebic southwestward at least to the Butternut,Wisconsin area, (LaBerge, 1997). This suggests that rifting, with associatedvolcanism and stagnant basin development, occurred after deposition of the PalmsFormation on the continental margin. On the main Gogebic range, then, the iron-formation accumulated on a shallow platform bounded on the east, south and westby rift basins with active volcanism.

II. Regional deformation/metamorhpic patterns.A. Metamorphism of continent margin sediments that increases in intensitysouthward associated with intense folding and north-directed thrusting. A zone ofhigh pressure metamorphism characterized by kyanite and a high temperature zone

30

REGIONAL PATTERNS ON THE PENOKEAN CONTINENTAL MARGIN IN THE SOUTHERN LAKE SUPERIOR REGION Gene L. M e r g e , UW Oshkosh , Oshkosh, WI 54901 and U. S. Geological Survey

Studies of outcrops and exploration drill cores during the past decade reveal several regional patterns in the deposition and subsequent deformation and metamorphism of the Earl Proterozoic Penokean continental margin rocks in northern Wisconsin and northern ~ichigan. These data help constrain models of the depositional and tectonic history of the region. . I. Regional depositional patterns.

A. Thinning westward of the basal arenite of the Chocolav Grouo. The Mesnard and Sturge& Formations are prominent arenite units in the ~ a r ~ b e t t e and Menominee districts. resuectivelv. and the Sundav Ouartzite forms a thin basal unit on the eastern Gogebic &nee. rfowever, a basal &&te is absent on the western Gogebic range, where the dolomite unit characteristic of the upper part of the Chocolay Group rests directly on Archean basement. Drill core data from the Park Falls - Mercer area of northern Wisconsin suggests that this pattern extends southward toward the proposed continental margin.

B. An increase in politic rocks southward toward the proposed continental marein. Drill cores from the Park Falls - Mercer area intersect abundant ~eli t ic schists, some of which contain interbedded carbonate units. Bedded sulfides are present locally within the politic rocks. The stratigraphic position of the politic units has not been ascertained, but they may be broadly correlative with the Chocolay Group (Cannon and others, 1998).

C. A consistent pattern of litholoeies within the Palms formation of the Menominee Group throughout the Penokean platform. The thin-bedded argillaceous lower member and the thick-bedded orthoauartzite uooer membea are r e c o b b l e from the Marenisco area westward to Lake ~amekaion. These membekare also recoenizable in drill cores from the Pine Lake Subterrane (Cannon and others. 1998) from the Lake Gogebic area southwestward at least to the Butternut, WI area, suggesting a uniform platf-1 depositional environment.

D. Rift-related volcanic rocks (Sims and others, 1989) are associated with iron- formation of the Menoninee ~ G u p on the proposed shelf. The Emperor Volcanics are interbedded with iron-formation on the eastern Gogebic (LaBeree and Klasner, 1994), and volcanics are present within the iron-formation & the w-&tern end of the Gogebic ( M e r g e and others, 1995). Volcanic rocks as well as carbonaceous sediments are associated with iron-formation south of the Gogebic range in a belt that extends from near Lake Gogebic southwestward at least to the Butternut, Wisconsin area, (JaBer e, 1997). This suggests that rifting, with associated volcanism and stagnant basin development, occurred after deposition of the Palms Formation on the continental margin. On the main Gogebic range, then, the iron- formation accumulated on a shallow platform bounded on the east, south and west by rift basins with active volcanism. -

11. Regional defonnation/metamorhmc patterns. " A. ~&orphism of &tin& margin sediments that increases in intensity southward associated with intense folding and north-directed thrusting. A zone of high pressure metamorphism characterized by kyanite and a high temperature zone

characterized by sillimanite(the Powell Subterrane and the Park Falls Subterrane,respectively of Cannon and others, 1998).

B. Emplacement of post-tectonic potassic, commonly pegmatitic, granites, some ofwhich are two-mica granites, or which contain sillimanite or garnet, typical ofanatectic granites.

C. A later deformational event with associated retrograde metamorphism. This isexpressed as widespread alteration of high grade minerals such as sillimanite andolivine (in carbonate rocks) to pyrophyllite and serpentine, respectively. Thisdeformation has locally affected the post-tectonic granites associated with the earlierdeformation.

References:

Cannon, W.F., LaBerge, G.L., Kiasner, J.S., and Schulz, K.J., 1998, Reinterpretationof the Penokean continental margin in part of northern Wisconsin and Michigan(Abstract): 44th Annual Institute on Lake Superior Geology, vol. 44, part 1, p.52-53.

Klasner, J.S., and LaBerge, G.L., 1994, Structural evolution of the eastern Gogebicrange, northern Michigan (Abstract): 40th Annual Institute on Lake SuperiotGeology, vol. 40, part 1, p. 23-24.

LaBerge, G.L., 1997, Early Proterozoic break-up of the Superior Craton: Implications ofdrill core and geophysical data south of the Gogebic range, northern Wisconsin(Abstract), 43rd Annual Institute on Lake Superior Geology, vol. 43, part 1, p.3 1-32.

LaBerge, G.L., Cannon, W.F., and Kiasner, J.S., 1995, New observations on thegeology of the western Gogebic range (Abstract): 41st Annual Institute on LakeSuperior Geology, vol. 41, part I, p. 3 1-32.

LaBerge, G.L. and Klasner, J.S., 1994, Tectonic implications of the Early Proterozoiclithostratigraphy on the eastern Gogebic range, northern Michigan (Abstract): 41stAnnual Institute on Lake Superior Geology, vol. 41, part 1, p. 33-34.

Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 1989, Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of thePenokean Orogen: Canadian Journal of Earth Sciences, v. 26, p. 2 145-2158.

31

characterized by sillimanitefthe Powell Subtenane and the Park Falls Subterrane, respectively of Cannon and others, 1998).

B. Emplacement of post-tectonic potassic, commonly pegmatitic, granites, some of which are two-mica granites, or which contain sillimanite or garnet, typical of anatectic granites.

C. A later deformational event with associated retrograde metamorphism. This is expressed as widespread alteration of high grade minerals such as sillimanite and olivine (in carbonate rocks) to pyrophyllite and serpentine, respectively. This deformation has locally affected the post-tectonic granites associated with the earlier deformation.

References:

Cannon, W.F., LaBerge, G.L., Klasner, J.S., and Schulz, K.J., 1998, Reinterpretation of the Penokean continental margin in part of northern Wisconsin and Michigan (Abstract): 44th Annual Institute on Lake Superior Geology, vol. 44, part 1, p. 52-53.

Klasner, J.S., and Merge , G.L., 1994, Structural evolution of the eastern Gogebic range, northern Michigan (Abstract): 40th Annual Institute on Lake Superior Geology, vol. 40, part 1, p. 23-24.

Merge, G.L., 1997, Early Proterozoic break-up of the Superior Craton: Implications of drill core and geophysical data south of the Gogebic range, northern Wisconsin (Abstract), 43rd Annual Institute on Lake Superior Geology, vol. 43, part 1, p. 31-32.

LaBerge, G.L., Cannon, W.F., and Klasner, J.S., 1995, New observations on the geology of the western Gogebic range (Abstract): 41st Annual Institute on Lake Superior Geology, vol. 41, part 1, p. 3 1-32.

LaBerge, G.L. and Klasner, J.S., 1994, Tectonic implications of the Early Proterozoic lithostratigraphy on the eastern Gogebic range, northern Michigan (Abstract): 41st Annual Institute on Lake Superior Geology, vol. 41, part 1, p. 33-34.

Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 1989, Tectono- stratigraphic evolution of the Early Proterozoic Wisconsin magmatic tenanes of the Penokean Orogen: Canadian Journal of Earth Sciences, v. 26, p. 2145-2158.

A GEOCHEMICAL AND PETROLOGIC STUDY TO DETERMINE THE ORIGIN OFTHE CROWDUCK LAKE GROUP, KENORA, ONTARIO: A PROBLEMATICMETACONGLOMERATE.

LOUGHRY, Joy E., JOHNSON, Matthew M. and COTTER, J.F.P.,Geology Discipline, University of Minnesota, Morris,Morris MN 56267

The Crowduck Lake- Group is a sequence of argillite and a poorlysorted conglomerate (Archean ? age) exposed near Kenora, Ontario. Smith(1988) suggested that the Crowduck Lake Group may be a debris flowdeposit, however lithology and geographic location suggest that it may bea correlative of the "Gowganda tillites' (polymictic conglomeratesdeposited during the Gowganda glaciation 1.8 billion years ago). The goalof this study is to determine if the Crowduck Lake Group is glacial inorigin.

Extensive research has been done on the deposits of the GowgandaGlaciation (Summarized by Young 1969, 1973. 1999). Outcrops of theGowganda are located in Chibougamau, Quebec, Wyoming. and NorthernMichigan. Geochemical analysis indicate that there are broad similaritiesin diamictite constituents. Additionally. Chemical Index Alteration studiesprovide evidence of regionally cold climatic conditions (Young, 1999).Young (1969), on the basis of outcrop distribution, suggests thatelsewhere in Canada there are polymictic conglomerates which on detailedanalysis may well turn out to be tillites.

For this study an outcrop at the type locality of the Crowduck LakeGroup was described and sampled. Hand samples were analyzed forsedimentary structures, clast provenance and clast shape. Geochemistryof matrix samples was determined using ICP. Thin sections of matrix wereused for petrographic analyis.

Geochemistry of the Crowduck Lake Group is similar to that of thetype Gowganda Fotmation of Ontario. However, interpretation of theseresults is restricted because little is known about the geochemistry of thebedrock at the time of deposition. Additionally, the age of the CrowduckLake Group is poorly constrained. Thus, several possible origins of theCrowduck Lake Group remain plausible: glacial tillite, turbidite/debrisflow, and basal conglomerate. Although the sedimentology. location andage of the Crowduck Lake Group make a glacial origin attractive, evidenceis not conclusive. Additional petrographic analysis are underway.

32

1 A GEOCHEMICAL AND PETROLOGIC STUDY TO DETERMINE THE ORIGIN OF THE CROWDUCK LAKE GROUP. KENORA, ONTARIO: A PROBLEMATIC

I METACONGLOMERATE. LOUGHRY, Joy E., JOHNSON, Matthew M. and COTTER, J.F.P.,

Geology Discipline, University of Minnesota, Morris, Morris M N 56267

The Crowduck Lake Group is a sequence of argillite and a poorly sorted conglomerate (Archean ? age) exposed near Kenora, Ontario. Smith (1988) suggested that the Crowduck Lake Group may be a debris flow deposit, however lithology and geographic location suggest that it may be a correlative of the "Gowganda tillites" (polymictic conglomerates deposited during the Gowganda glaciation 1.8 billion years ago). The goal of this study is to determine if the Crowduck Lake Group is glacial in origin.

Extensive research has been done on the deposits of the Gowganda Glaciation (Summarized by Young 1969, 1973, 1999). Outcrops of the Gowganda are located in Chibougamau, Quebec, Wyoming, and Northern Michigan. Geochemical analysis indicate that there are broad similarities in diamictite constituents. Additionally, Chemical Index Alteration studies provide evidence of regionally cold climatic conditions (Young, 1999). Young (1969). on the basis of outcrop distribution, suggests that elsewhere in Canada there are polymictic conglomerates which on detailed analysis may well turn out to be tillites.

For this study an outcrop at the type locality of the Crowduck Lake Group was described and sampled. Hand samples were analyzed for sedimentary structures, clast provenance and clast shape. Geochemistry of matrix samples was determined using ICP. Thin sections of matrix were used for petrographic analyis.

Geochemistry of the Crowduck Lake Group is similar to that of the type Gowganda Fofmation of Ontario. However, interpretation of these results is restricted because little is known about the geochemistry of the bedrock at the time of deposition. Additionally, the age of the Crowduck Lake Group is poorly constrained. Thus, several possible origins of the Crowduck Lake Group remain plausible: glacial tillite, turbiditeldebris flow, and basal conglomerate. Although the sedimentology, location and age of the Crowduck Lake Group make a glacial origin attractive, evidence is not conclusive. Additional petrographic analysis are underway.

THE GRATIOT CHALCOCITE DEPOSIT, KEWEENAW PENINSULA, MICHIGAN

MAKI, John C., Department of Geological Engineering and Sciences, Michigan TechnologicalUniversity. Houghton, MI 49931, [email protected]; and BORNHORST, Theodore J., Department ofGeological Engineering and Sciences, Michigan Technological University, Houghton, MI 49931,[email protected]

ABSTRACT

Michigans Keweenaw Peninsula is well known for world-class native copper deposits and the lackof significant amounts of copper sulfides. Twelve copper sulfide (chalcocite-dominated) deposits havebeen discovered in the Keweenaw Peninsula. Robertson (1975) described one small deposit near Mt.Bohemia. Chalcocite-dominated deposits are concentrated in the lower quarter of the Portage LakeVolcanics from Suffolk to Mt. Bohemia (Figure 1). The Gratiot chalcocite deposit (previously termed543-S) is the largest of the copper sulfide dominated deposits and contains about 4.5 million metrictons of ore with an average grade of 2.3% copper.

The Gratiot chalcocite deposit is hosted by the Portage Lake Volcanics and is about 1.4 km from theKeweenaw fault, a major reverse fault of regional extent. In vicinity of the ore, basalt lava flows arecut by two dacite-andesite dikes. These dikes are nearly subparallel to the orientation of the lava flows.Ore grade intercepts occur mostly in brecciated amygdaloidal flow tops, although lesser amounts arefound in flow interiors and dikes. The highest grade and tonnage exists where both dikes are presentand where both dikes are thick. Faulting and fracturing are most prevalent in rocks where both dikesexist. The deposit is within a 12 km long fault zone, the Gratiot-Suffolk fault, that has a stratigraphicdisplacement of about 7 meters and is sub-parallel to the attitude of the basalt flows and sub-parallel tothe Keweenaw fault. Two other significant faults that cut the deposit are the Cross-Gratiot fault that isroughly perpendicular to the attitude of the basalt flows with a stratigraphic displacement of about 15meters and the Gratiot fault that is sub-parallel to the attitude of the basalt flows with very littlestratigraphic displacement. The highest grades are associated with the intersection of the Gratiot-Suffolk fault, the Gratiot fault and the Cross-Gratiot fault.

A suite of secondary hydrothermal minerals fill amygdules and fractures within the basalt lavaflows and dikes. Chlorite was the earliest secondary mineral, followed by epidote, potassium feldspar,native copper and native silver, prehnite and quartz and calcite (approximate order). Deposition ofpyrite clearly follows native copper and was itself followed by chalcopyrite, bornite, chalcocite andhematite. The last phase of secondary mineral deposition was adularia, laumontite and more calcite.Chalcocite is by far the major copper-bearing mineral in the Gratiot deposit, other copper-bearingminerals are trace to rare in occurrence. The relative age of mineral deposition in the Gratiot depositmatches the relative age of mineral deposition in the large native copper deposits in the KeweenawPeninsula. Butler and Burbank (1929) documented an early suite of minerals associated with nativecopper followed by chalcocite and then late adularia, laumontite and calcite. Native copper in theKeweenaw Peninsula is interpreted to be temporally and genetically related to compressional tectonicsthat produced reverse motion along the Keweenaw fault (Bornhorst, 1997). The close temporalrelationship of native copper and chalcocite in the Gratiot deposit suggests that chalcocite is also

related to compressional tectonics. Native copper ore-depositing fluids are interpreted as verysulfur poor and derived from basalts within the rift (Bomhorst, 1997). Chalcocite ore fluids must

33

MAKI, John C., Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, MI 4993 1 [email protected]; and BORNHORST, Theodore J., Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, MI 49931, [email protected]

ABSTRACT

Michigan's Keweenaw Peninsula is well known for world-class native copper deposits and the lack of significant amounts of copper sulfides. Twelve copper sulfide (chalcocite-dominated) deposits have been discovered in the Keweenaw Peninsula. Robertson (1975) described one small deposit near Mt. Bohemia. Chalcocite-dominated deposits are concentrated in the lower quarter of the Portage Lake Volcanics from Suffolk to Mt. Bohemia (Figure 1). The Gratiot chalcocite deposit (previously termed 5434) is the largest of the copper sulfide dominated deposits and contains about 4.5 million metric tons of ore with an average grade of 2.3% copper.

The Gratiot chalcocite deposit is hosted by the Portage Lake Volcanics and is about 1.4 km from the Keweenaw fault, a major reverse fault of regional extent. In vicinity of the ore, basalt lava flows are cut by two dacite-andesite dikes. These dikes are nearly subparallel to the orientation of the lava flows. Ore grade intercepts occur mostly in brecciated amygdaloidal flow tops, although lesser amounts are found in flow interiors and dikes. The highest grade and tonnage exists where both dikes are present and where both dikes are thick. Faulting and fracturing are most prevalent in rocks where both dikes exist. The deposit is within a 12 km long fault zone, the Gratiot-Suffolk fault, that has a stratigraphic displacement of about 7 meters and is sub-parallel to the attitude of the basalt flows and sub-parallel to the Keweenaw fault. Two other significant faults that cut the deposit are the Cross-Gratiot fault that is roughly perpendicular to the attitude of the basalt flows with a stratigraphic displacement of about 15 meters and the Gratiot fault that is sub-parallel to the attitude of the basalt flows with very little stratigraphic displacement. The highest grades are associated with the intersection of the Gratiot- Suffolk fault, the Gratiot fault and the Cross-Gratiot fault.

A suite of secondary hydrothermal minerals fill amygdules and fractures within the basalt lava flows and dikes. Chlorite was the earliest secondary mineral, followed by epidote, potassium feldspar, native copper and native silver, prehnite and quartz and calcite (approximate order). Deposition of pyrite clearly follows native copper and was itself followed by chalcopyrite, bomite, chalcocite and hematite. The last phase of secondary mineral deposition was adularia, laumontite and more calcite. Chalcocite is by far the major copper-bearing mineral in the Gratiot deposit, other copper-bearing minerals are trace to rare in occurrence. The relative age of mineral deposition in the Gratiot deposit matches the relative age of mineral deposition in the large native copper deposits in the Keweenaw Peninsula. Butler and Burbank (1929) documented an early suite of minerals associated with native copper followed by chalcocite and then late adularia, laumontite and calcite. Native copper in the Keweenaw Peninsula is interpreted to be temporally and genetically related to compressional tectonics that produced reverse motion along the Keweenaw fault (Bomhorst, 1997). The close temporal relationship of native copper and chalcocite in the Gratiot deposit suggests that chalcocite is also related to compressional tectonics. Native copper ore-depositing fluids are interpreted as very sulfur poor and derived from basalts within the rift (Bomhorst, 1997). Chalcocite ore fluids must

have carried more sulfur than native copper fluids. However, the genesis of fluids generating thechalcocite deposits is currently speculative.

References

FIGURE 1: LOCATION OF COPPER SULFIDE DEPOSITS

Bornhorst, T. J., 1997, Tectonic context of native copper deposits of the North AmericanMidcontinent Rift System: Geological Society of America Special Paper 312, p. 127-136.

Butler, B. S. and Burbank, W. 5., 1929, The copper deposits of Michigan: U. S. Geol. Survey Prof.Paper 144, 238 p.

Robertson, J. M., 1975, Geology and mineralogy of some copper sulfide deposits nearMount Bohemia, Keweenaw County. Michigan: Economic Geology, v. 70, p. 1202-1224.

34

Gratiot-Suffolk Fault

Gratiot Lake

Mt BohemiaDeposit

SuffolkDeposit

Gratiot Deposit

Oronto GrOLQ

1111111 Jacobsville Sandstone

KXII I

0

Portage Lake Volcanics

20

km

have carried more sulfur than native copper fluids. However, the genesis of fluids generating the chalcocite deposits is currently speculative.

Gratiot Deposit

FIGURE 1: LOCATION OF COPPER SULFIDE DEPOSITS

,< References - .

Bomhorst, T. J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift System: Geological Society of America Special Paper 312, p. 127-136

Butler, B. S. and Burbank, W. S., 1929, The copper deposits of Michigan: U. S. Geol. Survey Prof. Paper 144,238 p.

Robertson, J. M., 1975, Geology and mineralogy of some copper sulfide deposits near Mount Bohemia, Keweenaw County, Michigan: Economic Geology, v. 70, p. 1202-1224.

CHEMICAL AND MINERALOGICAL COMPARISON OF BARABOO, BARRON, AND SIOUXARGILLITE, METAPELITE AND PIPESTONE

MEDARJS, L.G., Jr., and FOURNELLE, J.H., Dept. of Geology & Geophysics,Univ. ofWisconsin-Madison, 53706, inedarisgeology.wisc.edu, johnfgeo1ogy.wisc.edu;BOSZHARDT, RI., Mississippi Valley Archaeology Center, Univ. of Wisconsin-La Crosse,54601, [email protected]; BROII{AHM, J.H., State Historical Society of Wisconsin-Madison,53706, [email protected]

Early Proterozoic (1760-1630 Ma) red quartzites in the southern Lake Superior region place importantconstraints on the climatic, sedimentary, and tectonic history of North America, and fine-grainedsedimentary and hydrothermally altered rocks in the quartzite sequences have provided material for NativeAmerican artihcts. Although argillites and pipestones from several localities have been investigatedfreviously by X-ray diffiaction, little attention has been paid so far to their chemical compositions andtextural details. Representative samples of Baraboo, Barron, and Sioux argillite, metapelite, and pipestonehave been investigated by petrographic microscope, XRF,XRD, and EMP to determine their textural characteristics andchemical and mineralogical compositions. Such informationprovides a basis ibr evaluating possible regional variations inchemical compositions and metamorphic conditions amongthe quartzite localities and for determining artifactprovenances.

Bamboo: fine-grained metasedimentary layers, severalinches to several feet in thckness, are interbedded withquartzite and vary from pink argillite (metasiltstone) to grayphyllonite (metapelite), depending on the proportions ofquartz and pyrophyllite. Pipestone occurs in several smallpipestone quarries and one outcrop near the base of thequartzite sequence in the south limb of the Bamboo syncline.The pipestone is a dark purplish-red argillite, consisting ofquartz, pyrophyllite, and hematite, which is cut by thin whitehydrothermal veins containing pyrophyffite, muscovite, anddiaspore. In several pipestone samples, argillite itself hasbeen altered to an assemblage of muscovite-pyrophyllite-diasp ore-hematite, in which quartz has been replaced largelyby pyrophyllite (Fig. 1). Small amounts of kaolinite in themetasedimentary rocks and pipestone are retrograde in origin,replacing pyrophyllite or muscovite.

Barron: four samples of discarded pipestone werecollected in the vicinity of a pipestone quarry in the DoyleForest Unit, Barron County. The pipestone is a massive tolaminated red argillite, which contains quartz, hematite, andlarge vermicular grains of kaolinite (Fig. 2).

Sioux: two samples of pipestone from Pipestone NationalMonument and one of flaggy argillite from the Jasper quad-rangle (sec. 32, Tl 04N, R4W) were provided by DavidSouthwick and Tony Runkel. The pipestone samples aretypical catlinite, being fine-grained and deep red in color,

35

CHEMICAL AND MINERALOGICAL COMPARISON OF BARABOO, BARRON, AND SIOUX ARGILLITE, METAPELITE AND PIPESTONE

MEDARIS, L.G., Jr., and FOURNELLE, J.H., Dept. of Geology & Geophysics,Univ. of Wisconsin-Madison, 53706, [email protected], [email protected]; BOSZHARDT, RF., Mississippi Valley Archaeology Center, Univ. of Wisconsin-La Crosse, 54601, [email protected], BROBHAHN, J.H., State Historical Society of Wisconsin-Madison, 53706, [email protected]

Early Proterozoic (1760-1630 Ma) red quartzites in the southern Lake Superior region place important constraints on the climatic, sedimentary, and tectonic history of North America, and fine-grained sedimentaly and hydrothemially altered rocks in the quartzite sequences have provided material for Native American artifacts. Although argilhtes and pipestones from several localities have been investigated previously by X-ray diffraction, little attention has been paid so fir to their chemical compositions and textural details. Representative samples of Barabw, Ban-on, and Sioux argillite, metapelite, and pipestone have been investigated by petrographic microscope, XRF, XRD, and EMP to determine their textural characteristics and chemical and mineralogical compositions. Such information provides a basis for evaluating possible regional variations in chemical compositions and metamorphic conditions among the quartzite localities and for determining artifact provenances.

Barabw: fine-grained metasedimentary layers, several inches to several feet in thickness, are interbedded with quartzite and vary from pink argillite (metasiltstone) to gray phyllonite (metapelite), depending on the proportions of quartz and pyrophyllite. Pipestone occurs in several small pipestone quarries and one outcrop near the base of the

sequence in the south limb of the Baraboo syncline. The pipestone is a dark purplish-red aigillite, consisting of quartz, pyrophyllite, and hematite, which is cut by thin white hydrothermal veins containing pyrophyllite, muscovite, and diaspore. In several pipestone samples, argillite itself has been altered to an assemblage of muscovite-pyrophyllite- diaspore-hematite, in which quartz has been replaced largely by pyrophyllite (Fig. 1). Small amounts of kaolinite in the metasedimentary rocks and pipestone are retrograde in origin, replacing pyrophyllite or muscovite. m: fow samples of discarded pipestone were

collected in the vicinity of a pipestone quarry in the Doyle Forest Unit, Barren County. The pipestone is a massive to laminated red argillite, which contains quark, hematite, and large vermicular-grains of kaolmite (Fig. 2). M: two samples of pipestone from Pipestone National

Monument and one of flaggy aigillite from the Jasper quadrangle (sec. 32, T104N, R4W) were provided by David Southwick and Tony Runkel. The pipestone samples are typical catlinite, being fine-grained and deep red in color,

locally with small pale orange-red reduction spots and pale gray elliptical domains. The pipestone mineralassemblage is pyrophyllite-diaspore-muscovite-hematite, and the pale gray elliptical domains are largelypyrophyllite with minor diaspore and muscovite (Fig. 3). Small amounts of kaolinite are retrograde afterpyrophyllite and muscovite. The argillite contains quartz, hematite, pyrophyllite, and traces of detritalmuscovite.

Chemical Composition Six samples of Baraboo, Barron, and Sioux argillite and metapelite areessentially devoid of Na20 and CaO and contain less than0.39 wt% K20, attesting to their extreme chemical Rock Compositions in the q Baraboomatunty and contrasting markedly with the composition of ICASH System Baronaverage shale (Fig. 4). Relative to argiuite and average Projected onto tle KAS Plane Siouxshale, the K-rich and Si-poor compositions of Bamboo and

19 SiSioux pipestones (0.66-5.62 wt% lC,O and 40.90-54.20wt% 5i02) indicate that they may have formed fromargillite by addition of K and removal of Si through ppestone

hydrothermal activity. Such a process could account for kinthe replacement of quartz in Baraboo pip estone andsuggests that the elliptical pyrophyllite domains in Sioux K

pipestone may also be the result of quartzreplacement. 8

Metamorphic Conditions: the Barron argilliteassemblage, quartz-kaolinite, is limited to 6temperatures below 300°C at 2 kbar (Fig. 5). TheBamboo and Sioux argilhite assemblage, quartz-pyrophyllite, is stable between 300 and 390°C at 2kbar, and the pipestone assemblage, pyrophyllite-muscovite-diaspore, is more narrowly confined 2

between 315 and 360°C (Fig. 5). The Baraboo andSioux assemblages cannot be due to burial alone, obecause depths on the order of 18-19 km would be 250 300 350 400 450

required to reach 300°C for a typical stable craton T CFig. 5 Reactions in the KASH system and assemblages

conductive geotherm. The age and source of the u Barron, and Sioux rocksthermal pulse responsible for Baraboo and Siouxmetamorphism remains uncertain. In addition, kaolinite is widely reported in the Sioux Quartzite, and it isunclear whether metamorphism was ubiquitous in the Sioux Quartzite (as it was in the Baraboo quartzite),or restricted to zones of hydrothermal activity, perhaps related to faulting.

Conclusions: the chemical compositions of argillite and metapelite confirm the chemical maturity of allsedimentary members of the Bamboo, Barron, and Sioux sequences, consistent with derivation from adeeply weathered, low-relief source in an Early Proterozoic warm, humid climate. Comparable mineralassemblages in Bamboo and Sioux pipestones demonstrate that the promotmg hydrothennal activity doesnot coincide with the 1630 Ma thermal front in the southern Lake Superior region, postulated by Hoim etal. (1998); whether they are contemporaneous remains to be seen. Regarding artifact provenance, Barronpipestone is easily distinguished from Bamboo and Sioux pipestone by the presence of vermicular kaolinite.Comparable mineral assemblages and textures make discrimination between Baraboo and Sioux pipestonesmore difficult. Baraboo pipestone may contain quartz, is locally cut by thin, white veins, and is darkpurplish red in color; Sioux pipestone is devoid of quartz and veins, commonly has reduction spots andconspicuous, small, pale gray, elliptical pyrophyllite-rich domains, and is deep red to orange in color.

36

I locally with small pale orange-red reduction spots and pale gray elliptical domains. The pipestone mineral assemblage is pyrophyllite-diaspore-muscovite-hematite, and the pale gray elliptical domains are largely pyrophyllite with minor diaspore and muscovite (Fig. 3). Small amounts of kaolinite are retrograde after pyrophyllite and muscovite. The argillite contains quartz, hematite, pyrophyllite, and traces of detrital muscovite.

Chemical Comoosition Six samples of Baraboo, Barron, and Sioux argillite and metapelite are essentially devoid of Na,0 and CaO and wntain less than 0.39 wt% K,0, attesting to their extreme chemical maturity and contrasting markedly with the composition of average shale (Fig. 4). Relative to argillite and average shale, the K-rich and Si-poor wmpositions of Baraboo and Sioux pipestones (0.66-5.62 wt% K,0 and 40.90-54.20 wt% SiOJ indicate that they may have formed from argilhte by addition of K and removal of Si through hydrothermal activity. Such a process could account for the replacement of quartz in Baraboo pipestone and suggests that the elliptical pyrophyllite domains in Sioux pipestone may also be the result of quartz replacement.

Metamomhic Conditions: the Ban-on argiUite assemblage, quartz-kaolimte, is limited to temperatures below 300T at 2 kbar (Fig. 5). The Baraboo and Sioux argillite assemblage, quartz- pyrophyllite, is stable between 300 and 390T at 2

-. kbar, and the pipestone assemblage, pyropbyllite- muscovite-diaspore, is more narrowly confined 2 hetween 3 15 and 360Â° (Fig. 5). The Baraboo and Sioux assemblages cannot be due to burial alone, o because depths on the order of 18-19 km would be 250 300 350 400 450

T, "C quiredm reach 3000c 'raton Fig. 5 Reactions in the KASH system and assemblages conductive g ~ t h e n n . The age and source of the in Bamboo, Ban-on, and Sioux rocks thermal pulse responsible for Baraboo and Sioux metamorphism remains uncertain. In addition, kaolinite is widely reported in the Sioux Quartzite, and it is unclear whether metamorphism was ubiquitous in the Sioux Quartzite (as it was in the Baraboo quartzite), or restricted to zones of hydrothermal activity, perhaps related to faulting.

Conclusions: the chemical wmpositions of argillite and metapelite confirm the chemical maturity of all sedimentary members of the Baraboo, Barren, and Sioux sequences, consistent with derivation from a deeply weathered, low-relief source in an Early Proterozoic warm, humid climate. Comparable mineral assemblages in Baraboo and Sioux pipestones demonstrate that the promoting hydrothermal activity does not coincide with the 1630 Ma thermal front in the southern Lake Superior region, postulated by Holm et al. (1998); whether they are contemporaneous remains to be seen. Regarding artifact provenance, Barron pipestone is easily distinguished from Baraboo and Sioux pipestone by the presence of vermicular kaolinite. Comparable mineral assemblages and textures make discrimination between Baraboo and Sioux pipestones more difficult. Baraboo pipestone may wntain quartz, is locally cut by thin, white veins, and is dark purplish red in color; Sioux pipestone is devoid of quartz and veins, commonly has reduction spots and conspicuous, small, pale gray, elliptical pyropbylhte-rich domains, and is deep red to orange in color.

REPUBLIC WETLAND PRESERVE

John G. MeierDistrict Manager — Environmental Affairs

Cliffs Mining Services Company

Iron ore mining, by its very nature, causes large potential impacts to natural resources.Development of mine pits, rock stockpiles and tailings basins impacts wetlands from timeto time. Permits to impact wetlands are issued by the Michigan Department ofEnvironmental Quality if the impacts are "unavoidable" and are "minimized to thegreatest extent possible". Each permit requires that "compensatory wetland mitigation"be provided.

The Republic Wetland Preserve is being created at the former Republic Mine tailingsbasin to provide compensatory wetland mitigation for those unavoidable impacts towetlands at other mining operations. Instead of merely draining the water from thevarious tailings and water clarification ponds and converting the land to upland atRepublic, Cliffs, with MDEQ oversight, is constructing wetlands in these areas. Severalhundred acres of creditable wetland will be created. This wetland area, along withadjacent upland and external preservation area, will be placed in a ConservationEasement for the benefit of the State for long term protection.

37

REPUBLIC WETLAND PRESERVE

John G. Meier District Manager - Environmental Affairs

Cliffs Milling Services Company

Iron ore mining, by its very nature, causes large potential impacts to natural resources. Development of mine pits, rock stockpiles and tailings basins impacts wetlands from time to time. Permits to impact wetlands are issued by the Michigan Department of Environmental Quality if the impacts are "unavoidable" and are "minimized to the greatest extent possible". Each permit requires that "compensatory wetland mitigation" be provided.

The Republic Wetland Preserve is being created at the former Republic Mine tailings basin to provide compensatory wetland mitigation for those unavoidable impacts to wetlands at other mining operations. Instead of merely draining the water from the various tailings and water clarification ponds and converting the land to upland at Republic, Cliffs, with MDEQ oversight, is constructing wetlands in these areas. Several hundred acres of creditable wetland will be created. This wetland area, along with adjacent upland and external preservation area, will be placed in a Conservation Easement for the benefit of the State for long term protection.

POTENTIAL FOR STRATIFORM PGE MINERALIZATION INMAFIC LAYERED INTRUSIONS OF THE DULUTh COMPLEX

MILLER, James D., Jr., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114([email protected])

Economic concentrations of platinum group elements (PGEs) in meter-thick, stratiform, sulfide-bearinghorizons (reefs) have long been known to be associated with ultramafic-mafic layered intrusions such asthe Bushveld and Stillwater complexes. Recent discoveries of PGE mineralization in the SkaergaardIntrusion and related bodies in Greenland (Bird et al., 1991, 1995; Neilsen and Brooks, 1995; Aranson, etal., 1997; Andersen et al., 1998) have shown that stratiform PGE mineralization can also occur intholeiitic layered intrusions. The presence of many tholeiitic mafic layered intrusions associated with the1.1 Ga Midcontinent Rift suggests that the Lake Superior region is fertile ground for exploration of thisnewly recognized type of PGE mineralization. With the support of the State of Minnesota's MineralsCoordinating Committee the Minnesota Geological Survey has been assessing the potential for stratiformPGE mineralization in two intrusions of the Duluth Complex—the Layered Series at Duluth and theSonju Lake intrusion (Miller and Ripley, 1996).

Based on empirical observations of PGE mineralization in the Skaergaard and other tholeiiticintrusions, and on theoretical considerations of sulfur and PGE behavior in dynamic, differentiatingintrusions, the conditions that favor the formation of PGE-enriched horizons in tholeiitic layeredintrusions are:

1. The parent magma is initially sulfide undersaturated.2. The parent magma has a high initial PGE concentration and/or experiences a considerable amount

of fractional crystallization to build up noble metal concentrations prior to sulfide saturation.3. The initial segregation of sulfide melt is triggered by a process such as magma mixing,

decompression, assimilation, or simple differentiation that promotes a large R-factor(silicate/sulfide melt ratio).

These conditions imply that an intrusion that formed from an initially sulfide-undersaturated, tholeiiticparent magma has the best chance for significant PGE mineralization in the mid- to upper-level horizonthat marks the first "cumulus arrival" of immiscible sulfide melt. Theoretically, a gabbro cumulatecrystallized from a magma saturated in sulfide should contain greater than 400 ppm sulfur (Boudreau andMcCallum, 1992). Regardless of how sulfide saturation is triggered, or the way in which sulfide meltarrives at the magma chamber floor, it is likely that the first significant sulfide-saturated melt has thegreatest chance of encountering the most PGE-enriched silicate magma. Thus, exploration for a PGE-enriched horizon should attempt to seek out the stratigraphically lowest gabbroic cumulates that contain400 ppm S or more. The Cu/Pd ratio indicates the effectiveness of the sulfide-saturated melt inscavenging chalcophile metals from the silicate magma. Because Pd is several orders. of magnitude morecompatible with sulfide melt than Cu, an efficiently scavenged silicate melt should record a signficantincrease in Cu/Pd after a sulfide-segregation event.

These geochemical criteria were first applied to the Layered Series at Duluth (DLS, Miller, 1998).The DLS is a 3.5- to 4-km-thick, well-differentiated, layered intrusion emplaced into the basal section ofcomagmatic Keweenawan volcanics in the vicinity of Duluth. The DLS is composed of aunidirectionally differentiated suite of mafic cumulates that grades up from troctolite (P1+01) to gabbro(Pl+Aug+Fe0±0l±Ap), and displays a complementary cryptic variation in cumulus mineralcompositions. The transition from troctolitic to gabbroic cumulates occurs in an 1-km-thick intervalcalled the cyclic zone wherein P1+01 cumulates grade upward to Pl+Aug+Fe0,,+0l cumlates, and regressabruptly back to P1+01 cumulates. The cyclic zone is made up of as many as six such macrocycles,

38

POTENTIAL FOR STRATIFORM PGE MINERALIZATION IN MAFIC LAYERED INTRUSIONS OF THE DULUTH COMPLEX

MILLER, James D., Jr., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114 ([email protected])

Economic concentrations of platinum group elements (PGEs) in meter-thick, stratiform, sulfide-bearing horizons (reefs) have lone been known to be associated with ultramafic-mafic layered intrusions such as - the ~ u s h i e l d A d Stillwater complexes. Recent discoveries of PGE minerali&on in the Skaergaard Intrusion and related bodies in Greenland (Bird et al., 1991, 1995; Neilsen and Brooks, 1995; Aranson, et al., 1997; Andersen et al., 1998) have shown that stratiform PGE mineralization can also occur in tholeiitic layered intrusions. The presence of many tholeiitic mafic layered intrusions associated with the 1.1 Ga Midcontinent Rift suggests that the Lake Superior region is fertile ground for exploration of this newly recognized type of PGE mineralization. With the support of the State of Minnesota's Minerals coordinating ~ommi'ttee the Minnesota Geological Survey hasbeen assessing the potential for stratiform PGE mineralization in two intrusions of the Duluth Complex-the Layered Series at Duluth and the Sonju Lake intrusion (Miller and Ripley. 1996).

Based on empirical observations of PGE mineralization in the Skaergaard and other tholeiitic intrusions, and on theoretical considerations of sulfur and PGE behavior in dynamic, differentiating intrusions, the conditions that favor the formation of PGE-enriched horizons in tholeiitic layered intrusions are:

1. The parent magma is initially sulfide undersaturated 2. The parent magma has a high initial PGE concentration &h experiences a considerable amount

of fractional crystallization to build up noble metal concentrations prior to sulfide saturation. 3. The initial segregation of sulfide melt is triggered by a process such as magma mixing,

decompression, assimilation, or simple differentiation that promotes a large R-factor (silicatdsulfide melt ratio).

These conditions imply that an intrusion that formed from an initially sulfide-undersaturated, tholeiitic parent magma has the best chance for significant PGE mineralization in the mid- to upper-level horizon that marks the first "cumulus arrival" of immiscible sulfide melt. Theoretically, a gabbro cumulate crystallized from a magma saturated in sulfide should contain greater than 400 ppm sulfur (Boudreau and McCallum, 1992). Regardless of how sulfide saturation is triggered, or the way in which sulfide melt arrives at the magma chamber floor, it is likely that the first significant sulfide-saturated melt has the greatest chance of encountering the most PGE-enriched silicate magma. Thus, exploration for a PGE- enriched horizon should attempt to seek out the stratigraphically lowest gabbroic cumulates that contain 400 ppm S or more. The Cfld ratio indicates the effectiveness of the suicide-saturated melt in scavenging chalcophile metals from the silicate magma. Because Pd is several ordersof magnitude more compatible with sulfide melt than Cu, an efficiently scavenged silicate melt should record a signficant increase in CuIPd after a sulfide-segregation event.

These geochemical criteria were first applied to the Layered Series at Duluth (DLS, Miller, 1998). The DLS is a 3.5- to 4-km-thick, well-differentiated, layered intrusion emplaced into the basal section of comagmatic Keweenawan volcanics in the vicinity of Duluth. The DLS is composed of a unidirectionally differentiated suite of mafic cumulates that grades up from troctolite (Pl+01) to gabbro (Pl+Aug+FemÂ±O1Â±Ap and displays a complementary cryptic variation in cumulus mineral compositions. The transition from troctolitic to gabbroic cumulates occurs in an 1-km-thick interval called the cyclic zone wherein Pl+01 cumulates grade upward to Pl+Aug+Fe-+01 cumlates, and regress abruptly back to Pl+01 cumulates. The cyclic zone is made up of as many as six such macrocycles,

which implies that despite being well-differentiated, the DLS magma system was open to recharge andventing (Miller and Ripley, 1996).

Sulfur concentrations indicate that the DLS was initially sulfide undersaturated and did not reachsulfide saturation until the onset of cyclic zone crystallization. Cu/Pd ratios begin to show irregularthough significant increases upsection from the base of the cyclic zone. The irregular increase in Cu/Pdratios probably reflects the open nature of the DLS system. Interestingly, the anomolous concentrationsof sulfur and PGEs occur at macrocycle boundaries, which I interpret to indicate venting and/or rechargeevents. These observations suggest that a PGE-enriched sulfide reef may exist near the base of cycliczone, and may correspond to a horizon indicative of open-system perturbations to the DLS magma.

Geochemical studies were recently initiated to study the stratiform PCE potential of the Sonju Lakeintrusion (SLI) near Finland, Minnesota. The SLI is a 1-km thick, sheet-like intrusion that was emplacedbeneath a granophyre body. It is a completely differentiated intrusion that displays a very regular,unidirectional cumulus paragenesis: Ol—>Pl+Ol—>Pl÷Aug+Ol—>Pl+Aug+Fe—>Pl+Aug+Fe+Ol+Ap. This phase progression and a smooth cryptic variation in mineral composition indicate that the SLIcrystallized as an almost closed system. The SLI is the Keweenawan intrusion most similar to theSkaergaard. Geochemical analyses of sulfur and PGEs have not yet been completed for the SLI.Previous analyses included Cu which provides indirect evidence for the abundance of sulfide in the SLIcumulates. The Cu concentration shows a dramatic 5-fold increase (from cl0Oppm to >500 ppm) at alevel above the base of the intrusion approximately equal to 2/3 of its thickness. This change isapproximately midway through the Pl+Aug+Fe0, cumulate interval; notably, it is at the equivalentstratigraphic level, and within the same cumulate type as the Platinova reef of the Skaergaard Intrusion(Andersen et al., 1998). I anticipate reporting geochemical analyses for the SLI at the ILSG meeting.They will provide more direct evidence of the potential for a PGE-enriched sulfide-bearing horizon inthe upper part of SLI.

In conclusion, these studies suggest that significant PGE mineralization may exist in the medialorupper parts of the DLS and SLI intrusions, as well as in other tholeiitic layered intrusions of theMidcontinent Rift. Moreover, these studies demonstrate that geochemical criteria can be used (1) toevaluate the overall potential for stratiform PGE mineralization in tholeiitic mafic intrusions, and (2) toassist in identification of favorable intervals within individual intrusions. Because potential PGE-mineralized horizons are likely to be meter-scale in thickness, and contain only a few modal percentsulfide, core drilling and systematic geochemical sampling are necessary to confirm their existence.

References

Andersen, J.C.ø., Rasmussen, H., Neilsen, T.F.D., Rønsbo, J.G., 1998, The Triple Group and the Platinova gold and palladiumreefs in the Skaergaard Intusion—stratigraphic and petrographic relations: ECONOMIC GEOLOGY, v.93, p. 488—509.

Aranson, J.G., Bird, DX., Bernstein, S. and Kelemen, PB., 1997, Gold and platinum-group element mineralization in theKruuse Fjord Gabbro complex. East Greenland: ECONOMIC GEOLOGY, v.92, p. 490—501.

Bird, D.K., Brooks, C.K., Gannicott, R.A., Turner, P.A., 1991, A gold-bearing horizon in the Skaergaard Intrusion, EastGreenland: ECONOMIC GEOLOGY, v. 86, p. 1083—1 092.

Bird, D.K., Aranson, IC., Brandriss, ME., Nevle, It)., Radford, C., Bernstein, S., Gannicott, R.A., and Kelemen, P.R., 1995,A gold-bearing horizon in the Kap Edvard HoIm Complex, East Greenland: ECONOMIC GEOLOGY, v.90, p. 1288—1300.

Boudreau, A.E. and McCallum, 1.8., 1992, Concentration of platinum-group elements by magmatic fluids in layered intrusions:ECONOMIC GEOLOGY, v. 87, p. 1830—1848.

Miller, J.D., Jr., 1998, Potential for stratiform POE deposits in sulfide-undersaturated, tholeiitic layered intrusions of the DuluthComplex. Minnesota, USA. 8th International Platinum Symposium Abstracts, Rustenburg, South Africa, GSSA andSAIMM Symposium Series 518. p.263—266.

Miller, J.D.. Jr.. and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA, in Cawthorne, R.G., ed.,Layered Intrusions: Amsterdam, Elsevier Sci., p. 257—301.

Neilsen, T.F.D. and Brooks, C.K., 1995, Precious metals in magmas of East Greenland: Factors important to the mineralizationin the Skaergaard Intrusion: ECONOMIC GEOLOGY, v. 90, p. 1911—1917.

39

which implies that despite being well-differentiated, the DLS magma system was open to recharge and venting (Miller and Ripley, 1996).

Sulfur concentrations indicate that the DLS was initially sulfide undersaturated and did not reach sulfide saturation until the onset of cyclic zone crystallization. CuRd ratios begin to show irregular though significant increases upsection from the base of the cyclic zone. The irregular increase in CuIPd ratios probably reflects the open nature of the DLS system. Interestingly, the anomolous concentrations of sulfur and PGEs occur at macrocycle boundaries, which I interpret to indicate venting and/or recharge events. These observations suggest that a PGE-enriched sulfide reef may exist near the base of cyclic zone, and may correspond to a horizon indicative of open-system perturbations to the DLS magma.

Geochemical studies were recently initiated to study the stratiform PGE potential of the Sonju Lake intrusion (SLI) near Finland, Minnesota. The SLI is a 1-km thick, sheet-like intrusion that was emplaced beneath a granophyre body. It is a completely differentiated intrusion that displays a very regular, unidirectional cumulus paragenesis: 01->P1+01->Pl+Aug+01->Pl+Aug+Fe_->Pl+Ang+Fe_+Ol+ Ap. This phase progression and a smooth cryptic variation in mineral composition indicate that the SLI crystallized as an almost closed system. The SLI is the Keweenawan intrusion most similar to the Skaergaard. Geochemical analyses of sulfur and PGEs have not yet been completed for the SLI. Previous analyses included Cu which provides indirect evidence for the abundance of sulfide in the SLI cumulates. The Cu concentration shows a dramatic Sfold increase (from e100ppm to >500 ppm) at a level above the base of the intrusion approximately equal to 2.13 of its thickness. This change is approximately midway through the Pl+Aug+Fe cumulate interval; notably, it is at the equivalent stratigraphic level, and within the same cumulate type as the Platinova reef of the Skaergaard Intrusion (Andersen et al., 1998). I anticipate reporting geochemical analyses for the SLI at the ILSG meeting. They will provide more direct evidence of the potential for a PGE-enriched sulfide-bearing horizon in the upper part of SLI.

In conclusion, these studies suggest that significant PGE mineralization may exist in the medial orupper parts of the DLS and SLI intrusions, as well as in other tholeiitic layered intrusions of the Midcontinent Rift. Moreover, these studies demonstrate that geochemical criteria can be used (1) to evaluate the overall potential for stratiform PGE mineralization in tholeiitic mafic intrusions, and (2) to assist in identification of favorable intervals within individual intrusions. Because potential PGE- mineralized horizons are likely to be meter-scale in thickness, and contain only a few modal percent sulfide, core drilling and systematic geochemical sampling are necessary to confirm their existence.

: .,!,

Andersen, J.C.0.. Rasmussen, H., Neilsen, T.F.D., R~nsbo. J.G., 1998, The Triple Group and the Platinova gold and palladium reefs in the Skaergaard Intusion-stratigraphic and petrographic relations: ECONOMIC GEOLOGY, v.93, p. 488-509.

Aranson, J.G., Bird, D.K., Bernstein, S. and Kelemen, P.B., 1997, Gold and platinum-group element mineralization in the Kmuse Fjord Gabbro complex. East Greenland: ECONOMIC GEOLOGY, v.92, p. 490-501.

Bird, D.K., Brooks, C.K., Gannicott, R.A., Turner, P.A., 1991, A gold-bearing horizon in the Skaergaard Intrusion, East Greenland: ECONOMIC GEOLOGY, v. 86, p. 1083-1092.

Bird, D.K., Aranson, J.G., Brandriss. M.E., Nevle, R.J., Radford, G., Bernstein, S., Gannicott, R.A.. and Kelerotn, P.B., 1995, A gold-bearing horizon in the Kap Edvard Holm Complex, East Greenland: ECONOMICGEOLOGY, v. 90. p. 1288-1300.

Boudreau, A.E. and McCallum, IS., 1992, Concentration of platinum-group elements by magmatic fluids in layered intrusions: ECONOMICGEOLOGY, v. 87. p. 1830-1848.

Miller, J.D.. Jr., 1998, Potential for stratiform PGE deposits in sulfide-undersaturated, tholeiitic layered intrusions of the Duluth Complex, Minnesota, USA. 8th International Platinum Symposium Abstracts, Rustenburg, South Africa, GSSA and SAIMM Symposium Series S18, p. 263-266.

Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA, in Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Sci.. p. 257-301.

Neilsen, T.F.D. and Brooks, C.K., 1995, Precious metals in magmas of East Greenland: Factors important to the mineralization in the Skaergaard Intrusion: ECONOMICGEOLOGY, v. 90, p. 191 1-1917.

NEW GEOLOGIC MAP OF THE CENTRAL DULUTH COMPLEX

MILLER, James D., Jr., and CHANDLER, Val. W., Minnesota Geological Survey, University ofMinnesota, 2642 University Ave., St. Paul, MN 55114 (milleO66 [email protected])

The Minnesota Geological Survey is preparing a new interpretation of the geology of the central part ofthe Duluth Complex, which will be published as a 1:1 00,000-scale map this summer. The map will covera rectangular area of about 1,100 square miles bounded by long. 92°07'30" and 9101510011 and lat.47°37'30" and 47°15'OO". Both the southeastern corner of the map area, which is anchored to the LakeSuperior shoreline near Silver Bay, and the northwestern corner, which covers the basal contact of theDuluth Complex, have adequate bedrock exposure that has been mapped at the scale of 1:24,000 (Miller,1988; Miller and others, 1993; Severson and Hauck, 1990; Severson and Miller, 1999). However, theintervening area, roughly 90% of the map area, contains little or no bedrock exposure. Geologicinterpretations in this area largely depend on evaluation of aeromagnetic and gravity data. Geologiccontrol for the geophysical data is limited to widely and unevenly dispersed drill cores as well as scatteredareas of outcrop (Severson, 1995; Chandler and others, 1991; Bonnichsen, 1971).

The bedrock geology of the map area is dominated by intrusive rocks that were emplaced into thelower and medial portionS of the largely basaltic North Shore Volcanic Group (NSVG) during the mainstages of the Midcontinent Rift magmatism (Paces and Miller, 1993). The intrusive rocks can be grosslysubdivided into those belonging to the more plutonic Duluth Complex and those composing the morehypabyssal intrusions of the Beaver Bay Complex. The boundary between two intrusive complexescorresponds to a semi-continuous septa of strongly hornfelsed volcanic rocks, locally containing minorintrusions, that trends northeasterly through the middle of the map area (Fig. 1). Large-scale intrusions ofdifferentiated mafic cumulates, gabbroic anorthosites, gabbros, and granophyres compose the DuluthComplex northwest of the volcanic septa. Southeast of the volcanic septa, smaller scale, sheet-like bodiesand dikes of mafic cumulate rocks, gabbro, diabase. diorite and granophyre constitute multiple intrusionsof the Beaver Bay Complex that were emplaced into higher levels of the volcanic edifice.

A number of intrusive components can be distinguished within the Duluth Complex. The oldest is astructurally complex assemblage of mostly leucogabbroic to anorthositic rocks, small bodies of olivineoxide gabbro, and scattered inclusions of volcanic hornfels (collectively termed AGV in Figure 1A). Thisassemblage corresponds to a very heterogeneous aeromagnetic signature of variable amplitude in thewestern part of the complex but gives rise to a more subdued signature to the east (upsection). Thechange may indicate a transition to more consistently anorthositic compositions. Irregular masses ofgranophyre occur within the eastern part of the AGV, but contact relationships are not exposed in thestudy area. Aeromagnetic patterns suggest that the subcircular to lensoidal granophyre bodies wereemplaced into the AGV, but exposures outside the map area (Boerboom and Miller, 1994) indicate thatlarge granophyre bodies can be older than the anorthositic rocks.

More than half of the Duluth Complex in the map area is composed of six discrete mafic layeredintrusions which were emplaced beneath and within the AGV and, in one case, beneath a body ofgranophyre. The stratiform internal structure of these layered intrusions gives rise to a crudely bandedaeromagnetic pattern. Truncations of the patterns imply a general sequence of emplacement and suggestthat successive intrusions overplated previous intrusions. Although the igneous stratigraphy of eachintrusion is generally poorly known, some notable differences are evident. Apparently, the oldest layeredmafic intrusion is the Partridge River intrusion (PRI), which forms the basal contact in the northwesternpart of the map area. The preponderance of troctolite in the FRI suggests that it formed by frequentimpulses of magma and little internal differentiation. The northeast end of the PRI is truncated by theSouth Kawishiwi intrusion (SKI). Where it is more completely exposed in the Gabbro Lake quadrangleto the north (Green and others, 1966), the SKI is also dominated by troctolite. At its southern end, thePRI is overplated and ultimately cut out by another intrusive body that is almost entirely unexposedexcept for a few drill holes along its basal contact and some troctolite emplaced into the AGV hangingwall of the PRI. This intrusion, which we tentatively call the West-central margin intrusion, has asubdued aeromagnetic signature indicative of troctolitic cumulates. The subdued pattern passes upsectioninto a busy, high-amplitude signature that shows some banding and thus may indicate the presence ofupper gabbroic cumulates beneath an AGV roof zone. The Greenwood Lake intrusion (GLI) is asomewhat isolated intrusion that truncates the SKI but otherwise intrudes AGV and granophyre rocks.The GLI is unique in that it contains a complete differentiation sequence that grades from lower troctolite

40

MILLER, James D., Jr., and CHANDLER, Val. W., Minnesota Geological Survey, University of Minnesota, 2642 University Ave., St. Paul, MN 551 14 (mille066 or [email protected])

The Minnesota Geological Survey is preparing a new interpretation of the geology of the central pan of the Duluth Comnlex. which will be nublished as a 1: 100.000-scale man this summer. The mao will cover a rectangular area of about 1,100 square miles bounded by long. 92007'30 and 91Â°15'00 andlat. 47'37'30 and 47Â°15'00" Both the southeastern comer of the map area, which is anchored to the Lake Superior shoreline near Silver Bay, and the northwestern comer, which covers the basal contact of the Duluth Complex, have adequate bedrock exposure that has been mapped at the scale of 1:24,000 (Miller, 1988; Miller and others, 1993; Severson and Hauck, 1990; Severson and Miller, 1999). However, the intervening area, roughly 90% of the map area, contains little or no bedrock exposure. Geologic interpretations in this area largely depend on evaluation of aeromagnetic and gravity data. Geologic control for the geophysical data is limited to widely and unevenly dispersed drill cores as well as scattered areas of outcrop (Severson, 1995; Chandler and others, 1991 ; Bonnichsen, 1971).

The bedrock geology of the map area is dominated by intrusive rocks that were emplaced into the lower and medial portions of the largely basaltic North Shore Volcanic Group (NSVG) during the main stages of the Midcontinent Rift magmatism (Paces and Miller, 1993). The intrusive rocks can be grossly subdivided into those belonging to the more plutonic Duluth Complex and those composing the more hypabyssal intrusions of the Beaver Bay Complex. The boundary between two intrusive complexes corresponds to a semi-continuous septa of strongly homfelsed volcanic rocks, locally containing minor intrusions, that trends northeasterly through the middle of the map area (Fig. 1). Large-scale intrusions of differentiated mafic cumulates, gabbroic &orthosites. gabbros, &d pnophyres compose the Duluth Complex northwest of the volcanic septa. Southeast of the volcanic septa, smaller scale, sheet-like bodies and dikes of mafic cumulate rocks, gabbro, diabase, dionte and granophyre constitute multiple intrusions of the Beaver Bay Complex that were emplaced into higher levels of the volcanic edifice.

A number of intrusive components can be distinguished within the Duluth Complex. The oldest is a structurally complex assemblage of mostly leucogabbroic to anorthositic rocks, small bodies of olivine oxide gabbro, and scattered inclusions of volcanic homfels (collectively termed AGV in Figure IA). This assemblage corresponds to a very heterogeneous aeromagnetic signature of variable amplitude in the western part of the complex but gives rise to a more subdued signature to the east (upsection). The change may indicate a transition to more consistently anorthositic compositions. Irregular masses of granophyre occur within the eastern part of the AGV, but contact relationships are not exposed in the study area. Aeromagnetic patterns suggest that the subcircular to lensoidal granophyre bodies were emplaced into the AGV, but exposures outside the map area (Boerboom and Miller, 1994) indicate that large granophyre bodies can be older than the anorthositic rocks.

More than half of the Duluth Complex in the map area is composed of six discrete mafic layered intrusions which were emplaced beneath and within the AGV and, in one case, beneath a body of granophyre. The stratiform internal structure of these layered intrusions gives rise to a crudely banded aeromagnetic pattern. Truncations of the patterns imply a general sequence of emplacement and suggest that successive intrusions overplated previous intrusions. Although the igneous stratigraphy of each intrusion is generally poorly known, some notable differences are evident. Apparently, the oldest layered mafic intrusion is the Partridge River intrusion (PRI), which forms the basal contact in the northwestern part of the map area. The preponderance of troctolite in the PRI suggests that it formed by frequent impulses of magma and little internal differentiation. The northeast end of the PRI is truncated by the South Kawishiwi intrusion (SKI). Where it is more completely exposed in the Gabbm Lake quadrangle to the north (Green and others, 1966). the SKI is also dominated by troctolite. At its southern end, the PRI is overplated and ultimately cut out by another intrusive body that is almost entirely unexposed except for a few drill holes along its basal contact and some troctolite emplaced into the AGV hanging wall of the PRI. This intrusion, which we tentatively call the West-central margin intrusion, has a subdued aeromagnetic signature indicative of troctolitic cumulates. The subdued pattern passes upsection into a busy, high-amplitude signature that shows some banding and thus may indicate the presence of upper gabbroic cumulates beneath an AGV roof zone. The Greenwood Lake intrusion (GLI) is a somewhat isolated intrusion that truncates the SKI but otherwise intrudes AGV and granophyre rocks. The GLI is unique in that it contains a complete differentiation sequence that grades from lower troctolite

cumulates to upper oxide gabbro cumulates. The northern extent of the GLI is truncated by the southernmargin of the troctolitic Bald Eagle intrusion, apparently the youngest major mafic layered intrusion inthe map area. The Osier Lake intrusion is a small, plug-like body of troctolite cored by gabbro cumulatesthat is known from two drill core and a bull's-eye aeromagnetic signature. It was emplaced entirely intoanorthositic rocks of the AGV, and thus, its age relative to other layered intrusions is unknown.

Intrusions of the Beaver Bay Complex occur as several suites of east-dipping, half-saucer-shapedsheets, which produce very distinctive, concentric aeromagnetic anomaly patterns. The largest andearliest intrusive suite is the Cloquet Lake layered series (CLLS), which appears to be a nested set ofsheet-like intrusions. It is difficult to accurately determine the compositional variability of the CLLSbecause it is completely unexposed and penetrated by only a dozen or so drill core. In general, the lowerpart of the series appears to be composed of at least two differentiated sequences of troctolitic to gabbroiccumulates. The upper part of the series appears to contain more evolved intrusive rocks, includinggabbro, ferrodiorite, monzodiorite, and granophyre, as well as an unknown amount of volcanic hornfels.A complex aeromagnetic pattern emanating from the southern margin of the CLLS may be caused bynumerous satellite dikes and sheets emplaced into volcanic rocks. The eastern side of the CLLS is cut byanother sheet-like body—the Sonju Lake intrusion (SLI). This strongly differentiated mafic layeredintrusion is well exposed just to the east of the map area in the Finland quadrangle (Miller and others,1993) and is capped by ferromonzonitic to granophyric rocks of the Finland granite. Good exposure inthe southern Doyle Lake and Silver Bay quadrangles (Miller, 1988; Miller and others. 1993) shows thatthe Finland granite cuts an early suite of gabbroic to ferromonzonitic rocks of the Lax Lake gabbro. Boththe Finland granite and the Lax Lake gabbro may originally have been part of the CLLS but weredisassociated from it by the intrusion of the SLI. The youngest intrusions in the map area are theanorthosite-inclusion-bearing dikes and sills of the Beaver River diabase. This intrusive swarm, which iswell exposed in the southeastern corner of the map area, is bounded on the west by a prominent dike thatfeeds into southeast-dipping sheets. The upper parts of these sheets commonly contain ferrogabbroiccumulate bodies, collectively termed the Silver Bay intrusions.

The 1:1 00,000-scale map of the central Duluth Complex will represent a significant improvement onprevious regional geologic maps of the complex. Because this map will be digitally compiled, we intendto revise it periodically as new data become available. An important contribution of this new geologicmap is that it will identify many previously unrecognized intrusive bodies that should provide newexploration targets for stratiform PGE deposits.ReferencesBonnichsen, 8., 1971, Outcrop map of southern part of Duluth Complex and associated Keweenawan rocks, St.

Louis and Lake Counties, Minnesota: Minn. Geol. Surv. Misc. Map M-l 1, scale 1:24,000.Boerboom, T.J., and Miller, J.D., Jr., 1994, Geologic map of the Wilson Lake quadrangle and parts of the Silver

Island Lake and Toohey Lake quadrangles, Lake and Cook Counties, Minnesota: Minn. Geol. Surv. Misc.Map M-81, scale 1:24,000.

Chandler, V.W., Miller, J.D., Jr., and Venzke, E. A., 1991, Central Duluith Complex drilling project; Minn. Geol.Surv. OFR 91-4,2 sheets, accompanying text.

Green, J.C., Phinney, W.C., and Weiblen, P.W., 1966, Gabbro Lake quadrangle, Lake County, Minnesota: Minn.Geol. Surv. Misc. Map M-2, scale 1:24,000.

Miller, J.D., Jr., 1988, Geologic map of the Silver Bay and Split Rock Point NE quadrangles, Lake County,Minnesota: Minn. Geol. Surv. Misc. Map M-65, scale 1:24,000.

Miller, J.D., Jr., Green, J.C., Chandler, V.W., and Boerboom, T.J., 1993, Geologic map of the Finland and DoyleLake quadrangles, Lake County, Minnesota: Minn. Geol. Surv. Misc. Map M-72, scale 1:24,000.

Paces, JR., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions,northeastern, Minnesota: new insights for physical, petrogenetic, paleomagnetic and tectono-magmaticprocesses associated with 1.1 Ga Midcontinent rifting: J. Geophys. Res., v.98, p. 13997-14013.

Severson, M.J., 1995, Geology of the southern portion of the Duluth Complex: Univ. of Minn. Natural ResourcesRes. Inst., Tech. Rept. NRRITI'R-95126, 185 p.

Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge Riverintrusion: Univ. of Minn. Natural Resources Res. Inst., Tech. Rept. NRRIIGMIN-TR-89-l 1,236 p.

Severson, M.J., and Miller, J.D., Jr., 1999, Bedrock geologic map of the Allen quadrangle, Minnesota: Minn. Geol.Surv. Misc. Map M-91, scale 1:24,000.

41

cumulates to upper oxide gabbro cumulates. The northern extent of the GLI is truncated by the southern margin of the troctolitic Bald Eagle intrusion, apparently the youngest major mafic layered intrusion in the map area. The Osier Lake intrusion is a small, plug-like body of troctolite cored by gabbro cumulates that is known from two drill core and a bull's-eye aeromagnetic signature. It was emplaced entirely into anorthositic rocks of the AGV, and thus, its age relative to other layered intrusions is unknown.

Intrusions of the Beaver Bay Complex occur as several suites of east-dipping, half-saucer-shaped sheets, which produce very distinctive, concentric aeromagnetic anomaly patterns. The largest and earliest intrusive suite is the Cloquet Lake layered series (CLLS), which appears to be a nested set of sheet-like intrusions. It is difficult to accurately determine the compositional variability of the CLLS because it is completely unexposed and penetrated by only a dozenor so drill core. In general, the lower part of the series appears to be composed of at least two differentiated sequences of troctolitic to gabbroic cumulates. The upper part of the series appears to contain more evolved intrusive rocks, including gabbro, ferrodiorite, monzodiorite, and granophyre, as well as an unknown amount of volcanic hornfels. A complex aeromagnetic pattern emanating from the southern margin of the CLLS may be caused by numerous satellite dikes and sheets emplaced into volcanic rocks. The eastern side of the CLLS is cut by another sheet-like body-the Sonju Lake intrusion (SLI). This strongly differentiated mafii layered intrusion is well exposed just to the east of the map area in the Finland quadrangle (Miller and others, 1993) and is capped by ferromonzonitic to granophyric rocks of the Finland granite. Good exposure in the southern Doyle Lake and Silver Bay quadrangles (Miller, 1988; Miller and others, 1993) shows that the Finland granite cuts an early suite of gabbroic to ferromonzonitic rocks of the Lax Lake gabbro. Both the Finland granite and the Lax Lake gabbro may originally have been part of the CLLS but were disassociated from it by the intrusion of the SLI. The youngest intrusions in the map area are the anorthosite-inclusion-bearing dikes and sills of the Beaver River diabase. This intrusive swarm, which is well exposed in the southeastern comer of the map area, is bounded on the west by a prominent dike that feeds into southeast-dipping sheets. The upper parts of these sheets commonly contain ferrogabbroic cumulate bodies, collectively termed the Silver Bay intrusions.

The 1: 100,000-scale map of the central Duluth Complex will represent a significant improvement on previous regional geologic maps of the complex. Because this map will be digitally compiled, we intend to revise it periodically as new data become available. An important contribution of this new geologic map is that it will identify many previously unrecognized intrusive bodies that should provide new exploration targets for stratiform PGE deposits. References Bonnichsen, B., 1971, Outcrop map of southern part of Duluth Complex and associated Keweenawan rocks, St.

Louis and Lake Counties, Minnesota: Minn. Geol. Surv. Misc. Map M-11, scale 1:24,000. Boerboom, T.J., and Miller, J.D., Jr., 1994, Geologic map of the Wilson Lake quadrangle and parts of the Silver

Island Lake and Toohey Lake quadrangles. Lake and Cook Counties, Minnesota: Minn. Geol. SUN. Misc. Map M-81, scale 1:24,000.

Chandler, V.W., Miller, J.D., Jr., and Venzke, E. A,, 1991, Central Dululth Complex drilling project; Minn. Geol. S w . OFR 91-4,2 sheets, accompanying text.

Green, J.C., Phinney, W.C., and Weiblen, P.W., 1966, Gabbro Lake quadrangle. Lake County, Minnesota: Minn. Geol. Surv. Misc. Map M-2, scale 1:24,000.

Miller, J.D., Jr., 1988, Geologic map of the Silver Bay and Split Rock Point NE quadrangles. Lake County, Minnesota: Minn. Geol. SUN. Misc. Map M-65, scale 1:24,000.

Miller, J.D., Jr., Green, J.C., Chandler, V.W., and Boerboom, T.J., 1993, Geologic map of the Finland and Doyle Lake quadrangles, Lake County, Minnesota: Minn. Geol. Surv. Misc. Map M-72, scale 1:24,000.

Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern, Minnesota: new insights for physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with 1.1 Ga Midcontinent rifling: J. Geophys. Res., v. 98, p. 13 997-14 013.

Severson, M.J., 1995, Geology of the southern portion of the Duluth Complex: Univ. of Minn. Natural Resources Res. Inst., Tech. Rept. NRRI/TR-95/26, 185 p.

Severson, MJ., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Univ. of Minn. Natural Resources Res. Inst., Tech. Rept. NRRIIGMIN-TR-89-11.236 p.

Severson, M.J.. and Miller, J.D., Jr., 1999, Bedrock geologic map of the Allen quadrangle, Minnesota: Minn. Geol. Surv. Misc. Map M-91, scale 1:24,000.

A.

Footwall NSVG Duluth Complex Beaver Bay complexShale Miscellaneous Anohositic Rocks ftGranophyre Mafic GranophyreIron-formation Volcanic Rocks Gabbroic Rocks Gabbroic Cumulates Gabbro, Ferrodiotite, Monzodiorite

— Volcanic Homlels LilTroctolite Cumulates Diabase, 01 Gabbro, TroctoliteGrariute —-" Fault • Oxide UltramaticMetavolcanic Rocks

—fSBl

"BRD

10 Miles

=20 Kilometers

FIGURE 1. A) Preliminary generalized geologic map of the central Duluth Complex. Specific intrusive units are: AGV-Anorthositic-gabbroic-volcanic assemblage (rock types distinguished in the northwest area), PRI-Partidge River intrusion, WCMI-West-central margin intrusion, SKI-SouthKawishiwi intrusion,OLl-Oreenwood Lake intrusion. BEt-Bald Eagle intrusion, OLI-Osier Lake intrusion, CLLS-Cloquet Lake layered series. SLI-Sonju Lake intrusion, FO-Finland granite, LLG-Lax Lake gabbro, BRD-Beaver River diabase, SBI-Silver Bay intrusions. Crosses mark 7.5' quadranglecorners. B) First-vertical-derivative, reduced-to-pole image of

aeromaicdata, which forms basis for the geologic interpretation shown in A.

SEDIMENTOLOGY OF TWO DEEP WELLS IN THE KEEWANAWAN MIDGONTINENTRIFT SYSTEM NEAR MUNISING, UPPER PENINSULA, MICHIGAN

OJAKANGAS. Richard W., Department of Geology, University ofMinnesota-Duluth, Duluth, MN 55812, [email protected]

Amoco Production Company drilled the St. Amour stratigraphic test wellin 1987. six miles south-southeast of Munising, Ml. The well bottomed at7,241 ft. Eight mites to the southeast of the St. Amour welt, at MickeyCreek, Cliffs Mining Services Company had drilled a 5.345 ft deep well in1969. These wells are located on the southwest flank of the MidcontinentRift System (MRS) where the rift starts bending towards the south.

Except for the Pleistocene (110 ft thick in the St. Amour and 88 ft thick inthe Mickey Creek), both wells were cored from top to bottom. The youngestbedrock is the Ordovician Au Train Formation, about 280 ft thick in the St.Amour well and 235 ft thick in the Mickey Creek well. The Au Trainoverties the Upper Cambrian (?) Munising Formation, a quartz arenite unitthat is 135 ft thick in the St. Amour and 310 ft thick in the Hickey Creek.

The Keweenawan rocks in the St. Amour well from the base downwardinclude =2200 ft of feldspatholithic sandstone (N=8: QIF/L= 72.2/20.7/7.2) interpreted as Jacobsville Fm.; =3000 ft of lithofeldspathicsandstone (N=7: Q/F/L=58.1/19.3/22.6) interpreted as Freda Fm.; 725 ftof quartz sandstone (N=4: Q/F/L=93.6/1 .4/5.0): 400 ft of basalt flowswith interflow black shales and red to brown coarse clastics includingconglomerate and lithic sandstone (N=2: QIFIL65.0I0.5/34.5); and 300 ftof dacitic (?) ignimbritic flows.

The Keweenawan rocks in the Hickey Creek welt from the base downwardinclude =1720 ft of dominantly feldspatholithic sandstone (N=6:Q/F/L=78.2/16.7/5.1) interpreted as Jacobsville Fm. and —2845 ft ofdominantly lithofefdspathic sandstone (N=9: QIF/L= 54.8/19.0/26.3)interpreted as Freda Fm. The main two types of sand-sized rockfragments in the two wells are metamorphic and fetsic volcanic. In the St.Amour well, metamorphic rock fragments (schist and greenstone) are onaverage twice as abundant as felsic volcanic fragments, whereas in theMickey Creek well, fetsic volcanics are three times as abundant asmetamorphic rock fragments. Porosity decreases downward in both wells,with virtually no porosity present below 2750 ft.

43

SEDIMENTOLOGY OF TWO DEEP WELLS IN THE KEEWANAWAN MIDCONTINENT RIFT SYSTEM NEAR MUNISING, UPPER PENINSULA, MICHIGAN

OJAKANGAS, Richard W., Department of Geology, University of Minnesota-Duluth, Duluth, MN 55812, [email protected]

Amoco Production Company drilled the St. Amour stratigraphic test well in 1987, six miles south-southeast of Munising, MI. The well bottomed at 7,241 ft. Eight miles to the southeast of the St. Amour well, at Hickey Creek, Cliffs Mining Services Company had drilled a 5,345 ft deep well in 1969. These wells are located on the southwest flank of the Midcontinent Rift System (MRS) where the rift starts bending towards the south.

Except for the Pleistocene (110 ft thick in the St. Amour and 88 ft thick in the Hickey Creek), both wells were cored from top to bottom. The youngest bedrock is the Ordovician Au Train Formation, about 280 ft thick in the St. Amour well and 235 ft thick in the Hickey Creek well. The Au Train overlies the Upper Cambrian (?) Munising Formation, a quartz arenite unit that is 135 ft thick in the St. Amour and 310 ft thick in the Hickey Creek.

The Keweenawan rocks in the St. Amour well from the base downward include -2200 ft of feldspatholithic sandstone (N=8: QlF/L= 72.2120.71 7.2) interpreted as Jacobsville Fm.; -3000 ft of lithofeldspathic sandstone (N=7: QIF/L=58.1/19.3/22.6) interpreted as Freda Fm.; - 725 ft of quartz sandstone (N=4: QlFIL=93.611.4/5.0); 400 ft of basalt flows with interflow black shales and red to brown coarse elastics including conglomerate and lithic sandstone (N=2: QlFlL=65.010.5134.5); and 300 ft of dacitic (?) ignimbritic flows.

The Keweenawan rocks in the Hickey Creek well from the base downward include -1720 ft of dominantly feldspatholithic sandstone (N=6: Q/F/L=78.2116.715.1) interpreted as Jacobsville Fm. and 52845 ft of dominantly lithofeldspathic sandstone (N=9: Q/FlL= 54.8119.0126.3) interpreted as Freda Fm. The main two types of sand-sized rock fragments in the two wells are metamorphic and felsic volcanic. In the St. Amour well, metamorphic rock fragments (schist and greenstone) are on average twice as abundant as felsic volcanic fragments, whereas in the Hickey Creek well, felsic volcanics are three times as abundant as metamorphic rock fragments. Porosity decreases downward in both wells, with virtually no porosity present below 2750 ft.

The mineralogy in the two wells is quite similar for the upper 5300 ft andthe formations, not unexpectedly, correlate well. They also correlate wellwith Keweenawan outcrops in the Lake Superior region (Ojakangas, 1986;Adamson, 1997). However, the lower portion of the St. Amour does notcorrelate well with the exposed Bayfield and Oronto Groups. Quartzsandstones occur low in the well, in direct contrast to exposed sectionswhere they are near the top. Quartzose sediments deep in the rift are alsopresent in the Eischeid # 1 well in Iowa (Ludvigson et at, 1990). Clearly,the search for uniformity and correlation where deposition likely occurredin a series of sub-basins (haif-grabens) is a tenuous pursuit, as notedearlier by Mauk (1991). Soreghan and Cohen (1993) interpreted relativelyquartzose sands (Q to 79%) on hinged margins of modern Lake Tanganyikato be the result of reworking in shallow water. This may be a partialanalogue for these quartz sandstones, but eolian activity in thevegetationless Middle Proterozoic terrestrial environment is also likely.

Cliffs Mining Services Company kindly purchased the thin sections.

REFERENCES

Adamson, k.F, 1997, Petrology, stratigraphy, and sedimentation of theMiddle Proterozoic Bayfield Group. Northwestern Wisconsin:Unpublished M.S. thesis, University of Minnesota, Duluth, 203 p.

Ludvigson, G.A., Mckay, H.M., and Anderson, R.R., 1990, Pekrology ofKeweenawan sedimentary rocks in the MG. Eischeid # 1 drillhole: inAnderson, R.R., ed., The Amoco M.G. Eischeid # 1 Deep Petroleum Test,Carroll County, Iowa, Iowa Department of Natural ResourcesSpecial Report Series No.. 2, p. 77-112.

Mauk, J. L., 1991, Sub-basins in the Proterozoic Midcontinent Rift:Stratigraphic evidence from Upper Michigan (Abs.): GeologicalSociety of America Abstracts with Programs, San Diego, CA, p. A59.

Ojakangas, R.W., 1986, Reservoir characteristics of the keweenawanSupergroup, Lake Superior region: in Mudrey, MG. Jr., ed.,Precambrian Petroleum Potential, Wisconsin and Michigan.Geoscience Wisconsin, V. 11, p. 25-31.

Soreghan, M.J.. and Cohen, A.S., 1993, The effects of basin asymmetry onsand composition: Examples from Lake Tanganyika. Africa: inJohnsson, M.J., and Basu, A.. eds.. Processes Controlling theComposition of Clastic Sediments: Geological Society of AmericaSpecial Paper 284. p. 285-301.

44

I The mineralogy in the two wells is quite similar for the upper 5300 ft and the formations, not unexpectedly, correlate well. They also correlate well

I with Keweenawan outcrops in the Lake Superior region (Ojakangas, 1986; Adamson, 1997). However, the lower portion of the St. Amour does not correlate well with the exposed Bayfield and Oronto Groups. Quartz sandstones occur low in the well, in direct contrast to exposed sections where they are near the top. Quartzose sediments deep in the rift are also present in the Eischeid # 1 well in Iowa (Ludvigson et al, 1990). Clearly, the search for uniformity and correlation where deposition likely occurred in a series of sub-basins (half-grabens) is a tenuous pursuit, as noted earlier by Mauk (1991). Soreghan and Cohen (1993) interpreted relatively quartzose sands (Q to 79%) on hinged margins of modern Lake Tanganyika to be the result of reworking in shallow water. This may be a partial analogue for these quartz sandstones, but eolian activity in the vegetationless Middle Proterozoic terrestrial environment is also likely.

Cliffs Mining Services Company kindly purchased the thin sections.

REFERENCES

Adamson, K.F., 1997, Petrology, stratigraphy, and sedimentation of the Middle Proterozoic Bayfield Group, Northwestern Wisconsin: Unpublished M.S. thesis. University of Minnesota, Duluth, 203 p.

Ludvigson, G.A., McKay, P.M., and Anderson, R.R., 1990, Petrology of Keweenawan sedimentary rocks in the M.G. Eischeid # 1 drillhole: in Anderson, R.R., ed.. The Amoco M.G. Eischeid # 1 Deep Petroleum Test, Carroll County, Iowa, Iowa Department of Natural Resources Special Report Series No., 2, p. 77-112.

Mauk, J. L., 1991, Sub-basins in the Proterozoic Midcontinent Rift: Stratigraphic evidence from Upper Michigan (Abs.): Geological Society of America Abstracts with Programs, San Diego, CA, p. A59.

Ojakangas, R.W., 1986, Reservoir characteristics of the Keweenawan Supergroup, Lake Superior region: in Mudrey, M.G. Jr., ed., Precambrian Petroleum Potential, Wisconsin and Michigan, Geoscience Wisconsin. V. 11. p. 25-31.

Soreghan, M.J., and Cohen, A.S., 1993, The effects of basin asymmetry on sand composition: Examples from Lake Tanganyika, Africa: in Johnsson, M.J., and Basu, A., eds., Processes Controlling the Composition of Clastic Sediments: Geological Society of America Special Paper 284, p. 285-301.

EARLY DESCRIPTIONS OF THE NATURAL FEATURES ON THE MARQUETTEIRON RANGE

Doug Ottke, U.S. Geological Survey, 12201 Sunrise Valley Dr. Reston, Virginia 20192

The history of the Marquette Iron Range in the upper peninsula of Michigan is integrally linked tothe environment. The natural features of the Marquette Range defined the mode of production onthe Range, and conversely, as human systems became more adept in these modes of productionthey became the most important modifiers of the natural systems.

Settlement on the Marquette Iron Range was closely related to two factors of naturalenvironment: the iron ore formations and the hardwood forest. The distribution of three iron oreformations--principally the Negaunee, but also the Bijiki and other Iron-formations within theMichigamme Formation --determined the locations of the sites of extraction of iron ore in thearea. Stands of hardwood forest, particularly maple and yellow birch were the major factordetermining the location for processing operations that converted iron ore into pig iron. Thelocation of beehive kilns where the hardwood was converted into charcoal, the early forges, andthe blast furnaces were reliant foremost on the availability of hardwood. The iron ore formationsand the hardwood forest of the Range defined the physical subdivision of the land into tractsowned by individuals. Because a readily available workforce was needed at the mines, forges,kilns, and furnaces of the area, locations of settlement on the Range were within very closeproximity to the sites of extraction and processing operations. Transportation, from the earliesttrails to the plank road, strap railroad and the Iron Mountain Railroad completed in the 1857 weredefined by the natural location of the ore and the processes involved with extracting it.

In the mid 1850's, with advances in transportation associated with the completion of the railroadto the mining areas near Negaunee and lshpeming and the completion of the Sault Locks nearSault Saint Marie, the production of iron ore reached a level of 11,343 tons per year. By 1868 thatnumber had reached a level of 508,000 tons mined. The depression of 1873 caused a sharp dropin production. However, for the year before the drop one million tons had been withdrawn from40 pits carved into the ore bodies. In 1880 two million tons were mined and that number rose toseven million tons mined during the year of 1890. The quarrying operations progressed deeperinto the earth until by the 1880's shaft mining became a necessity, calling for advancement intechnological applications and forcing many of the less wealthy operations out of business. Thelarge amounts of capital involved in diamond drilling exploration, and the operationsunderground forced industrial consolidation of the many small operations into a few largecompanies commanding large resources. As mines went underground, the need for timber alsoincreased as supports for the underground workings became a necessity. Between .72 and .92cubic feet of timber was needed per ton of ore extracted.

In 1868 eleven blast furnaces were in operation on the Marquette Range. Twenty five furnacestotal were built in Marquette County. Numerous beehive kilns dotted the countryside. Thesekilns converted maple, yellow birch and, in some instances, hemlock, pine, and other softwoodsinto charcoal. The charcoal consumed per ton of iron produced in furnaces varied from 110bushels to 140 bushels, with a bushel consisting of 20 pounds of charcoal. The Pioneer Furnace,the original blast furnace in the area located in Negaunee, used the timber from 1,500 acres in theyear of 1869 to produce 9,500 tons of pig iron. One acre of wood could produce 14 tons of iron.It is estimated that by 1903 thirty acres of hardwood per day was needed to supply the kilns of theMarquette Range. In the first half century of iron production 330,000 acres were cut.

In order to understand more thoroughly how natural and human systems have modified each otheron the Marquette Range, the pre-major European settlement natural environment must be

45

EARLY DESCRIPTIONS OF THE NATURAL FEATURES ON THE MARQUETTE IRON RANGE -- --

-, Doug Ottke, U.S. Geological Survey, 12201 Sunrise Valley Dr. Reston, Virginia 20192 r

The history of the Marquette Iron Range in the upper peninsula of Michigan is integrally linked to the environment. The natural features of the Marquette Range defined the mode of production on the Range, and conversely, as human systems became more adept in these modes of production they became the most important modifiers of the natural systems.

Settlement on the Marquette Iron Range was closely related to two factors of natural environment: the iron ore formations and the hardwood forest. The distribution of three iron ore formations-principally the Negaunee, but also the Biiiki and other Iron-formations within the Michigamme Formation --determined the locations of the sites of extraction of iron ore in the area. Stands of hardwood forest, particularly maple and yellow birch were the major factor determining the location for pro&ssing operations that converted iron ore into pig iron. The location of beehive kilns where the hardwood was converted into charcoal, the early forges, and the blast furnaces were reliant foremost on the availability of hardwood. The iron ore formations and the hardwood forest of the Range defined the physical subdivision of the land into tracts owned by individuals. Because a readily available workforce was needed at the mines, forges, kilns, and furnaces of the area, locations of settlement on the Range were within very close proximity to the sites of extraction and processing operations. Transportation, from the earliest trails to the plank road, strap railroad and the Iron Mountain Railroad completed in the 1857 were defined by the natural location of the ore and the processes involved with extracting it.

In the mid 1850's, with advances in transportation associated with the completion of the railroad to the mining areas near Negaunee and Ishpeming and the completion of the Sault Locks near Sault Saint Marie, the production of iron ore reached a level of 11,343 tons per year. By 1868 that number had reached a level of 508,000 tons mined. The depression of I873 caused a sharp drop in production. However, for the year before the drop one million tons had been withdrawn from 40 pits carved into the ore bodies. In 1880 two million tons were mined and that number rose to seven million tons mined during the year of 1890. The quarrying operations progressed deeper into the earth until by the 1880's shaft mining became a necessity, calling for advancement in technological applications and forcing many of the less wealthy operations out of business. The large amounts of capital involved in diamond drilling exploration, and the operations underground forced industrial consolidation of the many small operations into a few large companies commanding large resources. As mines went underground, the need for timber also increased as supports for the underground workings became a necessity. Between .72 and .92 cubic feet of timber was needed per ton of ore extracted.

In 1868 eleven blast furnaces were in operation on the Marquette Range. Twenty five furnaces total were built in Marquette County. Numerous beehive kilns dotted the countryside. These kilns converted maple, yellow birch and, in some instances, hemlock, pine, and other softwoods into charcoal. The charcoal consumed per ton of iron produced in furnaces varied from 1 10 bushels to 140 bushels, with a bushel consisting of 20 pounds of charcoal. The Pioneer Furnace, the original blast furnace in the area located in Negaunee, used the timber from 1,500 acres in the year of 1869 to produce 9,500 tons of pig iron. One acre of wood could produce 14 tons of iron. It is estimated that by 1903 thirty acres of hardwood per day was needed to supply the kilns of the Marquette Range. In the first half century of iron production 330,000 acres were cut.

In order to understand more thoroughly how natural and human systems have modified each other on the Marquette Range, the pre-major European settlement natural environment must be

delineated. Beginning in 1838, the Michigan Geological Survey and the Linear Survey of theU.S. Land Office created descriptions of the natural environment of the Marquette Range. Thesedescriptions better enable the understanding of the pre-major settlement natural environment.The descriptions contained within these reports of the location of the original outcrops of iron oreand the stands of trees shows how important these factors were to the early settlement of theMarquette Range. The descriptions also allow us to see how influential that settlement was inmodi1'ing the natural systems of the Range.

46

delineated. Beginning in 1838, the Michigan Geological Survey and the Linear Survey of the U.S. Land Office created descriptions of the natural environment of the Marquette Range. These

I descriptions better enable the understanding of the pre-major settlement natural environment. The descriptions contained within these reports of the location of the original outcrops of iron ore and the stands of trees shows how important these factors were to the early settlement of the Marquette Range. The descriptions also allow us to see how influential that settlement was in modifying the natural systems of the Range.

DEVELOPMENT OF VOLCANOGENIC MASSIVE SULFIDE DEPOSIT EXPLORATIONTARGETS IN NORTHERN MINNESOTA FROM GIS SPATIAL ANALYSIS OF

GEOLOGICAL, GEOCHEMICAL, AND GEOPHYSICAL CRITERIA

DEAN M. PETERSON and DR. RONALD L. MORTON

Economic Volcanology Research Lab, Geology Department, University of Minnesota - Duluth,Duluth, Minnesota, USA 55812

Volcanogenic massive sulfide (VMS) deposits are syngenetic stratiform iron and base-metal (Cu-Zn-Pb)accumulations that form out of large geothermal systems in subaqueous volcanic environments. At the"bottom" of VMS-producing systems are subvolcanic intrusions, which produce focused heat sources thatprovide the energy for circulating hydrothermal fluids and leaching reactions, and may add base-metalsinto the geothermal system. Above the subvolcanic intrusions, the strata have undergone extensive high-temperature alteration, which can include metal depletion, alkali modification, and extensivesilicification.

Over the last twenty years, a VMS ore deposit model has been developed from the work of manyresearchers. Important features of the model include the following:

• VMS deposits are associated with submarine volcanic rocks, and typically are spatially associatedwith felsic volcanic rocks in greenstone terranes.

• VMS deposits occur with no obvious correlation with age or petrochemistry but do occur in distinctprovinces, camps or clusters.

• Deposits often occur at one or more distinctive favorable stratigraphic horizons within a camp. Thesehorizons represent quiescent periods in volcanic activity, tops of volcanic cycles, or a major volcanic-sediment interface. The distal extent of ore horizons may be marked by a siliceous, ferruginous, or abase metal bearing sediment and/or felsic tuff.

• Footwall subvolcanic intrusions are typically sill-like, with irregularly distributed porphyritic andnon-porphyritic phases. The intrusions may contain minor porphyry copper occurrences.

• Synvolcanic fault controls are recognized in some deposits, and may control the location of rhyolitedomes that occur in the footwall of some deposits. Proximal facies volcanic rocks associated withsynvolcanic faults are generally the most favorable horizons for ore deposition.Footwall alteration may include a pipe-like zone and/or widespread semi-conformable and cloud-likealteration.

• The massive-sulfide nature of the ore typically makes them conductors of electricity, and thereforecan be detected by airborne and ground-based electro-magnetic (EM) surveys.

Many exploration programs for VMS deposits have been based on only one feature of the ore depositmodel, specifically, the blind drilling of airborne EM conductors. Although this method of explorationhas discovered many major deposits in the past, strictly EM-based VMS exploration has manyshortcomings. The drawbacks include any or all of the following:

• Poor geologic control of conductor location• Poor ranking of conductors in a survey block• High cost of drilling tens to hundreds of barren or graphitic EM conductors prior to discovery• Poor rate of discovery (less than 1% of drilled EM conductors lead to mines)

47

DEVELOPMENT OF VOLCANOGENIC MASSIVE SULFIDE DEPOSIT EXPLORATION TARGETS IN NORTHERN MINNESOTA FROM GIs SPATIAL ANALYSIS OF

GEOLOGICAL, GEOCHEMICAL, AND GEOPHYSICAL CRITERIA

DEAN M. PETERSON and DR. RONALD L. MORTON

Economic Volcanology Research Lab, Geology Department, University ofMinnesota - Duluth, Duluth, Minnesota, USA 55812

Volcanogenic massive sulfide (VMS) deposits are syngenetic stratifm iron and base-metal (Cu-Zn-Pb) accumulations that form out of large geothermal systems in subaqueous volcanic environments. At the "bottom" of VMS-producing systems are subvolcanic intrusions, which produce focused heat sources that provide the energy for circulating hydrothermal fluids and leaching reactions, and may add base-metals into the geothermal system. Above the subvolcanic intrusions, the strata have undergone extensive high- temperature alteration, which can include metal depletion, alkali modification, and extensive silicification.

Over the last twenty years, a VMS ore deposit model has been developed from the work of many researchers. Important features of the model include the following:

VMS deposits are associated with submarine volcanic rocks, and typically are spatially associated with felsic volcanic rocks in greenstone terranes. VMS deposits occur with no obvious correlation with age or petrochemistry but do occur in distinct provinces, camps or clusters. Deposits often occur at one or more distinctive favorable stratigraphic horizons within a camp. These horizons represent quiescent periods in volcanic activity, tops of volcanic cycles, or a major volcanic- sediment interface. The distal extent of ore horizons may be marked by a siliceous, ferruginous, or a base metal bearing sediment and/or felsic tuff. Footwall subvolcanic intrusions are typically sill-like, with irregularly distributed porphyritic and non-porphyritic phases. The intrusions may contain minor porphyry copper occurrences. Synvolcanic fault controls are recognized in some deposits, and may control the location of rhyolite domes that occur in the footwall of some deposits. Proximal facies volcanic rocks associated with synvolcanic faults are generally the most favorable horizons for ore deposition. Footwall alteration may include a pipe-like zone andlor widespread semi-confmable and cloud-like alteration. The massive-sulfide nature of the ore typically makes them conductors of electricity, and therefore can be detected by airborne and ground-based electro-magnetic (EM) surveys.

Many exploration programs for VMS deposits have been based on only one feature of the ore deposit model, specifically, the blind drilling of airborne EM conductors. Although this method of exploration has discovered many major deposits in the past, strictly EM-based VMS exploration has many shortcomings. The drawbacks include any or all of the following:

Poor geologic control of conductor location Poor ranking of conductors in a survey block High cost of drilling tens to hundreds of barren or graphitic EM conductors prior to discovery Poor rate of discovery (less than 1% of drilled EM conductors lead to mines)

Flowchart for the 3-step GIS modelling of the VMS potential of the Archean VermilionDistrict and its western extension, northeastern, Minnesota. The three steps in the studyare: 1) Detailed geologic mapping and compilation of all available geological,geochemical, and geophysical data. Conversion of all data into primary GIS datasets;2) Extracting and enhancing the features of the primary datasets that are important forpredicting VMS deposits; and 3) Integration of the preliminary maps using GIS modellingtechniques that predict mineral potential.

This study formulates a GIS-based exploration model that incorporates all facets of the VMS ore depositmodel into a series of digital maps. These maps have ultimately led to the generation of specific VMSexploration targets in the study area of northern Minnesota. The study integrates a new 5880 kmgeologic compilation map with mapped alteration zones, over 5400 ground and airbome EM anomalies,and approximately 21,000 copper and zinc assays. Intermediate maps used to predict VMS potentialinclude stratigraphy, heat generation, alteration, Cu-Zn geochemistry, and EM conductors. Themethodology used to create the 015-based model is presented in Figure 1.

STEP I

Build SpatialData Base

Collect spatialData and Inputinto GIS

S S S S

Extraction of 015Features Relevantto the VMS OreDeposit Model

STEP 2

DataProcessing

ROCK, CORE, SOIL AIRBORNE AND& LAKE SEDIMENT GROUND EM

GEOCHEMISTRY CONDUCTORS

———--H—I. aassi& intervals I. Extnct ground EM2. Grid soil & lake aSs 2. Classil5, airborne EM3. Buffer rock & axe

Preliminary Mapsof Selected GISFeatures

BEDROCK MAPPEDGEOLOGY ALTERATION

ZONES

-H————t. Reclassii I. Extractqtz.cpidote2. Extract contacts 2. Extract chlorite3. Qeale buffers 3. CYeate buffers

;contsctstontacts

Volcanic-Sedin.tcontacts

Greanstone Subvókar.ac Qsarfl- Chlorite5eq lntrusio,.s Epidote

Assign Weightsof Evidencehny,gIits(O.on . t.)

Cu Zn Cu Zn Cu Znsoils Lake Rocks

STEP 3

IntegrationModelling

Map Overlayto ProduceInterrnedtateFactor Maps

AGromd tEM EM

Overlay ofIntermediateFactor Mapsto Produce theFinal Predictive Map

Figure 1.

STRATIGRAPHY HEAT ALTERATION GEOCHEMICAL EMFactor Factor Factor Factor Factor

VMS NERAL NJFEWflAL MAP

48

This study formulates a GIs-based exploration model that incorporates all facets of the VMS ore deposit model into a series of digital maps. These maps have ultimately led to the generation of s p e c i f i c p s exploration targets in the study area of northern Minnesota. The study integrates a new 5880 km geologic compilation map with mapped alteration zones, over 5400 ground and airborne EM anomalies, and approximately 21,000 copper and zinc assays. Intermediate maps used to predict VMS potential include stratigraphy, heat generation, alteration, Cu-Zn geochemistry, and EM conductors. The methodology used to create the GIs-based model is presented in ~ i & e 1.

STEP 1

luild Spatial Data Base

STEP 2

Data Processing

STEP 3

Integration Modelling

Figure 1.

Collect Spatial Data and Input into CIS

Extraction of CIS Features Relevant to the VMS Ore Deposit Model

Preliminary Maps of Selected GIs Features

Assign Weights of Evidence F-wriflte(0m-lm)

Map Overlay to Produce Intermediate Factor Maps

Overlay of Intermediate Factor Maps to Produce the Final Predictive Map

BEDROCK MAPPED ROCK. CORE. SOIL AIRBORNE AND GEOLOGY ALTERATION & LAKE SEDIMENT GROUND EM

ZONES GEOCHEMISTRY CONDUCTORS

I 1. RecUmfy 1.Classifymiervals 1-ExtractcrowdEM

2.Gndsoil&lateseds 2. Classify airbornem 2 . m - 2.Extractchlonu 3.0cÃ§Ebufiei 3 .Omebuf im 3.Bufferrock&core

A A- C"aC"a m a - Soik Lake Reeks EM EM

1 1 1 STRATIGRAPHY HEAT ALTERATION GEOCHEMICAL

1 EM

Factor Factor Factor Factor Factor

VMS MINERAL-POTENTIAL MAP L

Flowchart for the 3-step GIs modelling of the VMS potential of the Archean Vermilion District and its western extension, northeastern, Minnesota. The three steps in the study are: 1) Detailed geologic mapping and compilation of all available geological, geochemical, and geophysical data. Conversion of all data into primary GIs datasets; 2) Extracting and enhancing the features of the primary datasets that are important for p~dicting VMS deposits; and 3) Integration of the prelimhry maps using GIs modelling techniques that predict mineral potential.

LOW-GRADE METAMORPHISM AND HYDROTHERMAL ALTERATION OF THEUPPER KEWEENAWAN PORTAGE LAKE VOLCANICS, MICHIGAN

PUSCHNER, Ulrich, SCHMIDT, Susanne Th., Mineralogisch-Petrographisches Institut,Base!, Switzerland and BORNHORST, Theodore J., Department of GeologicalEngineering and Sciences, Michigan Technological University, Houghton, Michigan49931

The Portage Lake Volcanics in the Keweenaw Peninsu!a of Michigan consists of a thicksuccession of subaerial tho!eiitic basalt !ava flows with scattered interfiow sedimentary!ayers. These rocks have been subjected to low-grade metamorphic and hydrothermalalteration. The world-class native copper deposits were emplaced during this alterationevent. In the Keweenaw Peninsula, and elsewhere, the Portage Lake Volcanics are notwidely exposed at the surface because of overlying unconsolidated glacial sediments.However, subsurface drill core were obtained in the Keweenaw Peninsula duringexploration for native copper. Much drill core, some over 100 years old, still exists inexcellent condition and with original dri!l core logs and locations. Mr. Gordon Petersonof Calumet owns and continues to maintain much of the existing drill core and allowed usto sample the core for our research. Stoiber and Davidson (1959a, b), in part, used a drillcore cross-section to postu!ate metamorphic zonation within the Portage Lake Volcanics.Four drill core cross sections through the Portage Lake Volcanic have been studied usinga combination of microscopic investigation, XRD technique, and electron microprobeanalysis, including the drill core section studied by Stoiber & Davidson (1959a, b). Thesefour dril! core cross sections are located between Ahmeek and Copper Harbor andprovide coverage through 3000m of the Portage Lake Volcanics over a distance of 60 kmalong strike. Interlayered sediments and se!ected lava flows can be used as markerhorizons to corre!ate the stratigraphic zones between the different individual drill holes.

Initial results demonstrate that two main minera! associations can be distinguished withinthe amygdaloidal tops of the basaltic !ava flows of the upper part of the Portage LakeVo!canics (above the Greenstone flow) in the Copper Harbor to Ahmeek sections: (1) azeolite-dominated assemb!age with laumontite-chlorite-corrensite±wairakite±quartz±calcite, and (2) a pumpellyite-epidote-dominated assemblage with chlorite±quartz±calcite±prehnite±laumontite. The zeolite-dominated assemblage is more frequent in theupper part of the section, but wairakite occurs only below the Hancock Conglomerate.The pumpellyite-epidote-dominated assemblage is characteristic of the lower part of thesequence. Zeolite minerals are present throughout the whole vertical section. Alongstrike, a temperature gradient is observed with lower temperature conditions in theCopper Harbor section and higher temperature conditions in the Ahmeek section nearAhmeek to the southwest. This correlates with the distribution of native copper depositsincreasing in frequency and size from Copper Harbor towards Ahmeek.

The most abundant phyllosilicates in the upper part of the Portage Lake Volcanics arechlinochlore and corrensite. Chlinoc!ore is present in all morphological units of a lavaflow, i.e. in the amygdaloidal flow top and in the massive interior, and within allstratigraphic horizons. The mean of the interlayered cations as determined by electronmicroprobe analysis is around 0.1 indicating on!y very low mixed-layered components.Corrensite is more abundant in the amygdaloidal flow tops. In the eastern most Copper

LOW-GRADE METAMORPHISM AND HYDROTHERMAL ALTERATION OF THE UPPER KEWEENAWAN PORTAGE LAKE VOLCANICS, MICHIGAN

PUSCHNER, Ulrich, SCHMIDT, Susanne Th., Mineralogisch-Petrographisches Institut, Basel. Switzerland and BORNHORST. Theodore J.. Department of Geological ~ n ~ i n e e r i n ~ and Sciences, Michigan ~echnolo~ical university, ~ o u ~ h t o n , - ~ i c h i ~ a n 4993 1

The Portage Lake Volcanics in the Keweenaw Peninsula of Michigan consists of a thick succession of subaerial tholeiitic basalt lava flows with scattered interflow sedimentary layers. These rocks have been subjected to low-grade metamorphic and hydrothermal alteration. The world-class native copper deposits were emplaced during this alteration event. In the Keweenaw Peninsula, and elsewhere, the Portage Lake Volcanics are not widely exposed at the surface because of overlying unconsolidated glacial sediments. However, subsurface drill core were obtained in the Keweenaw Peninsula during exploration for native copper. Much drill core, some over 100 years old, still exists in excellent condition and with original drill core logs and locations. Mr. Gordon Peterson of Calumet owns and continues to maintain much of the existing drill core and allowed us to samnle the core for our research. Stoiber and Davidson (l959a. b), in part. used a drill core cross-section to postulate metamorphic zonation within the portage Lake Volcanics. Four drill core cross sections through the Portage Lake Volcanic have been studied using a combination of microscopic investigation, XRD technique, and electron microprobe analysis, including the drill core section studied by Stoiber & Davidson (1959a. b). These four drill core cross sections are located between Ahmeek and Copper Harbor and provide coverage through 3000m of the Portage Lake Volcanics over a distance of 60 km along strike. Interlayered sediments and selected lava flows can be used as marker horizons to correlate the stratigraphic zones between the different individual drill holes.

Initial results demonstrate that two main mineral associations can be distinguished within the amygdaloidal tops of the basaltic lava flows of the upper part of the Portage Lake Volcanics (above the Greenstone flow) in the Copper Harbor to Ahmeek sections: (1) a zeolite-dominated assemblage with laumontite-chlorite-corrensiteÂ±wairakiteÂ±quart calcite, and (2) a pumpellyite-epidote-dominated assemblage with chloriteiquartz* calciteÂ±prehniteÂ±laumontit The zeolite-dominated assemblage is more frequent in the upper part of the section, but wairakite occurs only below the Hancock Conglomerate. The pumpellyite-epidote-dominated assemblage is characteristic of the lower part of the sequence. Zeolite minerals are present throughout the whole vertical section. Along strike, a temperature gradient is observed with lower temperature conditions in the Copper Harbor section and higher temperature conditions in the Ahmeek section near Ahrneek to the southwest. This correlates with the distribution of native copper deposits increasing in frequency and size from Copper Harbor towards Ahmeek.

The most abundant phyllosilicates in the upper part of the Portage Lake Volcanics are chlinochlore and corrensite. Chlinoclore is present in all morphological units of a lava flow, i.e. in the amygdaloidal flow top and in the massive interior, and within all stratigraphic horizons. The mean of the interlayered cations as determined by electron microprobe analysis is around 0.1 indicating only very low mixed-layered components. Corrensite is more abundant in the amygdaloidal flow tops. In the eastern most Copper

49

Harbor section, smectites (saponite, montmorillonite) as well as paragonite andpyrophyllite are also present in the zeolite-dominated assemblage.

The Portage Lake Volcanics was subjected to a complex metamorphic and hydrothermalalteration event (Stoiber & Davidson, 1959, Bornhorst, 1997). The initial results of thiscontinuing research suggest that the abundant laumontite-chlorite assemblage in most ofthe upper part of the Portage Lake Volcanics was likely the result of regional zeolitefades metamorphism during burial. The pumpellyite-prehnite-epidote assemblage islikely related to a hydrothermal phase. In the Copper Harbor to Ahmeek section thehydrothermal event occurs only locally, but when present it is pervasive.

References

Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North AmericanMidcontinent rift system: Geological Society of America Special Paper, 312, p. 127-136.

Stoiber, R.E. and Davidson, E.S., l959a, Amygdule mineral zoning in the Portage LakeLava Series, Michigan Copper district. Part I: Economic Geology, v. 54. p.1250-1277.

Stoiber, R.E. and Davidson, E.S., l959b, Amygdule mineral zoning in the Portage LakeLava Series, Michigan Copper district. Part H: Economic Geology, v. 54, p. 1444-1460.

50

Harbor section, smectites (saponite, montmorillonite) as well as paragonite and pyrophyllite are also present in the zeolite-dominated assemblage.

The Portage Lake Volcanics was subjected to a complex metamorphic and hydrothermal alteration event (Stoiber & Davidson, 1959, Bornhorst, 1997). The initial results of this continuing research suggest that the abundant laumontite-chlorite assemblage in most of the upper part of the Portage Lake Volcanics was likely the result of regional zeolite facies metamorphism during burial. The pumpellyite-prehnite-epidote assemblage is likely related to a hydrothermal phase. In the Copper Harbor to Ahmeek section the hydrothermal event occurs only locally, but when present it is pervasive.

References

. Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North American Midcontinent rift system: Geological Society of America Special Paper, 312, p. 127- 136.

Stoiber, R.E. and Davidson, E.S., 1959a. Amygdule mineral zoning in the Portage Lake Lava Series, Michigan Copper district. Part I: Economic Geology, v. 54, p.1250- 1277.

Stoiber, R.E. and Davidson, E.S., 1959b. Amygdule mineral zoning in the Portage Lake Lava Series, Michigan Copper district. Part IT Economic Geology, v. 54, p. 1444- 1460.

Lake Superior's lUngs: Clues to the origin of Polygonal Fault Systems

Deborah E. Rausch and N/gel J. WattrusLarge Lakes Observatory, University of Minnesota

Duluth MN 55812email: drausch2(did.umn.edu

Polygonal Fault Systems (PFS) are a recently recognized class of soft-sediment structures. They resultfrom volumetric contraction triggered by syneresis during early compaction of ultra-fine sediments. PFSwere first described in Paleogene claystones from the North Sea basin (Cartwright, 1994). In 2D cross-sections, PES appear be small extensional faults and in map view, the faults have a polygonal appearance.Cartwright and Lonergan (1996) measured the extensional strain and determined that apparent extensionon any given horizon was uniform in all directions. They concluded from this that the fault systems couldnot have a tectonic origin and that the faulting was related to volumetric contraction of sediments duringearly burial and compaction. From 3D seismic data collected in the North Sea, Cartwright and Dewhurst(1998) identified specific criteria of PFS. These criteria are used to delineate systems in othersedimentary basins. Based on these criteria, PFS are typically close networks of normal faults with smallthrows that occur over a large area in sedimentary basins. The systems are layer bound and areoccasionally tiered. They only occur in very fine-grained sediments (<2 micron). There are no knownmodern analogues.

Doughnut-shaped rings and subcircular depressions have been observed in the side-scan records collectedin Lake Superior (Bergson and Clay, 1973; Johnson et al., 1984; Flood and Johnson, 1984; Flood, 1989;and Anderson, 1997). The described features are 100-300 meters in diameter. 10-3- meters wide and 5meters deep. Side-scan data collected by the Large Lakes Observatory (LLO) confirms the widespreaddevelopment of ring structures throughout much of the basin. The rings commonly occur wherever fine-grained glacio-lacustrine sediments are found. High-resolution seismic reflection data collected withboomer and echosounder systems show that the nngs are associated with deformation that typicallydisrupts the thifl Ilolocene surface sediments and glacio-lacustrine sediments below. The echosounderdata clearly shows that the underlying glacio-lacustrine exhibits extensive fracturing and small-scalefaulting. Some of these fractures and faults continue to the surface where they coincide with a ring orpartial ring structure. Many, however, are truncated at the Holocene boundary. Throws on the smallfaults are often less than 50 cm and the apparent displacement on these faults is typically normal. Someof the echosounder records show evidence of apparently genetically unique deformation sequences in thefine-grained sediments. These sequences are separated by a poorly resolved horizon with little coherentinternal reflections. The sequences above and below exhibit well developed internal reflections that donot appear to be concordant. Both systems show well-developed fracture and micro-faulting patterns.This observation suggests that these systems are confined to distinct intervals in the subsurface, and aretherefore genetically unique.

It is proposed that the rings in Lake Superior are surface expressions of PFS developed in the near surfaceclays. A comparison with the criteria outlined by Cartwright and Dewhurst (1998) show that the near-surface sediment and data from Lake Superior meet nearly all of the requirements for PFS development.The seismic data collected thus far indicate that there are basinwide layer bound normal fault systemswith small throws. The Lake Superior glacio-lacustrine sediments are characterized by fine grain size,high water content, and a clay fraction composed predominantly of illite.

51

Lake Superior's Rings: Clues to the origin of Polygonal Fault Systems

Deborah E. Rausch and NigeI J. Wattrus Large Lakes Observatory, University ofMinnesota

Duluth MN 55812

Polygonal Fault Systems (PFS) are a recently recoenized class of soft-sediment structures. They result from-volumetric contraction triggered by s&eresis-during early compaction of ultra-fine sediments. PFS were first described in Paleoeene clavstones from the North Sea basin (Cartwrieht. 1994). In 2D cross- - < - . sections, PFS appear be small extensional faults and in map view, the faults have a polygonal appearance. Camwight and Lonergan (1996) measured the extensional strain and determined that apparent extension on any given horizon was uniform in all directions. They concluded from this that the fault systems could not have a tectonic origin and that the faulting was related to volumetric contraction of sediments during early burial and compaction. From 3D seismic data collected in the North Sea, Cartwright and Dewhurst (1998) identified specific criteria of PFS. These criteria are used to delineate systems in other sedimentary basins. Based on these criteria, PFS are typically close networks of normal faults with small throws that occur over a large area in sedimentary basins. The systems are layer bound and are occasionally tiered. They only occur in very fine-grained sediments (<2 micron). There are no known modern analogues.

Doughnut-shaped rings and subcircular depressions have been observed in the side-scan records collected in Lake Superior (Bergson and Clay, 1973; Johnson et al., 1984; Flood and Johnson, 1984; Flood, 1989; and Anderson, 1997). The described features are 100-300 meters in diameter, 10-3- meters wide and 5 meters deep. Side-scan data collected by the Large Lakes Observatory (LLO) confirms the widespread development of ring structures throughout much of the basin. The rings commonly occur wherever fine- grained glacio-lacustrine sediments are found. High-resolution seismic reflection data collected with boomer A d echosounder systems show that the rings are associated with deformation that typically dismots the thir Holocene surface sediments and elacio-lacustrine sediments below. The echosounder . ~ ~ ~ ~~ - ~ ~

data clearly shows that the underlying glacio-lacusmne exhibits extensive fracturing and small-scale faulting. Some of these fractures and faults continue to the surface where they coincide with a ring or partial ring structure. Many, however, are truncated at the Holocene boundary. Throws on the small faults are often less than 50 cm and the apparent displacement on these faults is typically normal. Some of the echosounder records show evidence of apparently genetically unique deformation sequences in the fine-grained sediments. These sequences are separated by a poorly resolved horizon with little coherent internal reflections. The sequences above and below exhibitwell developed internal reflections that do not appear to be concordant: Both systems show well-developed fracture and micro-faulting patterns. This observation suggests that these systems are confined to distinct intervals in the subsurface, and are therefore genetically unique.

It is proposed that the rings in Lake Superior are surface expressions of PFS developed in the near surface clays. A comparison with the criteria outlined by Camwight and Dewhurst (1998) show that the near- surface sediment and data from Lake Superior meet nearly all of the requirements for PFS development. The seismic data collected thus far indicate that there are basinwide layer bound normal fault systems with small throws. The Lake Superior glacio-lacustrine sediments are characterized by fine grain size, high water content, and a clay fraction composed predominantly of illite.

The following model is proposed to explain the origin of the rings on the floor of Lake Superior. Highsedimentation rates associated with the retreat of ice out of the basin produced deposits with high watercontents, which behaved like a colloidal fluid. Sometime after deposition, syneresis in the sedimentsinitiated volumetric contraction. This process led to the development of fractures and faulting in theclays. The growth-fault appearance of some of the faults suggests that deformation and faulting wassyndepositional and continued for some time. Since many of the fractures and faults truncate against theHolocene boundary, we believe that most of the volumetric contraction ceased before the deposition ofthe Holocene sediment. Some fractures which continue to the surface appear to act as conduits for theexpressed water. This is supported by submersible observations (Flood, 1989) that appear to suggest thatwater venting from the lake floor removed or prevented the deposition of the fine Holocene material inthe vicinity of the ring.

REFERENCESAnderson, K.A., 1997, A seismic strati graphic study of western Lake Superior (M.S. thesis). University of

Minnesota, 74p.

Berkson, f.M., and Clay, C.S., 1973, Possible syneresis origin of valleys on the floor of Lake Superior:Nature, 245:89-91.

Cartwright, f.A.. 1994, Episodic basin-wide fluid expulsion from geopressured shale sequence in theNorth Sea basin: Geology, 22 :44 7-450.

Cartwright. .J.A., and Dewhurst, D.N., 1998, Layer-bound compaction faults infine-grained sediments:Geol. Soc. Am. Bull., 110:1242-125 7.

Cart-wright, .14. and Lonergan, L., 1996, Volumetric contraction during the compaction of mudroc/cy: Amechanism for the development of regional-s cale polygonalfault systems: Basin Research, v.8,p 183-193.

Flood. RD., 1989. Submersible studies of current-modified bottom topography in Lake Superior: .1Great Lakes Res., 15:3-14.

Flood. R. D. and Johnson, T C., 1984, Side-scan targets in Lake Superior- evidence for bedforms andsediment transport: Sedimentology, v. 31, p. 311-333.

Johnson, T. C.. Halfman, J.D., Busch, W.H., and Flood, R.D., 1984, Effects of bottom currents andfish onsedimentation in a deep-water. lacustrine environment.' Geol. Soc. Am. BulL, 95:1425-1436,

Mollard, ID., 1983, The origin of reticulate and orbicular patterns on the floor of the Lake Agassiz basinin Teller, IT. and Clayton, L., eds.. Glacial Lake Agassiz: GeoL Assoc. Can. Sp Paper, p. 355-374.

52

The following model is proposed to explain the origin of the rings on the floor of Lake Superior. High sedimentation rates associated with the retreat of ice out of the basin produced deposits with high water contents, which behaved like a colloidal fluid. Sometime after deposition, syneresis in the sediments initiated volumetric contraction. This pocess led to the development of fractures and faulting in the clays. The growth-fault appearance of some of the faults suggests that deformation and faulting was syndepositional and continued for some time. Since many of the fractures and faults truncate against the Holocene boundary, we believe that most of the volumetric contraction ceased before the deposition of the Holocene sediment. Some fractures which continue to the surface appear to act as conduits for the expressed water. This is supported by submersible observations (Flood, 1989) that appear to suggest that water venting from the lake floor removed or prevented the deposition of the fine Holocene material in the vicinity of the ring.

REFERENCES Anderson, K.A., 1997, A seismic stratigraphic study of western Lake Superior (M.S. thesis): University of

Minnesota, 74p.

Berkson, J.M.. and Clay. C.S., 1973. Possible syneresis origin of valleys on the floor ofLake Superior: Nature, 24589-91.

Cartwright, J.A., 1994. Episodic basin-wide fluid expulsion Â¥fro geopressured shale sequence in the North Sea basin: Geology, 22:447-450.

Cartwright, J.A., and Dewhursi, D.N., 1998. Layer-bound compaction faults infine-grained sediments: Geol. Soc. Am. Bull., 110:1242-1257.

Cartwright, J.A. and Lonergan. L., 1996, Volumetric contraction during the compaction of mudrocks: A mechanism for the development of regional-scale polygonal fault systems: Basin Research, v.8, p 183-193.

Flood, R.D., I989, Submersible studies of current-modified bottom topography in Lake Superior: J. Great Lakes Res., 15:3-14.

Flood. R. D. and Johnson, T.C., 1984, Side-scan targets in Lake Superior- evidence for bedforms and sediment transport: Sedimentology, v. 31, p. 311-333.

Johnson, T. C., Hal/man, J.D., Busch, W.H., and Flood, R.D., 1984, Effects of bottom currents and fish on sedimentation in a deepwater, lacustrine environment: Geol. Soc. Am. Bull., 95:1425-1436.

Mollard. J.D., 1983, The origin of reticulate and orbicularpatterns on the floor of the Lake Agassiz basin in Teller, J.T. and Clayton, L., eds.. Glacial Lake Agassiz: Geol. Assoc. Can. Sp. Paper, p. 355- 3 74.

GEOLOGY OF THE TILDEN MINE, MARQUEflE IRON RANGE, MICHIGAN

Glenn W. Scott and Helene M. Lukey, Mine Engineering Department, Tilden MiningCompany, P.O. Box 2000, Ishpeming, MI 49849

The Tilden Mine produces iron ore from the Negaunee hon Formation, Menominee Group,Marquette Range Supergroup in the Upper Peninsula of Michigan. As the base of the ironformation is not exposed at the Tilden Mine, the stratigraphic thickness is unknown. Thethickness exceeds 1000 feet and is probably underlain by and is gradational intoundifferentiated elastics and iron formation of the Empire Mine. The iron ore deposit at theTilden Mine has been divided into geologic domains based in part on lithology hut, moreimportantly on the metallurgical response based on bench tests and in the processing plant.The interaction of sedimentation and diagenesis within growth fault controlled basin,metamorphism and supergene oxidation has resulted in a complex suite of ore types, eachwith specific, if not entirely objective, blending characteristics and problems. Productionsince 1974 has been 126 million long tons of pellets from 355 million long tons of ore at astripping ratio of 0.94. As of January 1, 1999, the proven and probable long range (30 year)mine plan reserve was 628 million long tons of ore containing 233 million long tons ofpellets at a stripping ratio of 0.77.

Chlorite schist defines the footwall of the Tilden ore body. The Martite and main pitCarbonate domains domains lie stratigraphically below the CDffl Footwall and Main PitHanging wall intrusives. This is metallurgically "good ore" and is typically characterized byhigh weight recovery (35-45+%), low phosphorous, low slime iron and good grinding media.The West PitJCDffl Hematite domain in the west pit is stratigraphically between the CDfflhanging wall and footwall. In contrast to the martite domain, this is metallurgically 'poor'ore with higher slime iron, phosphorous and poor grinding. Mineralogically and texturally,this domain differs from the martite domain in being dominantly platey hematite with thin(mm scale) chert laminae. The Magnetite domain is the stratigraphically equivalent to theWest PitJCDIII. Mineralogically, the ore consists primarily of magnetite-siderite-chert withvariable hematite and silicates. This mineralogy, along with the thin laminations, appears toindicate a restricted basin and a reduced environment of deposition.

There are several igneous horizons at the Tilden Mine. The term 'intrusive' or 'dike' is usedfor diabasic to porphyritic to aphanitic bodies, which vary from (semi) conformable sill-likehorizons, usually interpreted as synsedimentary sills but may be flows, to obviouslycrosscutting bodies, interpreted as dikes that may be feeders. The sills and majority of thedikes appear to be of early Proterozoic age, related to the Clarksburg Volcanics and/or theHemlock and Emperor Volcanics, but there are several dikes that are interpreted to be ofKeweenawan age based on the magnetic signature.

53

GEOLOGY OF THE TILDEN MINE, MARQUETTE IRON RANGE, MICHIGAN

Glenn W. Scott and Helene M. Lukey, Mine Engineering Department, Tilden Mining Company, P.O. Box 2000, Ishpeming, MI 49849

The Tilden Mine produces iron ore from the Negaunee Iron Formation, Menominee Group, Marquette ~ a n ~ e ~ u p e r ~ r o u p in the Upper peninsula of Michigan. As the base of the iron formation is not exposed at the Tilden Mine, the stratigraphic thickness is unknown. The thickness exceeds 1000 feet and is probably underlain bfand is gradational into undifferentiated elastics and iron formation of the Empire Mine. The iron ore deposit at the Tilden Mine has been divided into geologic domains based in part on lithology but, more importantly on the metallurgical response based on bench tests and in the processing plant. The interaction of sedimentation and diagenesis within growth fault controlled basin, metamorphism and supergene oxidation has resulted in a complex suite of ore types, each with specific, if not entirely objective, blending characteristics and problems. Production since 1974 has been 126 million long tons of pellets from 355 million long tons of ore at a stripping ratio of 0.94. As of January 1,1999, the proven and probable long range (30 year) mine plan reserve was 628 million long tons of ore containing 233 million long tons of pellets at a stripping ratio of 0.77.

Chlorite schist defines the footwall of the Tilden ore body. The Martite and main pit Carbonate domains domains lie stratigraphically below the CDffl Footwall and Main Pit Hanging wall intrusives. This is metallurgically "good ore" and is typically characterized by high weight recovery (35-45+%), low phosphorous, low slime iron and good grinding media. The West Pit/CDin Hematite domain in the west pit is stratigraphically between the CDIII hanging wall and footwall. In contrast to the martite domain, this is metallurgically 'poor' ore with higher slime iron, phosphorous and poor grinding. Mineralogically and texturally, this domain differs from the martite domain in being dominantly platey hematite with thin (mrn scale) chert laminae. The Magnetite domain is the stratigraphically equivalent to the West Pit/CDffl. Mineralogically, the ore consists primarily of magnetite-siderite-chert with variable hematite and silicates. This mineralogy, along with the thin laminations, appears to indicate a restricted basin and a reduced environment of deposition.

There are several igneous horizons at the Tilden Mine. The term 'intrusive' or 'dike' is used for diabasic to porphyritic to aphanitic bodies, which vary from (semi) conformable sill-like horizons, usually interpreted as synsedimentary sills but may be flows, to obviously crosscutting bodies, interpreted as dikes that may be feeders. The sills and majority of the dikes appear to be of early Proterozoic age, related to the Clarksburg Volcanics andlor the Hemlock and Emperor Volcanics, but there are several dikes that are interpreted to be of Keweenawan agebased on the magnetic signature.

DYNAMIC AND STATIC STRENGTH OF AGGREGATES: AN ESTIMATE OF RATESENITIVITY OF GEOLOGIC MATERIALS

STANLEY J. VITTON, Department of & Environmental Engineering, GHATUPARTHJS UBHASH, Department of Mechanical Engineering—Engineering Mechanics, MichiganTechnological University, Floughton, Michigan 49931

The Michigan Department of Transportation (MDOT) has initiated research into the dynamic fracturecharacteristics of aggregates in Portland cement concrete (PCC) pavements. The main focus of thisresearch is to study the fracture and strength characteristics of the coarse aggregate portion in PCC.Due to the wide range of rock types in Michigan, there is a corresponding range of coarse aggregateused PCC. In addition, a significant amount of blast furnace slag is produced and used in PCC. Tostudy the static and dynamic strength of the aggregates. unconfined uniaxial compressive strength ofthree limestones, four dolomites. two basalts and three blast furnace slags were conducted. Inaddition, tests were also conducted under dry and saturated conditions. The studies revealed that theslag aggregates exhibited the lowest compressive strength, followed by the limestones and dolomites.The basalt aggregates exhibited the highest compressive strength. The compressive strength data ofdry and saturated rocks were found to have a strong correlation to the bulk densities. Both the thyand saturated aggregates revealed a higher compressive strength under dynamic loads compared tothe static loads. The percentage increase in dynamic compressive strength over its static strength isidentified as a measure of the rate sensitivity of these aggregates, which may have considerablerelevance in applications such as rock blasting, underground construction, and geologic interpretation.

The aggregate used in the study were obtained from quarries in Michigan, Ohio and Ontario, whilethe blast ftrnnace slags were obtained from steel mills in Sault Ste. Marie, Ontario and Detroit,Michigan. The three limestones investigated were from the Fibron Limestone Formation of middleSilurain age (Port Inland Quarry), the Roger City Limestone Formation of middle Devonian age(Presque Isle Quarry), and the Bay Port Limestone of late Mississippian age (Bay County RoadCommission Quarry). The dolomites were from the Engadine Dolomite Formation of middle Silurianage (Cedarville Quarry), the Raisin River Dolomite Formation of late Silurian age (France StoneQuarry). the Lucas Dolomite Formation of middle Devonian age (Rockwood Quarry), and astratigraphically later dolomite from the Lucas Formation (Denniston Quany). The igneous rockstested were from the Portage Lake Volcanics (Moyle Quarry) and the Paleoproterozic volcanics nearBruce Mines, Ontario (Ontario Traprock Quarry).

The aggregates were subjected to uniaxial compressive loads at two different strain rates. Low strainrate experiments (static) were conducted on a MTS servohydraulic machine and high strain rateexperiments (dynamic) were conducted on a modified split Hopkinson pressure bar (MSHPB). TheSplit Hopkinson pressure bar çalso called a Kolsky bar, Kolsky, 1949) technique has been widelyused for dynamic testing of materials at high strain rates in the range of IO'-l 4 s' (Ravichandran and

Subhash, 1995). It consists of a striker bar, an incident bar and a transmission bar, as shownschematically in Figure I. The specimen to be characterized is placed between the incident andtransmission bars. The free end of the incident bar is impacted by the striker bar, which is launchedfrom a gas gun, at a predetermined velocity. The impact generates a compression pulse in the incidentbar which travels towards the specimen, subjecting it to the required compressive loading. A part ofthis pulse is transmitted to the transmitter bar and the rest is reflected back into the incident bar as atensile pulse. Strain gages are mounted at the center of each bar to measure the magnitude andduration of these stress pulses. Based on one-dimensional calculations, it can be shown that themagnitude of the transmitted pulse gives a measure of the stress to which the specimen is subjectedand the magnitude of the reflected wave gives a measure of the strain rate within the specimen.

54

DYNAMIC AND STATIC STRENGTH OF AGGREGATES: AN ESTIMATE OF RATE SENITIVITY OF GEOLOGIC MATERIALS

STANLEY J. VITTON, Department of & Environmental Engineering, GHATUPARTHI SUBHASH, Department of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, Michigan 4993 1

The Michigan Department of Transportation (MDOT) has initiated research into the dynamic fracture characteristics of aggregates in Portland cement concrete (PCC) pavements. The main focus of this research is to study the fracture and strcngth characteristics of the coarse aggregate portion in PCC. Due to the wide range of rock types in Michigan, there is a corresponding range of coarse aggregate used PCC. In addition, a significant amount of blast furnace slag is produced and used in PCC. To study the static and dynan~ic strength of the aggregates, unconfined uniaxial compressive strength of three limestones, four dolomites, two basalts and three blast furnace slags were conducted. In addition, tests wcre also conducted under dry and saturated conditions. The studies revealed that the slag aggrcgates exhibited the lowest compressive strcngth, followed by the limestones and dolomites. The basalt aggregates exhibited the highest compressive strength. The compressive strcngth data of dry and saturated rocks were found to have a strong correlation to the bulk densities. Both the dry and saturated aggregates revealed a higher compressive strength under dynamic loads compared to the static loads. The percentage increase in dynamic compressive strcngth over its static strength is identified as a measure of the rate sensitivity of these aggregates, which may have considerable relevance in applications such as rock blasting, underground construction, and geologic interpretation.

The aggregate used in the study were obtained from quarries in Michigan, Ohio and Ontario, while the blast furnnace slam were obtained from steel mills in Sault Stc. Marie. Ontario and Detroit. Michigan. The threelimestones investigated were from the Fibron Limestone Formation of middle Silurain age (Port Inland Quarry), the Roger City Limestone Formation of middle Devonian age (Presquc Isle Quarry), and the Bay Port Limestone of late Mississippian age (Bay County ~ o a d Commission Quarry). The dolomites wcre from the Engadine Dolomite Formation of middle Silurian age (Cedarvillc Quarry), the Raisin River Dolomite Formation of late Silurian age (France Stone Quan-y), the Lucas Dolomite Formation of middle Devonian age (Rockwood Quarry), and a stratigraphically later dolomite from the Lucas Formation (Denniston Quarry). The igneous rocks tested were from the Portage Lake Volcanics (Moyle Quarry) and the Paleoprotcrozic volcanics near Bruce Mines, Ontario (Ontario Traprock Quarry).

The aggrcgates were subjected to uniaxial compressive loads at two different strain rates. Low strain rate experiments (static) wcre conducted on a MTS sc~ohydraulic machine and high strain rate experiments (dynamic) were conducted on a modified split Hopkinson pressure bar (MSHPB). The Split Hopkinson pressure bar (also called a Kolsky bar, Kolsky, 1949) technique has been widely used for dynamic testing of materials at high strain rates in the range of 10'-104 s"' (Ravichandran and Subhash, 1995). It consists of a striker bar, an incidcnt bar and a transmission bar, as shown schematically in Figure 1. The specimen to be characterized is placed between the incident and transmission bars. The free end of the incident bar is impacted by the striker bar, which is launched from a gas gun, at a predetermined velocity. The impact generates a compression pulsc in the incidcnt bar which travels towards the specimcn, subjecting it to the required compressive loading. A part of this pulse is transmitted to the transmitter bar and the rest is rcflected back into the incident bar as a tensile pulsc. Strain gages are mounted at the center of each bar to measure the magnitude and duration of these stress pulses. Based on one-dimensional calculations, it can be shown that the magnitude of the transmitted pulsc gives a measure of the stress to which the specimen is subjected and the magnitude of the reflected wave gives a measure of the strain rate within the specimen.

integrating the strain rate with respect to time yields the strain in the specimen. Thus, the stress-strainresponse of a material can be obtained at high strain rates.

A minimum often samples were tested for each aggregate type in each condition, e.g. thy, saturated,static, and dynamic conditions. The results of the static and dynamic testing for thy conditions arepresented in Figure 2, where the blast fi1rnace slags are identified as sample types 1. 2, and 3; the

lirnestones are sample types 4, 5, and 6; the dolomites are sample types 7 through 10; and the basaltsample types are II and 12. In addition, the bulk density of the rock types are also presented. It canbe seen from this data that the slags exhibited the lowest strength followed by the limestones anddolomites with the basalts having the highest strength. In addition, it can be seen that there is a goodcorrelation with bulk density. With the partial exception of the slags, all of the materials testedshowed an increase in dynamic strength over the static strength, indicating that these materials arerate sensitive.

The blast furnace slag showed relatively low strength both in satic testing and dynamic testing.However, it can be seen that the air cooled slag had both low strength and strength in the range of thelimestoncs and dolomites. The two data sets shown for the air cooled slag represent samples formtwo different blocks of slag material assumed to have been cooled at different rates depending onwhere it was deposited. The blast fijrnacc slag is formed from the gangue and secondary constiuentsin iron ore as well as coke residue and limestone and has a chemical composition of CaO, Si02, Al03and MgO with trace amounts of.sulphur and some alkalis (Smolczyk, 1980). The final structure ofthe slag depends on the cooling conditions of the molten slag. When it is allowed to cool slowly itforms into a dense crystalline structure with properties similar to rock. When the molten slag isquenshed quickly with limited amonts of water, it traps steam in the mass and produces a porous,glassy material with poor mechanical properties. However, when the air cooled slag is formed it alsohas different rates of cooling depending on its location when deposited for cooling. The molten slagexposed to the atmosphere (top) will cool more quickly then the slag at the bottom of the deposit.The statification when cooling is cieariy observed by a color straification and the appearance of pores.The test data shows this with a significant increase in strength with test samples from two differentblocks from the same deposit. i.e., top and bottom locations.

In the case of the basalt.s tested, it can be seen that the trap rock basalt has a higher strength then thePortage Lake volcanic basalt. in a simplied framework it is assumed that the cooling rate of the traprock basalt was longer than the Portage Lake voleanics, which were formed formed near or at thesurface. This is also seen in the crystalline structure of the two basalts, with the trap rock basalthaving a more uniform crystalline structure and the Portage Lake volcanics a more inconsistent andrandom crystalline structure.

The sedimentary limestones and dolomites also show rate sensitivity. It is interesting to note in thelimestones that the static strength for the three limestones tested are relatively close, while thedynamic strength are higher. In particular, the Silurian limestone (Port Inland) showed significantlyhigher rate sensitivity than does the Devonian limestone (Preque Isle) or the Mississippian limestone(Bay County). However, this same trend is not seen in the doloinites. With the exception of theSilurian dolomite (Cedarvillc) there is an increase in both the static and dyanmic strength withgeologic age.

Previous research has shown that for a brittle material, the damage evolution under appliedcompressive loading is intimately related to microeracking at defects such a pores, inclusions, secondphase particles, twin grain boundary intersetcions and triple point grain boundary junctions (Lanford,1977). However, under dynamic loading conditions above a critical strain rate the rate sensitivity of amaterial is atiributted to the inertia-dominated dynamic crack growth from pre-existing flaws. Thus,

55

Integrating the strain rate with respect to time yields the strain in the specimen. Thus. the stress-strain response of a material can be obtained at high strain rates.

A minimum of ten samples were tested for each aggregate type in each condition, e.g. dry, saturated, static, and dynamic conditions. The results of the static and dynamic testing for dry conditions arc presented in Figure 2, whcre the blast furnace sla&s arc identified as sample types I. 2, and 3; the limestones are sample types 4, 5. and 6; the dolomites arc sample types 7 through 10; and the basalt sample types are 11 and 12. In addition, the bulk density of the rock types are also presented. It can be seen from this data that the slags exhibited the lowest strength followed by the limestones and dolomites with the basalts having the highest strength. In addition, it can be sccn that there is a good correlation with bulk density. With the partial cxception of the slag!,, all of the materials tested showed an increase in dynamic strength o v a the static strength, indicating that these materials are rate sensitive.

The blast furnacc slag showed relatively low strength both in satic testing and dynamic testing. However, it can be seen that the air cooled slag had both low strcngth and strength in the range of the liniestones and dolomites. The two data sets shown for the air cooled slag represent samples form two different blocks of slag material assunicd to have been cooled at different rates depending on whcre it was deposited The blast furnacc slag is formed from the gangue and secondary constiuents in iron ore as well as coke residue and limestone and has a chemical composition of CaO, Si02, A103 and MgO with trace amounts of sulphur and some alkalis (Smolczyk, 1980). The final structure of the slag depends on the cooling conditions of the molten slag. When it is allowed to cool slowly it forms into a dense crystalline structure with properties similar to rock. When the molten slag is quenshed quickly with limited amonts of water, it traps steam in the mass and produces a porous, glassy material with poor mechanical propcriies. Howcvcr, when the air cooled slag is formed it also has different rates of cooling depending on its location when deposited for cooling. The molten slag exposed to the atmosphere (top) will cool more quickly then the slag at the bottom of the deposit. The statification when cooling is clearly observed by a color straification and the appearance of pores. The test data shows this with a significant increase in strcngth with test samples from two different blocks from the same deposit, i.e., top and bottom locations.

In the case of the basalts tested, it can be sccn that the trap rock basalt has a higher strength then the Portage Lake volcanic basalt. In a simplied framework it is assumed that the cooling rate of the trap rock basalt was longer than the Portage Lake volcanics, which were formed formed near or at the surface. This is also seen in the crystallinc structurc of the two basalts, with the trap rock basalt having a more uniform crystallinc structure and the Portage Lake volcanics a more inconsistent and random crystalline structure.

The sedimentary limestones and dolomites also show rate sensitivity. It is interesting to note in the limestones that the static strength for the three limestones tested are relatively close, while the dynamic strength arc higher. In particular, the Silurian limestone (Port Inland) showed significantly higher rate sensitivity than does the Devonian limestone (Prcque Islc) or the Mississippian limestone (Bay County). Howcvcr, this same trend is not seen in the dolomites. With the cxception of the Silurian dolomite (Cedarville) there is an increase in both the static and dyanmic strength with geologic age.

Previous research has shown that for a brittle material, the damage evolution under applied compressive loading is intimately related to microcracking at defects such a pores, inclusions, second phase particles, twin grain boundary intersetcions and triple point grain boundary junctions (Lanford, 1977). However, under dynamic loading conditions above a critical strain rate the rate sensitivity of a material is attributted to the inertia-dominated dynamic crack growth from prc-existing flaws. Thus,

response of a defect to loading is inertia dependent, i.e. the time it takes to overcome the inertia of themicrocrack, more defects are loaded in the material leading to higher strength. Therefore, it is

speculated that if dynamic loading stresses a larger portion of a material's defects, then it may bepossible that geologic mechanism such as formational history, compaction and lithification may bebetter observed under dynamic fracture testing.

References

Koisky, H., 1949, An Investigation of the Mechanical properties of Materials at Very High Rates of

Loading. Pmc. R. Soc. London, B62, pp. 676-700.

Ravichandran. G. and Subhash, G., 1995, A Micromechanical Model for High Strain Rate Behavior

of Ceramics, lifiernalional .Journal of Solids and Siructures. 32 [17/IS], pp. 2627-2646.

Smolczyk, H.G.. 1950, Slag Strcture and Identification of Slags, Inter. Congress on the Chcmistry of

Cement, 7" ed. 1980. Paris a editions Septima, pp. 3-17.

Striker Bar

Figure I Modifidied Split Hopkinson Pressure Bar wit Momentum Trap.

Figure 2

=

Q 22 dF V

H0 0 ID—

Dynamic and Static Strength Tests in Dry Conditions.

56

High Speed, 4 ChannelDigital Oscilloscope(Nicolet PRO4O)

Incident Bar Transmission Bar

0 0S

ii Ji 1'

Stat

Dens 0

mU.5C0

>0C0C,V

I

700 -

600

500

400

300

200

00

0

3.5

3.0C.,

E

.4:

2.5CC2'VA=0C

2.0

1

I

0

0

II I;

1

I

0 1 2 3 4 5 6 7 6 9

Sample Type

1.510 11 12 13

response of a defect to loading is inertia dependent, i.e, the time it takes to overcome the inertia of the microcrack, more defects are loaded in the material leading to higher strength. Therefore, it is speculated that if dynamic loading stresses a larger portion of a material's defects, then it may be possible that geologic mechanism such as formational history, compaction and lithification may be bcncr observed under dynamic fracture testing.

References

Kolsky, H., 1949, An Investigation of the Mechanical properties of Materials at Very High Rates of Loading, Proc. R. Sue. London, B62, pp. 676-700.

Ravichandran, G. and Subhash, G., 1995, A Micromechanical Modcl for High Strain Rate Behavior of Ceramics, International Journal of Solids and S/ruc/ure.~. 32 [I 711 81, pp. 2627-2646.

Smolczyk, H.G., 1980. Slag Strcture and Identification of Slags, Inter. Congress on the Chemistry of Cement, 7* ed. 1980. Paris aeditions Septima, pp. 3-17.

High Speed, 4 Channel Digital Oscilloscope (Nicolet PR040)

I I Wheatstone Bridge

Strain gage specimen - 1 m m - I Striker Bar Incident Bar Transmission Bar

Figure 1 Modifidied Split Hopkinson Pressure Bdr wit Momentum Trap.

Sample Type

Figure 2 Dynamic and Static Strength Tests in Dry Conditions.

56

Palmer Gneiss; a large, low-grade shear zone

'Webster, C. L., 'Cambray W. F., 2Scott, 0., 2Nordstrom, P. 'Wilson, E.'Dept. Geological Sciences, Michigan State University, East Lansing, MI 48824

2Cleveland Cliffs Mining Co, lshpeming, MI

The Marquette Trough is interpreted to be an asymmetric rift-related basin; truncated stratigraphy

on the southern margin and a rollover structure to the north provides evidence for this

interpretation, Marking the southern boundary, between the Archean gneiss and the Early

Proterozoic metasedimentary rocks of the Marquette Range Supergroup, is the Palmer Gneiss

(PG). Van Rise and Bayley (1897) first interpreted the PG as a comminuited, sericitzed and

partly silicified phase of the lower Precambrian gneiss. In 1968, Gair and Simmons concurred

with theft interpretation and offered three possible modes of genesis: (1) alteration and shearing

of Precambrian rock during faulting, (2) migration of fluids along the contact between lower and

middle Precambrian rocks, and (3) alteration of a regolith during folding. Since the time of

mapping by Gair and Simmons, Tilden open pit mining operations have exposed the PG. Large

shear bands, with approximately 2.5 meter spacing, indicate the importance of ductile

deformation during the formation of the PG in a low-grade shear zone (Figure 1). Foliation

measurements show a steep NNE dipping S-foliation being cut by a shallow NNE shear band,

indicating a reverse sense of shear (Figure 2). The folds measured in the adjacent iron formation,

at the Tilden and Empire mines, consistently plunge gently towards the WNW, reflecting that

they were formed under the same strain conditions as the shear zone. The NNE-SSW

compressive direction corresponds to previous studies in the area (Myers, 1984). Whole rock

chemical analysis of the PG suggests that these altered rocks are not granitic in origin, but rather

basaltic. The basaltic nature indicates that the PG is of Proterozoic age. The Penokeon Orogeny,

collision of an island arc with the Lake Superior Craton, caused closure of the Marquette Trough

that reactivated preexisting normal faults and resulted in a reverse dip slip. Strain was

concentrated in a portion of a metadiabase sill that was dragged into the shear zone causing

57

Palmer Gneiss; a large, low-grade shear zone 8 -

' 1 ~ e b s t e r , C. L., 'cambray W. F., '~cott, G.,'~ordstrom, P. 'Widson, E.

' ~ e ~ t . Geological Sciences, Michigan State University, East Lansing, MI 48824 2 Cleveland Cliffs Mining Co, Ishpeming, MI

The Marquette Trough is interpreted to be an asymmetric rift-related basin, truncated stratigraphy

on the southern margin and a rollover structure to the north provides evidence for this

interpretation. Marking the southern boundary, between the Archean gneiss and the Early

Proterozoic metasedimentary rocks of the Marquette Range Supergroup, is the Palmer Gneiss

(PG). Van Hise and Bayley (1897) first interpreted the PG as a comminuited, sericitzed and

partly silicified phase of the lower Precambrian gneiss. In 1968, Gair and Simmons concurred

with their interpretation and offered three possible modes of genesis: (1) alteration and shearing

of Precambrian rock during faulting, (2) migration of fluids along the contact between lower and

middle Precambrian rocks, and (3) alteration of a regolith during folding. Since the time of

mapping by Gair and Simmons, Tilden open pit mining operations have exposed the PG. Large

shear bands, with approximately 2.5 meter spacing, indicate the importance of ductile

deformation during the formation of the PG in a low-grade shear zone (Figure 1). Foliation

measurements show a steep NNE dipping S-foliation being cut by a shallow NNE shear band,

indicating a reverse sense of shear (Figure 2). The folds measured in the adjacent iron formation,

at the Tilden and Empire mines, consistently plunge gently towards the WNW, reflecting that

they were formed under the same strain conditions as the shear zone. The NNE-SSW

compressive direction corresponds to previous studies in the area (Myers, 1984). Whole rock

chemical analysis of the PG suggests that these altered rocks are not granitic in origin, but rather

basaltic. The basaltic nature indicates that the PG is of Proterozoic age. The Penokeon Orogeny,

collision of an island arc with the Lake Superior Craton, caused closure of the Marquette Trough

that reactivated preexisting normal faults and resulted in a reverse dip slip. Strain was

concentrated in a portion of a metadiabase sill that was dragged into the shear zone causing

alteration and shearing of the rock to form the PG. A 34% loss of volume is the calculated effect

of alteration from the metadiabase protolith to the PG (Gresens, 1967).

References:

Gair, J. E., and Simmons, G. C, 1968, Palmer Gneiss-an example of retrograde metamorphismalong an unconformity: U.S. Geological Survey Professional Paper, 600-D, p. D186-D194.

Gresens, R. L., 1967, Composition-volume relationships of metasomatism: Chemical Geology, v.2, p. 47-65.

Myers, G., 1984, Structural analysis of foliated Proterozoic metadiabase dikes in the Marquette-Republic region of Northern Michigan: Master's Thesis, Michigan State University.

Van Hise, C. R., and Bayley, W. S., 1895, The Marquette iron-bearing district of Michigan: U.S.Geological Survey Monograph 28, p. 608.

Fig. 1 Footwall Shear Zone, Tilden Mine

(Palmer Onees}

o Pole to mean C-foliation• Pole to mean S-foliation

A Slip direction

* Mean S-C. intersection

S-C' intersections

\ S-foliation plane

Top of shear zone

C-foliationplane

Fig. 2 Stereonet of Footwall Shear Zone, Tilden

58

East

2.5

N

alteration and shearing of the rock to form the PG. A 34% loss of volume is the calculated effect

of alteration from the metadiabase protolith to the PG (Gresens, 1967).

References:

Gair, J. E., and Simmons, G. C, 1968, Palmer Gneiss-an example of retrograde metamorphism along an unconfonnity: U.S. Geological Survey Professional Paper, 600-D, p. D186- D194.

Gresens, R L., 1967, Composition-volume relationships of metasomatism: Chemical Geology, v. 2, p. 47-65.

Myers, G., 1984, Structural analysis of foliated Proterozoic metadiabase dikes in the Marquette- Republic region of Northern Michigan: Master's Thesis, Michigan State University.

Van Hise, C. R., and Bayley, W. S., 1895, The Marquette iron-bearing district of Michigan: U.S. Geological Survey Monograph 28, p. 608.

West

Fig. 1 Footwall Shear Zone, Tilden Mine . \ -*- (Palmer Gneiss)

Pole to mean C'-foliation

Pole to mean S-foliation

Slip direction

Mean S-C' intersection

S-C' intersections

S-foliation plane

Top of shear zone

C'-foliation plane

Fig. 2 Stereonet of Footwall Shear Zone, Tilden

58

IS THE JANICE LAKE UNCONFORMITY A LINK BETWEEN THE HEARNECRATON AND THE LA RONGE - LYNN LAKE VOLCANIC ARC?

YEO. Gary M., Saskatchewan Energy and Mines, 1914 Hamilton St., Regina, SK,S4P 4V4, [email protected]

The Wollaston Domain, a NE-trending, 75km wide belt of 2076-1860? Ma paragneisses,overlying Archean orthogneisses along the eastern edge of the Heame Craton in Saskatchewan, isseparated from 1910-1875 Ma arc volcanics and sediments of the La Ronge Domain by (a)Archean orthogneisses, poorly known supracrustal rocks, and minor Proterozoic plutons of thePeter Lake Domain, (b) 1855 Ma monzogranite to quartz diorite of the Wathaman Batholith, and(c) the Rottenstone Domain, a migmatitic belt of sediments, similar to those of the La RongeDomain, intruded by tonalite. The boundary between Wathaman Batholith and Wollaston andPeter Lake domains is the Needle Falls - Parker Lake Shear Zone. While this is a fundamentalbreak between ancient crust to the west and juvenile crust of Trans-Hudson Orogen to the east,there is no evidence that it is a collisional suture.

Within the Wollaston supracrustal succession, the Janice Lake Conglomerate, which canbe traced discontinuously for 245 km along the eastern margin of the belt, marks a majorunconformity. The conglomerate is polymictic, with clasts derived from underlying stratapredominating. Locally, it includes fanglomerate. Although the conglomerate cannot be tracedwestward, younger calc-silicate bearing strata, locally resting on various older lithostratigraphicunits, mark the presence of a profound unconformity. A correlative unconformity has beenrecognized in the Manitoba segment of Wollaston Domain, another 200 km to the northeast.

In the absence of any evidence for thermal uplift (ie. igneous activity), uplift andextension in Janice Lake time must be due to mechanical flexure of the crust. A probable cause istectonic loading of the eastern margin of the Hearne craton as the La Ronge - Lynn Lake volcanicarc converged towards it over an east-dipping subduction zone. Volcanism in the La Ronge -Lynn Lake arc began about 1910 Ma and reached a peak about 1880 Ma. Following arc-cratoncollision at about 1860 Ma, the sense of subduction was reversed and the Wathaman Batholithwas emplaced at about 1855 Ma. Geochemical evidence suggests that the La Ronge segment ofthe arc developed in a transitional crust setting, close to or partly over marginal continental crust.

Tectonic loading of the outboard margin of the Heame Craton would have resulted in aninboard flexural bulge, with consequent normal faulting and erosion over its crest and depositionof coarse clastics on its flanks. As convergence proceeded, the bulge would have migratedcratonward and died out, creating a diachronous regional unconformity. Preservation of coarseclastics would be favoured only on the trailing, eastern flank of the bulge, however. Theapproaching volcanic arc would have become a bather to the open ocean, and accumulation ofevaporites and carbonates in the resulting restricted basin would have been increasingly favoured.

This hypothesis also explains the dramatic change in sedimentation recorded by theWollaston paragneisses. Following 2100 Ma rifting, the widespread Lower Sequence ofpsammopelites, pelites, and feldspathic psammites accumulated on a passive margin. As arc-craton convergence proceeded, coarse clastics were shed off a migrating flexural bulge ahead ofthe arc. Upper Sequence calc-silicate rich strata subsequently accumulated in the developingforeland basin.

59

IS THE JANICE LAKE UNCONFORMITY A LINK BETWEEN THE HEARNE CRATON AND THE LA RONGE - LYNN LAKE VOLCANIC ARC?

YEO, Gary M., Saskatchewan Energy and Mines, 1914 Hamilton St., Regina, SK,

The Wollaston Domain, a NE-trending, 75 km wide belt of 2076-1860? Ma paragneisses, overlying Archean orthogneisses along the eastern edge of the Hearne Craton in Saskatchewan, is separated from 19 10- 1875 Ma arc volcanics and sediments of the La Ronge Domain by (a) Archean orthogneisses, poorly known supracrustal rocks, and minor Proterozoic plutons of the Peter Lake Domain, (b) 1855 Ma monzogranite to quartz diorite of the Wathaman Batholith, and (c) the Rottenstone Domain, a migmatitic belt of sediments, similar to those of the La Ronge Domain, intruded by tonalite. The boundary between Wathaman Batholith and Wollaston and Peter Lake domains is the Needle Falls - Parker Lake Shear Zone. While this is a fundamental break between ancient crust to the west and juvenile crust of Trans-Hudson Orogen to the east, there is no evidence that it is a collisional suture.

Within the Wollaston supracrustal succession, the Janice Lake Conglomerate, which can be traced discontinuously for 245 km along the eastern margin of the belt, marks a major unconformity. The conglomerate is polymictic, with clasts derived from underlying strata predominating. Locally, it includes fanglomerate. Although the conglomerate cannot be traced westward, younger calc-silicate bearing strata, locally resting on various older lithostratigraphic units, mark the presence of a profound unconformity. A correlative unconformity has been recognized in the Manitoba segment of Wollaston Domain, another 200 km to the northeast.

In the absence of any evidence for thermal uplift (ie. igneous activity), uplift and extension in Janice Lake time must be due to mechanical flexure of the crust. A probable cause is tectonic loading of the eastern margin of the H e m e craton as the La Ronge -Lynn Lake volcanic arc converged towards it over an east-dipping subduction zone. Volcanism in the La Ronge - Lynn Lake arc began about 1910 Ma and reached a peak about 1880 Ma. Following arc-craton collision at about 1860 Ma, the sense of subduction was reversed and the Wathaman Batholith was emplaced at about 1855 Ma. Geochemical evidence suggests that the La Ronge segment of the arc developed in a transitional crust setting, close to or partly over marginal continental crust.

Tectonic loading of the outboard margin of the Heame Craton would have resulted in an inboard flexural bulge, with consequent normal faulting and erosion over its crest and deposition of coarse clastics on its flanks. As convergence proceeded, the bulge would have migrated cratonward and died out, creating a diachronous regional unconformity. Preservation of coarse clastics would be favoured only on the trailing, eastern flank of the bulge, however. The approaching volcanic arc would have become a barrier to the open ocean, and accumulation of evaporites and carbonates in the resulting restricted basin would have been increasingly favoured.

This hypothesis also explains the dramatic change in sedimentation recorded by the Wollaston paragneisses. Following 2100 Ma rifting, the widespread Lower Sequence of psammopelites, pelites, and feldspathic psammites accumulated on a passive margin. As arc- craton convergence proceeded, coarse clastics were shed off a migrating flexural bulge ahead of the arc. Upper Sequence calc-silicate rich strata subsequently accumulated in the developing foreland basin.

(a) Rift Sequence (Courtenay - Cairns Lake Belt): 2100 Ma

Riftsediments—_

* + t ÷ j* + %+ + L+ +- Hearne Craton

÷ + ÷ ÷

Stretched continental crust± + %+ + A+ + + ±1

Superior Cratori?4•

+ + +

Oceanic crust

(b) Lower Sequence: 2100 - 1880 MaLa Range

Volcanic Arc(1910- 1860 Ma)

Yeo, ci. 1998. A systems tract approach to the stratigraphy of paragneisses in the southeasternWollaston Domain: Saskatchewan Energy and Mines, Misc. Rep. 98-4, p36-47.

60

(c) Upper Sequence: 1880-1860 MaRestricted basinUplift and erosion (uncónforrMy)

(d) Cord illeran Margin: 1860 - 1830 MaForeland sedimentation

tharnan(1855 Ma)

Reference

(a) Rift Sequence (Courtenay - Cairns Lake Belt): 2100 Ma

,..,

(b) Lower Sequence: 2100 - 1880 I Volcanic Arc

(c) Upper Sequence: 1880 - 1860 Ma Uplift and erosion (uncbnformity) Restricted basin

Reference +'>, : <;s

Yeo, G. 1998 oach tot isses in the southeastern Wollaston Domain: Saskatchewan Energy and Mines, Misc. Rep. 98-4, p36-47.

LOW-RESOLUTION SEQUENCE STRATIGRAPHY OF EARLY PROTEROZOJCPARAGNEISSES, WOLLASTON DOMAIN, SASKATCHEWAN

YEO, Gary M., Saskatchewan Energy and Mines, 1914 Hamilton St., Regina, SIC, S4P 4V4,gary.yeo @sem.ov.sk.ca

The Wollaston Domain is a NE-trending, 75 km wide belt of of early Proterozoic paragneisses,overlying Archean orthogneisses along the eastern edge of the Hearne Craton; west of the Trans-Hudson Orogen. Bimodal volcanics near the base of the paragneiss succession give a U-Pb zirconage of 2.08 Ga. while minor gabbro and granitic plutons range from 1.81 to 1.84 Ga. A three-year, 1:20,000 scale mapping project was begun in 1997 in the southern part of WollastonDomain to improve understanding of the stratigraphy and structure of the guesses, which underliethe unconformity-type uranium deposits of Athabasca Basin. A basic strategy of the project is tomap outward from Archean inliers so as to be able to work out the stratigraphic succession inspite of complex deformation and high metamorphic grade.

Four episodes of regional deformation can be distinguished. Dl involved isoclinalfolding, local basement overthrusting, and transposition of primary layering into the SI foliation.D2 involved tight to isoclinal folding about NE-trending axes, superposition of a strong 52foliation, and local thrust faulting. D3 involved local open folding about SW to NW axes, andlocal brittle-ductile fault movement. D4 involved minor sinistral offset on a series of N to NNWtrending brittle faults.

Two episodes of high T - low P (upper amphibolite - lower granulite facies)metamorphism are associated with Dl and 1)2 deformation. Peak P-T conditions, reached duringD2, were about 750°C and 5 kbar. Along the eastern margin of the belt, however, metamorphicgrades are relatively low (upper greenschist - lower amphibolite facies).

Two widespread paragneiss successions are distinguished in Wollaston Domain: a lowerPelite Unit, and an overlying Psammite Unit. These can be subdivided into at least eight subunitsin the Foster - Daly lakes area. Two restricted successions are also recognized: a basal package(Courtenay Lake and Souter Lake Formations), preserved locally along the eastern margin of thedomain, and the heterogenous Hidden Bay Assemblage of interlayered quartzites, pelites.amphibolites. calc-silicates, and marbles, which likely overlies or is correlative with thePsammite Unit near Wollaston Lake. The Courtenay Lake Formation has been demonstrated tobe a rift succession, while a passive margin setting is inferred for the overlying Pelite andPsanunite units. Attempts to determine the depositional setting of individual lithologic units,however, have been restricted to the low-grade rocks along the eastern margin of the domain.

Although sequence stratigraphy has become a standard technique for analysis of platformand craton margin strata, there have been few attempts to apply it to metamorphosed anddeformed rocks. As demonstrated here, it has great potential in interpreting strata in whichobliteration of primary features limits application of facies models or comparison with modernsedimentary environments.

Two major sequence boundaries subdivide the succession. The unconformity on Archeanbasement can be recognized without difficulty. The base of the Janice Lake Conglomerate (UnitWo) marks a second unconformity. To the west, where the conglomerate is absent, thisunconformity corresponds to the base of the calcareous rocks that overlie it. Within eachsequence, the mapped lithofacies units can be asigned to a succession of systems tracts.

61

LOW-RESOLUTION SEQUENCE STRATIGRAPHY O F EARLY PROTEROZOIC PARAGNEISSES, WOLLASTON DOMAIN, SASKATCHEWAN

YEO, Gary M., Saskatchewan Energy and Mines, 1914 Hamilton St., Regina, SK. S4P 4V4,

The Wollaston Domain is a NE-trending, 75 km wide belt of of early Proterozoic paragneisses, overlying Archean orthogneisses along the eastern edge of the Hearne Craton; west of the Trans- Hudson Orogen. Bimodal volcanics near the base of the paragneiss succession give a U-Pb zircon age of 2.08 Ga, while minor gabbro and granitic plutons range from 1.81 to 1.84 Ga. A three- year, 1 :20,000 scale mapping project was begun in 1997 in the southern part of Wollaston Domain to improve understanding of the stratigraphy and structure of the gnesses, which underlie the unconformity-type uranium deposits of Athabasca Basin. A basic strategy of the project is to map outward from Archean inliers so as to be able to work out the stratigraphic succession in spite of complex deformation and high metamorphic grade.

Four episodes of regional deformation can be distinguished. D l involved isoclinal folding, local basement ovenhmsting, and transposition of primary layering into the S l foliation, D2 involved tight to isoclinal folding about NE-trending axes, superposition of a strong S2 foliation, and local thrust faulting. D3 involved local open folding about SW to NW axes, and local brittle-ductile fault movement. D4 involved minor sinistral offset on a series of N to NNW trending brittle faults.

Two episodes of high T - low P (upper amphiboIite - lower granulite facies) metamorphisk are associated with Dl a n d ~ 2 def&mation. Peak PIT conditions. reached during D2, were about 750Â° and 5 kbar. Alone the eastern margin of the belt. however, metamorphic - - grades are relatively low (upper greenschist - lower amphibolite facies).

Two widespread paragneiss successions are distinguished in Wollaston Domain: a lower Pelite Unit, and an overlying Psammite Unit. These can be subdivided into at least eight subunits in the Foster - Daly lakes area. Two restricted successions are also recognized: a basal package (Courtenay Lake and Souter Lake Formations). preserved locally along the eastern margin of the domain. and the heterogenous Hidden Bay Assemblage of interlayered quartzites, pelites, amphibolites, calc-silicates, and marbles, which likely overlies or is correlative with the Psammite Unit near Wollaston Lake. The Courtenay Lake Formation has been demonstrated to be a rift succession, while a passive margin setting is inferred for the overlying Pelite and Psammite units. Attempts to determine the depositional setting of individual lithologic units, however, have been restricted to the low-grade rocks along the eastern margin of the domain.

Although sequence stratigraphy has become a standard technique for analysis of platform and craton margin strata, there have been few attempts to apply it to metamorphosed and deformed rocks. As demonstrated here, it has great potential in interpreting strata in which obliteration of primary features limits application of facies models or comparison with modem sedimentary environments.

Two major sequence boundaries subdivide the succession. The unconformity on Archean basement can be recognized without difficulty. The base of the Janice Lake Conglomerate (Unit Wo) marks a second unconformity. To the west, where the conglomerate is absent, this unconformity corresponds to the base of the calcareous rocks that overlie it. Within each sequence, the mapped lithofacies units can be asigned to a succession of systems tracts.

Lower Sequence: Ferruginous (garnet), anoxic (graphite), muddy sediments of UnitWna fine upward into aluminous (clay-rich) muddy sediments of Unit Wnps. Such retrogradationis consistent with a transgressive systems tract. The quartzite beds in Unit Wna may be storm-generated flows. Less aluminous muddy sediments of Unit Wns are transitional through UnitWnsp to Unit Wrn sands. Such a progradational sequence is consistent with a highstand systemstract.

Upper Sequence: Polymict conglomerate of Unit Wo marks a period of uplift anderosion. Most clasts are derived from the underlying sedimentary succession. Althoughrestricted to the eastern margin of Wollaston Domain, the Janice Lake Conglomerate can betraced along strike for at least 245 km. This unit is interpreted as alluvial fan deposits. Non-marine conditions indicate that it is a lowstand systems tract. An upward decrease in clastic inputis suggested between calc-silicate bearing Unit Wrnc and calc-silicate rich Unit Wnc. Thissuggests another transgressive systems tract. The calc-silicate rich rocks may be evaporiticdeposits.

Delaney. GD.. Jankovic, Z., MacNeil, A.. McGowan, J.. and Tisdale, D., 1997, Geological investigations of the Courtenay Lake.Cairns Lake Fold Belt and the Hill Lake Embayment, Johnson River Intier, Wollaston Domain, northern Saskatchewan:Saskatchewan Energy and Mines, Misc. Rep. 97.4. p90-1O1.

Tran, H.T. and Yeo, G.M., 1997. Geology of the Burbidge Lake' northern Upper Foster Lake aica. eastern Wollaston Domain(NTS 74A-14): Saskatchewan Energy and Mines. Misc. Rep. 97.4. p72-89.

Tran, H.T., Yco, G., Bradley. S. and Lowry, Jl*, 1998. Geology of the Daly-suttle-Middle Foster lakes asea, eastern wollastonDomain (NTS 74A-5, -Il. and .12): Saskatchewan Energy and Mines, Misc. Rep. 98-4. p36.47.

Yeo, G., 1998. A systems tract approach to the stratigraphy of paragneisses in the southeastern Wollaason Domain: SaskatchewanEnergy and Mines, Misc, Rep. 98.4. p36.47.

62

LIthelpay lnterDretatipnC,0CC,

C.C,Cl)I-C,C.a

C,0CC,=C.C,

Cl)

C,

0-I

Wnc

Wrnc

WoUnconformity

Wrn

Wnsp

Wns

Wnps

Wna

Unconformity (and high

Gale-silicate rocksTransgressiveSystems Tract

Sauna or restricted basincalcareous sediments andevaporites

Carbonate-cementedcoastal sands

Gale-silicate -bee ringarkose

Gale-silicate-bearing Lowstandconglomerate and arkose Systems Tract

Arid zonealluvial fan deposits

Uplift and erosion

Arkose and subarkose

HighstandSystems Tract

Coastal sands

Transitionat shell sandsand muddy sands

Biotite psarn miteand psammopelite

Shelf muddy sandsBiotite psammopeliteand psammite

Sillimanite-cordierite-biotule psammopeliteand psammite Transgressive

Systems Tract

Outer shell muddy sandsand sandy muds

Anoxic shell mudsand sands

Graphite-garnet-biotutepsammopelite and pelitewith quartzite interbeds

strain zone)

Granitoid rocks with localBasement amphibolite, metased.

xenoliths, and chsrnockie

References

Uplift and erosion

Lower Sequence: Ferruginous (garnet), anoxic (graphite), muddy sediments of Unit Wna fine upward into aluminous (clay-rich) muddy sediments of Unit Wnps. Such retrogradation is consistent with a transgressive systems tract. The quartzite beds in Unit Wna may be storm- generated flows. Less aluminous muddy sediments of Unit Wns are transitional through Unit Wnsp to Unit Wm sands. Such a progradational sequence is consistent with a highstand systems tract.

Upper Sequence: Polymict conglomerate of Unit Wo marks a period of uplift and erosion. Most clasts are derived from the underlying sedimentary succession. Although restricted to the eastern margin of Wollaston Domain, the Janice Lake Conglomerate can be traced along strike for at least 245 km. This unit is interpreted as alluvial fan deposits. Non- marine conditions indicate that it is a lowstand systems tract. An upward decrease in clastic input is suggested between calc-silicate bearing Unit Wmc and calc-silicate rich Unit Wnc. This suggests another transgressive systems tract. The calc-silicate rich rocks may be evaporitic deposits.

Unconformity

Unconformity (and high strai

LuhtMY

- I 1 coastal sands I

Gale-silicate rocks Transgressive Systems Tract

Calc-silicate-bearing

Calc-silicate-bearing Lowstand 1 Arid zone Systems Tract alluvial fan deposits

Salina or restricted basin calcareous sediments and evaporites

Carbonate-cemented

Uplift and erosion

Arkose and wbarkose

Highstand Systems Tract

Biotite psammopeiite and psammite

i imanite-cordierite- biotite ~sammo~e i i t e

Transgressive

psammopeiite and pelite

Coastal sands

Transitional shelf sands and muddy sands

Shell muddy sands

Outer shelf muddy sands and sandy muds

Anoxic shelf muds and sands

Uplift and erosion

Granitoid rocks with local amphibolite, metased. xenoliths, and charnockile

Delaney. G D . Jankovic, Z . MacNol. A . McGowan. J .and Tisdale. D . 1997. Geological investigations of the Counenay Lake. C s Lake Fold Belt and the Hi l l Lake Embayment, Johnson River Intier. Wollasion Domain. northern Saskatchewan. Saskatchewan Energy and Mines. Misc Rep 97-4, p90-101

Tran, H T and Y e o , O M . . 1997. Geology of the Burtudge Lake - northern Upper Foster Lake area, eastern Wollasion D o m n N T S 74A-14)- Saskatchewan Energy and Mines. Misc. Rep. 97-4. p72-89.

Tran. H.T.. Yeo. G.. Bradley. S. and Lewry. J.F. 1998. Geology of the My-Suti le-Middle Foster lakes area. eastern Wollasion Domajn (NTS 74A-5, -I I, and -12) Saskatchewan Enem and Mmcs, Misc Rep 98-4. p36-47.

Yeo, G.. 1998, A systems tract approach to the stratigraphy of para&sses i n the southeastern ~ o l i a s t o n Domain: Saskaichewal Energy and Mines. Misc. Rep. 984. p3647.

institute on lake superior geology - lakehead...

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