immobile element geochemistry of altered volcanics and
TRANSCRIPT
Immobile Element Geochemistry
of Altered Volcanics and Exhalites
at the Thalanga Deposit,
North Queensland
by:
Waiter Henmann B.Sc.(Hons.) c/- PO Box 873 Charters Towers QLD 4820
Submitted in fulfIlment of the requirements for the degree of M.Econ.GeoL University ofTasmania
December, 1994.
ABSTRACT
Thalanga is a defonned and metamorphosed volcanic hosted massive sulphide deposit
consisting of several thin, stratiforrn semi-connected lenses located at a sul:>-vertically
dipping contact between rhyolite and dacite formations. Rocks composed of quartz
magnetite-barite and chlorite-tremolite-carbonate, which have previously been
interpreted to be exhalites, exist in intimate stratabound association with sulphides,
particularly in the western lenses. Rhyolites stratigraphically beneath the deposit are
extensively hydrothennally altered with a stratabound stringer zone of intense
silicification containing 4-20% pyrite in the immediate footwall of the sulphide lenses,
grading outwards and downwards through progressively weaker zones of qu~rtz
chlorite-sericite and quartz-sericite alteration.
Titanium, aluminium and zirconium remained chemically immobile during alteration,
and subsequent metamorphism. These immobile elements permit identification of the
volcanic precursors of altered rocks and quantitative estimation of the chemical changes
due to alteration. Large gains of silica, iron, sulphur and loss of sodium are indicated
for the zone of most intense footwall alteration.
Chlorite-tremolite-carbonate rocks associated with sulphides have immobile element
contents and ratios identical to those of altered footwall rhyolites and their chemistry is
consistent with a derivation from rhyolite involving large gains of calcium,
magnesium, C02 and losses of silica and sodium. They are re-interpreted to be
metamorphosed chlorite-carbonate alteration assemblages which probably formed in
permeable rhyolitic volcanic1astics by hydrothermal fluid and sea water mixing at the
upper and outer parts of a mineralising sul:>-rnarine hydrothermal system..
Magnetite-quartzites have very low immobile element contents and may be the only true
exhalites at Thalanga, other than massive barite and base metal sulphide assemblages.
2
Declaration
This thesis contains no material which has been accepted for the award, to me, o(any
other higher degree or graduate diploma in any tertiary institution. To the best of my
knowledge and belief this thesis contains no material previously published or written
by another person except when due reference is made in the text of the thesis.
t 112/94
WaIter Hemnann
ACKNOWLEDGEMENTS
The Managers of Pancontinental Resources (Base Metals) Pty Ltd are acknowledged
for providing the opportunity to study the Thalanga deposit and funding for chemical
analyses and petrographic sections.
It is my pleasure to sincerely thank all the following people for their contributions to
the work.
A great deal of knowledge and helpful, stimulating discussion was volunteered by
Pancon geologists Andrew Blair, Haydn Hadlow, Anthea Hill, Mel Jones, Colin
Kendall, lan Levy, Rodney Sainty andlan Warland.
Kim Burleigh, Krista Davies and Harold Webber cheerfully provided invaluable
technical assistance in dri:llcore handling and sampling. Robin Voss expedited colour
photocopying of photographic figures.
Ron Berry, Bruce Gemmell, Ross Large and Joo Stolz, of the University of Tasmania
- Centre for Ore Deposit and Exploration Studies, are acknowledged for their guidance
and interest in the project.
Not least, the encouragement and forbearance of Danielle; Alina and Eva Hemnann is
great!y appreciated.
3
....... . .....
TABLE OF CONTENTS
Page . i
1 ABSTRACf 2 ,., .... INfRODUCIlON 7
i
3 GEOLOGY 9
3.1 Regional setting 9 3.2 Local geology 10
4 SAMPLING, ANALYTICAL MEfHODS and UTHO- 21 GEOCHENUCALGROUPINGS
5 HOST ROCKALTERATION AND GEOCHEMISTRY 23
5.1 Imfnobile element geochemistry 23 5.2 Mass changes in altered rhyolites 39 5.3 Synthesis 43
6 GEOCHEMISTRYof EXHALITES, PSEUDO-EXHALlTES 44 AND SULPHIDE ASSEMBLAGES
6.1 Immobile element geochemistry of exhalites, pseudo- 44 exhalites and sulphides
6.2 Mass ~gesrelative to a rhyolite precursor 49
7 DISCUSSION and GENErIC INTERPRETATION 54
7.1 Magnetite quartzites 54 7.2 Siliceous pseudo-exhalites 54 7.3 Chlorite-tremolite-carbonateassemblages 55 7.4 Chlorite-carbonate alteration in other deposits and .
comparisons with Thalanga 61
8 CONCLUSIONS 70
9 REFERENCES 72
10 APPENDICES 75
I AnalyticalData
II Descriptions of Rhyolite, Exhalite and Pseudo-exhalite litho-geochemical groups.
III QCf Normative calculations
4
LIST OF FIGURES
Figure No.
1
2
3
4
5
6
7a&7b
8a
8b
9a
9b
lOa lOb
lIa& lIb
12a& 12b
13a& 13b
14a 14b
15
16a & 16b
17a, b, c & d
18
19
20
Title Scale
Thalanga sub Campaspe Geological Plan and Longitudinal Projection 1:20,000
Thalanga Mine Grid and Borehole Location Plan 1:20,000
East Thalanga; geological cross section, 32320E 1:2,500
East Thalanga; geological cross section, 32120E 1:2,500
West Thalanga; geological cross section, 2011OE 1:2,500
West Thalanga; geological cross section, 19750E 1.:2,500
Harker diagrams: Thalanga volcanic rocks, Si02 vs: Ti02, A1203, Fe203, MgO, CaO, Na20, K20, P20s.
X-Y scatterplots: Ti02-A1203 and Zr-Nb; Thalanga volcanics. X-Y scatterplots: Zr-Y and Y-Nb Thalanga volcanics.
X-Y scatterplots: incompatible-compatible elements, ZrTi02 and Zr-A1203; Thalanga volcanics. X-Y scatterplots: incompatible-compatible elements, NbTi02 and Y-Ti02; Thalanga volcanics.
Photograph: altered and unaltered andesite core specimens. Photograph: strong domainal quartz-epidote alteration in dacite.
X-Y scatterplots: immobile elements for altered/unaltered andesite and dacite sample pairs.
Photograph: epidote-albite alteration in porphyritic dacite and micro-diorite.
X-Y scatterplots: Zr-Ti02; samples from TH406.
Column graphs: absolute mass changes of altered rhyolites. Column graphs: relative mass changes of altered rhyolites.
X-Y scatterplots: Zr-Ti02 and A1203-Ti02; quartz-hematite and quartz-magnetite rocks.
X-Y scatterplots: Zr-Ti02 and A1203-Ti02; Thalanga exhalites, psuedo-exhalites and sulphide assemblages.
Column graphs: absolute mass changes of psuedo-exhalites and massive pyrite relative to rhyolite. Graphic Log: TH245, 670-680m.
Graphic Log: W201INED42,102-128m.
Photograph (in Appendix II): weakly altered rhyolites.
5
21 Photograph (in Appendix IT): moderately altered rhyolite and intensely silicified rhyolite with pyritic stringer veins.
22 Photograph (in Appendix II): silicified "white" rhyolites.
23 Photograph (in Appendix II): Group 6 siliceous "hanging wall" rhyolites.
24 Photograph (in Appendix II): Group 7 foliated altered rhyolites.
25 Photograph (in Appendix II): magnetite-quartzites in TH40.
26 Photograph (in Appendix II): patch of clinozoisitecarbonate-actinolite alteration in rhyolite.
27 Photograph (in Appendix II): chlorite-tremolite assemblages, TH390B.
. !
28a&b Microphotograph (in Appendix II): sphalerite grains included in tremolite.
29 Microphotograph (in Appendix II): carbonate replacing tremolite-chlorite.
30 Microphotograph (in Appendix II): carbonate replacing tremolite.
31a& b Microphotograph (in Appendix II): carbonate with ragged relict patches of tremolite-chlorite.
32 Geological cross section of the South Hercules deposit, (from Khin Zaw and Large, 1992).
33 Schematic section of genetic model for South Hercules deposit, (from Khin Zaw and Large, 1992).
34 Schematic sections of three hydrothennal systems on Valu Fa Ridge, Lau Basin, (from Fouquet et al., 1993).
35 Schematic sections of genetic model for West Thalanga sulphide lens and associated alteration zones.
Table No. Title LIST OF TABLES
Page
1 Major and trace element analyses - Thalanga. (in Appendix I)
2 Litho-geochemical Groups. 22
3 Compositions of Precursors and Means of rhyolite and pseudo-exhalite
groups. (in Appendix 1)
6
2 INTRODUCTION
Metamorphic assemblages of carbonate, tremolite, actinolite, chlorite, epidote, barite, quartz and magnetite are spatially associated with stratabound volcanic hosted m~ssive
sulphide mineralisation at the Thalanga deposit in North Queensland. These interesting rocks have been traditionally termed "exhalites" following the original interpretation of Gregory et al. (1990) that they represent metamorphosed Ca-Mg carbonate and iron-silica chemical sediments which precipitated on the Late Cambrian sea floor from the same hydrothermal system responsible for massive base metal sulphide deposition; perhaps from the lower temperature and more oxidised
hydrothermal fluids exhaled during early or waning stages of mineralisation.
This study is an attempt at discriminating the various assemblages previously grouped as "exhalites" and interpreting their modes of formation. The method is largely geochemical with a focus on immobile element geochemistry to identify the precursors
and chemical changes of altered volcanic host rocks and their chemical relationships with so-called exhalites.
Wills (1985) and Gregory et al. (1990) observed that the "exhalites " associated with sulphide ores are composed of Mg, Ca, Ba and carbonate rich minerals and the most distal types are dominated by cherty quartz-magnetite assemblages. Wills (op. cit.) used a threefold classification as follows:
carbonate-tremolite exhalite =low SiOz, high Mg/Fe+Mg ratio; shaleyexhalite = moderate Si02, moderate Mg/Fe+Mg; siliceous exhalite = high SiOz, low Mg /Fe+Mg ratio.
He speculated that these chemical trends, particularly the MglFe+Mg ratio, reflected proximal and distal settings and could be useful exploration vectors.
A detailed study by Duhig et al., (1992) of the distal siliceous types, found that they
are composed essentially of silica and iron oxides with <0.5% alumina and that less metamorphosed equivalents from further east in the Mt Windsor Volcanics contain fIlamentous microbial fossils. These provide good evidence that the ironstones formed from crystallisation of silica-iron oxyhydroxide gels deposited in sea floor
depressions by mixing of seawater and low temperature hydrothermal fluids in distal or waxing and waning-proximal settings, more or less as envisaged by Gregory et al.
(1990). This type of siliceous ironstone is reasonably classified as exhalite - a term which carries a strong genetic connotation.
At Thalanga, however, the term "exhalite" has evolved into a large basket into which any odd rocks (i.e. anything not recognisably like the massive sulphides, metavolcanic host rocks and younger intrusive dykes) have been uncritically placed. This misuse was mentioned by Wills (1985, p37) who recognised that some rocks classed as exhalites were really myolitic "pyroclastics" that had been strongly replaced by "exhalite" type minerals in a form of primary Mg metasomatism. He nevertheless
7
applied the term exhalite extensively and not only in situations closely associated with
stratiform sulphides.
Rivers (1985) in studying the footwall alteration system, interpreted cross-cutting, pipe
like or sheet like zones of actinolite-carbonate-epidote+/-chlorite which previous workers had classed as exhalites but which she considered to represent pathways for
primary C02 bearing hydrothermal-mineralising fluids, essentially perpendicular to stratigraphic layering in the footwall of the West and Central Thalanga orebodies.
The enigma was also noted by Stolz (1989,1991) who found petrographic evidence for carbonate-actinolite alteration in footwall vitric rhyolites, suggested that the
assemblage represented thermally metamorphosed carbonate-chlorite alteration and observed that the high Ca, Mg, CO2 and lack of Na depletion in these rocks starkly
contrasted with the chemistry of the more widespread quartz-sericite-chlorite style of footwall alteration. Stolz (1991, p.55) stated that many of the actinolite rich rocks
which had previously been classified as exhalites were "clearly due to alteration processes" but had reservations about some other well banded carbonate-actinolite
barite and quartz-barite assemblages which he conceded might be genuine exhalites or mixtures of exhalites and volcaniclastic materials.
The close spatial association of carbonate-tremolite-chlorite assemblages with ore in some parts of the deposit, notably West Thalanga, has led to the presence of these
minerals being used for identification of the favourable horizon. This is a fundamental concept of exploration for stratabound sulphide mineralisation. In view of the uncertainty of origin of the carbonate-tremolite-chlorite assemblages it is important,
however, to discriminate between true exhalites which may be the lateral equivalents of stratiform sulphide ore and alteration assemblages which may fit into zoned
hydrothermal alteration patterns and provide exploration vectors of a different kind.
8
3 GEOLOGY
3. 1 Regional Setting
The Thalanga deposit lies near the western end of an E-W trending narrow belt of
deformed Cambro-Ordovician marine sediments and volcanics known as the Mt
Windsor Subprovince, (Berry et aI., 1992; Gregory et aI., 1990). The stratigraphic
sequence is grossly conformable and has an exposed thickness of upto lOkm, faces
south and comprises (in ascending order) the following major formations:
Puddler Ck Fmn. - up to 9000m of continent and volcanic derived, massive to
laminated wackes and siltstones with minor basaltic-andesitic
volcanics concentrated in the upper few hundred metres; the
volcanics have geochemical signatures typical of int~plate
volcanism (Stolz, 1994) and have been related to lithospheric
thinning and incipient back-arc basin development.
Mt Windsor Fmn. - .a 400m to 5000m thick sequence of subaqueously deposited rhyolites dominated by passively extruded thick flows, domes
and high level intrusives with subordinate volcanic1astics and
mass flow breccias. Isotopic evidence (Stolz, op. cit.)
suggests they were derived from melting of continental crust.
Trooper Ck Fmn. - a 500-2000m thick sequence of highly variable basaltic
andesitic, dacitic and rhyolitic volcanics, mass flow
volcaniclastic breccias, volcanogenic siltstones and vitric ash
units with minor calcareous metasediments and probably
exhalative siliceous ironstones. Stolz (op. cit.) interpreted the
volcanics to have been derived from melting of a variably
subduction modified, sub-arc mantle wedge and erupted during
back arc extension.
Rollston Range Fmn. - Lower Ordovician fossiliferous sandstones and siltstones of
volcanic provenance and uncertain thickness which
conformably overlie the Trooper Ck Fmn.
The rocks have experienced at least one, possibly two, major deformation event(s) of
which the most recognisable is termed D2 (after Berry et aI., 1992). This deformation
produced a large E-W trending upright fold (or folds?) and a near vertical, axial plane
penetrative cleavage or foliation called S2. The greater part of the Mt Wmdsor
Subprovince represents a single south facing limb of a large D2 antiform. D2 appears
to have coincided with or followed the intrusion ofthe Mid-Late Ordovician? gneissic,
early phases of the Ravenswood Granodiorite Complex and it produced relatively low
pressure regional metamorphic assemblages ranging from prehnite grade in the east to
upper greenschist andalusite-biotite grade in the west.
9
A locally developed ENE trending cleavage, S3, occurs in association with steep NE to ENE trending faults with south side up displacements which Berry et aI., (1992) suggested were related to emplacement of unfoliated (post D2) granitoids of Silur~
Devonian age. Post kinematic granitoids produced local contact metamorphic aureoles up to cordierite grade which are most recognisable in the eastern areas where prograde contact metamorphism exceeded the grade of the earlier regional metamorphism.
3.2 Local Geology
The Thalanga massive sulphide deposit consists of several semi-eonnected, thin stratiform lenses of pyrite-sphalerite-galena-chalcopyrite lying conformably at the near vertically dipping contact of the Mt Windsor Formation (MWF) and the Trooper Creek Formation (TCF). The geological and structural features of the deposit have been described by Wills (1985), Berry (1989), Gregory et al. (1990), Hill (1991) and Berry
et al. (1992). There is consensus that it is a synvolcanic, volcanic hosted massive sulphide deposit of the thin, extensive, layered sheet style as discussed by Large (1992).
In longitudinal projection (Figure 1) the sulphide lenses show a semi continuous, broad arcuate distribution, partly attributable to faulting, over a known strike extent of about 3000m. The maximum vertical dimension is about 400m and thickness ranges from <lm to about 2Sm with an average of about Srn. The total mineral resource is estimated at 8.6Mt consisting of:
mined production to June 1994: 2.5Mt in situ resources at June 1994: 6.1Mt @ 1.6%Cu, 3.0%Pb, 93%Zn, 77g1tAg
and OAgltAu.
The deposit is located at the foot of the eastern end of the Thalanga Range - a low strike ridge flanked to the north, east and south by an extensive peneplain which covers the basement with upto lOOm of semi consolidatedTertiary alluvial sediments known as the Campaspe Beds. Surface exposure in the vicinity of the deposit is poor and most of the geological interpretation is based on observations from RABlPercussion holes, diamond drill holes and mine development. Figure 2 shows the locations of significant drillholes mentioned in the text and Table 1; Figures 3 to 6 illustrate typical cross-sections of the deposit. The following presents a brief description of the main geological units which are further subdivided and described in greater detail in the discussion of alteration
geochemistry in Section 4 and in Appendix H.
Footwall Rhyolites Rocks in the footwall consist of massive rhyolitic lavas with probably subordinate rhyolitic breccias and volcaniclastic units. In the vicinity of the ore deposit they are
1 I
10
... lQJ
~[1::dJ~=(g~~~~~~~@j~(QJ~[QJ@3W
"'",,... "'",, THALANGA AREA SCALE: 1:20,000
"''''",,...... Chartn ro.~...
""- ... ... <";,>-.., r;;l-~53i-------- I, .. ... / /, " '.....,""",J... ... " I
"" ... ... ,~"
,19&",,,, ,'"
I '" '"
LEGEND
1ERTIAAY LJ Campaspe Beds
UIOOLE '" ~ RlM!IlIn'ood CrandalClrit.OROOIilCWl
CJ Sediments and VQlcanics
o '" Ande:!liUc: Volconict
o F'oliatecl RhyoUte }I .".~"o Oodtlc: VolQ:lni=
UPPER ~:::.:Qi :,::1 Quartz Eye TutI'C'lIBRWI ........... ~ ...... ~ Ore Horizon.; &/to(lt. to J,!Q$'Ji\'e
LO~ ..~ Sulphidu (Co$5(l" at surloco)
ORIlOIi1CWl o NSemi J.lCl!lJlW Sylphides
TO
M' ~ ------PyrilIc Rhy.rltic Volcona (Stronqal }EJ ,oct'o<!l NlO'aIlon) ;;! ~ .................. ~ ......
o Rhyorrtic Volcanie:r. Hydrathennolly ~
ottered in FoobtoU ~ ...~ ......WhIl. RhyolitaG , .... - - -- Thin- Ore Horizon Of rlnrllql"Clphic: po.Mon
WEST THALAN GA CENTRAL THALAN GA I EAST THALANGA SURFACE OPEN PIT VOMAGKA SURFACE FAR EAST
I I_ _ _. ••••••• • I.. ---_-L T~ __.
900R!. ._ , WEST EXTENDED I , , ' : ' "';"';'01/' .' I~ I , --------_900Rl. I • - ; I" , • '
aooRl. _ I _1 ' 9, • • F .' • -t-" • _ _800Rl. : n~ 't .,).~. :1 /
700R!. _ J: .- .---"",L' • ~ S • ~/ _700R!. .\It' "
I L" ..('• , : 1.....-1 - • " 600R1. _ ." '_1-"'" ,~ _soaR!.
r I
500Rl. _ i I I I ~ I
I~ I~
IN IN IN II:l
OE:\'UOP><eNT _ I (') 1(')
ORE R£S£R\I'E: BlOCl<S _ ..-'_ IlINEAAL RESOURCE: OU'ltlNE. r----....... 1110 POINT ORE ~C110N ~~[g1uO[g~~ ~(QJ~~~[g][QJe:D~[guO©J~
Figure 1
18~
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13
Geological Legend for Figures 3, 4, 5 & 6
~ ~
o o
Tc: Campaspe Beds, unlithified Tertiary terrestrial sediments.
Dol: dolerite dyke.
Dv: Dacite, mainly coherent.
. ,,
~ ~
HWF: Hangingwall Fragmental unit, sericitic siltstone with dacite clasts.
- QFP: coherent quartz feldspar porphyry.
1;~;~~~~!;!t;tit;t!1 :::::::::::;:::::::::::::::::~
QEV: quartz crystal rich volcaniclastics and breccias
Rv: Footwall rhyolitic rocks with weak quartz-sericite alteration.
Rv: Footwall rhyolitic rocks with moderate quartzchlorite-sericite alteration, 1-4% pyrite.
Rv: Footwall rhyolitic rocks with intense quartz-(sericite) alteration and 4-20% disseminated and veiny pyrite, minor chalcopyrite.
LLJ Massive sulphides.
wR: intensely silcified "white" rhyolite with <2% pyrite.
Chlorite-tremolite-carbonate rocks.
lOOORl.. lOOORL
o o Tc
900RL 900RL wR
Dv Rv
800Rl
.~.,.:.. "!••.t"
~ \
700RL
Dv
600RL 600RL
? 500RL
EAST THALANGA SECTION 32320 E 1:2500
Figure 3. Geological cross section, East Thalanga 32320E
14
lOO<lRl
r-........,...._O__ ~ o o o
Te
c
lOO<lRl
900RL
Dv
800RL
700RL Dv
?
700RL
600RL
?
500RL
Dv Rv
?
EAST THALANGA SECTION 32120 1:2500
500RL
E
Figure 4. GltogiCal cross section, East Thalanga32120E
I I
I
15
lOOORL lOOORL
900RL
Dv
WEST THALANGA SECTION 20110 E
800RL 800Rl
1:2500
?
~ ;c, Figure 5. Geological cross section, West Thalanga 20110E
16
lOOORL
z 0 0 (\J 0 (\J
z 0 0 re') 0 (\J
z 0 0 <r 0 (\J
z 0 0 If) 0 (\J
WEST THALANGA EXTENDED SECTION 19750 E
900RL
1:2500 \ 900RL
800R!.. Rv 800R!..
QEV
100R!..
Dv
600RL 600Rl
500RL 500RL
Dv
~ o If) o
400RL(\J
Figure 6. Geological cross section, West Thalanga 19750E
17
zonally altered to quartz, sericite, chlorite and phlogopite dominant assemblages with extensive minor disseminated sulphides (mainly pyrite) and pyrite-chalcopyrite
stockworks in the most intense proximal zones. Alteration and subsequent metamorphism produced considerable textural modification and often the only convincing relict fabric is of sparse, small (<.5%, <2mm) phenocrysts or crystals of quartz. Common angular-fragmental, wispy and mottled fabrics are interpreted to be
largely due to devitrification and hydrothermal alteration effects in formerly coherent lavas as described by Stolz (1989) and AlIen (1988). A systematic footwall stratigraphy, distribution of coherent and volcaniclastic units, or possible permeability control of the latter on hydrothennal alteration zones, has not yet been fully mapped
out.
Favourable Horizon Most of the sulphide lenses are associated with a group of distinctive crystal rich volcaniclastic mass flow sediments and felsic porphyries characterised by abundant, large bluish grey quartz crystals/phenocrysts. In West Thalanga a Ca-Mg rich suite of carbonate-actinolite/tremolite-chlorite rocks and quartz-magnetite-barite rocks termed Itexhalites" are closely associated with ore. These collectively comprise the Thalanga favourable horizon, of a few metres to a few tens of metres thickness, which is sandwiched between the altered MWF rhyolites of the footwall and relatively unaltered TCF dacites and mafic volcanics in the hanging wall.
Rocks composed of carbonate-tremolite-chlorite (CfC) are virtually ubiquitous associates of ore in the West and West Extended lenses, of limited occurrence in Central and absent or minor in the Vomacka and East Thalanga lenses. They range from <lm to a maximum of about 20m in thickness and are co-extensive with ore but usually do not extend laterally and vertically more than about 1.5Om beyond ore. Massive barite, with or without quartz-magnetite, of upto a few metres in thickness is
common in the favourable horizon in the West Extended, updip fringes of West, parts of Vomacka and Far East lenses, but is largely absent from Central and East Thalanga lenses.
The coarse quartz phyric rocks, traditionally known as variations on "quartz-eye" tuff/porphyry/sandstone, have been described by Wills (1985) and Gregory et al. (1990). They partly overlie and partly host the sulphide lenses and associated Ca-Mg carbonate/silicate and barite where present, at West Extended, West and East Thalanga
but are poorly developed at Central and Vomacka. Detailed observations by Sainty and Hill (in prep.) have enabled a twofold subdivision as follows. * Quartz eye volcaniclastics (QEV): massive to bedded, sometimes graded,
volcaniclastic sandstones and polymictic breccias representing mass flow deposits of pyroclastically or hyaloclastically disaggregatedvolcanogenic detritus. Oasts of altered rhyolite, massive sulphide, barite and magnetite-quartzite occur in the polymictic breccias and indicate sedimentary reworking of previously formed
massive sulphide mineralisation.
18
* Quartz-feldspar porphyry (QFP): massive, coherent, coarse quartz and feldspar phyric porphyry probably formed as flow/dome complexes or cryptodomes partly
intrusive into co-magmatic QEVs within the favourable horizon and also occurring as thick intrusive sills within the footwall and hanging wall sequences.
Within the favourable horizon the "quartz-eye" rocks extend laterally and vertically well beyond the limits of the sulphide lenses and have very variable thickness. QEV units range from <2m upto about 30In thick; QFP bodies are rarely <Srn thick and range upto SO-I5Om thick in the areas updip of the eastern parts of the East Thalanga and Far
East sulphide lenses and along the Thalanga Range to the west of the deposit. Thick coherent QFP at the favourable horizon is negatively correlated with occurrence of ore. Alteration intensity in QEV and QFP is variable but generally much weaker than that of
the footwall; the QFPs often contain fresh feldspar and may be essentially unfoliated. In some parts, notably the down dip fringes of the West Thalanga lens,strong footwall style quartz-sericite-pyrite alteration transgresses stratigraphically up into QEV and may mark foci where hydrothermal discharge continued after the broad unfocussed zone of the footwall had been partly capped off by deposition of QEV units.
Hangingwall Sequence Over most of the deposit the hanging wall is defmed by unmineralised and relatively
unaltered feldspar phyric to aphyric dacite conformably overlying the QEV/QFP unit of the favourable horizon or in direct contact with massive sulphides. Dacite units show some variations in feldspar phenocryst content but are usually massive and
interpretable as thick coherent flows. In Central and West Thalanga, the hanging wall sequence includes one or more thick
units of meta basalt and basaltic andesite which are essentially conformable, massive and appear to be sills or coherent flows. Narrow intervals of defonned mafic rocks of similar composition occur sporadically in the footwall and are interpreted to be feeder dykes to the hanging wall units. Following the traditional usage, these mafic rocks are
collectively called "andesites" in this thesis.
Volcaniclastic sediments of mostly dacitic derivation are a minor component of the hanging wall sequence in the mine area but increase in proportion westward. In drill holes TH45 and TH406 (on Section 18600E,-lkm west of West Extended) the hanging wall is a southward fining and facing sequence of dacitic lithic mass flow breccias, sandstones and massive to laminated siltstones.
A distinctive volcaniclastic sandstonelbreccia unit known as "Hangingwall Fragmental" (HWF) is widespread in the Vomacka to East Thalanga area as a layer up to 15m thick between the favourable horizon and the base of the dacite. It is characterised by coarse, matrix supported, clasts of white aphyric dacite and sparse fresh feldspar crystals (but a general absence of quartz crystals) in a foliated sericitic matrix. It is interpreted as an unsorted mass flow deposit of mixed dacitic and rhyolitic detritus,
possibly from an exotic source and introduced in a single flow unit. Like the dacites
19
and andesites of the hanging wall, it is essentially devoid of pyrite and other sulphides and does not contain clasts of massive sulphides or other lithologies of the favourable horizon.
Alteration in the coherent dacites and andesites of the hangingwall sequence is generally weak or patchy with apparent fracture control of alteration assemblages including epidote, clinozoisite, chlorite, carbonate, quartz and hematitestained albite but without significant sulphide. This marked change, from widely distributed
sulphides (mainly pyrite) in the footwall and favourable horizon, to an essential absence of sulphides in the hanging wall, is a striking feature of the Thalanga deposit. It suggests that the hanging wall sequence was not in place at the time of local massive sulphide deposition and that the contrasting alteration styles are due to different
hydrothermal systems.
Maficdykes Two kinds of post tectonic (post D2) mafic dykes occur at Thalanga.
"Microdiorites" are medium to coarse grained holocrystalline rocks occurring as near vertical dykes upto a few metres in thickness, most abundant in the East Thalanga to
Far East area. They contain primary hornblende and plagioclase with poikilitic to porphyritic textures and have sharp and often fine grained chilled margins with mainly north to north easterly trends which transect the stratigraphy and S2 foliation. Microdiorite dykes lack the S2 schistosity of the enclosing volcanics and their textures and contact relationships convincingly indicate that they were intruded after massive
sulphide deposition and ductile deformation. Wills (1985) considered them to be related, on mineralogical grounds, to post tectonic phases of the Ravenswood
Granodiorite Batholith. Microdiorite dykes commonly display fracture controlled to semi pervasive, green and pink epidote-actinolite-albite-carbonate alteration which is megascopically similar to that in the hangingwall dacite, (Figure 12). The coincidence supports the interpretation that this style of alteration is post tectonic and unrelated to the footwall alteration and massive sulphide mineralisation. It is contrary to Stolz' (1991, p.62) observation that it forms a proximal zone confined to
the hangingwall of the orebodies and the inference that it is synvolcanic alteration and a useful exploration vector.
Black, fme grained ophitic/amygdaloidal "Dolerite" occurs in one continuous dyke of about three metres thickness and associated minor dykelets which run semi parallel to stratigraphy within or in close proximity to the favourable horizon in West Thalanga and extending west along the Range. The main dyke cuts through the West Extended lens at an acute angle and, like the microdiorites, clearly post dates massive sulphide
deposition and ductile deformation. The dolerite is thought to be younger than the microdiorites; Wills (1985) noted that the dolerite dyke cuts across a group of NNE trending brittle faults and quoted a K-Ar radiometric age of ,..,,340 Ma for it.
20
4 SAMPLING, ANALYTICAL METHODS and LITHOGEOCHEMICAL GROUPINGS
This investigation of host rock and favourable horizon geochemistry is based on major
and trace element data from rocks analysed for Pancontinental Resources in 1994 and substantially augmented by data presented by Wills (1985) and Stolz (1989, 1991). Manipulation of these diverse analyses has produced a useful data set comprising of
220 volcanic/intrusive rocks and 71 favourable horizon lithologies including "exhalites" and massive sulphides. The analyses are listed in Table 1 of Appendix I with explanatory notes on methods of analyses and the nature of recalculations applied
to bring them into compatible form.
The majority of analytical samples are of unoxidised, half sawn 50mm or 36n;un diameter drill core segments, mostly of 0.3 to l.Om length, from exploration diamond drill holes in the immediate vicinity of the Thalanga deposit. A few are rock chip samples from outcrops in the railway cutting -5km west of the mine. The spatial
relationships and mineralogical and textural observations are based on detailed megascopic core logging backed up by microscopic examination of a suite of38
representative thin sections prepared for this study.
To facilitate geochemical comparisons of volcanic units and alteration types the 291
analyses have been subdivided into 28litho-geochemical groupings into which the analyses in Table 1 (Appendix 1) have been sorted. The separations are made on megascopically visible mineralogical/textural differences and, in the case of rhyolites, also on location, whether from footwall or apparent hangingwall settings.
The litho-geochemical groups are numbered (not strictly sequentially) from 1 to 45, briefly listed here in Table 2 and described in greater detail in Appendix II .
21
Table 2 Litho-geochemical Groups
Group No. Description
RHYOLITES 1 25 Least altered-weakly altered footwall rhyolites, <1% pyrite. 2 18 Moderately Qtz-sericite-chlorite altered footwall rhyolites with
mottled, foliated or pseudo-fiamme fabrics; 1-4% pyrite. 4 13 Intensely alteredfootwall rhyolites [QSeChlPy} commonly
with pyrite stringers; >4% pyrite. 5 19 Siliceous White and Grey Rhyolites in footwall; low Na, high K;
usually low pyrite. 6 21 Siliceous QFpR and Buff Rhyolite in hangingwall settings; > 1% Na20,
variable NafK, low pyrite. , 7 20 Foliated Buff and [Chl-Ser} altered Rhyolite in hangingwall settings; Si
depleted?, low pyrite. 9 11 Railway cutting (outcrop) Rhyolites.
DACITES, HWF, OFP-OEV and MAFICS 10 36 Fresh looking Fs phyric or aphyric Dacites. 12 10 Pervasively altered Dacites. 13 3 Hangingwall Fragmental unit (HWF) - bulk rock. 14 2 Clasts from within HWF unit. 15 9 QFP or undifferentiated "Qtz eye" rocks. 16 3 QEV Breccias and volcanic1astic sediments. 17 21 UndifferentiatedMafic rocks; mainly hangingwall (TCF) meta-
Andesites. ' 18 8 Microdiorite.
OTZ-MAGNEfITE-BARITE ROCKS 21 3 Magnetite-quartzite. 22 3 Quartz dominant: Qtz-Ba, Qtz-Mt with relict rhyolitic fabrics (e.g.
TH5, 158.Om). 23 6 Qtz-Ba-Mt-Chl-Act+/-Cb+/-Sulphides; in very variable proportions.
ALTERITES (not in favourable horizon) 26 2 Ac-Ep/Czo-Qtz+/-Cb alteration of rhyolite.
CARBONATE-TREMOLITE-CHLORITE ASSEMBLAGES 31 5 Chl-(Phl-Ser) schist (+/- pyrite, sulphides). 32 2 Chlorite> tremolite; <5% carbonate 33 7 Tremolite> chlorite; <5% carbonate 34 6 Tremolite + chlorite (variable); 5-50% carbonate 35 10 Carbonate >50% > tremolite + chlorite 37 2 Ep-Cb> Ac; mainly in QEV in mid-upper parts of favourable horizon.
SULPHIDES 41 5 Massive to semi massive Barite » Sulphides +/- minor Cb. 43 12 Massive Sulphide, Sp+Gn> Py+Cp. 45 ~ Massive pyrite, +/- minor Cp, Sp. 28 groups.
291 analyses.
22
5 HOST ROCK ALTERATION AND GEOCHEMISTRY
The procedures outlined by Finlow-Bates and Stumpfl (1981) and expanded by
MacLean and Kranidiotis (1987) and Macl.ean and Barrett (1993), have been us~d to
examine the host volcanics for chemically immobile elements and mass changes
associated with hydrothermal alteration as a basis for interpreting the chemistry and
origin of chlorite-tremolite-carbonate assemblages and other possible exhalites.
5. 1 Immobile Element Geochemistry
The major element data for most components (plotted as Harker diagrams in Figures 7a
& 7b) shows a broad range of concentrations within, and overlap between, the felsic
volcanic groups. The rhyolites and dacites, for instance, have the following ~anges:
Si02% MgO% Na2O% K20%
Rhyolites 56-86 0-10 0-6 1-9 Dacites 55-80 0.2-8 1.5-9 0.1-5.5 There is no relationship between Si and Mg or Na+K to indicate that these rocks
represent part of a magmatic fractionation trend; the variations point to mobility of
these elements during hydrothermal alteration or metamorphism.
However, an examination of the data suggests that Ti, AI, Zr and, to a lesser degree, Y
and Nb have remained relatively immobile during alteration at Thalanga.
MacLean and Barrett (1993) recommended that element immobility betested on
variably altered and fresh samples from an identifiable volcanic unit which can be
traced laterally through an alteration zone. In such a single precursor system, X-Y
scatter plots of immobile element pairs will form highly correlated linear trends due to
mass gains and losses of the mobile components in the altered parts. Ideally, the lines
of regression pass through the origin of the plot which represents infinite mass gain;
relative mass loss is represented by points which plot on the "alteration line" farther
away from the origin than the unaltered precursor composition. Application of this
test to alteration zones at a number of Canadian volcanic hosted massive sulphide
deposits in greenstone belts (Macl.ean and Barrett, 1993; Barrett et al., 1993;
MacLean and Kranidiotis, 1987; Skirrow and Franklin, 1994) has shown that Ti, Al
and the high field strength elements (HFSE): Zr, Nb and (to a slightly lesser degree) Y,
were essentially chemically immobile during hydrothermal alteration and greenschist
facies metamorphism.
Plots of immobile-incompatible* element pairs such as Zr-Y and Zr-Nb are useful for
identifying rocks of magmatic affInity within a volcanic sample set: they produce linear
magmatic enrichment (fractionation) trends which pass through the origin and which
are co-incident with the alteration lines for each magmatic suite. *RJOTNOTE: COli: et aL, (1979) defined an incompatible element as one which is largely excluded from the lattices of minerals crystallising in a magma and therefore becomes concentrated in (is fractionated into) the liquid. MacLean and Barren, (1993) cautioned that in felsic calc-alkaline suites where K-feldspar may be a phenocryst phase, the HFSE may become semi-compatible.
23
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Rocks from different magmatic fractionation series fall on separate
fractionationJalteration lines and the element ratios may provide a discriminant of
magmatic suites and tectonic setting (MacLean and Barrett, 1993).
Plots of immobile compatible-incompatible or compatible-compatible element pairs, on
the other hand, form separate alteration lines for each chemically distinct rock unit and
are therefore useful for discrimination and correlation of individual homogenous
volcanic units within and through zones of alteration.
The plots of incompatible pairs Zr-Nb, Zr-Y and Y-Nb, illustrated in Figures 8a & b,
show a fair amount of scatter but the linearfractionationJalteration trends referred to by
MacLean and Barrett, (1993) are discernible.
The Trooper Creek Formation (TCF) series metabasalt to dacite forms crude linear
arrays with calculated regression lines passing close to the origin and having
correlation coefficients ranging from r = 0.61 (Y-Nb) to 0.83 (Zr-Y).
If it is accepted that these TCF volcanics from the Thalanga hangingwall stratigraphy
do form a single magmatic differentiation series then the significantly lower r-value for
Y-Nb compared to Zr-Y and Zr-Nb suggests that of these three, Zr was possibly least
chemically mobile,
Two interesting asides, concerning magmatic affinities and origins, may be gleaned
here from the immobile-incompatible plots. The range of Zr concentration ofTCF
dacites is similar to that of the rhyolites, most of which are from the Thalanga footwall
and are without argument assigned to the Mt Windsor Formation (MWF). Although
substantial overlaps and a few scattered outliers exist, the rhyo1ites in general have
significantly higher values of Nb and Y than the dacites; i.e. they have higher Nb/Zr and YIZr ratios which can be taken as an indication that the two sets are not co
magmatic. This accords'with the interpretation of Stolz, (1994) that the MWF and
TCF have different partial melting sources; namely continental crustal and subduction
modified mantle melting, respectively. Furthermore, there is no indication in these
plots that the so-called Hangingwall Rhyolites (Groups 6 & 7) have any closer affinity
to the TCF dacites than do the other rhyolites (Groups 1 to 5) which are undoubtedly
from the footwall and MWF. This is consistent with, but not proof of, the
interpretation that the Hangingwall Rhyolites of the Central and West Thalanga
deposits are equivalent to less altered parts of the footwall rhyolites and have been
structurally emplaced.
The plots of compatible-incompatible element pairs Zr-Ti, Nb-Ti, Y-Ti and Al-Zr,
(shown in Figures 9a & b) display to varying degrees the radial alteration lines
satisfying MacLean and Barrett's (1993) test for chemical immobility.
Each of the plots shows a relatively tight alteration line for the collective rhyolites and
rather broader scatter, sometimes barely recognisable as linear alteration trends, for the
dacites, metabasalts and 'other groups.
26
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0.4 - --- - - - - - -- --- -- - - ---- - ----- -- - -- --- ~ - ,- ------m-" <-- Jd!I -- - --- - -- -------- - --- --
[ EB",,~db x 1 0 ~ "" ~' .: All Rhyolites
";" x -0 CD x: r = 0.6320.2 ----------------- ----------------/~-r------ -~------'-----------------------------------
... "" : 10 • :p :.0 0 ,,+~A6. • _ oort0
" .. 0.0
0 5 10 15 20 A120J % 25
• Gp I wk alt Rhyolite 0 Gp 7 ser-cW alt 'HW' Rhy ~ Gp 17 Andesites 0 Gp 2 mod all Rhyolite T Gp 9 Railcut Rhy ~ Gp 18 Microdiorites
+ Gp 4 intensely alt Rhy Ell Gp 10,12 Dacites 6. Gp 5 Si alt WRhy x Gp 13,14 HWF & c1asts
• Gp 6 Si 'H\V' Rhy 0 Gp 15,16 QFP & QEV
30 Nb ppm
All Rhyolites o 0 0
25 ----------------------~---------------~-------~------------------ ___ ~---------!-7-Q·~----, ,
o 0.- 0.: - - - - - - - - - - - -- -- - -- - - - - ~ -,- - - -+- ......... -- -0-". -- -'- ----- -- -.. --<>- -- l._ - -,7' .....::. - - -- - - - ---- -- -20
: '. • 0 :- .0.•
_---::. A .. All TCF 10-17 , c -- : r=O.nl : ."00 -,
15 ----------------------~----~ ;trOl-----T---~)f------~~-------------~----------------------+AA : 0lM. 0 T II:8Ell ~ _--
, ~+T .6. _--\- Ell : + + ~ lDEHIIBB_-/ EIB ,
~..... 00-" : : - - - - - -!Y--:f) - -BE> ~ -w-----)(- ------Hl- -- - -- - - - - - -- - - -7 - ---- - - - - - - - -- -- - - - -- 10
6. 'Ell _"'Ell m Ell m , , ~ + 6. :j-H- 0 x m IIIlII ~ '* ~ _--,~ ~x Ell Ell Ell
$ $ -- : , , 5 ----~-;~::------~-------iB---------------~-----------------------~----------------------
o$$ -' , ,-$....-: : :
----'ft i j
0 0 100 200 300 Zr ppm 400
• Gp I wk alt Rhyolite 0 Gp 7 ser-chl alt 'HW' Rhy $ Gp 17 Andesites 0 Gp 2 mod alt Rhyolite T Gp 9 Railcut Rhy ~ Gp 18 Microdiorites + Gp 4 intensely alt Rhy Ell Gp 10,12 Dacites A Gp 5 Si alt WRhy X Gp 13,14 HWF & c1asts
• Gp 6 Si 'HW' Rhy 0 Gp 15,16 QFP&QEV
Fi&ure Sa. Ti02-Al203 and Zr-Nb scatterplots for Thalanga volcanics with linear regression lines and "r" values for rhyolites and Trooper Creek Formation rocks.
27
100 r-------r----r-----r---r-----,----r---- ------. Y ppm
; I ,
75 ----------------------;------------------------:-----~-----------------;----------------------
, , .
50
25
ol:.-~~...:I::L. ____'~..L_ __....L. .........____J ~____J
o 100 200 300 Zr ppm 400
• Gp 1 wk alt Rhyolite 0 Gp 7 ser-chl alt 'HW' Rhy $ Gp 17 Andesites [] ..,Gp 2 mod alt Rhyolite Gp 9 Railcut Rhy ~ Gp 18 Microdiorites + Gp 4 intensely alt Rhy IB Gp 10,12 Dacites 6 Gp 5 Si alt WRhy X Gp 13,14 HWF & clasts + Gp 6 Si 'HW' Rhy 0 Gp 15,16 QFP & QEV
3Or----T""-......---T""-..-----r-..---.--..---.---r-.....,....-~.....,....-...__.......,...-T__.......,...-..__.......,...__... , //
Nb ppm :AII TCF 1O-~7///
o 0 : 9 0: r=0.6}J/ _- - - - ~ - - - - _- - - - - - - - - - - ~ e:I- - - - - - - - - - - - - - - - - - - - - - -:- - - - - - - - - - - - - - - - - - - - - - - - ~ - - -....-? c!' - - - - - - - - - - - - - - - 25
~ .... /
............. : : @ c. O. : 0 //,
------------- ------- --~o-.-.-,- -.-------.- __ -: -~ 0 - -- - - - - - ~ / ~~-- -- - - - -t --------_·~U3·!lY5~1!~~~20 , •• o· ,r=0.4270+6;"'. • IIC .. :
, 0 [].., , I .... I I
C .., c.'•• 0 .. ClY'. , _____________ - - I _ -.:J-tJ=I6---J:t~~-::: _ ~ _+ __:
15 • ~ IB EBJ/lI:l / / 'IQ : .
+ +: 6 'ft9.~/ .: , o I!iIB + c:c ~E() <:IB .-r. IB : :
+ $:A:r6. : : ---- -----E9G1+!Bl-~lS-- - -"'--IB- - ------ ---- - -;------ --- - ---- - --- --- - --~- ------ - -ilI-- --- - - - --- 10
61B IB // IBIB IB IB ~ ~ / 6 m:B::$ IB !BC IB $~~~. IB IB ID(
$ .$'~: , .:-: - $ - - - - - - - - - ea- --- ~ ------------------------:- -----------------------;--------------- ------ 5 -
$$:~
' , : $ ~ ,
0 0 25 50 75 Y ppm 100
• Gp 1 wk alt Rhyolite 0 Gp 7 ser-chl alt 'BW' Rhy $ Gp 17 Andesites ..,c ~Gp 2 mod alt Rhyolite Gp 9 Rai1cut Rhy Gp 18 Microdiorites + Gp 4 intensely alt Rhy IB Gp 10,12 Dacites 6 Gp 5 Si alt WRhy X Gp 13,14 HWF & clasts + Gp 6 Si 'HW' Rhy 0 Gp 15,16 QFP & QEV
Figure 8b. Zr-Y and Y-Nb scatterplots for Thalanga volcanics showing linear regression lnes and "rn values for rhyolites and Trooper Creek Formation rocks.
28
25...-------,r-------r-----r---~---__._---__r_---.....,....---.,.,
Al203 %
20
15
10
5 +
OL-----'-----"'-----""'--------'-----'-----'-----L----......l o 100 200 300 zr ppm 400
• Gp 1 wk alt Rhyolite o Gp 7 ser-chl alt'HW' Rhy Gp 17 Andesites o Gp 2 mod alt'Rhyolite T Gp 9 Railcut Rhy Gp 18 Microdiorites + Gp 4 intensely alt Rhy lB Gp 10,12 Dacites l1 Gp 5 Si alt WRhy X Gp 13,14 HWF & clasts
Gp 6 Si 'HW' Rhy o Gp 15,16 QFP&QEV• 1.2 r------,..-----.-----r-----r------,------r----r-------,
Ti02 %
1.0 ----------------------~------------------------------------------------+----------------------
v Vv , :
0.8 - -- --:-'!'l----------~~, ---- -------- -------- ----- ---- ---- ------- -- -- -~ ----- ---- -- ----- -----$$' ,
$$ '. lB lB , ______________________ ~------r------------------------- ~--- _0.6 lB '
$ $t $ lB !, ?\lB lB $ : ilImlB am :
0.4 -----~---------------~-----()-~--~--- JB---------------------------- 7---------------------
: lB 0 81B ~)( 0 ' All Rhyolites '" 0 ;0 lB Ox .' r=O.766
0.2 ----------------------~------~---(}~--------()~--------------------_: ------------------00 • '
T· :
0.0 0 100 :::WO 300 Zr ppm 400
0• Gp 1 wk alt Rhyolite Gp 7 ser-chl alt 'HW' Rhy '!' Gp 17 Andesi tes 0 Gp 2 mod alt Rhyolite T Gp 9 Railcut Rhy v Gp 18 Microdiorites + Gp 4 intensely alt Rhy lB Gp 10,12 Dacites l1 Gp 5 Si alt WRhy X Gp 13,14 HWF & clasts
• Gp 6 Si 'HW' Rhy 0 Gp 15,16 QFP & QEV
Figure 9a. Incompatible-compatible element plots Zr-Ti02 and Zr-Al203 for Thalanga volcanics, showing linear regressions for all rhyolites combined.
29
- - - - - - - - - - - -- - --
----- -- ---
------ ---- ---- --
---
Ti02 % , ..
1.0
0.8
0.6
0.4
0.2
0.0
, , --------------~---------------~----------------~---------------, , , ..• -- -- -~-~-- -- ----~--~--
,
--- --------$+--- --t- -- -- -----+- -- .~' , ,""'"
4> 40 ~
-$---
$
" , : Ell $ 40 i;I : Ell
- - - - - - - -, - --- -~ - - - - -- - - ~ - - - - - - - - - - - - - - - -:- - - - - , ,Ell,
$: Ell Ell: $4o$ I::
, 1I:l!!l --- - - - --"- -- -- H1- - - -- -- -(5--
ffi ,Ell W Ell Ell
. e Ell 6 : x 0 8------ --,- ----- ---------, -,
All Rhyolit;es r= 0.593
0 5
• Gp I wk alt Rhyolite a Gp 2 mod alt Rhyolite
+ Gp 4 intensely alt Rhy A Gp 5 Si alt WRhy
• Gp 6 Si 'HW' Rhy
1.2 Ti02 %
• , ,
+,
10
I i EIl;EIl
'" ' --"'- - -- -11- -:- ----m 0
~
<> ... Ell
x 0
Ell ' ' : x----0 - -,- - - -- --- -a
I ; ...
15
o
0
20
1.0 ----------------------~------------------------I------
, . , . , ,
~
+
.~ ,
---------------~--------------
Gp 7 ser-chl alt 'HW' Rhy Gp 9 Railcut Rhy Gp 10,12 Dacites Gp 13,14 HWF & clasts Gp 15,16 QFP & QEV
, , ,
Ell
25 Nb ppm 30
Gp 17 Andesites Gp 18 Microdiorites
_
0.8 ------~--~--------~-----------------------~-----------------------~----------------------$ $> , . ; :'*' . ,
t ;Ell Ell. :
0.6 -------~-------------~------------------------~-----------------------~----------------------Ell, , , , ajE ,
~ Ell EBB Ell Ell : ,m ' E!l . , ~ Ell EJPl :'\! Ell: :
0.4 ----t 5-----~ ---~ ---_ElL _ElL - - - - - - - - - - - - - - -:- - - - - - - - - - - - - - - - - - - - - - - --:- - - - - - - - - - - - - - - - - - - - - -
~ ill EIlffi!!!BEIl . .
$ ?Jao ~ x e • : : All Rhyolites
--------------------E)~-~---~------~---()-----~--------- ~----------J-~-Q,~Z9---0.2 <> <> ... ...: :y t<>~a.l.:J~~~-1:~<>~::>--.-----.-~<>~:--------l<> 6t-6 <>~ '. !ill'" • ~ +AT Ao.0 L:!:+~.L...___'____::l"'_____'__....L...___''__.........._.L...___'__...I._......._"''''____'_......._L...___'__...l._........_"''''____l
o 25
• Gp 1 wk alt Rhyolite c Gp 2 mod alt Rhyolite + Gp 4 intensely alt Rhy A
• Gp 5 Si alt WRhy Gp 6 Si 'HW' Rhy
50 75 Y ppm 100
<>
... Ell
x 0
Gp 7 ser-chI alt 'HW' Rhy Gp 9 Railcut Rhy Gp 10,12 Dacites Gp 13,14HWF & clasts Gp 15,16 QFP & QEV
$,. Gp 17 Andesi tes Gp 18 Microdiorites
Figure 9b. Incompatible-compatible element plots Nb-Ti02 and Y-Ti02 for Thalanga volcanics, showing linear regressions for all rhyolites combined.
30
The correlation coefficients for calculated (least squares) linear regression lines of the combined rhyolites (Groups 1 to 9) range from r =0.766 for Zr-Ti down to r =0.326 for Y-Ti. This tends to suppOrt the earlier inference from incompatible pairs thatZr was less mobile than Nb and Y. On the shaky basis of rhyolite alteration line correlation coefficients alone, it would seem that Zr, Ti and Al have been less
chemically mobile than Nb and Y although this could be partly due to the relatively low concentrations and consequent lower analytical precision of the latter two.
Even so, the "r" correlation values of rhyolite alteration lines for the Zr, Ti and AI combinations fall considerably short of the values anticipated by MacLean and Barrett (1993) of r = 0.90-0.99 for single precursor data sets. The Thalanga rhyolites, however, are not from a single precursor homogenous volcanic unit. This rhyolite data set includes samples from about 6.5km of strike length (from Thalanga r~lway
cutting to East Thalanga) and about 3DO-4OOm of stratigraphic thickness and undoubtedly combines various lava and volcaniclastic units of the footwall stratigraphy which has not yet been unravelled due to the alteration overprint and lack of exposure. Given this, it is remarkable that the rhyolites fall on fairly tight alteration trends and suggests considerable magmatic uniformity in the footwall rocks whatever their lithofacies variation may be.
The broader scatter for lithotypes of the TCF suggests greater variations in magma compositions which may be attributable to fractionation. In general, the TCF volcanics, existing as they do in the hangingwall sequence, are less hydrothermally altered than the rhyolites of the footwall and, having suffered lesser mass gains and losses, are less radially strung out along their alteration lines. Two or more compositional groups can be inferred for the dacites from the immobile element data but the spatial distribution of these samples has not been examined to check if they do split into stratigraphically sensible units.
The nearest approach to testing for immobility in a single precursor system, which has been attempted in this study, involves pairs of adjacent altered and unaltered segments of core from TCF dacite and andesite in which hangingwall style epidote-quartz-(albitecarbonate) alteration is strong and pervasive but patchy on a several centimetres to metres scale and is confidently interpreted to occur within previously homogenous volcanic units. The two pairs tested are;
Sample Nos: 60209 & 60210 from 279.0-279.9m and 280.1-2803m in hole TH407; these are, respectively, epidote-albite-(actinolite-carbonate) pervasively altered and relatively unaltered metabasalt (Figure lOa) in a >3Om thick unit stratigraphically overlying the favourable horizon updip of the West Thalanga
Extended sulphide lens.
Sample Nos: 60224 & 60225 from 7935-79.65m and SO.3-80.4m respectively, in drill hole W2007NED70; (depicted before sampling in Figure lOb). These
31
Figure lOa.� Photograph of altered and unaltered "andesite" samples 60209 & 60210
from TH407 showing sJ1arp boundary of pervi:lsive epidote-aJbite
(carbonate-act! noli te) alte ration..
Figure lOb.� Diamond drjlJ Cafe from W2007NED70 (bl:'fort' sawing of si:lmples 60224 &
60225) showing strong, dO!11Clinaf, partly fracture contmlled, epidote
quartz <lJteration in fresh dacite.
32
samples are from unaltered and pervasively epidote-quartz altered dacite, situated
about 30In stratigraphically above the West Thalanga ore horizon at -530RL on 20060E.
The two sample pairs are highlighted on X-V plots for supposed immobile elements in Figures lla & 11b. They fall close to radial alteration lines passing through the origin on the Zr-Ti02, Zr-A1203 and (less consistently) A1203-Ti02 plots and appear to satisfy the MacLean and Barrett (1993) test for chemical immobility. The altered samples always plot closer to the origin implying bulk mass gains and relative dilutions of the immobile elements by alteration. The plots with combinations of Nb and Y, on
the other hand, show some very peculiar alteration lines connecting these sample pairs suggesting, as inferred above, that these elements have not been immobile or are
subject to analytical/sampling errors due to the low concentrations, or both. Consideration of the major element data for these sample pairs (Table 1, Appendix I) even without calculation of mass changes, points to major addition of Ca and C02 with consequent relative dilution of most other components as the main chemical changes associated with this type of hangingwall alteration. It is quite different to the Ca, Na depletion and moderate to strong Si, Fe and S addition characteristic of the much larger scale of footwall alteration.
There are two lines of evidence to suggest that the hangingwall style of Ca metasomatism postdates the footwall alteration and massive sulphide mineralisation. 1 In the dacite it often occurs as selvedges to veinlets which appear to cut across the
main foliation (S2 of Berry et al., 1992) and the selvedges sometimes have feathery outer margins suggesting that alteration has been controlled by fluids pervading outwards along the pre-existing cleavage; e.g. Figure 12a.
2� Megascopically similar, green and pink, vein associated to pervasive epidote-(albitecarbonate) alteration is common in microdiorite dykes which cut across and clearly
postdate the footwall pyritic stringer alteration zone, massive sulphide lenses and S2 forming deformation event; e.g. Figure l2b.
The contrasting styles offootwall and hangingwall alteration are, therefore, not necessarily comparable in terms of fluid chemistry and mobility or immobility of elements.
Nevertheless, it seems reasonable to conclude that: *� Precursor Zr, Ti and Al ratios remain well preserved despite strong hangingwall
alteration involving substantial chemical, and virtually total mineralogical changes, and can be reliably used to infer the parent rock type in rocks otherwise altered
beyond recognition. * Nb and Y behave erratically in the same kind of alteration system and are best
excludedfrom rock classification and alteration mass balance calculation exercises.
33
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VI
Figure 1221� Sawn core specin18n5 or porphyr·jtic: dacite showing vein epidote-aJbite alteration se]vedges witll fea.tl1ery margills suggesting alteration pervaded outw;wds along pre-existing \Neak S2 cleavage
Figure'12b� Semi pervasive Ciod vein selvedge epidote-a Ibitei:llteri.1tion SilllilL~r to that in dacite (above) but here oc:cuning in post ])2 deformabcm microdiOlite dyke. Also shows nn.1hip]e phases of mafic magma injection with f<:=ldspar porhyritic type intruded by finer gTained, more mafic phase
36
Mention has been made of the difficulties of applying the single precursor test for immobility in the footwall alteration zones and the fair, but not excellent, correlation
coefficients obtained for alteration lines regressed from the Thalanga wide rhyolite data set. However, it has been found that narrowing down the areal extent of a foot~all
rhyolitic sample set will obtain considerably higher correlations. This is exemplified by the Zr-TiOz data from TH 406 which is an 804.7m deep hole drilled southwards under Thalanga Range, 2km along strike to the west of the mine (Figure 2). Every tenth metre of the core was sampled and analysed for Zr, Ti (and a number of other major elements including Na and K but full major element analyses, notably Si and Al, are not available). It yielded a population of 54 footwall rhyolitic rocks ranging from weakly altered to strong pyritic stringer zone alteration types (lithological Groups 1,2,4 & 5), 5 samples of coherent Quartz-Feldspar Porphyry
(Group 15),3 samples of associated foliated quartz-eye volcaniclastics (Gro~p 16) and 21 samples from the stratigraphically overlying dacitic vo1caniclastic and siltstone sequence.
The Zr-TiOz scatterplots in Figures Ba & b are notable for the significantly higher correlation coefficient of r =0.883 for the combined footwall rhyo1ites (compared to r =0.766 for the 127 rhyolites plotted on Figure 9a). The TH406 rhyolites do not necessarily represent a single precursor unit either; the samples are derived from a stratigraphic interval estimated at about 200m and a down dip extent of about 500m but their tight correlation and clear separation from the other lithotypes, despite significant quartz-sericite alteration, supports the conclusion that Zr and Ti have remained essentially immobile during footwall alteration and metamorphism.
There is nothing in the accumulated geochemical data to suggest otherwise and TilZr ratios have been used with confidence for the classification of volcanic rocks and interpretation of magmatic affinities in the Thalanga area; (e.g. Stolz, 1991; Sainty and
Hill, in prep.)
In general, footwall MWF rhyolites are easily discriminated by their low TilZr and TilAl ratios and the tendency to fall close to consistent alteration lines on immobile compatible-incompatible plots. TCF dacites have greater scatter and are clearly separated from rhyolites on all plots having TiOz as one variable.
Rocks such as the quartz-feldsparporphyries (QFP), quartz-eye volcaniclastics (QEV) and the distinctive Hangingwall fragmental unit (HWF), which mainly occur in the favourable horizon between the MWF and TCF, have immobile element element concentrations which partly overlap with the dacites and partly fill the gap between dacites and rhyolites. This is consistent with the interpretation that these may be hybrids originating by mixing of magmas, in the case of the porphyries, or mixing of detritus from separate volcanic sources in the case of volcaniclastics. The relationships are well illustrated by the Zr-TiOz plot for TH406 (Figure 13a) which shows the coherent QFPs (of which two samples are from a lOm thick, sill like unit enclosed by
rhyolites in the footwall) plotting between the dacite and rhyolite fields. The quartz
37
1.2 .-----....---r---.~)___r_-""'T'""-__r--....--..,.....-....,..-__,,.--.....--..,
Ti02 % , "
------------- ---------------1--------------- --------------,---------------,------------~-1.0 , "
, " , , , , , ,
, " ----- - -- --- -- - -- ------ ------._-- ------ - --- -- - --- --- -- ----- ... -- - -- - --- -- -- --'----- -- --- --- 0.8 , "
·· .· ,"
· . 0.6 ------------- -------------~--------------- --------------~---------------:--------------
o 0
~ 00 : :0 - - - - - - -- - - - - - --- -- - - ---- -- --:---0<>------ - -- - - -- ----- --- - - - ,-- ------ - -- -- --1- --- - - - - - --- -0.4 o o. : All Rhyolites :
<0 . r=O.883 : --- ----- ---- ---- ----- -- ---cP---- 0 -- --, -- - - --- -- --: ---- -- - --- -----;- ------ ----- -0.2 , , , ,
0.0 '--_=-_---'-__"'--_...1-_--'__"""'--_........__'--_...1-_---1.__"'--_.....
o 100 200 300 400 500 Zr ppm 600
Rhyolite Gps. 1.2,4,5 o QFPGp 15 o - HW Dacitic V/clastics $ QEVGp 16
Figure 13a. Zr-Ti02 plot for analyses from TH 406, Thalanga Range; showing linear regression and correlation factor "r" for all rhyolites combined.
0.6 .---r----.--.-----.--'"-r---r----.-....-o--r""-or---.----,r---r---r!-....---..-...---.-__., Ti02 %
,. ,
0.5 ----------------------~----------------------~----------------------~---o---------~------
, ~ 0
O.~ ----------------------~----------------------~--------~--o---------~---------------------
o o :
0.3 f---- --- ---- ------ ---- -~--- -------- - - --- ----- -~-- - --- i ----- ---- -----~--- -- -- ------ -------.-: 00 : : : : 0 ; .
0.2 ~---------------------~-----------------()---~---------------------~----------------------
, $ $, , I I IC
0.1 ~-- --- -~ ~-~-\-[~Uf~B.--A--¥----t~-"----- -- -~ ---------- -i------------ ------- --tAN" • ,
: : : Zr ppm I i i0.0 I-..__ .........._...a_.......L__'___-'-........._-'--....I---I._-'---'---I_...a_........._'___-'-.....L._~_'_
100 150 200 250 300 Gp 1 wk alt Rhy 6. Gp 5 Silicified white Rhy o QFPGp 15-c Gp 2 mod alt Rhy 0 HW Dacitic V/clastics $ QEVGp16
+ Gp 4 intensely alt Rhy
Figure 13b. Same data as Figure 13a with Rhyolites subdivided into litho-geochemical Groups and axes stretched for clarity.
38
. ~ •...:.-
eye bearing volcaniclastics between 644-678m, which apparently overlie coherent
QFP, have Ti/Zr ratios similar to footwall rhyolite implying that the latter was the
dominant source of volcaniclastic materials.
The volcaniclastic sediments further "up" in the hangingwall sequence mostly have the
Ti/Zr signature of dacite. The exceptions include two samples of unusually high Zr
content which approach the rhyolite alteration line (~5OOppm Zr) and one of 1.2% Ti02 content from near the end of the hole, which appears to reflect an input of mafic volcaniclastic materials near the top of the intersected sequence.
5.2 Mass changes in altered rhyolites
The immobile compatible-incompatible plots (Figures 9a,b) show systematic
geochemical variations for the rhyolite groups. Despite substantial overlaps between
groups, there are qualitative drifts, progressive along the alteration lines towards the
origins of the plots, from the least altered to the most intensely altered footwall rhyolitic
rocks (Groups: 1,2 & 4) which imply significant mass gains associated with this type
of alteration. Group 7 foliated rhyolites, on the other hand, plot with higher immobile
element concentrations (away from the origins) relative to the Group6less altered
rhyolites with which they are closely associated. This indicates relative mass loss
associated with Group 7 type phyllosilicate alteration.
In single precursor systems, the relative mass changes can be quantified by calculations
based on the dilution or concentration of a single immobile component "monitor"as outlined by Maclean and Barrett (1993). Alternatively, the Isocon method employed
by Gemmell and Large (1992), after Grant (1986), provides a graphic means of
estimating mass changes based on several immobile elements. Both methods rely on
determination of an unaltered or least altered equivalent rock - the "precursor" of the
altered rocks.
.Such calculations are not straightforward for the Thalanga footwall alteration zones
because of the difficulty in identifying a single precursor unit which can be traced from
unaltered into altered zones. Nonetheless, the apparent primary geochemical
uniformity of the footwall rocks (despite probable variation of volcanic facies) suggests
that the mass change calculations may be employed in a general way to provide at least
approximate indications of gains and losses of the mobile components.
Three possibilities fora rhyolite precursor composition are obvious:
* Least altered footwall rhyolites in Group 1. * Freshest of Group 6 "hangingwall rhyolites" which are interpreted to be more or
less equivalent to Group 1 representing structurally emplaced, weakly or unaltered,
footwall rhyolite. * Group 9 railway cutting rhyolites which are from the footwall sequence several
kilometres west of the Thalanga deposit and outside its alteration zone.
39
In this study, three separate precursor compositions, designated Precursors: 1,6 and 9
respectively, have been constructed by averaging the compositions of several more or
less subjectively chosen samples from each of the likely groups. The main selection
parameters included presence of relict feldspar, lack ofNa depletion, Alterationlndex
<70 (Ishikawa et al., 1976), Na20lK20 ~ 1 and moderate levels of A1203 and Zr.
Table 3, in Appendix I, lists the three estimated possible precursor compositions, the
samples from which they were derived and the mean compositions and standard
deviations of the seven rhyolite groups.
The absolute and relative mass changes of all major components have been calculated
by the MacLean and Barrett (1993) method, using Zr* as the immobile monitor
element, and are shown graphically in Figures 14a &14b.
The calculated absolute mass changes are presented in units of g/100g, (i.e. grams of a
component added to or removed from an original precursor bulk mass of 100grams).
The relative mass changes are presented as multiples of the amounts of components
assumed to have been present in the unaltered precursor, (i.e. if an unaltered bulk
mass of lOOg of precursor contained, say, 60g of Si02 a relative mass change of 0.5
would indicate that an additional30g of Si02 had been added to the bulk rock; a
negative relative mass change factor indicates removal of a component).
The mass changes show similar patterns for all three model precursors in most major
components except AI203, suggesting that the estimates of the three precursor
compositions are equally valid. A1203 shows small gains for all groups with respect
to Precursors 1 and 9 and, contrarily, small losses with respect to Precursor 6.
However, the approximate constancy of A1203 relative mass changes across all rhyolite
groups supports the previous interpretation that it was essentially immobile during
alteration and hence the problem is likely to lie in the choice of precursor compositions.
A possible cause of the discrepancies could be plagioclase phenocryst fractionation. Plagioclase alumina contents range from 19.4wt% (albite) to 36.6wt% (anorthite)
which are significantly higher than the average concentrations in the rhyolites and
therefore variations in plagioclase phenocryst contents may account for the calculated
alumina mass changes of about 2-4 g/100g.
Notwithstanding that, it is apparent that the main chemical changes in the progressive
footwall alteration (Groups 1,2 and 4) involve substantial additions of Si, Fe and S
and depletion of Na, and that these changes are consistent for all precursors.
Footnote: The Maclean and Barrett (1993) method has been applied here with confidence in the immobility of Zr. It may be argued. that the Isocon method has an advantage in that it is not solely dependent on the altered/unaltered ratio of a single immobile "monitor" element but takes for its mass change calculations the gradient of the line of best fit through a series of plotted points which ideally lie on a straight line passing through the origin if the points represent immobile elements, (Grant, 1986). In this study, however, it was found that the elements suspected of being immobile did not clearly plot on single lines and a degree of SUbjectivity was required to place the isocons despite Grant's (1986) assurance that they could readily be determined by inspection. It is considered that the tests for immobility outlined MacLean and Barn:tt (1993) are more useful in that they are able to simultaneously examine a large suite of geochemical data from alteration zones of varying intensity.
40
!>Il 50 8 Absolute mass changes of Rhyolite 00 ___g~~_,!p_s__~()~~~!"_e_f! _~~__~_~~~~_~~~ __~ . Cl ~p_! ~ __ .c:rp_~ _1--40
~ Gp2 ~ Gp7 30 f-- -----------~---------------------------------------------------------------------------------
• Gp4 g Gp9
20 f-- -- ---- ----- ----- -- ----- --- ----- ----- ----' ---- ------ -- --- -- ------.-.- Gp-5- --- ------ -- --- ---
10 !:
11 ,:' -iillllfkr !]' m. ~ ~1lI "0 I-' --!lFU
-10 ,I I I I I I I I I I ,...,N N ,..., '.,J'""' '-' '""' '""' '""' V'l :zl c Olj0 Q ?::l ?a
,..... ~ '""' e;Z; ?::l 2 2 ~ z'" ~ :< 2:
C>Il 50 8 Absolute mass changes of Rhyolite Qo- _______g...~_,!p~__<:()!J11!~~_f! _~~__~_~~~~_~ ...__~ Cl __ .c:rp_ ~ ~ __ .c:rp_~ _
4O
!iJ Gp2 ~ Gp7 30 --------------------------------------------------------------------------------------------
• Gp4 ~ Gp9
20 ----------------------------------------------------------------~--Gp-5-------------------
10
o
-10
!>Il 50
§ Absolute mass changes of Rhyolite 00 __ ~t:"~~p~__~l?!1l1!~~_f! _~~__J.>_~~~~~~_ ~ __ . Cl __ .c:rp_! ~ __ .c:rp_~ .
40
rn Gp2 ~ Gp7 30 ---------------------------------------------------------~-----------------------------------
• Gp4 ~ Gp9
20 ---------------------------------------------------------------~--(}~-5-------------------
10
o
-10
Figure 14a. Absolute mass changes of Rhyolite Groups 1 to 9 compared with "Precursors 1, 6, and 9". Mass change units are g/100g.
41
----------------------------
----------------------------
----------------------------
5 Relative mass changeso Gp 1 ~ Gp6 . Qf__RbY_Qlite__ g1:011PS _
4 ---------------------------[ill Gp 2 ~ Gp 7
• Gp4 g Gp9
o
-1 N N M M Q 0 0c 0 on0 l~N;;j ?2 N -' ~ ~ ~
<:)'< ....
5 Relative mass changes ~lo Gp 1 ~ Gp6 . . oLRhy.nl.i1e._ groups )~
4 ---------------------------compared to Precursor 6 11
~ Gp2 ~. Gp7Cl ~ 3
£ • Gp4 ~ Gp9
'02 -1IIt---------------------- Q) IIIIIl Gp5 0.. ";:I
:; 1 8
o
-1
5
Relative mass changeso Gp 1 ~ Gp6 ___oLRhy.ol.i1e. _group£. _4 ---------------------------
compared to Precursor 9 Cl mill Gp 2 ~ Gp 7
£ ~ 3
• Gp4 E3 Gp9
'02 -lIIr--Cip:s---------------Q)
0...;:1
:; 1 8
o
-1 o ~
Figure 14b. Relative mass changes of major components for Rhyolite Groups 1 to 9 compared with "Precursors 1, 6, and 9".
42
Taking the Precursor 1 case as an example: Si02, Fe203 and S combine to produce an
absolute gain of 62g/100g for the average Group 4 intense pyritic stringer zone alteration. This implies a substantial volume increase which could be partly, but not entirely, accounted for by the stockwork of pyrite veins. Sodium depletion only
slightly offsets this gain with a mass reduction of3g/100g which in relative terms represents a virtually complete loss of this element reflected in feldspar destruction. It is notable that the Na loss is also essentially complete in the Group 2 zone of moderate alteration even though this has gained only about 17g/100g from Si02, Fe203, MgO and S.
The Group 5 rhyolites on average appear to have gained even more silica than Group 4 footwal1 stringer zone alteration (35 to 50 g/lOOg depending on choice of precursor
composition), but negligible Fe and S (pyrite) and are alone in having gained significant K20 (4 to 5 g/lOOg) which has remained essentially immobile in the other rhyolite groups. Group 5 rhyolites also appear to have undergone major Na depletion (-D.8 in relative terms) similar to Groups 2 and 4 which are recognised as parts of a
zoned footwall hydrothermal alteration system. Although the Group 5 rocks including the "white rhyolites" of East Thalanga, do not unambiguously fall into this alteration
zonation, and despite their apparent megascopic "freshness", coherence, sometimes sharp contact relationships and ellipsoidal forms which have led to previous speculations of their origin as small volcanic domes, the geochemical features are consistent with them being alteration products derived from massive si1icification of
fairly ordinary footwall rhyo1ites.
S.3 Synthesis The data presented and discussed in this chapter has demonstrated several important aspects of the geochemistry of the Tha1anga host rocks and hydrothermal alteration
system which are fundamental to an examination of the "exhalites".
* The elements Ti, Al and Zr have been chemically immobile despite strong hydrothermal alteration and subsequent metamorphism.
* The immobile element ratios clearly discriminate between the major volcanic lithotypes and can be used to identify the precursors to otherwise unrecognisable, very altered volcanic rocks.
* The immobile element concentrations andjudiciously selected least altered precursor compositions can be used to estimate the mass gains and losses of chemical components which were mobile during alteration. Absolute mass gains of silica, iron and sulphur totalling -60 g/lOOg, and complete removal of sodium,are
indicated for the most intense zone offootwall alteration.
43
6 GEOCHEMISTRYof EXHALITES, PSUEDO-EXHALITES AND SULPHIDE ASSEMBLAGES
Rocks which have been previously classed as "exhalites" have a remarkable dive~sity
of mineral assemblages and range of chemical compositions. Wills (1985, p.48) tabulated the following extremes (from 0.5mdrill core samples) for some major components:
Si02 6.6 - 91.9%
A1203 0.41 - 15.6%
MgO 0.26 - 22.6% FeS2 0.13 - 5.03%
CaC03 0.2 -73.2%
BaS04 0.01 - 28.0%
On the basis of differences in Si02 content and MgfFe ratios, Wills (1985) identified three main groups which he tenned siliceous, shaley and carbonate-tremolite exhalites. I regard that as an oversimplification in that the carbonate-tremolite category includes near monomineralic end members of those minerals with or without chlorite. Accordingly, in this study, these rocks have been further subdivided to reflect the proportions of chlorite-tremolite-carbonate and other important phases into a total of 13 mineralogical groups covering all rocks which have been previously interpreted as exhalites. These are (non-sequentially): Groups 21 to 45 in Table 1 of Appendix I.
The modes of occurrence and essential mineralogical and textural features of the groups are presented in Appendix II as a background to the following examination of their chemistry.
6. 1 Immobile element geochemistry of exhalites, psuedo-emalites and sulphides
It has been shown that Al, Ti and Zr remained chemically immobile in the footwall rhyolites despite significant mass changes of other components during alteration and metamorphism. In other words, the footwall rocks have chemically interacted with presumably great volumes of hydrothermal fluids which have not been able to dissolve and transport Al, Ti and Zr. It therefore follows that such fluids, when "exhaled" from seafloor hydrothermal vents, would not be carrying these elements and could not precipitate them into any chemical sediments - "exhalites" - which might be deposited from solution.
True exhalites may be expected to contain very low amounts of the immobile elements.
The immobile element data presented by Duhig et al. (1992) and graphically reproduced here in Figure 15, from siliceous ironstones which have been demonstrated
44
1.2 r--r--...---.....,....--,...---..";--r----.--r----r-.,.~-.,----r--r---.--T",, --r----r--.......--r---,
Ti02 % ,
1.0 ... ---------------------~----------------------~----------------------~---------------------
0.8 - ---- --- --- ------- ---- ~ ---------------------- ~ ---- --- -- ------------- ~ - --- ---- -------- ----, . ,. . ,
, , 0.6 ~-----------.---------~----------------------~-------.--------------~----------------------, ,
0.4 ~ --- ---- ---- ------ ----~ ----- -------- -- .-- --- - ~- --- ---- ------- --- --- - ~ --- - -- -- -- ------e -- - -
SS15C: : :0.2 ~--.------------------~----------------------~----------------------~----------------------
0.0 ...L..'-L-_'---'---''--...J1L...-..........----'_-4-----'_........ ...L-........._-'--_.LI' _.L-........_.L-........--l
o 100 200 300 Zr ppm 400
1.2 r-~-__r-.......-_r-....,!r-----.-__r-__._-....,..-..,~r-----r----r--.,..--r--..,
Ti02 %
1.0 - ------ --------- ------ ----- -- -;- - -------- -------- ------ - ----- ~ ----------- ---- -------- --. - --
,
0.8 ~ ------ ----- ---------- ---- ----;- -- -- ---- -- --- --- ----------- -- ~ ------ ----- ---- ------- ----- --
0.6 ~-- --- --. ---- --- -- --- ---- --- - -;---. - ------ -- ------- -- -- --- --- ~ -------- - ---- ------------- --
0.4 ~ --------- ---------------- ----;- ----------- ----- --------- ---- ~ ----- ---- -- -- ----------- ----, '
SS15C 0.2 ... -----.- ---. ------- --- ------- -;- ---- ------- ------------------ ~ ---------- -------------- ----
,
0.0 ."..;:;::_"--_"--_"'--_"""-_...;_........._---"_---"_--''--_'--_'--_''--_'''--_...1------1
o 10 20 A1203 % 30
Figure 15. Zr-Ti02 and A1203-Ti02 plots of 13 quartz-hematite and quartz-magnetite exhalites analysed by Duhig et al., 1992. All except sample SS15C, have very low concentrations of the immobile elements. SS15C probably contains minor detrital volcaniclastic materials.
45
on relict textural grounds to be true exhalites, shows that this is the case. Most of their data points plot close to the origins of the immobile element graphs with ::;0.06% Ti02, ::;O.8%Alz03 and :::;;11ppm Zr. The only exception is sample SS15C which, it may be
supposed, contained some clastic material; probably from a mafic source judging by the anomalous Ti02.
Similar graphical treatment of immobile component data from the chlorite-tremolitecarbonate rocks and other so called exhalites and sulphide assemblages in Groups 21 to 45 (Figures 16 a & b) shows that only two samples fall within this low Ti-Zr-Al field. Both are magnetite quartzites (Group 21) which Duhig et al. (1992) recognised as metamorphically recrystallised equivalents of the silica-iron exhalites.
Most of the remaining data points form crude linear arrays which approximate to the rhyolite "alteration lines" discussed in the preceding sections. Furthermore, the data are systematically arranged with the most chloritic types (Groups 31 & 32) plotting furthest away from the origin and samples with greater tremolite and carbonate (Groups 33 to 35) plotting progressively nearer to the origin. This pattern imitates the mass losses and gains associated with alteration in the footwall rhyolite. In terms of immobile element geochemistry, these rocks are interpretable as altered rhyolites.
There are, however, a number of outliers and litho-geochemical groups which plot away from the "rhyolite alteration line" and there are plausible reasons, outlined below, for these deviations.
* The base metal sulphide rich samples of Group 43 have surprisingly high Zr values and form a linear array which intersects the "rhyolite line" at about 80ppm Zr. On the other hand they have low Ti02 and Al203 values, which would be expected in mainly exhalative sulphides, indicating a unique (for Thalanga) disassociation ofAl and Zr. There is an imperfect but distinct correlation between high Pb and Zr values: sample 71068 contains 1.45% Pb and 67ppm Zr whilst at the other end of the spectrum sample 61473 contains 17.6% Pb and 477ppm Zr. It is notable that the massive pyrite samples of Group 45, which have even greater sulphur levels (up to 34%) but low lead, lie on the Ti02-Zr plot on a linear array which is very close to
the "rhyolite line" and has an r =0.92 correlation factor. There is a strong suspicion that the Zr values for Group 43 are spuriously high, caused by XRF "line overlap" of the Pb and Zr spectra as noted by Dunn (1994) and in Appendix le.
* Some of the barium rich samples of Groups 23 and 41 have moderately high TiOz (to 0.11%) coupled with low Zr andAlz03 values and may similarly be affected by
Ba and Ti XRF "line overlap". The origin of massive sulphides is not a major focus of this study and the problem has not been further pursued. However, it is clearly a caution that the use of standard XRF immobile element analyses may lead to misinterpretation of volcanic
affinity in mineralised samples containing high levels of lead and barium.
46
1.20
I
Tl0
2
% i
i ,
I ,
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i , , i
I
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tzit
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t w
ith r
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hy f
abri
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ixed
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and
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76
6
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bona
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bona
te
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altd
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arbo
nate
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l M
assi
ve B
arit
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assi
ve s
Ulp
hide
s S
p+G
n> P
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p
+
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ve P
yrit
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:!:J
Fig
ure
16a.
Im
mob
ile
com
pone
nt Z
r-T
i02
plot
of
Tha
lang
a ex
hali
tes,
psu
edo-
exha
lite
s an
d su
lphi
des
(Gro
ups 21~45).
The
rhy
olit
e li
near
reg
ress
ion
(as
in F
ig. 9
a) i
s sh
own
for
com
pari
son;
all
oth
er T
hala
nga
volc
anic
s ar
e sh
own
as s
mal
l do
ts,
rhyo
lite
dot
s ar
e re
mov
ed f
or c
lari
ty.
The
Gp
21 m
agne
tite
-qua
rtzi
tes
have
ver
y lo
w Z
r-T
i co
ncen
trat
ions
; m
ost
othe
r gr
oups
fol
low
~he
rhy
olit
e al
tera
tion
tre
nd.
Not
e th
e li
near
reg
ress
ion
for
mas
sive
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.
Oth
er v
olca
nics
/int
rusi
ves
All
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1.20
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37
Ep-
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ltd Q
EV
o 10
20
A
I20
3
'1l
30
o
Op4
3 M
assi
ve s
ulph
ides
S
p+G
n> P
y+C
p
~
Fig
ure
16b.
Im
mob
ile c
ompo
nent
s A
1203
-Ti0
2 p
lot
of T
hala
nga
exha
lite
s, p
sued
o-ex
hali
tes
and
sulp
hide
s (G
roup
s 21
-45)
, D
ispl
ay p
aram
eter
s ar
e si
mil
ar to
Fig
ure
16a.
+
Gp4
5 M
assi
ve P
yrit
e
Oth
er v
olca
nics
/int
rusi
vcs
All
Rhy
olil
es
(exc
ept
rail
CU
lling
s3n
ll»e
s)
* Wills' (1985) sample of "carbonate exhalite" from TH40, 260.8m; contains 0.26% Ti02 and plots well away from the "rhyolite line". This is not surprising given its
location amongst feldspar phyric dacite well up in the hanging wall stratigraphy at West Thalanga Extended; there are clear gradational alteration boundaries up and down core into dacite and sheared dacite and there are relict feldspar phenocrysts even in the relatively massive carbonate-epidote assemblage. The setting, relict texture and immobile element geochemistry support the interpretation that this is an altered volcanic rock - probably a dacite or andesitic dyke.
* Another of Wills' (1985) samples which plots significantly off the "rhyolite line" is TH43A,405.8m. It is an assemblage of chlorite-tremolite-carbonate (CTC) from
the favourable horizon immediately below 2m ofbaritic massive sulphide and it contains some relict qtz crystals. Patchy to spotty chlorite-carbonate alteration occurs in the quartz-eye volcanic1astic unit immediately overlying the sulphide and it is interpreted that the analysed CTC from below the sulphide represents a more pervasively altered equivalent ofthe QEVabove.
Apart from these significant outliers the remaining immobile element data points lie
within the range of scatter of altered footwall rhyolites despite radically different modal and major element composition in most groups.
The samples from Group 26, and some from Group 23, retain modal quartz and relict rhyolitic or volcanic1astic fabrics and can be interpreted as examples of partial alteration, transitional to more complete chlorite-tremolite-carbonate assemblages of Groups 32, 33,34 & 35 which generally do not contain quartz and relict volcanic
fabrics.
6.2 Mass Changes relative to a rhyolite precursor
In view of the immobile element trends, it is interesting to treat the so-called"exhalites" as altered rhyolites and estimate the major component mass changes using the procedure of MacLean and Barrett (1993) as has been done for the variously altered footwall rhyolites (ref: Section 5.2)
In calculating mass changes for rhyolites, Zr was used with confidence as the immobile
monitor. The "exhalite" assemblages, however, sometimes carry anomalous Pb and Ba which, as noted above, are suspected of causing analytical errors in Zr and Ti respectively. To insure against this potential problem, mass changes have been separately calculated for three immobile monitors: Alz03, TiOz and Zr, as a check for consistency and these values then averaged to arrive at mean mass changes. "Precursor 1" (fabricated from the least altered of footwall rhyolites) has been used as the precursor composition, as before. The results of this approach are presented graphically in Figure 17a-d as absolute mass changes expressed in gl100g units.
49
225
o Gp22 Qtzite+/-Ba,Mt ~ Gp33 Trem>Cbl, .200 -------------------------~(h-rell~tFUiyfabIlc----------~~o(;Ut)OJJak:--------------------
g Q
175 ---~--------------I2J---Cfp2(5j(~t-ElJ~zo~---~---(}I>3~1Lrern~CJbf-----------------
alteredrhvolite 5-50%carbonate 150 ---~-----------------------------~-------------------------------------------------------
OIl
125 ___ jg= JII ~J?~l_~~_~~~~ ~ ~~~~?J[~~_~ _
U ~
100 ---~--------------Ei§---c;p3:i(Sbi;1fr~~-------~---(}j;4~};1a-~~~-~~t~-------------
$ <5%caIbonate 75 ---~----------------------------------------- ------------------------------------------
] 50 ---~----------------------------------------- ---------------------------------
25
o
-25 -------.---------------------------------------------------------------------------
-50 -----------------------------------------------------------------------------------Fig 17a. Immobile monitor: Zr
-75
225
o Gp22 Qtzite+/-Ba.Mt ~ Gp33 Trem>Cbl,200 ---~------------·-----·-W1tlirellctFOiylabIic·--------~~~Iiite--------.-------------
Cl Q
175 --~--------------~---------------------------~----------_._------.------------------Cb G Gp26Act-Ep-Czo-Cb w Gp34 Trem+ CbI alteredrhyolite 5-5O%carbonate
150 ---~------------------_._----------------------------------------------------------------OIl .
; • Gp31 Chi Schist § Gp35Cb>Trem+Cbl125 ---~--------._----------------------------------------------------------.---------------
U ~
100 ---~--------------Er--Gp32rn;Tr~-------.---(}j;45Evla~~~-pYrit~-------------
$ d%caIbonate75 ---~------------------------------------------------------------------------------------
---~----------------------------------------- ------------------------------------------50 ]
25
o
-25
-50 Fig l7b Immobile monitor: Ti02
-75
Figures 17 a & b. Graphic representation of absolute mass changes of Thalanga psuedo-exhalites and massive pyrite relative to rhyolite composition "Precursor I", calculated with Zr (a) and Ti02 (b) as immobile monitors.
50
225
o Gp 22 Qtzite+/-Ba,Mt ~ Gp33 Trem>Cbl. .200 -.-----------------------------------------------------------------------------------------
Oil with relict Rhy fabric <5%carbonate
8175 ---~--------------[2]---~26-J\~t=E1l~z~~---~---Crj;3~lrr~~~c:bi------------------
alteredrhyolite 5-50%carbonate150 ---1P------------------------------------------------------------------------------------
~
~ • Gp31 ChI. Schist ~ Gp35Cb>Trem+Cbl125 ---C)-------------------------------------------------- ----------------------------------
~ 100 ---~--------------e---Gp32Chl;T~~-------mI---Gp45~f;~~·~-~t-.;--------------
;i <5 %carbonate 75 ---~---------------------------------------- ------------------------------------------
50 ---ce--------------------------------
25 ---~--------------------- ---------- ----~ ---------------------------
f+.Jdi~r__----.....~~IUoJ;--..c.....I:~- ~ nl ~-.. __ .Jlo n
-25 -----------------------------------------------------------------------------------
-50 ~------------------------------------------------------------------------------------------
Fig. l7c. Immobile monitor: A1203 -75 I I , I I I I I I I
N V) er.>o oo an o [::: :::i 3 ~ g
~ §~ ~
225
o Gp22 Qtzite+/-Ba,Mt ~ Gp33 Trem>Cbl. 200 ---~--------------------~t&re1ictFQiytalJnc----------<:5~caiEK)na~---------------------
8175 ---~--------------[]]---CiP2(5.~c~~~zo~---~---Ci])3~~reurr~(;bf------------------
alteredrhyolite 5-50%carbonate 150 ---~-----------------~-------------------------------------------------------------------
1; • Gp31 ChI. Schist ~ Gp35Cb>Trem+Cbl125 ---J:------------------------------------------------------------------------------------
U fa
100 ---~ --------------ea---Gp32Cbi>lrrem:-------11---Cij;45 ~f~~v~-rynte ----~ ------- -$ <5%carbonate
75 ---~----------------------------------------- ------------------------------------------i ,Q
50 ---ce---------------------------------------- ---------------------------------
25
o
-25
_______________________ ElgJ.7~LM.E.~ __~L~~~~_~J!~J:~ __~~!~!lJ...~~~4Lt~!l! __-so immobile monitors Zr, A1203 & Ti02
-75
Figures 17 c & d. Graphic representation of absolute mass changes of Thalanga psuedo-exhalites and massive pyrite relative to rhyolite composition "Precursor 1; calculated with (c) A1203 as immobile monitor and (d) Means of mass changes calculated with Zr, Ti02 and A1203 immobile monitors.
51
It is apparent that although there are slight to significant differences in the quantities calculated with respect to the three different immobile monitors, the mass change
patterns are similar for most litho-geochemical groups and major components. I~
general, theA1203 monitor tends to exaggerate the mass changes relative to those calculatedbyTi02 and Zr.
There is an anomaly in Groups 33, 34 &35 for Si02 which is calculated as a mass gain according to A1203 , as a mass loss according to Zr and some each way according to Ti02. However, the means of the three sets of calculated mass changes provide acceptable
average values for discussion, and reduce the Si02 anomaly to insignificance. In the following paragraphs the calculated mass changes are simply referred to, for brevity, as "mass changes" but it must be kept in mind that these are hypothetical mass
changes, based on an artificial precursor composition and, in some cases, with no evidence other than the linear immobile element trends to suggest that they were derived by alteration of rhyolite. The approach is partly experimental leading to a test
of whether these assemblages could have been derived from alteration of rhyolite.
Group 22 Siliceous "exhalites"
The mass change for this group is dominated by a vast gain of almost 200g/100g Si02 overshadowing small gains of Fe, S and Ba and depletion of Na with other components essentially stable. This pattern is like a more extreme, silica dominated,
fOITIl of strong footwall rhyolite alteration as represented by Groups 4 and 5 from pyritic stringer zones and the "white" rhyolites (which these rocks most resemble). Rocks of this class in TH5 were interpreted by Wills (1985) and Stolz (1991) as siliceous exhalites on the basis of the silica, barite and magnetite content. Close
examination during this study has shown, however, that the barite and magnetite occurs largely in veins anq there are sparse but distinct relict quartz phenocrysts which
support the immobile element and mass change data in suggesting an origin by silicification of a rhyolitic precursor. An alternative which cannot be ruled out, is that these rocks fOITIled by limited rhyolitic volcaniclastic input into siliceous exhalative sediment.
Group 26 The two samples in this group both have relict rhyolite fabrics and occur intra-footwall. The main chemical changes (relative to "Precursor 1") are
moderate depletion of Si02 and slight gains in CaO and MgO to produce a net loss of about 8g/100g.
Groups 31 & 32 These are chlorite dominated assemblages with moderate to strong depletion of Si02 offset by moderate gains of MgO to produce a net mass loss of ~36g/100gin Group 31 and near parity in Group 32. CO2 is insignificant and reflected in low carbonate contents. Wills, (1985) referred to rocks of this type as "shaley exhalite" and, as noted in Appendix I-a, some of his samples of that description have been recognised as altered dacite and placed with Group 12.
52
Groups 33,34 and 35 These are chlorite-tremolite-carbonate (CfC) assemblages with
low, moderate and high carbonate contents respectively. They are characterised by
relatively minor mass changes in Si02 but strong enrichment in MgO, CaO and
progressively increasing C02. They have net mass gains of around 60, 140 and ..
1909/100g respectively with a contribution from CO2 of about 6Og/ lOOg in the
carbonate rich Group 35.
Group 45 Massive Pyrite
These are not directly relevant to the hypothesis that crCs are formed from altered
rhyolite but are shown in Figures 17a-d to emphasise the difference in the pattern of
mass changes. Massive pyrite has strong depletion in Si02 and strong enrichment in
Fe203, MgO and S, relative to rhyolite, but no significant changes in CaO and C02.
This suite of major mobile components shows that the massive pyrite lenses have a
chemical affinity with the footwall style of alteration.
It is noteworthy that Na20 is significantly depleted in all of the groups charted, with
most groups reporting a loss of ~3g/100g Na20. In relative terms this represents
virtually all of that component originally present in the parent rhyolite, ("Precursor 1"
contains 3.17% NCl.20). Sodium depletion by feldspar destruction is the unifying
characteristic of the footwall hydrothermal alteration system. The group with the least
amount of sodium loss here is Group 26 which agrees with the interpretation that this
style of intra rhyolite actinolite-c1inozoisite alteration may be an incipient stage
transitional to more pervasive and complete chlorite-tremolite-carbonate assemblages.
53
7 DISCUSSION and GENETIC INTERPRETATION
7 . 1 Magnetite quartzites (Group 21)
The data introduced above shows that magnetite quartzite samples, from the dacite and
mixed volcanic1astic dominated hanging wall sequence at West Thalanga, have
distinctive silica-iron rich compositions and low immobile element contents. Their composition is similar to siliceous ironstones from elsewhere in the Mt Windsor
Subprovince which are less metamorphosed and have textures which Duhig et al. (1992) have convincingly shown to be of exhalative origin. The Thalanga magnetite
quartzites are entirely recrystallised to a fine granuloblastic texture but the chemical
similarity and absence of relict volcanic phenocrysts leaves little doubt that they also
originated as exhalites.
Although the only two analyses available are from a single unit in TH40 there are other occurrences of mineralogically and texturally similar rocks within the favourable
horizon at West Thalanga, Vomacka and, rarely, East Thalanga. They occur as thin
bands, lenses and medium sized c1asts within volcanic1astic sediments and breccias in
the stratigraphically upper part of the favourable horizon, in apparently distal settings and often in association with barite. These observations are consistent with Duhig et
al.'s (1992) interpretations of deposition from relatively cool fluids discharged by
weak or waning hydrothermal systems, or from the cooled remnants of fluids which
had already precipitated sulphides. The survival of c1asts within volcanic1astic
breccias suggests that the magnetite-quartzite protoliths were reasonably lithified soon
after formation.
7 .2 Siliceous psuedo-exhalites (Group 22)
Rocks in this category may be as siliceous as the magnetite quartzites and contain some
magnetite and barite but they differ by containing higher levels of immobile elements
including Al, Ti, Zr and Y, significant tracesofMg and K and relict quartz
phenocrysts. These belie a purely exhalative origin and imply formation by gross silicification of a rhyolitic precursor or, at least, rhyolitic volcanic1astic input into
otherwise nearly pure siliceous exhalite.
However, in the most sampled occurrence in TH5, the siliceous rock is quite massive,
absolutely without relict stratification or compositionallayering, the contacts tend to be gradational or sheared and the associated magnetite and barite is concentrated in brittle
style veins rather than layers. The siliceous rock is not restricted to the favourable horizon but also occurs in patches for about 30m down the hole, in association with
quartz-barite-minor sulphide veins and patchy actinolite-chlorite-carbonate alteration
gradational into less altered quartz-feldspar phyric rhyolite. The overall impression is
of domainal alteration with the relationships confused by structural complexity.
54
The structural set up is not well understood, but thought to be related to D3 brittle
ductile shearing, (Hill, 1991), where the favourable horizon is displaced 120m
southwards, adjacent to the down dip temrination of the Central Thalanga massive sulphide deposit.
The other analysed example, from hole E3184SD31, contains ~2% relict quartz
phenocrysts in a fine matrix of granuloblastic quartz with subordinate flaky muscovite
which has strong preferred orientation anastomosing around O.2-0.5mm ovoid bodies
in a texture which could be inherited from perlitic fractures, (England, 1994). It is,
without doubt, a silicified and metamorphosed rhyolite. Magnetite at -2vol% occurs
as fine euhedra disseminated amongst the granuloblastic quartz matrix and in fine
quartz veins with minor barite and galena. The veins are internally recrystallised,
irregular, may be folded and boudinaged, and are certainly of an early, pre
metamorphic, generation.
It is conceded that the requirement for an Si02 mass gain by alteration in coherent
rhyolites of the order of 100-200g/100g is a daunting one, but this is merely about
twice the net mass gain confidently interpreted for the most intensely quartz-pyrite
(sericite) altered footwall rhyolite. The possible relicts ofperlitic fracturing suggest a
mechanism for allowing silicifying fluids to thoroughly pervade the rocks and
considerable Si02 may have been added as open vein fillings. The large volume
increases implied by such large Si02 gains may be possible if the alteration occurred at,
or at shallow depths below, the sea floor, unconstrained by great lithostatic loads - a
setting which is consistent with the location of the occurrences at, or not far below, the
favourable horizon and the association with iron oxides and barite which infer
relatively cool, oxidised fluids. It is inferred that shallow sub-seafloor alteration may
have occurred by reaction with fluids similar to those which precipitated the silica
hematite precursors of magnetite-quartzites above the sea-floor.
7.3 Cblorite-tremolite-carbonate assemblages (Groups 26 & 31-35)
These groups are collectively considered here because they all contain Mg andlor Ca
bearing phases such as chlorite, tremolite, actinolite, clinozoisite, dolomite and calcite.
Some of the unifying factors and distinctions are:
Groups 31 to 35 occur in close association with each other in the favourable horizon
and also intra-footwall rhyolite (e.g. Section 20110E, Figure 5) but apparently not
more than about 40m stratigraphically below the favourable horizon. They appear
to form a modal mineral continuum from chlorite dominant without carbonate, through chlorite-tremolite mixtures with low carbonate, to carbonate dominated
assemblages with variable chlorite-tremolite. Some examples of the chloritic types
in Groups 31 and 32 contain sparse relict quartz crystals resembling phenocrysts
but the carbonate-tremolite rich types are dominated by metamorphic and veiny
fabrics without relict volcanic features.
55
Group 26 includes incipiently altered rhyolites, with unambiguous relict volcanic
textures, which occur quite widely in the footwall in lateral? zones which have not undergone strong quartz-sericite, Ca-Na depleting, alteration.
Section 6.2 presented hypothetical mass changes, relative to a rhyolitic precursor composition, which suggested that the Group 31 and 32 chloritic assemblages could be produced from a rhyolite by relatively modest chemical changes mainly involving loss
of Si, Na and gain of Mg. lbis pattern of chemical change is similar to that in Group 26 (but lacks the slight Ca gain) and, most significantly, involves a net mass loss which produced enrichment of the immobile components.
Mass loss is significant because it eliminates Wills' (1985) "shaley exhalite" concept of sedimentary mixing of exhalative and volcaniclastic materials as a mode of formation. As postulated above, true exhalites should have very low immobile element
concentrations; addition of volcaniclastic detritus could increase the concentrations up toward the level of the source magma - but not beyond. To achieve that would require an unlikely chemical or sedimentary "winnowing" process to enrich the immobile elements above magmatic concentrations and then add them - in exactly the same ratios as in the footwall rhyolites - to the immobile poor exhalite.
These chlorite dominated assemblages have immobile element ratios very close to those of rhyolites and could only have been formed from rhyolitic precursors by alteration which resulted in a net mass loss. This conclusion is consistent with their modes of
occurrence. For instance: in TH245 (Figure 5 and Figure 18) the zone of tremolite-carbonate in the
interval 672.5-675.9m is flanked at both contacts by narrow zones of Group 31 type chlorite schist which grade up and down core into quartz-sericite-(chlorite) altered rhyolite containing patches of (perhaps incipient) actinolite-clinozoisite-carbonate
alteration similar to that of Group 26. Similar chlorite dominated rocks, with or without relict quartz phenocrysts and sometimes significant pyrite, commonly occur in narrow zones at the contact between quartz-sericite altered footwall rhyolite and massive pyrite or massive base metal
sulphides as in drill holes W2011NED42 (Figure 5 and Figure 19) and TH390B (Figure 3). In short, they often mark the boundary between "normal" quartz-sericite
alteredfootwall rhyolite and tremolite-carbonate ormassive sulphide assemblages and are best interpreted as altered rhyolite.
The loss of Si02 implied for this alteration (ref: Section 6.2) could be of either hydrothermal or metamorphic origin. If, as discussed below, the tremolite-carbonate rocks were formed by metamorphic reaction of dolomite, quartz and water to produce tremolite, calcite and CO2 then a local excess of dolomite may have caused scavenging of Si02 from surrounding rhyolite and formed narrow alteration fronts of silica depletion.
56
Figure 18.
TH245 670-680m Graphic Log
DEPTH (m)
670 ,.' /' .' ." /' -'" ,. /'- ". ". '.. ~. ". .... " ~/""'/"""//
".. ..... .... -.. "" .... -'" ~. .~ .•. / ..'~./.~."
",',', --, '-. "", ". ••" ;' .E .,- /' ,.- .,- ,
'- '\ ", \. '< '- "'.... .. ,. ..' ..- " ~...- , .... >, .... ".. '.', '" ..." ..' .,' ,. ..' ," ,. ..' .'"
' .. --••~ ' .. <', "
,." I' ./ .," ,. /' .,- / - ·w. -' \....
~ "It" '- -....' f'.: '" _.r ..,"'. ••' .' ,. , /' .< ..671 , •••. ' .•/ '/ "'", ",.. '." <•••••.• ,,
.. ' /' ~.....,'~.; '\ ""./"'/ .... ~.SAMPLE - ,." -. '\. -'."'. '\." .,' ,. .,- ..' /' ..' .... .....: .• / "'/ :.. / ~ .•-'-./NUMBER
'- .... ", '.". >. >. ". '"• Rhyolitic volcaniclastic ••••, / -._~ •••: .. -.. " ".~"" ••> -,,;' "~~' ..- ,.' ./ ."" ..- /' ....'
... ' '". < <. ", breccialsst; moderate-strong
... "'_." ." '\..' / .•:. '....' ... r .... ~••-' ." ,,' ,.> ." ,;' qtz-ser alteration with -5% Py.
-' ~ " ~ :" " .-- '\. .,:672 ........-.•"' ' : .•" : .' .. / '\. ". " \. "54359 .. / ~"J".'''~. .'..... ', .........
.,' .,' " ..' ~. " ..' ..' .... ". ,'
' ' ..
54360 =:i2~" ./P$i'.',/" tI'·./ Chlorite schist, minor Tremolite, sphalerite. 54361
673 Massive carbonate with marbly bands and patches
54362 of Tremolite. Minor Ba, dissem Sp,Py,Gn.
674
54363 Massive Trem>Chl with -30% carbonate.
-~ff'rl¥JjU)J,1-1 Sulphides <1%. 675
54364 Massive Tremolite, minor chlorite, <10% Cb; local minor ba rite, diopside. 54365 Sulphides.1-3%.
676 54366 Fine grained fetted
massive chlorite with subordinate Trem-Czo; 54367
••••,'••>•••,' minor barite, sphalerite. •w ' w .... ......, ', ~..~677 .'" ... ...... ~ .... ........................... ...
,·w ... .'. "' "' \, ........................'."" ...
."'''' "..'- . ... .,. ... ... ~~ ... ... ~ ... Siliceous quartz phyric.'. ... '. .... ~
~. .'
................. "" '" .................' . Rhyolite; scattered ........................ ..... ....... "" ...
.'. "" '. "' .... "cloudy" patches of ~••> .... ,,_ .'" ... J'• ...678 .. ... .... .. A ................................ incipient Act-Qtz-Czo-Cb ... ,. '" ..... ...
.' ... ............ ~
.., .. alteration with traces of"" ... ...... • w w .
,.. .................. "' ........ Ba, Gn,Py. ......... . '""
....> '" "" ~."' ................ " . ... .... • > ~ " •
... A .... .' ..
.," ' ~ .......................... ................ . ... .... .... ... .... .. .... ... .... ... . .. ... ... ......... .".-.. .... ... ... ... . ,..~ ....................... ...... .'..............'...................... ....
560
DEPTH (m) 102
104
106
108
110
112
114
116
118
120
122
124
126
128
Sample
61468
61469
61470
61471
61472
61473
61474
61475
61476
61477
61478
61479
61480
61481
61482
61483
61484
61485
61486
61487
61488
61489
61490
61491 =, >
61492
61493
Figure 19. W2011 NED42 Graphic Log
Dacite
QEV breccialSst minor veiny Py-Cp and clasts of massive Py and Ba+/-Cp,Gn
Massive SUlphide: SP»Gn-Py 20% gangue of qU, chi, Cc, minorTr.
Sheared contact
Massive felted dk Chi with bands Tr (Ep,Cc, spotty Czo, Py, Gn, Cp)
70% Tr > Chi, minor Cc and Sp, Gn, Cp, Py
Carbonate: Cc-Do»Tr-Ghl very minor sulphides
Tr» Chi, Cb; 10% SpCpGn
80% Cb»>Tr,ChI
CID> Tr,ChI; 15% Sp(GnCpPy)
Do>Cc =ChI,Tr; 2"kSp
Tr-Q)/:>Q); 30% Sp>GnPyCp
SemiM assivePy, 40% TrChl,5%Sp
Massive Pyrite 80%
Massive Pyrite 65%; with patches of dark Chi sometimes with spotty Czo or Cb?
Ser-Chl schist
Qtz-Ser-Py altd Rhy Qxtal volcaniclastic
Q-Ser-Py altd Rhy volcaniclastic
57
The origin of tremolite-carbonate rich assemblages is not so neatly constrained. These
have immobile element ratios suggesting rhyolitic parentage but in view of the
significant net mass gains due to addition of Mg, Ca and CO2, it is not clear whether
the relationship was by alteration of a rhyolitic precursor or by volcaniclastic input into
carbonate exhalite. Certainly, there are near monomineralic end members of
carbonate and tremolite with minor chlorite, and all gradations between, which could
indicate fluctuations in exhalative vs. volcaniclastic deposition.
There is no convincing small scale compositional banding to suggest bedding but then,
such features are not likely to be well preserved in these extensively recrystallised
rocks where the grainsize of tremolite often exceeds 2Omm. As noted in Appendix II
carbonate, dominantly calcite, appears to stand late in the (metamorphic) paragenetic
sequence. It occurs as veins and ragged sparry patches which clearly cut across,
replace and corrode other phases including tremolite, chlorite and sulphides. This
form might suggest a late stage, post metamorphic introduction of calcite - it is, co
incidentally, the dominant phase in tensional veins within dolerite dykes and in
association with brittle faults which are younger than deformation and metamorphism.
Chlorite and tremolite, on the other hand, are probably metamorphic phases; the
typically random orientation of tremolite suggests that it crystallised after the peak of
deformational strain.' Wills (1985) and Gregory et al. (1990) appear to have regarded
tremolite/actinolite in these rocks as "primary" exhalative phases but this would have
required temperatures in excess of ~300°C which are unlikely in the hydrothermal fluid
- sea water mixing model they envisaged.
An alternative origin, postulated by Rivers (1985) and Stolz (1991), involves
metamorphic reaction of quartz and dolomite to produce tremolite and calcite as in the
simple reaction:
(1) 8SiOz + 5Cat.Vlg(C03)2 + H20 = Ca2MgsS iB022(OH)2 + 3CaC03 + 7C02.
8Qtz + 500 + H20 = Yremolite + 3Cc + 7COz
According to the xC02 vs. Temperature phase diagrams presented by Rivers (1985)
and kindly supplied by Berry (pers. comm.) this reaction would proceed at about 350
4SOOC at 2kb pressure and low to moderate activities of CO2. These physical
conditions are consistent with the grade of regional metamorphism estimated by Wills
(1985) and Berry (1991).
Footnote: It is noteworthy that modal quartz is generally not present in the cblorite-tremolite-carbonate assemblages which implies that dolomite was in excess or neatly balanced to consume all quartz in the reaction stated. The only known exception to this observation is in Sample No. 54365 which contains a patch of coarse skarn-like diopside-quartz-barite. The diopside implies a higher temperature. some 5O-7O"C higher than the formation of tremolite at moderate xC02, and the local availability of excess Si02 to allow the reaction: (2) 3Cc + 2Qtz + Yr = SDi + H2O = 3C02 to proceed. Elsewhere the shortage of Si02 may have maintained the stability of tremolite even at the higher temperature range.
58
The question then arises: would quartz-dolomite-(cblorite) precursor assemblages have
the appropriate balance of components to produce the assemblages now observed?
The chemical feasibility of this process has been investigated by devising a simple
normative calculation after the style of the well known CIPW Nonn (explained by Cox
et al., 1979) using the minimal normative components of quartz, cblorite, dolomite for
the precursor and chlorite, tremolite, dolomite, calcite for the metamorphic product.
This normative system has been informally called the "QCI' Norm" - (an acronym for
quartz-carbonate-tremolite).
Some fundamental assumptions of the QCI' Norm calculations are:
*� ThatAl203, which has been shown to be immobile in this system, resides in
chlorite before and after metamorphism and that the main changes involve
quartz+dolomite transforming to tremolite+calcite+C02; any excess Mg would be
expressed as dolomite in the metamorphic assemblage.
*� The chlorite is an Mg rich clinochlore with an assumed composition of
(MgsFe2A12)(Si6A12)020(OH)16. This is not entirely arbitrary, the composition is
a rounded average composition of seven micro-analyses of Mg rich chlorites from
the footwall, presented by Rivers (1985).
*� Other phases used in the normative calculation have simple end member
compositions.
Dolomite (CaMg)(C03)2
Calcite CaC03
Tremolite Ca2MgsSis022(OH)2
Quartz Si02
*� The molecular weight of water in the hydrous phases is ignored, it being assumed
that the metamorphic system was not dry.
The procedure followed and the results of the QCT Norm calculations for mean
compositions of the various litho-geochemical groups are presented in Appendix ill. The concept being tested is that if the metamorphic system was isochemical, or closed,
(except for volatile components H2O and CO2) the present bulk compositions of
cblorite-tremolite-carbonate assemblages ought to be re-calculable into pre
metamorphic normative quartz-dolomite-chlorite assemblages and the major
components ought to balance out. If they do not balance, then the postulated
precursor assemblage is inappropriate or there have been significant mass changes
during metamorphism; (i.e. the metamorphic system was not closed). It is a way of
testing ifquartz-dolomite-cblorite (alteredrhyolite) assemblages could be direct pre
metamorphic precursors of the assemblages now observed.
The Groups 33,34 & 35, rather surprisingly, pass the test with only minorirnbalances
in chemical components.
The average of Group 34 compositions (n=5), for instance, has near perfect
component balances for Si02, FeO+MnO, MgO and CaO and a deficit of C02
59
equivalent to ~18wt%. The C02 deficit, in molecular proportions, is almost exactly in
the ratio 7:8 (C02:QtZ) as predicted by reaction (1), and is geologically sensible in that
C02, being volatile, would be lost from the system during metamorphism.
Similarly, the average composition of Group 35 carbonate rich assemblages balances
well in the QCT Norms, shows original calcite with dolomite in the precursor
assemblage, residual dolomite in the metamorphic assemblage and a C02 deficit of
~ lOwt%. This also is geologically interpretable as indicating an excess of dolomite
(i.e. insufficient quartz in the system to consume all precursor dolomite) and a lesser
loss of volatile C02 reflecting the lesser production of metamorphic calcite.
Group 33, representing tremolite rich - carbonate poor assemblages, behaves slightly
differently in the QCT Norm. It shows a slight deficiency in MgO equivalent to
~3wt%, (i.e. there is not quite enough MgO to balance the Si02 remaining, after an
allocation to chlorite, to make pure tremolite) which suggests that the AI-Si proportion
in the chosen chlorite composition may be inappropriate for this assemblage or that
other silicate phases are present.
The MgO imbalances are even more pronounced in Groups 31 & 32 chlorite rich
assemblages, which have very little or no carbonate, and support the implication that
the assumed chlorite composition may be inappropriate. A more aluminous chlorite,
say: ~Mg4Fe3A15)Si6Al2)02o(OH)16 may be nearer the mark. The problem does not
arise in the carbonate rich Groups 34 & 35 probably because of the relative
unimportance of chlorite in those assemblages.
Nevertheless, the QCT Normative relationships suggest that quartz-chlorite-dolomite
(calcite) in various proportions does constitute a chemically and geologically reasonable
pre-metamorphic assemblage for the existing chlorite-tremolite-carbonate rocks.
Furthermore, the QCT Norms of the weighted average of all Group 32, 33, 34 & 35
compositions, are nearly balanced. It suggests that, although there may have been
internal small scale mass transfers, the overall metamorphic system was closed with
respect to non-volatile components and the present assemblages could have been
derived from metamorphism ofa quartz-chlorite-dolomite-(calcite) precursor. There is
no requirement for invoking significant syn or post metamorphic calcite input, despite
what might be inferred from the replacive textural relationships of calcite and other
phases.
60
7 •4� Chlorite-carbonate alteration in other deposits and comparisons with Thalanga
Chlorite-carbonate alteration assemblages are known in association with several . western Tasmanian VHMS deposits, including Hellyer (Gemmell and Large, 1992), Que River (Offier and Whitford, 1992), Rosebery (Lees et al., 1990) and South Hercules, (Khin Zaw and Large, 1992). Other mainland deposits with documented
carbonate alteration include Mt Chalmers in central Queensland (Hunns et al., 1994) and the Mons Cupri, Teutonic Bore, Gossan Hill and Scuddles deposits in Western Australia, (Barley, 1992).
In the Western Australian examples the chIorite and carbonate are iron rich types
occurring in the footwall zones proximal to massive sulphides, sometimes associated with stockwork mineralisation "in poorly developed, or laterally extensive, feeder zones", (Barley, 1992). These seem unlike the Thalanga style where chIoritetremolite-carbonate rocks are mainly stratiform in the favourable horizon and the footwall feeder systems are distinctly siliceous.
A geological cross section ofMt Chalmers presented by Hunns et al. (1994, Figure 3) shows a stratiform zone of "massive dolomite alteration" underlying and laterally equivalent to a massive sulphide lens and immediately overlying a zone offootwall
stringer pyrite-chalcopyrite mineralisation.
At Hellyer, chlorite-carbonate alteration appears to be restricted to small zones immediately below massive sulphides at the margin of a siliceous alteration pipe under the deposit; (Figure 6 of Gemmell and Large, 1992). The carbonate is reportedly
mostly dolomite occurri~g as small crystals and rounded nodular forms in massive chlorite. Analyses given by Gemmell and Large, (1992, Table 1) show that the chIorite-carbonate zone contains the immobile components Ti, AI and Zr in concentrations very close to those of the adjacent unaltered footwall andesite. This supports the interpretation that the assemblage is an alteration product of andesite
involving a loss of Si and Na balanced by gains of Fe, Mg, Ca and CO2 with minimal net mass change.
Chlorite and carbonate of varying compositions are sporadically distributed in the Que River alteration system. Offier and Whitford, (1992) attributed this heterogeneity to successive influxes of variable hydrothermal fluids and mixing with seawater with the latter as the main source of magnesium. They identified both hydrothermal and
metamorphic chlorites.
Lees et al. (1990) referred to carbonate alteration at the Rosebery deposit in a "loosely defined halo of variously textured carbonate rocks" surrounding the orebodies and confined to the host rockunit. They suggested that the spatial association indicated a relationship to ore formation and postulated several modes of carbonate origin
61
including feldspar alteration and sub-sea floor precipitation from depleted hydrothermal
fluids. Recent studies by Orth and Hill, (1994) have recognised relict volcanic
textures indicating that carbonate alteration nucleated on pumice clasts and feldspar
crystals and that it formed by replacement in the uncompacted shallow substrate of the
sea floor - not by exhalative deposition.
Khin Zaw and Large (1992) described carbonate alteration at South Hercules overlying
and laterally interfmgering with a stratabound, zoned sulphide deposit. The sulphide
facies is dominated by disseminated and veiny, low grade, Pb-Zn mineralisation in a
silica-sericite altered volcaniclastic sediment package about 20m thick which is
underlain by felsic pumiceous volcanics and overlain by a shale-siltstone-quartz phyric
volcaniclastic sequence. Thin stratiform lenses of semi-massive sphalerite-galena
pyrite occur toward the base of the sulphide facies and lenses of massive pyri~e-barite
quartz occur at the top. Massive carbonate and cherty carbonate alteration zones occur
around, laterally interfingering with, and overlying the upper part of the sulphide
facies, (Figure 32). The cherty carbonates are fine grained, intergrown with
cryptocrystalline silica, and appear to be spatially related to, perhaps lateral equivalents
of, the massive pyrite-barite lenses. Carbonate-chlorite lenses with a distinctive blebby texture occur near the top of the carbonate facies and within the overlying
hanging wall siltstones. No whole-rock chemical data is available but the carbonates are reported to be consistently Mn rich (kutnahorite and rhodochrosite) with minor Fe
andMg.
The deposit and alteration zones were interpreted to have formed by sub-seafloor
replacement from relatively low temperature, moderately acidic hydrothermal fluids
which cooled and increased to near neutral pH by mixing with sea water in the
permeable volcaniclastic ,substrate. Khin Zaw and Large (1992) considered that local
"variations in permeability controlled the shape and outline of the mineralised zone with
the ore fluids mixing both vertically and laterally along the permeable horizon.
Sulphide facies mineralisation was formed in the central (high temperature) zone of
mixing followed by the carbonate facies in the cooler, lateral and stratigraphically
higher zones." The genetic model is illustrated in Figure 33. Khin Zaw and Large (1992) were unable to define the depth at which mineralisation and alteration occurred
below the sea floor but noted that the presence of carbonate-chlorite alteration lenses in
the hanging wall sediments could indicate a sediment cover of at least IQ-2Om at the
time of mineralisation.
A detailed study of the"Area of Active Venting" (AAV) in the Middle Valley of the
northern Juan de Fuca Ridge, by Goodfellow et al. (1993), has revealed a vertically
and laterally zoned alteration system in Holocene, hemipelagic and turbiditic sea floor
sediments. It consists of an upwardly convex inner zone, about 200x4OOm in area, of
Mg smectite-chlorite-silica-barite-pyrite-gypsum alteration enriched in Mg, S, Ba and
depleted in Ca and C02 relative to unaltered sediments. An outer zone, up to 200m
wide, consists of Mg smectite-carbonate-barite-minor pyrite which is enriched in Ca,
62
I
'50mc.
~ Blebby carbonele - chlorite zone
§ Massive carbonate zone
I22l Cherty cart>onate zo_
Et! Massive pyrlle ± berlte zone
[J SlIIceQus ± strlngery sUlphide zone
I!l!iI Sphalerite - galena ± pyrite zone
.' Fluorite occurrence
5630mN
SOUTH HERCULES
Figure 32.� Geological cross section of South Hercules deposit showing distribution of sulphide and carbonate alteration zones, (from Khin Zaw and Large, 1992)
i
!
-Hanglngwall Eplclasltcs
more permeable
~::--';' "'Tuflaceous
~I~~~~~ shales
Footwall Rhyolillc massflows +
low temperature hydrothermallluld (-200"C)
Carbonate-altered zones
Massive pyrlte-barlte zone
Sphalerlte-galena ± pyrite zone
Siliceous ±stringer sulfide zone
Figure 33.� Schematic section illustrating a model for formation of the South Hercules deposit, (from Khin Zaw and Large, 1992). Carbonate alteration occurred in permeable volcaniclastic units by mixing and cooling reactions of hydrothermal fluids and sea water in the upper and outer parts of the system.
63
C02 and Ba Elevated levels of most chalcophile elements occur in both zones, probably as sulphides.
It is interpreted that the alteration minerals are forming in the near sea floor substrate
«lOm depth) from an evolving hydrothermal fluid as it moves upward and outward
from a central conduit. In the inner zone, biogenic carbonates are dissolved by high
temperature hydrothermal fluids (120-270°C) and Mg smectites deposited due to
combination of hydrothermal silica with Mg from the down welling sea water. Carbonate concretions are deposited at the transition between the zones, and in the
outer zone, at temperatures below -100°C by laterally migrating evolved fluids.
Isotopic evidence indicates that a major part of the carbon in the carbonate is derived by
bacterial oxidation of hydrothermal methane. Goodfellow et al. (1993) interpret that
lateral flow of the evolving hydrothermal fluids along sandy layers is promoted by
poor cross-stratal permeability, in the outer zone, during episodic "throttling'~ of
fracture permeability in the upflow zone by mineral precipitation.
Turner et al. (1993) suggest that areas of diffuse venting of hydrothermal fluids in the
AAV may "eventually create impermeable crusts, restrict discharge, and pond
hydrothermal fluids below an impermeable cap. Rupture of these caps may result in
discharge at discrete centres, prerequisite for the initiation of chimney growth and
mound development."
Similarly, a recent interpretation of present day sea floor hydrothermal activity at the
Valu Fa Ridge in the Lau Basin, by Fouquet et al. (1994), proposed that formation of
an impermeable crust (of Fe-Mo oxides in that case) occurs at an early stage by diffuse
low temperature venting in highly permeable volcaniclastics which permit mixing of
hydrothermal fluids and sea water. Sea water mixing is reduced by progressive
sealing of the sea floor surface and the resulting higher temperature subsurface fluid
circulation causes intense, alteration and Cu-Fe sulphide precipitation in horizontal
layers beneath the crust (Figure 34). Continued tectonic activity may create fault
controlled permeability leading to focussed discharge and formation of sulphide
mounds on the sea floor.
This brief review of a number of ancient and modem deposits shows that carbonate
and carbonate-chlorite (or Mg-smectite) assemblages are not unusual in VHMS
alteration systems. Several of the occurrences have such assemblages restricted to
thin stratiform zones and the authors cited invariably refer to them as hydrothermal alteration products. This supports the assertion, made in Section 73, that quartz
carbonate-chlorite (or Mg-smectite) assemblages, produced by hydrothermal alteration
of myolitic rocks, are feasible precursors to the chlorite-tremolite-carbonate
metamorphic assemblages associated with mineralisation at Thalanga. On the other
hand, there is little evidence apart from their thin stratiform distribution, that they are
exhalativesedlinen~.
South Hercules and the AAV carbonate bearing alteration zones appear to be
analogous to West Thalanga in tenns of their broad lateral extent and thin stratabound
64
A ~ HINE HINA 1850m
~ . .,...~;::.:;;;::::::::::..... 1vOLCANIC STAGE
.:-:...... 200m J� \. ".' ~ --.J
B
1700m
C
1900m TECTONIC STAGE
Hi:.::.::! 2 bY::;:;:J 3 [::::=J 5
Figure 34.� Schematic cross sections of the three hydrothermal deposits on Valu Fa Ridge, Lau Basin; (from Fouquet et al., 1993). Hine Hina (top) represents an early stage of activity in which diffuse discharge of low temperature hydrothermal fluids mixing with sea water in the upper zones of permeable volcaniclastics causes precipitation of an Fe-Mn crust. Sea water mixing is reduced as the crust develops and seals the sea floor; this forces lateral flow of higher temperature fluids to produce a stratiform zone of intense alteration and deposition of Fe-eu sulphides in horizontal layers beneath the crust. Tectonic activity may create fault controlled permeability leading to focussed fluid discharge, development of stockworks along the faults and formation of sulphide mounds and chimneys on the sea floor; e.g. Vai Lili (centre). Lithological reference: 1= Fe-Mn crust and sulphide impregnation in vo1caniclastics; 2= massive sulphides; 3= brecciated andesite; 4= pillow lavas; 5= massive andesite.
65
character.
Possibly significant differences are:
*� the extremely low aspect ratio of the West Thalanga chlorite-tremolite-carbonate
zone which has a thickness range of O.5-2Om, up to 350m dip extent and,...8OOm
strike extent compared with 20-4Om thickness and ~300m strike extent at South
Hercules,
*� the high Mu contents of carbonates at South Hercules which may be a district scale feature since similar compositions occur at Rosebery,
*� the prominence of volcaniclastics and fine grained sediments in the host sequences at South Hercules and AAV contrasting with apparently coherent (?) felsic lava
dominated sequences at Thalanga.
South Hercules and the AAV alteration zones have both been interpreted to have
formed by sub-sea floor replacement of permeable sediments in the mixing zones of
upwelling relatively low temperature hydrothermal fluids and sea water. In both cases
there is lateral and vertical alteration mineral zonation, considered to have been strongly
influenced by local permeability contrasts, with carbonate deposited in the outer and
upper, cooler, parts of the hydrothermal system.
The presence of a cap rock, either of impermeable shaley sediment or a hydrothermal
mineral crust formed by diffuse sea floor venting, seems to be an important factor in
forcing hydrothermal fluids to migrate laterally in the sea floor substrate.
At Thalanga, the extensive, more or less stratiform, zone of footwall quartz-sericite
(chlorite-pyrite) alteration suggests lateral hydrothermal flow, at least in the upper parts
of the footwall. The strongest pyritic footwall alteration at West Thalanga is partly
confined within a crudely bedded unit of silty-sandy-quartz crystal rich rhyolitic
volcaniclastics and form~. a conformable zone, no where much greater than about 25m thick and thinning up dip, in the immediate footwall of the favourable horizon (Figures
5 & 6). At Central and East Thalanga this style of alteration is more intense,
characterised by stringer pyrite and minor chalcopyrite mineralisation and it forms broadly stratiform zones upto .5Om thick which also suggest some type of layered
permeability contrast. The rocks in adjacent less altered parts ofthe footwall appear to
be largely coherent rhyolites although the obscuring effect of alteration is undeniably
strong and some of the footwall rocks have a relict or pseudo? fragmental fabric and
may have originated as hyaloclastite breccias with presumably high permeability.
Sea floor capping presents a possible explanation for development of extensive
stratiform high temperature alteration zones in the substrate and may also partly control
the mixing/cooling reactions thought to be responsible for carbonate-chlorite alteration.
A sudden rupture of the cap by tectonic disturbance, leading to discrete venting of ponded hydrothermal fluid, could promote entrainment of cold sea water from the
lateral fringes of the capped area and deposition of Mg-silicates and carbonates at
retreating mixing fronts below the cap. Alternatively, a cap of limited permeability, as
for instance in clastic sediments, could act as a diffuser creating broad areas of diffuse
66
venting and developing a patchwork of small overlapping hydrothermal upflow and sea
water recharge zones with Mg-silicates and carbonate alteration minerals deposited at
waxing and waning mixing fronts eventually leading to massive chemical modifi~ation
of the substrate. In an areally extensive permeable substrate, progressive sealing of
the surface by carbonate and chlorite alteration could lead to lateral hydrothermal flow
and incremental growth around the fringes of the alteration cap and, as the cap spread
outwards, development of the lower stratiform suIphide and silica-pyrite alteration
zones.
At Thalanga the "quartz-eye volcaniclastic" (QEV) unit(s) in the upper part of the
favourable horizon may have acted as cap rocks but this concept, standing alone, has
difficulty explaining why chlorite-tremolite-carbonate rocks are ubiquitous in West
Thalanga and fairly minor in East Thalanga when QEV exists at both. Gen~rally, the
QEV is thickest over the East Thalanga ore body, thickening up dip where it is
intruded by co-magmatic quartz-feldspar porphyry (QFP) sills but thinning sharply to
east and west along strike off the fringes of the deposit (compare Figures 3, 4 & 5). It may be argued that the thicker zones of QEV were more effective cap rocks, inhibiting
influx of sea water thereby excluding the source of Mg, maintaining the system at high
temperature and suppressing production of Mg carbonates and silicates. If so,
chlorite-tremolite-carbonate zones would be expected around the fringes of the deposit,
where the QEV thins out, but that is not borne out by observations. Replacement
mineralisation exists within the QEV at East Thalanga in the form of stratabound and
disseminated "hanging wall lenses" (mine terminology), and some, particularly in the
stratigraphically upper parts, is associated with patchy chlorite-tremolitelactinolite
carbonate alteration. This contrasts with West Thalanga where most sulphide
mineralisation and chlorite-tremolite-carbonate rocks lie stratigraphically below the
QEV unit. There is evidence in the occasional massive sulphide clasts within QEV,
to suggest that QEV was emplaced by mass flows which covered a largely preformed
sheet or low mound of massive sulphide. Accordingly, the QEV hosted sulphides and
associated minor chlorite-tremolite-carbonate alteration, at EastThalanga may represent
late stage mineralisation from hydrothermal circulation which persisted after QEV mass
flows covered the main, footwall, sulphide lens.
The hanging wall dacite is not considered a likely cap rock; it also is ubiquitous over
the orebodies as thick coherent flow(s?) of rather uniform composition and character
and, as noted previously, its emplacement probably post dated alteration and sulphide
mineralisation and perhaps was a factor in "shutting down" the hydrothermal system.
The difference between West and Central to East Thalanga may have been controlled
by the temperature and flow rates of hydrothermal fluids. Central and EastThalanga
are regarded, on the basis of their more extensive zones of intense quartz-(sericite)
pyrite footwall alteration and centres of elevated copper grades, as areas of high
temperature and high fluid flux promoted by high permeability in presumably intensely
fractured or brecciated but formerly coherent and glassy rhyolitic lavas. The rapid,
diffuse discharge of high temperature fluids from multiple vents may have produced
67
overlapping mounds and hydrothennal crusts and allowed no opportunity for sea water
influx thus inhibiting the cooling-mixing reactions which form carbonate alteration.
West Thalanga, in contrast, is interpreted as an area of lower temperature hydrothermal
discharge (analogous to the AAV and South Hercules), draped with a veneer of bedded
rhyolitic volcanic1astics which partly controlled lateral migration of upwelling fluids
and provided a permeable unconsolidated surface sediment permitting gradual diffuse
seepage of hydrothermal fluids and mixing with continually replenished pore space sea
water. Carbonate precipitation and Mg-chlorite or smectite alteration of rhyolitic
volcanic1astic material occurred in the zone of mixing and cooling in the few metres of
sediment immediately below, and perhaps right up to, the sea floor and sulphides were
progressively deposited in the deeper, higher temperature zones of the vertically
condensed thermo-chemical system. Lateral propagation of the system, caused by
progressive sealing of the surface layers, produced the thin but extensive, stratiform
sulphide and alteration zones.
This model, schematically presented in Figure 35, accounts for the thin, extensive
stratiform and stratabound distribution of the West Thalanga chlorite-tremolite
carbonate rocks, their intimate association with sulphides and it also accommodates the
following features of the deposit.
*� The zone of pyritic quartz-sericite footwall alteration is relatively thin, weak and not
fully co-extensive with the chlorite-tremolite-earbonate and sulphide lens. It thickens and intensifies down dip reaching an (apparent) maximum below 600RL
on Section 20110E (Figure 5) where it is more siliceous, contains blebby
disseminated sphalerite in addition to pyrite and may represent a low temperature
feeder zone. By contrast, pyritic footwall stringers in East Thalanga contain minor
chalcopyrite, but seldom sphalerite, indicating higher temperature.
*� Most of the QEV is relatively unaltered and unmineralised except for pervasive
quartz-epidote-actinolite-chlorite andfootwall style quartz-sericite alteration which is
localised stratigraphica1ly above the inferred feeder system, at the down dip fringe
ofthe ore body. This may reflect a focus of upwelling fluids which persisted
during the waning stages ofhydrothermal activity after mass flow emplacement of
the QEV.
*� Semi massive barite and magnetite-quartzite exist in the upper part of the favourable horizon and are best developed in the distal up dip and western parts of the ore body
consistent with expected low temperature hydrothermal activity and fluid-sea water
nnxmg.
*� Massive sulphide and particularly massive pyrite lenses tend to be concentrated
toward the footwall of the chlorite-tremolite-carbonate zone. Some massive pyrite
lenses seem to be enclosed by chlorite-tremolite-carbonate rocks but they are partly
enveloped by shear zones and may be fault emplaced slices.
*� Six narrow intervals of chlorite-tremolite-earbonate alteration separated by zones of
less altered rhyolite and rhyolitic volcaniclastics occur in drill hole TH245 where the
68
I
seaftOOfshallow convecting cold sea water
--~~\- /_~ / ~~*,&:;ffi '/ - -----~w~~~:~1»<""",.":%""~=,,:~,,,.= ..� ,..r- carbooa:chlorite~
permeable rhyolitic volcaniclastics .- disseminated Zn-Pb sulphide deposition in hydrotherrnalJsea water miXing zones
\ ..... /'/" /� I
coherent or less permeable rhyolite /�
/
, /I \ \ \
/ 1 ascending hydrothermal ftuids, un-focussed, relatively lowtemperature
carbonate-ehlorite alteration seals upper mixing zone of permeable zone causing lateral flow of hydrothermal fluid
---'...----
" '"
massive sulphide deposition and pyrite stringer zone alteration \ in lower parts of permeable layer as system heats
up and spreads laterally (
2\(
deposition of QEV mass1Iows and intrusion of co-magmatic sills
\.
, /"" ,� I
I shallowsea water convection substantially reduced,
,./ continuing hydro1hermal activityextends footwaJl type
\ " I silicification and stringer zone into QEV wtIh deposition of minor sulphides in upper layers
Figure 35.� Sequential schematic cross sections illustrating genetic model for West Thalanga type sulphide deposit and associated thin, laterally extensive chlorite-carbonate and siliceous-pyritic stringer alteration zones.
3
69
8
favourable horizon is offset southwards about l50m down dip of the ore lens,
(Figures 5, 18). These alteration lenses or bands are not specifically at the
favourable horizon but are within the footwall rocks upto about 40m stratigraphically below the favourable horizon. They may represent zones of
lateral hydrothermal and sea water mixing around the outer margin of the system
similar to the lateral interfingering of carbonate facies and sulphide facies reported at South Hercules (Khin Zaw and Large, 1992).
CONCLUSIONS
Titanium, aluminium and zirconium remained chemically immobile in Thalanga
volcanic rocks during episodes of hydrothermal alteration and subsequent upper
greenschist facies metamorphism.
Immobile elements can be used to identify the volcanic parentage of rocks otherwise
altered beyond recognition and to estimate the mass changes of mobile chemical
components. The most intense alteration, of rhyolites in the footwall of the massive
sulphide deposit, involved complete loss of sodium and major absolute mass gains of
silica, iron and sulphur, which together added about 6Og/100g to the rocks.
Alteration mass gains of this order indicate interaction with great volumes of
hydrothermal fluids. It is evident, by the preservation of primary TilAl and Ti/Zr ratios and the systematic variations in their concentrations relative to alteration intensity, that Ti, Al and Zr were not dissolved and transported by the fluids which
altered the footwall rocks. It follows that any exhalative deposits precipitated from
these fluids would have very low concentrations of the immobile elements.
The majority of rock types formerly interpreted as exhalites have immobile element
concentrations and ratios remarkably similar to the range of altered footwall rhyolites.
It is consistent with their immobile element geochemistry and textural-mineralogical
gradations, to interpret the chlorite-tremolite/actinolite-carbonate-etc. assemblages of
Groups 26 and 31-35, as altered rhyolitic rocks. The chlorite rich types in Groups 31
and 32 could have formed by alteration dominated by loss of Si and gain of Mg
effectively by progressive chloritisation of aluminous phases in the rhyolite.
Tremolite-carbonate dominated assemblages of Groups 33, 34 and 35 could have
formed by metamorphism ofcarbonate-quartz-chlorite precursors derived from
70
rhyolite by hydrothermal additions of (progressively increasing) Mg, Ca and CO2.
Comparable chlorite-carbonate assemblages are known, though perhaps not well .
documented, in the wall rock alteration systems of a number of (less metamorphosed)
Palaeozoic and Holocene volcanic hosted massive sulphide deposits. They appear to be developed in relatively cool, or the cooler parts of, submarine hydrothermal
systems; in the mixing zones of sea water and upwelling hydrothermal fluids, not far below the sea floor.
It is inferred that West Thalanga, in contrast to Central and East Thalanga, was formed
in a cool, low intensity hydrothermal system, thereby accounting for the distribution of
chlorite-tremolite-carbonate (meta) alteration assemblages. The thin stratiform
character of the chlorite-tremolite-carbonate and quartz-sericite-pyrite alteration zones,
and associated sulphide lenses, may be attributable to diffuse hydrothermal discharge
through a permeable volcaniclastic sea floor substrate and lateral hydrothermal flow
promoted by progressive sealing of the surface layer.
Thalanga magnetite-quartzites (Group 21) contain very low concentrations of
immobile elements, have a simple mineralogy and major component chemistry similar
to less metamorphosed exhalative quartz-hematite ironstones from elsewhere in'the Mt
Windsor SulrProvince and are reasonably interpreted to be thermally metamorphosed
equivalents. Magnetite-quartzite exists at several stratigraphic horizons in the
Thalanga area, forms but a minor component of the main mineralised horizon, and is
the only rock type (possibly excluding massive sulphides and barite) which is
unequivocally of exhalative origin.
Very siliceous rocks (Group 22) which share some chemical and mineralogical
similarity with magnetite-;:quartzites and silicified rhyolite may have formed by
sedimentary contribution ofminor rhyolitic volcaniclastic material into silica-iron
exhalites. A tentative alternative is that they originated by massive silicification of
high porosity rhyolitic rocks by sub sea floor reaction with cool hydrothermal fluids
similar to those which precipitated the exhalative silica-ironstones at surface.
71