petrology of graphitic sulfide-rich schists from south ... · occur with prograde metamorphism. a...

American Mineralogist, Volume 66, pages 908-930, IggI

Petrology of graphitic sulfide-rich schists from south-central Maine: an example ofdesulfidation during prograde regionar metamorfhi.r,

--_ - --

JoHN M. FrnRy

Department of GeologyArizona State UniversityTempe, Arizona 85281

Abstract

Graphitic sulfide-rich schists from a Buchan-type metamorphic terrain contain assem-blages of silicate, sulfide, oxide, and carbonate minerals meiamorphosed from chloritethrough sillimanite zone conditions. Sulfidic marine black shales, the protoliths of the graph-itic sulfide-rich schists, contain pyrite but either only traces ofpyrrhoiite or no pyrrhotite atall' The apP€arance of abundant pyrrhotite in the graphitic sutnOe-rich schists, therefore,documents the conversion of pyrite to pyrrhotite during metamorphism. Mineralogical, min-eral chemical, and whole-rock chemical data for the sulfide-rich schists indicate th;t the py-rite + pyrrhotite transition occurs by desulfrdation at constant whole-rock iron contentswithout participation of silicate minerals. The metamorphic reaction that converts pyrite topyrrhotite is unusual in two respects: (a) products and reictants are quenched in at least somespecimens in all metamorphic zones studied and (b) the reaction has gone to completion insome samples from each metamorphic zone. Increasing temperature (lrade) is thus unlikelyto exercise a dominant control over the reaction. Consideration of the amounts of iron sul-fides in the schists and the composition of C-O-H-S fluids with which the rocks were in equi-librium leads to the conclusion that enormous volumes of fluid flushed through the schistsduring the metamorphic event (volumetric fluid-rock ratios > l). The flow of large volumesoffluid through the schists serves as a mechanism that drives the desulfidation reaction.

Introduction

Iron sulfides, pyrite and pyrrhotite, are constitu-ents of many pelitic schists, and the sulfides reactboth with each other and with other phases in schistsduring prograde metamorphism. Carpenter (1974),for example, mapped an isograd based on the con-version of pyrite to pyrrhotite in pelitic schists fromTennessee and North Carolina. Robinson and Tracy(1976) and Guidotti et al. (1976) found systematicrelationships between the compositions of Fe-Mgminerals and the identity of coexisting iron sulfidesin schists from Massachusetts and Maine, respec-tively. Guidotti (1970a) used sulfide-silicate equilib-ria to estimate the composition of fluid in equilib-rium with pelitic rocks during metamorphism. Inspite of the common occurrence of iron sulfides inschists, there is little detailed knowledge of the min_eral reactions in which pyrite and pyrrhotite partici-pate over a wide range of metamorphic grades. Thisreport presents mineralogical and chemical data for a

suite of graphitic sulfide-rich schists from chloritethrough sillimanite grades. The first goal of the re-port is to examine mineral equilibria in these rocks asa function of grade and, specifically, (l) to determinewhether the commonly observed pyrite-pyrrhotitetransition may be mapped as an isograd; (2) to deter-mine the mineral reaction that is associated with thepyrite-pyrrhotite transition; and (3) to deterrninewhether any other reactions involving iron sulfidesoccur with prograde metamorphism.

A second goal of this report is to use mineralogicaland chemical data from the graphitic sul_fide-richschists to evaluate whether significant mass transportof sulfur occurs during prograde metamorphism.Mass transfer of sulfur during metamorphism is acontentious issue. Some have found evidence fortranspoft of sulfur around metamorphosed sulfidedeposits (Fullagar et al., 1967; popp, 1977), whileothers have argued that mass transfer of sulfur doesnot normally occur during metamorphic events(Thompson, 1972; Robinson er al., 1976).Iron sul-

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FERRY: SULFIDE GRAPHITE SCHISIS 909

fides are the principal repository for sulfur in peliticschists. Consequently the petrologic behavior of sul-fides is important because they are the main clue forelucidation of ryhether mass tansport of sulfur oc-curs during metamorphisn.

Geological seaing

Samples were oollected from the Silurian Water-ville and $engerville Formations in south-centralMaine (Fig. l). Samples from the Sangerville Forma-tion were collected within 100 m of its contact withthe Waterville Formation. The fonnations are oom-posed of interbedded shale, argillaoeous limestone,and argillaceous sandstone and their metanorphicequivalents. Compmiti,onal layering is on a scale ofl-8 cm. Most samples of graphitic sul-fide-rich schistwere collected from discontinuous but mappable ho.rizons. Three sarrples (l%A" 7llB, 9l9G) were col-lected from graphitic sulfide-rich beds in outcrops ofothcrwise normal pelitic schist. The three samples arenot unusual in composition rel,ative to samples col-lected from mappable horizons of graphitic sulfide-rich schist.

The area in Figure I is a Buchan-type metamor-phic terrain, and Osberg (persond *66'rnicatio41976) has mapped isograds in the Waterville Forma-tion based on the first appearancc of biotite, garnet,staurolite + andalusite, and sillinanite in peliticschists. Metasediments are folded into isoclindly re-folded recumbant folds (Osberg 1979) and are in-truded by two-mica qvarlz monzonite stocks. Por-phyroblasts of garnet, staurolite, andalusite,cordierite, and sillimanite cut across schistosity asso-ciated with the isoclinal folds in the area. The peakof regional metamorphism followed almost all thedeformation. Ferry (1978) has argued that meta-morphism and intrusion of the granitic rocks were atleast broadly synchronous. Geochronologic work onboth the granitic and metamorphic rocks indicatesthat the metamorphism and granite emplacement oc-curred during the Devonian Acadian Orogeny (Dall-meyer, 1979). Evidence for three metamorphic epi-sodes (one well to the west of the present area) hasbeen reported for rocks of the western part of the Au-gusta Quadrangle by Novak and Holdaway (1981).They suggest that low-variance assemblages and lim-ited textural evidence of disequilibrium inply pres-ence of two metamorphic episodes (possibly M, andM, of Guidotti, 1970b) in the area immediately westof the present map area. If there were two metamor-phic events in the present area, they were closely-spaced in time and P-T conditions and the rocks in

a{:m'

Fig. l. Geologic stetch map of study area. Data from Osberg(196E; pcrsonal communication); Barker (1964); Ferry (1978).

question appear to have entirely equilibrated to thelast and most pervasive event (possibly Mr).

Pressure and temperature

Estirnates of pressure and temperature attainedduring metamorphism in the study area are summa-rized by Ferry (1980b). Pressure was 3500+200 barsnear the sillimanite isograd. Temperatures rangedfrom - 375"C at the biotite isograd to 550"-575'C inthe sillimanite zone. Mean temperatures for themetamorphic zones in Figure l, based on the biotite-garnet Fe-Mg exchange geothermometer, are: garnetzone,460oC: staurolite + andalusite zone, 490oC; sil-limgile zone, 550oC.

M et ho ds of inv e sti gation

Nineteen samples of graphitic sulfide-rich schistswere collected. Sulfide-rich schists were differenti-ated from normal pelitic schists by their greater sul-fur content (arbitrarily taken as > I weight percentsulfur). The locations of samples used in the reportare given in Figure l.

i[/|.

3r''

/ g..Phitic ldfido-

/ r i ch .ch la la

Sv: Vaaslboo Fofdtion

Ss. Watarvilla & SangatvillaFomationr, sndittatdliattd

-- - - airatig.rplric lendary lEisanwaldvilL & \baalboto Fdmaliona

---- i.ogr.d. h p.litic !chi.t.,h.chs..l on hlghg.aata rffi

FERRY: SULFIDE GMPHITE SC//ISIS

Thin sections were prepared from rock specimensthat correspond in outcrop to individual beds ofgraphitic sulfide-rich schist. At some outcrops morethan one bed was sampled. Most samples appear ho-mogeneous in hand specimen. A few specimens con_tain sulflde-rich laminations with spacings of approx_imately 2-5 mm. Thin sections, prepared from slicesthat cut across the laminations, were sufficiently largeto sample a statistically homogeneous volume oflaminated rock. Mineral assemblages were identifiedpetrographically using transmitted and reflectedligh1. 11 was arbitrarily assumed that a group of min-erals in the same 2 mm diameter circular domain of afhin 5s6fie1 were in equilibrium. Obvious cases oftextural disequilibrium such as inclusions of onemineral in another or mantling of one mineral by anaggregate of another were exceptions to this assump-tion.

Corrpositions of minerals were obtained with anautomated MAC electron microprobe at the Geo_physical Laboratory, Carnegie Institution of Wash-ington. Data for silicates were reduced by the methodof Bence and Albee (1968) with correction factors byAlbee and Ray (1970). Data for sulfides and carbon-ates were reduced using the MAGIC IV computer pro-Eram, a zAF correction scheme (Colby, l97l).

Modal amounts of selected minerals were deter-mined by counting 200H000 points in thin section.In general, modal analysis of sulfides in reflectedlight posed no problems. Products of surficial weath-ering, limonite and marcasite, were counted as freshpyrite or pyrrhotite according to the kind sf sulfidethat it was associated with. Sample 240,{ contains asubstantial amount of secondary marcasite. The mar-casite was counted as pyrite and pyrrhotite in theproportions in which fresh pyrite and pyrrhotite oc-cur in the rock. Modal analysis of most silicates washampered in all samples except those from the sil-limanite zone by the fine grain size of the rocks andby finely divided graphitic material. The ubiquitousgraphitic material tends to obscure all silicates exceptbiotite. Volume amounts of minerals were convertedto molar amounts (per reference volume of rock) us-ing molar volume data (Robie et al., 1967; Hewittand Wones, 1975).

Chemical analyses of rock samples for major ele-ment metal oxides were performed using inductivelycoupled argon plasma methods by Technical ServiceLaboratories, Mississauga, Ontario (A. Debnam, an-alyst). Samples for chemical analysis, approximately5 cm' in volume, were cut from hand specirrens asclose as possible to where chips were removed to pre-

pare thin sections. Table I lists analyses of a MountHood andesite which is used as a secondiry standardin the XRF laboratory in the Department of Chemis-try at Arizona state University. Reliability of theanalyses may be assessed by comparing the composi-tion of the Mount Hood andesite with the analysis ofthe same material performed by Technical ServiceLaboratories. Whole-rock samples were analyzed forsulfur by Technical Service Laboratories. Sampleswere fused at over l800oc in a teco combustion fur-nace. During fusion all sulfur was evolved as SO,gas. The amount of sulfur was measured by a vol-umetric titration of the SO, gas. Whole-rock sampleswere also analyzed for total carbon and sulfur byC. F. Lewis in the laboratory of C. B. Moore at Ari-zona State University. Analytical methods used forsulfur at A.S.U. are those of Moore et aI. (1972). Forthe determination of carbon, samples were com-busted in a flowing oxygen atmosphere in an induc-tion furnace at over l80O.C to form COr. After puri-fication of the efluent gases, the amount of CO, wasmeasured using a LEco762-0W carbon analyzer withan infrared detection system.

Mineralogr and mineral chemistryThe mineralogy of all samples and selected com-

positional parameters for minerals are compiled inTable 2. Table 3 presents modal proportions of sul-fides in all samples and of biotite in selected samples.

Sulfides

All samples contain pyrrhotite which occurs aselongated anhedral grains that poikolitically encloseqrrurtz and feldspar near their margins. Grains havelong dimensions that vary within each specimen from30 to a maximum of 50G-2000 microns. There is no

Table l. Comparison of composition of andesite from MouatHood determined in the XRF laboratory, Department ofChemistry, Arizona State University with conposition of same

material determined at Technical Service Laboratories.

% difference relativeto ASU value

S i0zA l z 0 :Fe203Ca0MgoNa20(20Ti0zMn0Paos

60 . 5618 . 09

o - 4 2

6 . 1 52 . A 84 . 3 21 . 0 90 . 9 40 . r zo , 2 2

5 8 . 9 91 7 . 8 97 . 0 a6 . 5 52 . & 34 . 5 40 . 9 11 . 0 00 . 1 00 . 2 1

2 . 61 . 19 . 06 . 2r . 75 . 1

1 6 . 56 . 4

l o . /

FERRY: SULFIDE GRAPHITE SCI//SIS

Tablc 2. Mincral asscmblage and mineral composition data for graphitic sulfide-rich schists{

9 l l

Maj orMineral s

Chlorite Zone2728 111 rA

Biotite Zone Garnet Zone274A 2748 196A tL22c

Staurol i te + Andalusi te

quart zpyr i tepyrrhotitefb iot i te Idoloni te 5

- + +Prag loc l ase | |K- fe ldspars 9nuscovite I I

AccessoryMineral,s

chlor i teSprehni terut i lesphenechalcopyr i tegraphi te**ca l c i t e9sider i tet

x xx o

0 . 8 9 8 0 . 8 9 5o o

0 . 1 0 0 0 . 1 s 80 . 0 1 0 . 0 1

o o0 . 9 5 0 . 9 5

o oo ox xo ox xx xo 0 . 2 4o o

x xx o

0 . 8 9 2 0 . 9 0 60 . 0 5 5 0 . 1 1 90 . 0 6 4 o0 . 3 7 0 . 3 2

o oo 0 . 9 7

0 . 0 6 3 oo ox xo ox xx xo o

F e O . 8 8 F e O . 9 1MnO.02 Mn0 .04CaO. 10 CaO.05

x xx xx 0 . 9 1 6

0 . 0 s 6 0 . 0 4 7o o

0 . 4 0 0 . 4 7o 0 . 9 5

0 . 9 8 o

x xx o

0 . 8 9 9 0 . 9 1 60 . 0 4 7 0 . 3 7 8

o o0 . s 1 0 . 4 20 . 9 5 o

o 0 . 8 5

oxoxxxoo

ooxoxxoo

x x xx x x

0 . 9 0 0 0 . 8 9 6 0 . 9 0 2o . 0 4 2 0 . 0 4 6 0 . 0 3 5

o o o0 . 5 7 0 . 5 5 0 . 7 s

o 0 . 9 5 o0 . 9 6 o o

oooxxxoo

oxxxXXo

oxxoxxoo

oxxxxxoo

oXxxxxoo

Maj orMinerals

Si l l inani te Zone71 1B 965A 967A 970A 1103 -1 1106 -1 1107 -1 LL07 -2

quart zpyritepyrrhot i te t

b i o t i t e 3 "dolomite '

- -L+p lag loc l ase ' IX- fe l dspar s ernuscoviieS 5

AccessoryMineral s

chlor i te 5prehniterut i lesphenechalcopyritegraphi te**ca l c i t essider i te#

x xo o

0 . 9 4 0 0 . 8 9 20 . 3 1 1 0 . 0 7 4

o o0 . 7 2 0 . 8 80 . 9 6 x0 . 9 8 o

x x xx o x

0 . 9 0 9 0 . 9 2 0 0 . 8 9 40 . 0 1 8 0 . 4 6 7 0 . 0 4 8

o o o0 . 9 3 0 . 9 2 0 . 8 s

o 0 . 9 3 0 . 9 3o o o

x x xo o o

0 . 9 1 8 0 . 9 1 2 0 . 9 1 80 . 1 5 7 0 . 1 8 1 0 . 1 4 8

o o 00 . 4 1 0 . 4 4 0 . 8 90 . 9 4 o o0 . 9 7 0 . 9 5 o

oxxxxxo

ooxoxxoo

ooxoxxoo

oooxxxoo

oooxxx

o

ooxoxxoo

ooxxxxo

ooxxxxoo

x wpnesent; o=abeent; ** gzaphite oz' graphitie naterial;

E Ee/(Fe+Mg); 59 K/(K+Nd; # Molar proportione of Fe, Itn,

t z, Fe"S; +t Xan;

and Ca cornponents.

systematic ch4nge in grain size with increasing meta-norphic grade. In some samples large pyrrhotitegrains tend to be aligned in discontinuous layers withthe distance between layers 2-5 mm. Pyrrhotiteranges in composition from Fen.rrS to Feo*S. Al-though sulfides were analyzed for Fe and S only, thesum Fe * S suggests that elements other than Fe andS, if present, occur in concentrations less than Iweight percent. Comparison of the measured compo-

sition of pyrrhotite coexisting with pyrite (Table 2)with th€ d41a ef feulmin and Barton (1964) indicatesthat pynhotite compositions have changed followingmetamorphism as is commonly observed elsewhere(e.9., Guidotti, 1970a). Some pyrrhotite in mosr spec-imens is slightly altered to marcasite along fractures(cl Ramdohr, 1969, Fig. 415). The alteration is at-tributed to surficial weathering and affects less than3Vo of pynhotite in all samples except 240A.

et2 FERRY: SULFIDE cRAPHITE SCI//SfS

Table 3. Modal percent pyrite, pynhotite, and biotite in graphiticsulfide-rich schists

were observed in thin section that would suggest dis-equilibrium either between pyrite and pyrrhotite orbetween sulfides and silicates.

All samples contain chalcopyrite which occurs asanhedral equant grains 10-30 microns in diameterand in close association with pyrrhotite.

Graphitic material

All specimens contain graphitic material. In thechlorite, biotite, garnet, and staurolite * andalusitezones it occurs as minute (< 5 microns) grains of un-known crystalline character along grain boundariesbetween silicates and sulfides. The graphitic matter isdistributed evenly throughout most samples, but in afew specimens it is concentrated in 5-10 rricron lay-ers that are separated by approximately 50 microns.In the sillimanite zone, rocks contain well-crystal-fized grains of graphite which have long dimensionsof ap*proximately 50 microns.

Sheet silicates

All rocks in the biotite zone and at higher gradescontain biotite. In specimens from the biotite, garnet,and staurolite + andalusite zones, biotite is evenlydistributed throughout specimens but occurs in twosizes. Most biotite is subhedral with a long dimensionneat 70 microns. Near sulfide grains, however, biotitegrains are much larger, typically 200 microns long. Insamples from the sillima6l" zone, however, biotite isof roughly uniform size everywhere in thin sectionwith a long dimension of 300-1000 microns. Repre-sentative chemical analyses of biotite are given inTable 4. All analyses in Table 4 represent averages of2-6 individual determinations from a thin section ofone sample. Analyzed biotites are distinctive in theirwide range in composition: Fel(Fe*Mg) ranges from0.024.47.

Muscovite occurs in all samples from the chloritezone and in seven samples collected from the otherzones at higher grades. In rocks in the chlorite, bio-tite, garnet, and staurolite + andalusite zones mus-covite is evenly distributed as subhedral grains 100microns long. No large muscovite grains were ob-served near sulfide grains. In sillimanite zone sam-ples the long dimension of muscovite is rruch larger,500 microns. Representative muscovite compositionsare listed in Table 4.

One sample (274A) contains chlorite whose com-position is listed in Table 4. Chlorite forms subhedralgrains, 2(n x 70 microns in size, and is always assosi-ated with large biotite crystals near grains of sulfide.

Six samples contain traces of prehnite that occurs

SpecimenNunber

PercentPyr i te

PercentPyrrhot i t e

PercentBiot i te

2 7 2 81 1 1 1 4

27 4A2 7 4 8

196A1122G

240L919G100sA100781 0 1 1 A

7t789654967A970Ar l 0 3 - 11106 - 11107 - 1r roT -2

1 . 00

1 4 . sL O . 7

0 . 60

R O

8 . 68 . 1

b . u) 7

to .4z . )

t r3 . 8

4 . 83 . 1J . J

4 . 47 , 0

5 . 5

1 1 . r4 . 6

1 0 . 95 . 96 . 54 , 3

nd*

2 3 . 8nd*nd*

20 .6Z J . J

2 4 . 9nd*

J 4 . O

t 4 . 75 . 0nd*

1 . 70 0

nd*nd*

00

0 . 30

0 . 2000

* rnt deternined

Although both monoclinic and hexagonal pynho-tite are reported from the study area (Williamsonand Meyer, 1969), no systematic attempt to charac-terize pyrrhotite in this manner was made. Petro-graphic examination of selected polished samplescoated with a fine-grained magnetite precipitate(Scott, 1974) indicated that all or almost all pyrrho-tite is hexagonal.

Ten samples contain pyrite which oocurs as sub-hedral or euhedral grains that frequently containstrings of inclusions that are oriented parallel to fo-liation. Euhedral grains form squares or rectangles inthin section with dimensions that vary in each speci-men from 30-3000 microns. The grain size of pyritedoes not systematically change as a function of meta-morphic grade. Some but not all pyrite occurs as in-clusions within pyrrhotite. No rims of pyrite aroundgrains of pyrrhotite, such as those described by Guid-otti (1970a), were observed in rocks of this study.Several analyses of pyrite indicate that it is stoichio-metric FeS, with less than one weight percent ele-ments other than Fe and S. Approximately 5Vo of thepyrite in two sections (196A and 10078) is partiallyaltered to limonite (cl Ramdohr, 1969, Fig. 480).

Pyrite, pyrrhotite, and the silicate and carbonateminerals are intimately integrown in all samples ofgraphitic sulfide-rich schist. No textural features

FERRY: SULFIDE GRAPHITE SCI//STS

Table 4. Compositions ofselected sheet silicates from graphitic sulfide-rich schists

9 1 3

Biot i tes Muscovi tes2144 196A 7 t2ZG 10078

' l t tB 967A 1 l u 5 - l l t 0 / - r 2728 2748 1005A 1107 - I

TiOz *

lvtn0Fe0Na2OMgoA l a O sS i0 zKzoCa0Sun

K* *Na

7 . 7 90 . 5 37 . 0 10 . 1 5

7 7 . 7 21 9 . 7 13 9 . 7 69 . 7 90 . 0 1

9 6 . 4 7

0 . 8 7 80 . 0 1 9

0 . 4 1 11 . 8 6 00 . 0 3 00 . 0 9 30 . 4 3 2

t . 2 0 22 . 7 9 8

0 . 4 20 . 0 32 . 2 s0 . 361 . 2 6

3 3 . 6 049 . 05

9 . 2 30 . 0 0

9 6 . 1 7

FeMoMnTi

A IS i

u . 5 l u - 5 20 . 1 3 0 . 2 62 . 3 9 2 . 3 90 . t 2 0 . 0 5

2 2 . 8 1 2 2 . 5 6t 8 . 1 2 7 8 . 2 74 2 , 6 s 4 7 . 4 5

9 . 5 7 9 . 8 70 . 0 0 0 . 0 0

9 6 . 2 8 9 5 . 3 6

0 . 8 3 6 0 . 8 7 40 . 0 l s 0 . 0 0 6

0 . 1 3 6 0 . 1 3 82 . 3 3 7 2 . 3 3 70 . 0 0 6 0 . 0 1 40 .026 0 .0260 . 3 8 6 0 , 3 7 7

1 . 0 7 7 1 . 1 1 92 . 9 2 3 2 . 8 8 1

0 . 3 9 1 . 0 40 . 1 2 0 . 7 51 . 9 6 1 . 9 10 . 0 3 0 . 0 4

2 2 . 4 6 2 2 . 1 51 8 . 8 3 1 9 . 1 34 2 . 1 r 4 1 . 1 7

9 . 5 4 9 . 7 60 . 0 0 0 . 0 0

9 s . 4 1 9 5 . 9 5

0 . 8 4 0 0 . 8 5 90 . 0 0 4 0 . 0 0 4

0 . 1 1 2 0 . 1 1 02 . 3 0 9 2 . 2 8 00 . 0 0 6 0 . 0 4 30 . 0 2 0 0 . 0 5 30 . 4 3 4 0 . 3 9 9

1 . 0 9 6 1 . 1 5 72 . 9 0 4 2 . 8 4 3

r . 8 7 1 . 1 9 7 . 2 30 . 6 2 0 . 2 0 0 . 5 1

1 L . 7 t 0 . 8 0 1 . 9 80 . 0 4 0 . 0 8 0 . 1 0

1 4 . 5 0 2 3 . 5 3 2 7 . 9 718 . 75 L8 .24 18 . 493 8 . 9 6 4 7 , 7 8 4 1 . 8 99 . 8 4 9 . 9 8 9 . 8 80 . 0 0 0 . 0 0 0 . 0 0

96 . 30 9s . 80 96 . 05

0 . 9 0 6 0 . 8 7 4 0 . 8 6 90 . 0 0 4 0 . 0 0 9 0 . 0 1 2

0 . 7 0 6 0 . 0 4 5 0 . 1 1 31 . 5 6 2 2 . 4 r 2 2 . 2 5 70 . 0 3 6 0 . 0 1 0 0 . 0 2 80 . 1 0 0 0 . 0 5 9 0 . 0 6 20 . 4 t 2 0 . 3 5 r 0 . 3 8 8

1 . 1 8 4 r . 1 . 2 7 1 . 1 1 32 . 8 L 6 2 . 8 7 3 2 . 8 8 7

0 . 7 6 9 0 . 8 6 7 0 . 8 9 2 0 . 9 1 80 . 0 4 s 0 . 0 2 7 0 . 0 3 8 0 . 0 5 0

0 . t 2 2 0 . 0 7 3 0 . 0 4 4 0 . 0 3 10 .122 0 . 561 0 . 185 0 . L470 . 0 0 1 0 . 0 0 0 0 . 0 0 2 0 . 0 0 00 . 0 2 0 0 . 0 1 6 0 . 0 8 4 0 . 0 8 4r . 7 9 6 1 . 5 6 3 1 . 7 1 4 1 . 7 5 0

0 . 3 4 1 . 6 9 t . 7 00 . 0 4 0 . 0 3 0 . 0 11 . 3 3 0 . 7 9 0 . 5 90 . 2 2 0 . 3 0 0 . 3 93 . 6 5 1 . 8 7 1 . 4 8

26 .71 33 .06 34 . 055 2 . 1 3 4 7 . 4 2 4 6 . 2 81 0 . 2 2 1 0 . 5 7 1 0 . 8 20 . 0 0 0 . 0 0 0 , 0 0

9 4 . 7 8 9 5 . 7 1 9 5 . 3 3

0 . s 3 2 0 . 8 6 4 0 . 9 2 05.468 3 . 136 5 . 080

0 . 7 9 33 . 2 0 7

Chlor i te2 7 4 A

Prehni test122G 1005A 10078

T i O z * 0 . 0 0l" tn0 0.19F e O 5 . 6 5N a z O 0 . 0 1MgO 30.33A 1 z O g 2 3 . 0 8S i O z 3 0 . 3 5K z O 0 . 0 7CaO 0 .00Sun 87 .68

T i O z * 0 . 0 7MnO 0 .07F e O 0 . 2 8N a 2 O 0 . 0 0M g O 0 . 2 2A l z O s 2 4 . 6 0S i O z 4 3 . 7 2K z O 0 . 0 2CaO 26 .90Sun 95 .85

0 . 4 9 0 . 1 50 . 3 1 0 . 4 50 . 6 4 0 . 3 20 . 0 3 0 . o z0 . 3 2 1 . 5 4

2 3 . 8 9 2 4 . 1 34 3 . 2 4 4 3 . 6 80 . 0 4 0 . 0 3

2 5 . 9 4 2 5 . 0 39 4 . 8 9 9 s . 4 0

1 . 9 2 8 1 . 8 4 20 . 0 3 4 0 . 1 5 70 . 0 3 7 0 . 0 1 80 . 0 1 8 0 . 0 2 50 . 0 2 6 0 . 0 0 71 . 9 5 4 I . 9 5 33 . 000 3 . 001

FeMgMnTiAIS i

0 . 2 8 34 . 2 1 90 , 0 1 30 . 0 0 02 . 5 5 t

2 . 8 3 1

Ca* *Mo

MnTiAISi

1 . 9 7 40 . 0 2 L0 . 0 1 50 . 0 0 20 . 0011 . 9 8 62 .996

* oride ueight pe"centsl x* eations/11 anhydrous otAgenatotns; # eations/l4 anhgdrous oxUgen atons'

as fan-like aggregates. Representative chemical anal-yses of prehnite are given in Table 4. The origin ofthe prehnite is uncertain. It only occurs in rocksmetamorphosed to garnet zone conditions or tohigher grades. Its peculiar crystal habit, however,suggests that it may be a retrograde product.

Feldspars and quartz

All specimens contain plagioclase feldspar. Inrocks from the chlorite, biotite, garnet, and staurolite+ andalusite zones plagioclase occurs as anhedral,

equant grains 10-30 microns in diameter. In silli-manite zone rocks plagioclase is larger, 50-70 mi-crons in diameter. All plagioclase in chlorite zonesamples is almost pure albite (Table 2). Plagioclasebecomes increasingly calcic in composition with in-creasing metamorphic grade; some specimens col-lected in the sillimanite(Anro-"').

Seven samples contain

zone contain anorthite

microcline and thev allwere collected in the garnet zone or at higher meta-morphic grades. In samples from the garnet and

9t4 FERRY: SULFIDE GRAPHITE SCI//STS

staurolite + andalusite zones microcline occurs inovoid spots 500-1000 microns in diameter. Somespots are single feldspar crystals while some are ag-gregat€s of 5-10 feldspar grains. The ovoid spots in-clude other minerals, usually some combination oflarge biotite grains, quartz, pyrrhotite, and pyrite. Insamples collected in the sillimanite zone, microclineis comparable in size and shape with plagioclase feld-spar (equant and 5G-70 microns in diameter). Ana-lyzed microclines are K-Na compounds but with avery restricted composition, )(, : 0.93-0.96 (Table2).

Quartz occurs in all samples. In specimens fromthe chlorite, biotite, garnet, and staurolite + andalu-site zones it is observed in two size ranges. Mostquartz is anhedral, equant, and 10-30 microns in di-ameter. Some larger quartz grains (100-300 micronsin diameter) are closely associated with pyrite andpyrrhotite. Sillimanite zone samples contain quartzof more uniform size, 50-100 microns in diameter.

Carbonates

Both samples collected in the chlorite zone andone in the biotite zone contain ferroan dolomite.Dolomite is evenly distributed through the rock asgrains 3G-50 microns in diameter. Compositionallythe dolomite is close to a Ca(Fe,MgXCOr), solid so-lution with measured Fel(Fe*Mg) : 0.06-0.16(Table 2). One sample (ll l lA) contains calcitegrains 20-30 microns in diameter.

Both specimens collected in the biotite zone con-tain siderite which occurs in aggregates of equant 50-70 micron diameter grains which partially rim somepyrrhotite grains and which are intergrown withlarge biotite and chlorite crystals. The siderite occursin small amounts (<l modal percent). It appears toreplace pyrrhotite and may be a retrograde product.Siderite is a (Fe,Ca,Mn)CO3 solid solution with com-position listed in Table 2.

Ti-rich minerals

a[ 5amples contain rutile andlor sphene. Rocks inthe chlorite and biotite zone only contain rutilewhich occurs as anhedraf poikolitic grains lG-30 mi-crons in diameter. Rutile retains the same crystal sizeand shape in garnet and staurolite + andalusite zonesamples. In the sillimanite zone specimens rutileforms subhedral grains with long dinensions of ap-proximately 100 microns. Sphene occurs in somesamples collected in the garnet, staurolite + andalu-site, and sillimanite aones as subhedral grains 40-60microns in their long dimensions. The composition ofrutile and sphene was not determined.

Mineral assemblages; changes in mineraloglt withmetamorphic grade

Mineral assemblages observed in the graphitic sul-fide-rich schists in each of the metamorphic zones aresummarized below. Accessory minerals are in paren-theses.Chlorite Zone: muscovite + ferroan dolomite +

quartz + albite + pyrrhotite + pyrite (+ rutile +chalcopyrite + graphitic material).

Biotite Zone: biotite + quartz * andesine + pyrrho-tite + pyrite t muscovite t ferroan dolomite (+rutile * chalcopyrite + graphitic material + sider-ite + chlorite).

Garnet Zone: biotite + quartz + andesine * pyrrho-tite + pyrite a muscovite f microcline (* sphene* chalcopyrite + graphitic material t rutile Iprehnite).

Staurotte + AndalusiteZone: biotite + quartz + an-desine/bytownite + pyrrhotite + pyrite t mus-covite t microcline (+ chalcopyrite + graphiticmaterial + rutile + sphene t prehnite).

Sillimanite Zone: biotite + quartz * andesine,/anor-thite + pyrrhotite + pyrite t muscovite t micro-cline (+ chalcopyrite * graphite * rutile + sphene+ prehnite).The principal changes in mineralogy of the graph-

itic sulfrde-rich schists occur over the transition fromchlorite to garnet zone conditions. In this interval themetamorphic rocks develop biotite, microcline, andsphene while ferroan dolomite is eliminated. At gar-net zone conditions and at higher grades the schistscontain a very simple mineralogy (ignoring acoessoryphases) that does not systematically change as afunction of grade: plagioclase + biotite + quartz +pyrrhotite + pyrite t muscovite t microcline. Thecompositions of some minerals change with meta-morphic conditions. Plagioclase systematically be-comes more calcic with increasing grade of meta-morphism. Although magnesian biotites are found insome rocks at all grades, biotite with Fel(Fe+Mg) >0.2 is found only in rocks from the staurolite * an-dalusite and sillimanite zones. Proportions of someminerals change as a function of metamorphic grade.The most significant of these changes is the ratio ofpyrite to pyrrhotite; the ratio is very low (0.00-0.02)in all samples collected in fts 5illimanite zone com-pared to the ratio in one or more samples collected atlower grades.

Whole-rock chemistry

Chemical analyses of the 19 graphitic sulfide-richschists are listed in Table 5. During analysis of the

FERRY: SULFIDE GMPHITE SCI'ISTS 915

whole-rock samples, iron is oxidized to FerOr; con-sequently total iron is reported as FetOr.

Much iron in the rock samples, however, is boundin sulfide rather than in oxide compounds. Represen-tation of total iron in Table 5 as Fe'O, artificiallyadds oxygen to the analysis that actually is not con-tained in the rock (the same would occur if total ironwere represented as FeO). Consequently, for manyrocks in Table 5, the sum of metal oxide, sulfur, andcarbon weight percents is greater than 1007o. Sumsgreater than lNVo are an artifact of representing totaliron as an iron oxide rather than as a combination ofiron oxide and iron sulfide. It would have been mostaccurate to represent total iron in Table 5 as an ap-propriate combination of FerOr, FeO, FeS, and FeSr.This was not possible both because the rock sampleswere not analyzed for oxygen and because whole-rock ferric-ferrous ratios were not determined. For-tunately, the arguments that follow require only aknowledge of total whole-rock iron and sulfur con-tents and not of how the iron is distributed betweenvarious sulfde and oxide compounds.

Analysts at Technical Service Laboratories ob-tained poor yields during loss on ignition for samplesthat contain appreciable carbonate (A. Debnam, per-sonal communication, 1980). Consequently the sumsfor samples 2728, 274A, and I I I lA are slightly low(97-98Vo). Analysis of the metallic element constitu-ents by inductively coupled argon plasma methods,however, is not dependent on a determination ofLOI. The measured concentrations of metals in sam-ples 2728,274A, and I I I lA, therefore should be re-liable.

Values for sulfur in Table 5 represent an un-weighted average of two duplicate analyses by C. F.Lewis at Arizona State University and one analysisby A. Debnam at Technical Service Laboratories.The individual measurements deviated from the av-erage usually by no more than +0.5 weight percent.Sulfur analyses for four samples in Table 5 are un-likely to be reliable. Samples 196A, 10078, and I107-2 contain some sulfide grains partially converted tolimonite during surficial weathering. Consequentlytheir given sulfur contents are likely to be too lowrelative to the unaltered metamorphic rock. Sample240A contains an appreciable number of pyrrhotitegrains partially converted to marcasite during surf-icial weathering. Consequently its stated sulfur con-tent is likely to be too high relative to the unalteredmetamorphic rock.

Student's t test (Leabo, 1972) was used in con-junction with the data in Table 5 to evaluate whetherthe schists underwent any statistically significant

changes in their content of major metal oxides (SiO',Al2O3, FerOr, MgO, CaO, NarO, K'O, TiOr) as afunction of metamorphic grade. Differences weretaken as significant if they occurred at a )95Vo con-fidence level. With the exception of NarO, averagemajor metal oxide contents of rocks (on a volatile-free basis) within a particular metamorphic zone arenot significantly different from average metal oxidecontents in each of the other four zones. The differ-ence in average NarO content between chlorite andsillimanite zone rocks is significant at a>99.9%o e;on-fidence level. Ferry (1980c) noticed a si.urilar, statisti-cally significant, decrease in the sodium content ofmetamorphosed carbonate rocks with increasingmetamorphic grade in the study area of Figure l.The sample base in Table 5 may be too small, how-ever, to confdently conclude that the schists lost so-dium during the metamorphic event. It is safe to saythat, within the context of the sample population, therocks collected for this study are at least broadlyisochemical with respect to major non-volatile ele-ments except possibly sodium.

Attainment of equilibrium

It will be assumed in later sections that chemicalequilibrium was attained among major minerals inthe graphitic sulfide-rich schists during the peak ofmetamorphism. A variety of observations are consis-tent with that assumption. Silicate and carbonatemineral grains appear fresh in thin section. Some sul-fide grains are altered but alteration products areusually minute in volume compared to the volume offreshlooking sulfide. Mineral grains have sharp,smooth boundaries. With the exception of pyrite in-clusions in pyrrhotite, there is no textural evidencefor reaction relationships between minerals in thesamples. Individual mineral grains are homogeneousin composition, and there is little or no variation inmineral composition from one portion of a thin sec-tion to another.

If an equilibrium relationship can be writtenamong chemical potentials of components in Fe-Mgsilicates, sulfides, and other coexisting phases, thencompositions of Fe-Mg silicates and sulfides can berepresented on a diagram such as Figure 2 (Popp etal., 1977). Minerals, whose compositions are por-trayed in Figure 2, are part of the assemblage biotite+ microcline + pyrrhotite t pyrite. I'he requisiteequilibrium relationship is:

lrKFcaAlsi3oro(oH)2,brorirc + 3trlurs,nrio :

pxntsi,or,x-r"la"o.. f 3pr'"s,pr.,trorirc * 4trl"ro'ouro (lA)

916 FERRY: SULFIDE GRAPHITE SCI/ISTS

Table 5. Whole-rock chemical data for graphitic sulfide-rich schists

Chlor i te Zone2 7 2 8 1 1 1 1 A

B io t i t e Zone774-E--2:7{E

Garnet Zone196A tL22c

Staurol i te + Andalusi te Zone

S iOz *

A l z O gFe20 3 * *

CaOMgoNa20Kz0T i0zMn0P zOsroI+Sum

5 8 . 5 7 6 7 . 0 21 4 . 4 0 9 . 4 39 . 2 5 4 . 5 02 . 5 4 4 . 8 13 . 0 6 2 . 6 61 . 8 1 2 . 0 73 . 1 9 1 . 7 10 . 7 2 0 . 6 10 . 0 7 0 . 0 70 . 1 0 0 . 0 63 . 6 6 4 . 6 0

9 7 . 3 7 9 7 . 5 3

3 . 5 1 1 . 3 01 . 5 4 2 . 3 7

5 4 . 6 1 7 7 . 2 01 3 . 2 4 9 . 6 47 3 . 1 , 2 4 . 6 42 . 6 1 0 . 7 03 . 4 6 2 . 9 80 . 7 3 0 . 2 03 . 5 7 2 . t 00 . 7 3 0 . 5 40 . 0 8 0 . 0 30 . 0 8 0 . 0 74 . 9 6 1 . 9 8

9 7 . 1 9 1 0 0 . 0 8

5 . 4 0 t . 4 61 . 9 1 0 . t 2

5 5 . 4 0 s 3 . 2 31 3 . 2 2 1 3 . 0 91 1 . 3 s 1 3 . ? 22 . 4 3 3 . 7 33 . 5 4 4 . 2 3r . 0 6 1 . . 5 23 . 3 0 2 . 4 50 . 7 8 0 . 8 30 . 0 5 0 . 0 60 . 0 5 0 . 0 98 . 5 0 7 . 5 7

9 9 . 6 8 1 0 0 . 0 2

8 . 1 2 5 9 . 9 21 . 4 6 0 . 7 5

s 5 . 2 8 5 s . 8 9 6 0 . 3 7 s 5 . 7 3 5 7 . 3 01 6 . 2 4 1 8 . 6 6 1 2 . L I 1 3 . 3 1 7 0 . 2 3s . 1 9 8 . 8 9 1 2 . 7 3 1 2 . 3 2 1 4 . 9 84 . 0 9 2 . 7 0 7 . 6 2 3 . 5 0 3 . 5 05 . s l 3 . 8 4 2 . S s 3 . 2 5 3 . 5 8r . 9 2 2 . 1 7 0 . 4 5 1 . 1 3 0 . 6 74 . r 2 3 . 5 2 2 . 9 5 3 . 4 9 1 . 5 10 . 8 2 0 . 9 5 0 . 6 9 0 . 8 0 0 . 5 80 . 7 7 0 . 1 0 0 . 0 8 0 . 1 1 0 . 1 20 . 0 7 0 . 0 7 0 . 0 7 0 . 0 7 o . 0 6s . 2 4 1 . 7 2 7 . r 3 s . 6 6 7 . 4 3

9 8 . 6 s 9 8 . 5 1 1 0 0 . 1 3 9 9 . 3 7 9 9 . 9 6

S 2 . 9 s 5 1 . 5 7 7 . 7 2 5 . 8 0 92 . 3 s 0 . 0 3 1 . 6 8 0 . 9 0

9 . 2 62 . 0 4

71 lB 965AS i l l iman i t e Zone

ffi 9 7

S i O z * 5 5 . 4 1A 1 2 0 3 1 7 . 6 8F e 2 0 3 * * 1 0 . 5 6CaO 6 .06M g O 3 . 4 3N a z o 0 . 2 9K z O 3 . 4 8T i O z 0 . 8 8M n O 0 . 1 4P z O s 0 . 1 0L O I t L . 7 2Sum 100.25

s 3 . 2 6c 0 . 3 3

7 6 . 6 1 5 8 . 6 19 . 6 8 7 2 . 9 64 . 2 5 1 1 . . 5 93 . 8 6 4 . 7 02 . 1 3 6 . t 30 , 2 2 0 . 1 40 . 8 5 2 . 5 00 . 8 6 0 . 7 20 . 0 8 0 . 1 10 . 0 7 0 . 1 1L . 7 7 2 . 6 2

9 9 . 7 8 9 9 . 5 9

n d 5 5 4 . 9 6n d 5 5 0 . 5 4

s 2 . 9 4 5 9 . 0 71 0 < ? 1 ) < A

1 0 . 8 0 1 2 . 2 58 . 2 8 4 . 7 03 . 4 1 3 . 7 80 . 2 4 0 . 4 02 . 5 9 2 . O l1 . 0 0 0 . 6 80 . 1 6 0 . r 20 . L 2 0 . 0 81 . 3 3 4 . 4 7

1 0 0 . 2 0 1 0 0 . 1 2

2 . 4 8 5 . 9 20 . 5 8 1 . 6 0

s 3 . 8 5 6 0 . 3 02 0 . 9 8 1 6 . 6 s8 . 9 9 9 . 7 01 . 8 5 1 . 5 67 . 7 8 2 . 5 81 . 2 8 0 . 9 16 . 2 9 4 . 4 71 . 0 7 0 . 9 90 . 0 4 0 . 0 80 . 1 0 0 . 0 83 . 0 6 2 . 7 7

9 9 . 2 9 1 0 0 . 0 9

2 . 9 30 . 4 3

5 3 . 9 018.26

6 . / 5

s . 8 66 . l 50 . 4 2J . J J

0 . 9 60 . 1 70 . 1 52 . O 7

r 0 0 . 0 2

3 . 8 7 2 . 4 3 5o . 4 7 0 . 5 4

* ueight pereent;

ueathez,ed sulfide;

** aLL Fe as Feror; I Loss on ignition; E eontains59 not deterinined.

Data for samples from the garnet, staurolite + an_dalusite, and sillimanite zones are all plotted. Thereis a systematic relation between the composition ofbiotite and the identity of iron sulfides in the rocks.All rocks with pyrite * pyrrhotite + microcline con_tain biotite with measured Fel(Fe+Mg) : 0.046_0.048. All rocks with pyrhotite * microcline butwithout pyrite contain more iron-rich biotite with

crossing of two tie lines in Figure 2 is probablycaused by adjustment of pyrrhotite compositions topost-metamorphic conditions.

A number of features that are evidence for a de-parture from chemical equilibrium should be men-tioned. As noted above, pyrrhotite grains do not havethe compositions they did during the metamorphicevent. Pyrrhotite rarely, if ever, retains peak meta-morphic compositions. Sulfides in almost all samplesshow signs of surficial weathering. Secondary marca-site, however, constitutes <3Vo of the sulfides in allsamples but 2404; secondary limonite occurs in vol-umes (l7o of sulfide volumes in all samples but196A, 10078, and 1107-2. The textures of sideriteand prehnite suggest that they may have grown afterthe peak of metamorphism. The evidence for dis-equilibrium is not taken as a serious objection to theassumption of chemical equilibrium among majorminerals in the graphitic sulfide-rich schists. Marca-

FERRY: SULFIDE GMPHITE SC^H/S?S 9t7

{FeSz/FeSztFeSF

Fe-biotite Mg-biot i te{FelFe+MgF

* K'feldspar

@oFig. 2. Chemographic relations among coexisting biotite and

iron sulfrdes in samples of graphitic sulfide-rich schist that containK-feldspar. a. Solid lincs are tie lines for specimens collected inthe study area (sample numbers refer to Fig. l). Filled syrnbolsreprescnt the bulk composition of the samples determined frommodes and nineral composition data. Circles: sillimanite zonerocks; squares: staurolite + andalusite zone rocks; triangles: garnetzone rocks. Vertical dotted lines separate samples from differentmetamorphic zones. Samples 240A, 10078, ll03-1, and ll22Gdefine the same pyrite-pyrrhotite-biotite triangle within error ofmeasurement. b. and c. Hypothetical change in the position of thepyrite-pyrrhotite-biotite triangle with increasing grade ofmetamorphism if the pyrite-pyrrhotite transition were to involvesulfi de-silicate reactions.

site, lime1i1e, siderite, and prehnite occur in verysmall amounts (<l7o). Furthermore, there is abun-dant evidence for attainment of equilibrium amongminerals in metacarbonate rocks interbedded withthe graphitic sulfide-rich schists in the area of FigureI (Ferry, 1976a,1979). It seems reasonable, therefore,to assume that chemical equilibrium was closely at-tained in the graphitic sulfrde-rich schists as well.

Fluid compositions

The composition of a C-O-H-S fluid in equilib-rium with the assemblage biotite + graphite + K-feldspar + pyrite + pyrrhotite waF calculated for fourspecimens (Table 6). Calculations are based on pro-cedures developed by Eugster and Skippen (1967)and Skippen (1971) for C-O-H fluids. Conditions ofPn.ia : P,oor : 3500 bars and ideal mixing of com-ponents in the fluid were assumed. Sources of fugac-

Table 6. Calculated composition of C-O-H-S fluids in equilib-rium with graphitic sulfide-rich schists during m€tamorphism

spec inen T foc) - ln f " , -Ln fo , * r .o *co , *ann * r . , *n ,NUnDel

1122G1 0 0 7 824041 1 0 3 - I

ity coefficients are those listed in Ferry (1976b)'Thermodynamic data for HrO, CO2, CO, HrS, andCHo were taken from the compilation of Stull andProphet (1971). The temperature of equilibrium wasestimated using biotite-garnet geothermometry inne4rby pelitic schists (Ferry, 1980b). Sulfur fugacitydata were taken from Touhnin and Barton (1964) atthe estimated temperature of equilibrium and cor-rected for a pressure of 3500 bars. The final con-straint needed to calculate fluid composition is therelation between /",o and /",, prescribed by equilib-rium lA. Following the analysis of Ferry Q976b), anumerical expression for the equilibrium representedby equation lA may be derived:

28,563 - 69.850r("K) - 0.233P(bars) + RnnK+ 4RTlnfH,o - 3RZl4fn,s : 0, (lB)

where

K" : (4..",oo)3 (a",,t) / (a^,,,;").

The first two terms in (lB) were derived from the ex-perimental data of Tso et al. (1979) using proceduresoutlined by Ferry (1976b). The third term was de-rived from data in Robie et al. (1967). Activities wereeither estimated from mineral composition data(Table 2) afi simple activity-composition models(biotite, Ferry, 1976b), or for sulfides, taken fromdata in Toulmin and Barton (1964) corrected for apressure of 3500 bars.

The principal species that compose the fluids inequilibrium with the grpphitic sulfide-rich schists areCIL, H,O, and COr. Graphitic sulfide-rich schists inthe garnet and in the staurolite + andalusite zoneswere in equilibrium with nearly binary Clt-HrOmixtures. The ratio H2O/CIJ' appears to have in-creased with increasing grade (temperature). Fluidsin the sillimanite zone appear to be close to binaryCOr-HrO mixtures with very little CHo.

Pyrite-pyrrhotite relations

Iron sulfides in sediments and sedimentary rocks

Some ensuing arguments regarding the behavior ofiron sulfides during metamorphism are based on ob-

4 s o o47504 8 o o5 4 o o

L 2 7 2 3 6 3 . 4 8 2 0 . 2 4 2 0 . 0 0 0 0 7 3 8 0 ' 0 1 5 0 . 0 0 s1 1 o 5 o 6 l - 7 4 5 0 . 4 4 2 0 . 0 0 1 0 . 5 2 0 0 0 3 3 0 . 0 0 4r o . 7 2 7 6 0 . 8 4 2 0 . S 0 7 0 . 0 0 1 0 . 4 5 0 0 . 0 3 7 0 . 0 0 47 . 2 0 6 5 0 . 9 0 6 0 7 0 9 0 2 2 9 0 . 0 1 1 0 0 s 0 0 . 0 0 1

c AA

c

9 1 8 FERRY: SULFIDE GRAPHITE SCIIISTS

served occumences of iron sulfides in sediments andsedimentary rocks. It is useful, therefore, to brieflyreview the origin of iron sulfides in sediments andtheir occurrence in sedimentary rocks. The protolithsof the graphitic sulfide-rich schists were almost cer-tainly black shales which, in turn, represent lithifiedanoxic muds. The formation of iron sulfide in marineanoxic muds has been thoroughly studied (Kaplan eral., 1963; Berner, 1964; Bemer, 1970; Sweeney andKaplan, 1973; Goldhaber et al., 1977; Betner et al.,1979). Iron and sulfur first combine during diagen-esis in anoxic muds to form various amorphous andmetastable iron monosulfides (e.g., mackinawite andgreigite). The iron monosulfides react with sulfide,produced by the reduction of seawater sulfate, toform pyrite. Reduction of sulfate to sulfide is usuallyaccomplished by anaerobic bacteria but apparentlymay also occur abiotically (Sweeney and Kaplan,1973). The conversion of iron monosulfides is usually9S-lNVo complete at depths greater than one meterbelow the sediment-seawater interface (Berner, 1964;Goldhaber et al., 1977). The complete conversion ofiron monosulfide to pyrite, observed in modernanoxic muds, must also have occurred at least as farback as the beginnfug of the Paleozoic Era. Standardtexts on sedimentary rocks (e.g., Milner, 1962;Greensmith et al.,l97l) as well as comprehensive re-views in journals (e.9., Love and Amstutz, 1966; Val-lentyne, 1963) are testimony to the abundance of py-rite in Phanerozoic marine sedimentary rocks. Incontrast, detailed studies ofpyritic black shales usu-ally report the complete absence of pyrrhotite (e.g.,Bates and Strahl, 1957; Elverhdi, 1977). Reviews ofthe occurrence of pyrrhotite in sedimentary rocks(Rosenthal, 1956; Kaplan et al.,1963) conclude that,while pynhotite is observed in marine sedimentaryrocks, it is very rare. Examination of the actual pub-lished descriptions reveal that pyrrhotite is found insedimentary rocks in two kinds of occurrences. Oneis pyrrhotite in black shales and marine limestoneswhere pyrite occurs in far greater abundance (Ofte-dahl, 1955; Rubey, 1930). The second is pyrrhotite asinclusions yithin calcite crystals that cover quartz-lined geodes in marine limestones (Smyth, 19l l; Erdet al., 1957\. From studies of anoxic muds and blackmarine shales in the literature, I conclude that all oralmost all iron sulifde in marine black shales is py-rite. This conclusion is supported by studies of low-grade metomorphic rocks. Carpenter (1974) observedthat black phyllites, studied from a chlorite zone,contained pyrite only. Although French (1968) inves-tigated a different rock type, he similarly observed

only pyrite in low-grade metamorphosed iron-forrra-tion.

The graphitic sulfide-rich schists in the study areacontain graptolites (Osberg, 1968), and are probablymetamorphic equivalents of pyritic marine blackshales. Because there is no evidence that pynhotiteexists in more than trace quantities in marine blackshales, I will assume that all iron sulfide originally inthe rocks was in the form of pyrite. The studies ofsediments and sedimentary rocks, cited above, areevidence that pyrrhotite observed in rocks from thechlorite zone of Figure I and at higher grades isformed by metamorphic mineral reactions.

A final noteworthy characteristic of pyritic marineblack shales is that all or almost all iron in the rock iscombined with sulfur as pyrite. Conant and Swanson(1961) report the chemical analysis of a sample of theChattanooga shale, a marine black shale seal4ining10-15 modal percent pyrite and only traces of othersulfides (e.9., chalcopyrite and sphalerite). If it is as-sumed (as seems appropriate) that all sulfur in therock is in the form of pyrite, then the chemical analy-sis requires that 90Vo of the rock's iron is bonded tosulfur as pyrite. If the stated values for iron and sul-fur in the analysis are in error by *sqo relative, then8l-99%o of the rock's iron resides in pyrite. Pettijohn(1973) reports the chemical composition of six blackshales, four of which are sulfur-rich (>l wt.7o S). Onesample is a Devonian black shale from Georgia.Even allowing a lSVo relative error for stated valuesof sulfur and iron in the analysis, 1007o of the iron inthe shale must be combined with sulfur as pyrite. Al-lowing t57o relative error for stated values of sulfurand iron, the mole percent iron that resides in pyritein the other three samples is:- 84-87Vo (Devonianblack shale from Ohio); 90-92Vo (Precambrian gra-phitic slate from Michigan); and l00%o (Posi-dienschiefer, Germany). One sample of graphitic sul-fide-rich schist from the present study area (196A)contains only traces of sutfrde other than pyrite.From the chemical analysis of sample 196,4, (Table5), allowing +5Vo rclative error for stated iron andsulfur values, 8l-98Vo of the rock's iron apparentlyresides in pyrite. The percentage derived from chem-ical analysis of sample 196A is probably too low be-cause, as discussed above, the stated sulfur contentfor the sample is fikely too low. A more accurate esti-mate of the amount of iron combined with sulfur inthe rock can be derived from modal and mineralcomposition data. Sample 196,{ contains 14.5 modalpercent pyrite and 35.7 modal perc€nt muscovite +biotite (it is difficult to distinguish fine-grained mag-

FERRY: SULFIDE GRAPHITE SCHISTS 9 1 9

nesian biotite from muscovite during modal analy-sis). If all mica is biotite, then using the chemicalanalysis of biotite in Table 4, a maximum of 0.33moles iron/1000 cm3 rock resides in mica. The rockcontains no other Fe-Mg silicates. Pyrite contains6.06 moles iron/1000 cm' rock. More than 95Vo ftonin sample 196A therefore resides in pyrite. Based onchemical analyses of pyritic black shales and on theanalysis of sample 196A, I will assume that all iron inthe unmetamorphosed equivalents of the graphiticsulfide-rich schists was bonded to sulfur as pyrite.

Pyrit e -pynhotit e is o gr ads

Carp€nter (1974) reported a pyrite-pyrrhotite iso-grad which almost exactly coincided with a biotiteisograd mapped in pelitic schists. At grades lowerthan the pyrite-pyrrhotite isograd, rocks contain py-rite and lack pyrrhotite. At grades higher than theisograd the rocks contain pyrrhotite without pyrite.

Data in Tables 2 and 3 demonstrate that a pyrite-pyrrhotite isograd cannot be mapped in the studyarea. All rocks, including those in the chlorite zonecontain substantial amounts of pyrrhotite. Somerocks in all zones contain pyrite. Guidotti (1970a)similarily observed that pyrite + pyrrhotite arewidely distributed in schists collected from differentmetamorphic grades.

The pyrit e -pyrrho tit e ff ansitio n

In order to determine whether there exists a reac-tion relationship between pyrite and pyrrhotite in thegraphitic sulflde-rich schists, the molar ratios of py-1i1sl(pyrite + pyrrhotite) were calculated frommodal data (Table 3) and plotted as crosses (Fig. 3)as a function of metamorphic grade. There is, ofoourse, a wide variation in pyrite/(pyrite + pyrrho-tite), but the ratio does appear to be lower, on the av-erage, for samples collected in the sillimanite zonethan for samples collected in the other zones. Thedata in Figure 3 for the graphitic sulfide-rich schistsare, in part, unrevealing because so few samples con-stitute the data base. The relationship between Pyriteand pyrrhotite during progressive metamorphism ofpelitic rocks can be more readily seen by examina-tion of data for 59 normal schists collected in thestudy area as well as the 19 graphitic sulflde-richschists (small dots, Fig. 3). As with data for thegraphitic sulfide-rich rocks, there is a wide variationin molar pyrite/(pyrite + pyrrhotite) in each zone (asexpressed by the large standard deviations in Fig. 3).The average pynte/{Oyr:rte f pyrrhotite), for eachzone, however, coatinuously decreases with increas-

o.8

o 0.6Ct+CI

E o.c

o.2

o.o

Fig. 3. Change in measured molar pyrite/ (pyrite + pyffhotite)

in schists as a function of increasing grade of metamorphism.

Crosses are graphitic sulfide-rich schists from the study area.

Smalt circles are normal pelitic schists and the graphitic sulfide-

rich schists (combined) from the study area. Large circles repre-

sent the mean pyrite/(pyrite + pyrrhotite) of all samples within a

metamorphic zone; vertical Iight line from circle to cross-bar rep-

resents one standard deviation about that mean. Small numbers

identify the number of samples that plot at the same value of pyl

(py + po), if more than one.

ing metamorphic grade from chlorite zone to sillima-nite zone conditions. Some differences in mean py-ritel(pynte + pyrrhotite) from one zone'to anothera1s highly significant. For example, using Student's ttest, it can be demonstrated that the difference inmean pyrite /Gryite * pyrrhotite) between all rocksin the chlorite zone and all those in the sillimanitezone is significant at a >99.9Eo confdence level. Evi-dently some mineral reaction converts pyrite to pyr-rhotite during progressive metamorphism. The con-version of pyrite to pyrrhotite by metamorphicreactions is, ofcourse, also indicated by the observa-tion that pyrite is abundant in marine black shaleswhile pyrrhotite either occurs in trace amounts or isabsent entirely.

The reaction that converts pyrite to pyrrhotite dur-ing metamorphism is unusual in two respects' First,the reaction is "smeared out" essentially over all

za

E:g ' 9Eaa

o

FERRY: SULFIDE GMPHITE SCIIIS?S

metamorphic grades: the quenched reaction is ob-served in at least some rocks from all metamorphiczones. Second,judging from data in Figure 3, the re-action has gone to completion in at least some sam-ples from all metamorphic zones. Both observationssuggest that increasing temperature is not the solemechanism that drives the pyrite-pyrrhotite transi-tion. Some phenomenon other than increasing tem-perature must be an important factor.

The pyrite-pyrrhotite reaction

Three possible types of mineral reactions could beresponsible for the conversion ofpyrite to pyrrhotiteobserved in rocks from the study area. One type in-volves the reaction of pyrite to pyrrhotite withoutloss or gain of either sulfur or iron from the rock.Such a reaction can be represented schematically as:

2 F e S r * 2 F e O + Cpyrite oxide or graphitic

silicate material (2)

: 4FeS + CO,pyrrhotite fluid

Reactions of type 2 require that some element be oxi-dized to balance the reaction. Carbon was chosen forthis role in reaction 2 because all graphitic sulfde-rich rocks contain abundant elemental carbon. Meth-ane may also be oxidized in other reactions like 2.Other possible elements and species seem less likelyto be involved in reactions like 2. Elemental hydro-gen did occur in the metamorphic fluid (Table 6) butonly in small concentrations. Iron as FeO could beoxidized to FerO, but no minerals soalaining sub-stantial amounts of ferric iron were observed in pyr-rhotite-bearing rocks.

Regardless of which element is oxidized, there arethree important consequences of reaction 2 that canbe tested by data from rocks undergoing the pyrite-pyrrhotite reaction: (a) depending on the composi-tion of pyrrhotites, approximately two moles of pyr-rhotite form for every mole of pyrite consumed; (b)whole-rock sulfur and iron contents are unaffectedby the reaction; and (c) as pyrrhotite forms, partici-pating oxides and/or silicates decrease in modalabundance. If participating silicates are Fe-Mg solidsolutions, the silicates become depleted in iron com-ponents. Reactions oftype 2have been proposed byThompson (1972) and Robinson and Tracy (1976).

A second type of reaction converts pyrite to pyr-rhotite at constant whole-rock sulfur content by add-ing iron to the rock. Such reactions can be repre-

sented schematically as:

2FeS,+ 2FeC l ,+2H,O + Cpyrite fluid graphitic

material (3)

: 4 F e S + 4 H C l + C O ,pyrrhotite fluid

Consequences of reaction 3 are: (a) approximatelytwo moles of pyrrhotite forrr for every mole of pyriteconsumed: (b) whole-rock sulfur contents are unaf-fected by the reaction but whole-rock iron contentsincrease with reaction progress; and (c) as pyrrhotiteforms, iron oxides and/or Fe-Mg silicates will belikely to increase in modal abundance through min-eral-fluid reactions. Silicates that are Fe-Mg solidsolutions will likely become enriched in iron com-ponents.

The third type of reaction converts pyrite to pyr-rhotite at constant whole-rock iron content by desul-fidation. Such a reaction can be represented sche-matically as:

2 FeS, + 2H2O + Cpyrite fluid graphitic

material(4)

: 2 F e S + 2 H r S + C O ,pyrrhotite fluid

Consequences of reaction 4 are: (a) approximatelyone mole of pyrrhotite forms for every mole of pyriteconsumed; (b) whole-rock iron contents are unaf-fected by the reaction but whole-rock sulfur contentsdecrease as the reaction progresses; and (c) as pyr-rhotite forms, oxides and/or silicates are unaffectedin either their modal abundance or their composi-tion.

Mineralogical, mineral chemical, and whole-rockchemical data on the graphitic sulfide-rich schists canbe used to test for the most plausible reaction amongtypes 2, 3, and 4. Selected mineralogical, mineralchemical, and whole-rock chemical parameters forall samples are plotted in Figures 4-7.Data in Figure3 demonstrate that a pyrite-pyrrhotite reaction oc-curred in at least some rocks at all netamorphicgrades. In plotting data from samples in all zones onthe same diagram (Figs. zt-7), it is implicitly assumedthat the pyrite-pynhotite transition occurs by thesame reaction type in each zone regardless of meta-morphic conditions. Such an assumption is justffiedfor samples from the garnet, staurolite + andalusite,and sillimanite zones, because the rocks have verysimilar mineralogies. For samples in the three highest

FERRY: SULFIDE GRAPHITE SCIT/STS 921

I..

Iaro1 t 1 19G5

o.o o.2 0.4 0.6 0.8 1.opylpy+po

Fig. 4. Measured relationship between molar pyritcl(pyrite +pyrrhotite) and total whole-rock iron for the graphitic sulfide-richschists. Sample numbers refer to Figure l. Dimension of boxesrepresent !5Vo relative error in the total iron value and twostandard deviations of measured py/(py + po). Hcavy line at leftmargin ofdiagram shows range in whole rock iron (considering ar.59o enor\ for pyrite-free rocks.

grade zones there is no suggestion that different reac-tion types are responsible f,or the pyrite-pyrrhotitetransition in different zones. In Figures 4-7 reactionprogress for the pyrite-pyrrhotite reaction is mea-sured by the molar ratio of pyrite/(pyrite + pyrrho-tite). For unreacted rocks the ratio is l; for rocks inwhich the reaction has gone to completion, the ratiois 0.

Figure 4 is a plot of whole-rock iron contentagainst reaction progress. There is no systematic rela-tionship between these two variables. If any relationcan be inferred to exist, it is a correlation of decreas-ing whole-rock iron contents with increasing reactionprogress. Data in Figure 4 are consistent with reac-tions 2 or 4 but not with reaction 3.

Figure 5 shows molar whole-rock S/Fe against re-action progress. The advantage of monitoring whole-rock molar S/Fe is that it should change in a quan-titatively predictable fashion depending on whichtype of reaction, 2, 3, ot 4, occurred during meta-morphism. If all iron in the unmetamorphosedequivalents of the graphitic sulfide-rich schists were

combined with sulfur as pyrite (as was argued above)and if reactions 3 or 4 occurred, data points for py-rite-bearing samples should lie along an inclined linedepending on the composition of pynhotite pro-duced. The line marked "FeS" corresponds to a reac-tion like 3 or 4 that produces stoichiometric FeS. Theline marked "FeeeS" corresponds to a reaction like 3or 4 that produces pyrrhotite with compositionFeorS. If reactions like 2 occurred, then data pointsfor pyrite-bearing samples should lie along the hori-zontalline labelled "constant sulfur and iron." Thereis a considerable scatter in the data points, and someof it reflects the practical difficulties in obtaining ac-curate chemical data on sulfur-rich rocks. As in-dicated by the long arrows, measured S/Fe values forsanples 196A and 1007B are probably too low be-cause the rocks contain some pyrite altered to limo-nite by surficial weathering. On the other hand, as in-dicated by the short arrow, measured S/Fe forsample 240A is probably slightly loo high because

o.o o.2 0.4 0.6 0.8 1.oPYIPY+Po

Fig. 5. Measured relationship between molar pyrite/(pyritc +pyrrhotite) and whole-rock molar S/Fe for the graphitic sulfide-rich schists. Samplc numbers refer to Figure l. Dimensions ofboxes represent t5Vo rclative error in thc total iron and sulfurvalues and two standard deviations of measured pyl(py + po).Hcavy line at left margin of diagram shows range in whole-rockS/Fe (considering *57o errors) for pyrite-free rocks. Straight linesand inset explained in text.

14

970711

1t07-r

11069t9

l10t -2

310 ;Q

to

I

922 FERRY: SULFIDE GRAPHITE SCIIISTS

o.o o.2 0.4 0.6 0.8 1.Opvl pv + po

Fig. 6. Measured relationship between molar pyrite,/(pyrite +pyrrhotite) and total moles pyrite + pyrrhotite for a referencevolume of graphitic sulfide-rich schist. Sample numbers refer toFigure L Dimensions of boxes represent two standard deviationsof the measured modal quantities and a t|Vo rclative enor on totalwhole-rock iron values. Heavy line at left margin of diagramshows range in total moles pyrrhotite (considering appropriateerrors) for a reference volumc of pyrite-free rock. Straight linesand insct explained in text.

ment that there is no obvious systematic change ofwhole-rock iron content with reaction progress. Thearray of data points in Figure 5 for pyrite-bearingsamples is interpreted, therefiore, as evidence for aprogressive loss in whole-rock sulfur with increasingprogress of the pyrite + pyrrhotite reaction. The ar-ray of data points in Figure 5 for pyrite-free rocks re-sults from mineral reactions unrelated to the pyrite-pyrrhotite transition and will be discussed below.

Figure 6 shows total moles of iron sulfide in allsamples against reaction progress. Adopting the ap-proach that Brimhall (1979) has used in a relatedproblem, the total moles of iron sulflde were mea-sured relative to a reference volume of rock that ini-tially contained one mole of pyrite. The referencevolume for each sample was determined in the fol-lowing fashion. Sample 196A, the only graphitic sul-fide-rich schist virtually free of pyrrhotite, contains Imole pyrite/165 cm3 rock (Table 3). The sample alsocontains 0.1422 moles total iron/100 g rock. If, aswas argued above, all iron in the unmetamorphosedequivalents to the schists resided in pyrite, thenwhole-rock iron serves as a measure of the original,premetamorphic pyrite content for each sample.Samples with iron contents greater than sample 196,4,contained one mole of pyrite in a rock volume lessthan 165 cm3; sarrples with iron contents less thansample 196,4' contained one mole of pyrite in a rockvolume greater than 165 cm3. The quantitative rela-

QBiotite Zone A.A GarnGt Zonc

E. I Stsurolitc+Andatusit. Zonc

O.O sii lmanit. zonc

A.l.O with K-toldspar

O. tr, A.O without K-fcld3par

o.o o.2 0.4 0.6 0.8 1.Opylpy+po

Fig. 7. Measured relationship between molar pyrite,/(pyrite +pyrrhotite) and biotite composition for the graphitic sulfide-richschists. Sample numbers refer to Figure l. Horizontal line marksbiotite composition of Fe,/(Fe+Mg) : 9.045.

1 .6oIr

1.4 66-o.tto

1.2 @g

do

t.u ;oot

o.8 3oor

0.6 at3o

o.8.ll

o.n

^ ^ ou.o +

3@

^ . 5 '-'t r

6'

othe rock contains a substantial amount of pyrrhotitealtered to marcasite by surficial weathering. After thespecial considerations for samples 196,4,, 2404, and1007B are taken into account, the data in Figure 5are reasonably consistent with the hypothesis that therocks initially contained all iron as pyrite and thatpyrrhotite was produced by either reactions 3 or 4.The data in Figure 5 are inconsistent with a suite ofrocks of similar initial bulk composition that werelater affected by reaction 2. Data in Figure 4 docu_

o 1

FERRY: SULFIDE GRAPHITE SClt/SrS

tionship between the reference volume (V) of anysample i, the reference volume of sample 196A, andthe whole-rock iron content of the two samples is

(165 cm3)(0.l422moles Fel100 g sample 196A)Y i - (X moles Fel100 g samPle i)

(X moles Fel100 g sample i)(s)

where X is determined from data in Table 5. Thevalue for F actually may be in the range 2l-26 basedon errors in modal analysis and an assumed t5Vo rel'ative error in the iron content of sample 196A' Aver-age F was rounded to the nearest value of5 and F :

25 was chosen in the calculation of reference vol-umes. Equation 5 will predict the volume of rock thatcontained one mole of pyrite even if all iron initiallydid not reside in pyrite, providing thatthefraction ofwhole-rock iron that resided in pyrite was the samefor all samples prior to metamorphism.

If reactions like 4 occurred, then data points forpyrite-bearing samples should lie along a horizontalor near-horizontal line depending on the compositionof pyrrhotite produced. The line marked "FeS" cor-responds to a reaction like 4 that produces stoichio-metric FeS. The line marked "Fe6nS" corresponds toa reaction like 4 that produces pyrrhotite with com-position FeorS. If either a reaction like 2 or 3 oc-curred, data points for pyrite-bearing samples wouldlie along the inclined line. Total moles of sulfide areunchanged during the pyrite-pyrrhotite reaction inthe reference volume of rock. Approxinately onemole of pyrrhotite therefore is produced for everymole of pyrite consumed. The data are consistentwith a suite of rocks that was affected by a reactionlike 4 but not by reactions like 2 or 3. The array ofdata points in Figure 6 for pyrite-free rocks resultsfrom mineral reactions unrelated to the pyrite-pyr-rhotite transition and will be discussed later.

Figure 7 shows the composition of biotite in thegraphitic sutflde-rich schists against reaction prog-ress. In all samples bat 274A biotite occurs as theonly Fe-Mg silicate. The only oxide mineral in theserocks is rutile. There is no relation between biotitecomposition and reaction progress for pyrite-bearingspecimens. Because reactions like 2 and 3 predict

changes in the composition of Fe-Mg silicates withreaction progress, data for pyrite-bearing samples inFigure 7 are inconsistent with reactions like 2 or 3.Data in Figure 7 are consistent with a reaction like(4) that does not affect the compositions of Fe-Mgsilicates in rocks undergoing the pyrite-pyrrhotite

transition. The lack of correlation between biotitecomposition and reaction progress is particularly in-portant for those pyrite-bearing specimens repre-

sented by filled symbols in Figure 7. These rocks

contain the assemblage biotite + K-feldspar + pyrite

+ pyrrhotite + graphitic material. A sulfide-silicatereaction is possible in these rocks such as

2KFe.AlSi,O,o(OH),+ 6FeS' + 3Cbiotite

: 2 KAlSi,O' *K-feldspar

pyrite graphiticmaterial (2A)

1 2 F e S + 2 H 2 O + 3 C O ,pyrrhotite fluid

There is no evidence, however, that a reaction like

2A took place in the pyrite-bearing samples. The ar-

ray of data points in Figure 7 for pyrite-free rocks re-

sults from mineral reactions unrelated to the pyrite-

pyrrhotite transition and will be discussed later'Only desulfidation reactions like 4 are consistent

with a pyrite-pynhotite transition and the data in

Figures 4-7. Considering the major species in themetamorphic fluid (Table 6), it is likely that two de-

sulfidation reactions occurred in the study area dur-

ing metamorphism. At chlorite through staurolite +

andalusite zone conditions, the pyrite-pyrrhotite

transition probably dominantly occurred by

2FeSr+ CI t : 2FeSpyrite fluid pyrrhotite (4A)

+ 2 H , S + cfluid graphitic

material

In the sillimanite zone, fluids were COr-HrO mix-

tures, and the pyrite-pyrrhotite transition probably

dominantly occurred by reaction 4. Both reactions 4

and 4A, howevet, proceeded at least to some small

degree at all metamorphic conditions.Because reactions 4 and 4A involve elemental car-

bon, there should be a relationship between the re-

duced carbon content of pyrite-bearing rocks andprogress of the pyrite-pyrrhotite reaction. Examina-iion of the data in Table 5 reveals that no such sys-

tematic relationship exists' Several explanations arepossible. First, for carbonate-bearing samples, total

carbon reported in Table 5 is not equivalent to re-

duced carbon. Second, desulfldation reaction 4 con-

sumes carbon while reaction 4,{ produces it' Con-

sequently the two reactions will affect the reduced

carboo content of rocks in opposite ways. Third, a

systematic relationship between reduced carbon con_tent and reaction progress is expected only for suitesof pyrite-bearing rocks undergoing the same reactionand with the same pre-metamorphic contents of ele_mental carbon. Chemical analyses of black shales(Pettijohn, 1975) indicate that this would be an un_likely circumstance. Unlike the case for pyrite, thereis no obvious way to estimate a referenci volume ofgraphitic sulfide-rich schist that initially containedone mole of reduced carbon prior to metamorphism.

Because reactions like 2 have been invoked to ex_

tions about pyritic black shales seem to indicate thatreactions like 2 could not proceed to a significant de_gree during metamorphism. Black shales contain py_rite and either only trace amounts of pyrrhotite or, asis more common, no pyrrhotite at all. All or alnostall iron (85-l007o) in the black shales is bound in py_rite. Reactions like 2 convert pyrite to pyrrhotite byextraction of iron from oxides and/or silicates andcombining it with pyrite. Consequently, if all the py-rite is converted to pyrrhotite by a reaction such as i,at least 40-50 mole percent iron in a rock must residein oxides and/or silicates prior to metamorphism.Data on pyritic black shales indicate that there sim_ply is not sufficient iron in silicates or oxides in theserocks for reaction 2 to be quantitatively effective.Samples of graphitic sulfide-rich schist illustrate, aswell, the problem of insufficient iron in sitcates.Modal data in Table 3 and mineral composition datain Tables 2 and 4 can be used to estimate the totalamount of iron in silicates in rocks 1007B, l0l lA,and ll22 G, which is 0.153, 0.146, and 0.193 molesFe/1000 cm' rock, respectively. The rocks contain3.57,3.36, and 4.45 moles pyrite/1000 cm3 rock re_spectively (Table 3). Even if all ironin the silicates inthe three rocks had been used to convert pyrite topyrrhotite by reactions kke 2, only 4-5Vo ofthi pyritewould have been consumed. A proponent of reaction2 could argue that the low iron content ofthe silicatesin the three samples prevented the conversion of py_rite to pyrrhotite. The mineralogy of the graphiticsulfide-rich schists from the sillimanite zone, how_ever, suggests that the pyrite-pyrrhotite transition isessentially completed in all rocks at least at highgrades. Ifthe low iron content ofsilicates can prohi_bit completion of the pyrite-pyrrhotite reaction, thenit is surprising that not one sample of graphitic sul_

FERRY: SULFIDE GMPHITE SCI'IS?S

fide-rich schist from the sillimanite zone containsmore than 0.3 modal percent pyrite while in each ofthe lower grade zones one or more samples contain1.0-14.5 modal percent pyrite.

Second, if reactions like 2 are to occur, some reac-tion relationship between sulfides and silicates mustexist in the rocks. Such a reaction relation is missingin many pyrite-bearing samples. For example, speci-men l0l lA contains, as major minerals, pyrite +pyrrhotite + biotite * plagioclase + quartz. No reac-tion can be written between components of the sul-fide minerals and the silicates. Added considerationof the accessory phases does not reveal any reactionrelationship among sulfides, major minerals, and ac-cessory minerals. A proponent of reactions like 2might argue that the lack of a sulfide-silicate reactionrelationship in specimens like l0l lA is what pre-serves pyrite in the rock. If this were true, however, itis surprising that all samples of schist collected in thesillimanite zone show evidence of an essentially com-plete pyrite-pyrrhotite reaction. Using a similar ar-gument as above, if reaction could be limited by lackof a sulfide-silicate reaction relation, it is difficult tounderstand why not one sample of sillimanite zoneschist contains more than a trace of pyrite.

Third, graphic analysis of the pyrite-pyrrhotitetransition seems to favor a reaction like 4 rather thana reaction such as 2. Figure 2 illustrates the composi-tion of coexisting sulfides and biotite in the assem-blage biotite + K-feldspar + graphitic material *pyrrhotite -r pyrite. Compositions of minerals are re-lated by reaction relation 2A, and pyrrhotite couldconceivably have been produced through this reac-tion. Graphically the effects of hypothetical reaction2A are illustrated in Figures 2b and 2c. If reaction2,A' were to occur with increasing grade, the biotite-pyrite-pyrrhotite triangle would sweep to the right inthe diagram. As reaction proceeds and the trianglesweeps to the right: (a) pyrite-bearing rocks (compo-sition A) develop pyrrhotite; (b) the ratio pyrite/(py-rite + pyrrhotite) decreases in rocks with pyrite +pyrrhotite but rich in pyrite (composition B); and (c)rocks with pyrite + pyrrhotite but rich in pyrrhotite(composition C) lose pyrite. The actual data from thegraphitic sulfide-rich schists (Fig. 2a) might be inter-preted as a state reached after the three-phase tri-angle swept across the diagram to the composition ofbiotite at Fel(Fe+MB) : 0.047. The principal argu-ment against such an interpretation is that there is noevidence that the biotite-pyrite-pyrrhotite triangleever swept across the diagram. All samples with py-rite + pyrrhotite + K-feldspar * graphitic material

FERRY: SULFIDE GRAPHITE SCI/ISTS

contain biot i te with essent ial ly the same Fel(Fe+Mg) : 0.046-0.048. The samples were collectedfrom metamorphic grades ranging from garnetthrough sillimanite zone conditions. If the three-phase triangle did indeed sweep across the diagramin Figure 2 it is surprising that not one sample"catches" the triangle in a position other than at Fe/(Fe+Mg) : 0.047. It seems fortuitous that the sweepof the triangle would have stopped at the same bio-tite composition in all samples from three differentmetamorphic zones. There is no evidence that thebiotite-pyrite-pyrrhotite triangle ever was in a posi-tion other than Fel(Fe+Mg) : 0.047 during meta-morphism of the graphitic sulfide-rich schists. I con-clude that the triangle always was positioned at Fe/(Fe+Mg) : 0.047+0.001 and that reaction 2A neveroccurred in the rocks.

I attribute the regular array of tie lines and mineralcompositions in Figure 2a to a close approach tochemical equilibrium among a suite of samples thatdiffer in bulk composition. Samples that plot to theleft in the diagram have lower molar S/Fe relative tosamples that plot in the righthand portion of the dia-gram. What is the cause of the variation in bulk rockcomposition? The difference in bulk compositionamong the samples could reflect differences in pre-metamorphic rock composition acquired during sedi-mentation and/or diagenesis. If this were true, thennatural processes have provided remarkable demon-stration of foresight. Dotted vertical lines in Figure2a segregate the samples by metamorphic zone. Thesamples lowest in molar S/Fe were collected in thesillimanite zone. The sample highest in S/Fe was col-lected in the garnet zone. Samples of intermediate S/Fe were collected in the intervening staurolite + an-dalusite zone. If the S/Fe of the metamorphic rockswere entirely controlled by sedimentation and diage-nesis, then sedimentary and diagenetic processeswere apparently able to deposit low S/Fe rocks withperfect accuracy precisely where later high grademetamorphism was to occur. Sedimentary and diage-netic processes were apparently also able to deposithigh S/Fe rocks only where later low grade meta-morphism would occur. I adopt an interpretation ofthe array of data points in Figure 2a lhat involvesless coincidence. If rocks experience desulfidationduring metamorphism, high grade rocks will, on theaverage, have lower S/Fe than lower grade rocks.Progressive metamorphism of a suite of rocks, ini-tially of similar bulk composition, will produce asuite of metamorphic rocks with variable S/Fe thatwill plot in a subhorizontal array across Figure 2a.

High grade rocks will plot further to the left in the ar-ray; lower grade rocks will plot to the right. I inter-pret the range in bulk rock compositions illustratedin Figure 2a as caused by desulfidation during meta-morphism. The graphical analysis of the graphiticsulfide-rich schists requires that if pyrrhotite formedby reaction 2A, then an implausible number of for-tuitous events occurred. The graphical analysis isconsistent, on the other hand, with development ofpyrrhotite in the graphitic sulfide-rich schists by de-sulfidation reactions such as 4 and 4A.

Sulfide-silicate relations

Mineral reactions between sulfides and silicatesmay explain some of the compositions of silicateminerals observed in the graphitic sulfide-richschists.

Composition of biotile coexisting with pyrite+ pyrrhotite

A striking aspect of the mineral composition datain Figure 7 is the regular relationship between biotitecomposition and the assemblage of iron sulfides inthe graphitic sulfide-rich schists. In rocks containingpyrite + pyrrhotite, Fel(Fe+Mg) of biotite is 0.018-0.056. For rocks containing pyrite + pyrrhotite *K-feldspar, the biotite is even further restricted incomposition, Fel(Fe+Mg) : 0'046-0'048' All rockswithout pyrite contain biotite with Fel(Fe+Mg) :

0.075-0.46'1. The composition of biotite coexistingwith pyrite + pyrrhotite is fixed over the entire rangeof metamorphic conditions from the biotite throughsillimanite zones. Furthermore, the relationship ap-pears to be a general one: Robinson and Tracy(1976) report that Fel(Fe+Mg) of biotite in pyrite +pyrrhotite-bearing schists from a sillimanite + K-feldspar zone in central Massachusetts has nearly thesame value, 0.05.

The unique composition of biotite coexisting withpyrite + pyrrhotite in biotite zone rocks may be aconsequence of a low variance mineral assemblagethat existed in the graphitic sulfide-rich rocks whenbiotite first formed. Pyrite-bearing phyllites of thechlorite zone contain dolomite * muscovite + albite+ quartz * rutile + graphitic material * chalcopyrite+ pyrite + pyrrhotite i calcite. At higher grades, py-rite-bearing schists contain biotite * plagioclase (X'"> 0.3) + quartz * rutile + graphitic material + chal-copyrite + pyrite + pyrrhotite + K-feldspar + mus-covite t chlorite + siderite. Only two samples werecollected from the biotite zone, and neither of themappears to contain a complete record of what miner-

als were involved in the reaction that formed biotite.By combining the assemblages found in chlorite zonerocks with those in higher grade rocks, the probablemineral assemblage that existed when biotite firstcrystallized may be estimated: muscovite * dolomite* biotite + albite + oligoclase + quartz + rutile +graphitic material * chalcopyrite + pyrite + pyrrho-tite f K-feldspar t chlorite + calcite t siderite. Co-existing albite and oligoclase are included because ofthe peristerite gap in the plagioclase solid solution se-ries and because coexisting albite (An",) and oligo-clase (Anro) have been observed in normal peliticschists from the biotite zone in the area of Figure I(Ferry, unpublished data). Compositions of mineralsin the combined mineral assemblage can be ex-pressed by the components KrO, NarO, CaO, FeO,MgO, CuO, TiO2, SiO2, Al2Or, C, O, H, and S. MnOwas not included because it occurs in concentrationsof less than 0.3 weight percent in minerals from rockscollected within the chlorite, biotite, and garnetzones. In the presence of a C-O-H-S-CI fluid ofconstant chloride concentration, at a given temper-ature and pressure, the assemblage muscovite +dolomite * biotite + albite + oligoclase + quartz +rutile + graphitic material + chalcopyrite + pyrite +pyrrhotite is univariant. If one of the four phases,chlorite, K-feldspar, calcite, or siderite is added, theassemblage is invariant. If MnO and Cl are consid-ered as chemical variables, participation of any othertwo of the minerals, chlorite, K-feldspar, calcite, orsiderite, would preserve the invariant state. It seemslikely that the graphitic sulfide-rich rocks were achemical system of very low variance, possibly in-variant, when biotite first crystallized. If the systemwere invariant, the composition of all phases wouldbe fixed, and the unique composition of biotite inrocks containing pyrite * pyrrhotite could be ex-plained.

The exact reaction that formed biotite cannot bespecified with great confidence. If K-feldspar is in-volved, a speculation that uses actual compositions ofminerals in rocks of the biotite and chlorite zones(Table 7) and major species in the fluid phase (Table6), might be

2.689 muscovite + 4.648 albite + 1.990 quartz +1.567 dolomite + 0.029 pyrrhotite + 0.030 rutile+ 3.139 CHo : 1.000 biotite + 6.269 oligoclase+ 1.600 K-feldspar + 6.273 C + 0.029 H,S +7.937 HrO (6)

The composition of the fluid and mineral solid solu-tions would all be fixed at a given pressure, remper-

FERRY: SULFIDE GRAPHITE SCHISTS

Tablc 7. Typical compositions of minerals from graphitic sulfide-rich schists collected in the biotite and chlorite zones.

nuscov i te

b io t i te

do I onite

pyrrhotite

a lb i te

o l igoc lase

K- fe I dspar

pyr r re

ru t i le

graph i t i cna t e r ia I

K 6 . e N a 6 . 9 5 M g s . 3 A I 2 . 3 5 S i 3 . 3 5 O 1 6 ( O H ) 2

K 6 . e F e 6 . 1 2 l " l g 2 . 2 6 T i s . 6 3 A 1 1 . 5 3 S i 2 . 9 q O 1 s ( O H ) 2

C a F e g . 9 5 M g 6 . e a ( C O 3 ) 2

F e 6 . 9 5

NaAl Si 3Os

C a q . 2 5 N a 9 . 7 5 A 1 1 . 2 5 S i 2 . 7 5 O 6

K s . 9 5 N a 9 . q 5 A l S i 3 O s

FeS2

si02

Tio2

c

ature, and chloride content of fluid by the additionalpresence ofpyrite.

At grades highsl than the biotite zone, biotite isthe only Fe-Mg mineral that occurs in major abun-dance (>2Vo) in the graphitic sulfide-rich schists. Theobservation suggests that the unique composition ofbiotite, coexisting with pyrite and pynhotite, was es-tablished in rocks when biotite first crystallized dur-ing biotite-zone conditions. The unique compositionof biotite preserved in pyrite + pyrrhotite-bearingrocks may reflect that biotite subsequently never par-ticipated in any other mineral reactions when therock was subjected to higher-grade metamorphicconditions.

Composition of biotite in pyrite-free rocks

The composition of biotite in rocks without pyriteis more iron-rich than Fel(Fe+Mg) : 0.05. After py-rite is eliminated from the graphitic sulfide-richschists by desulfidation reactions 4 and 4A, the moreiron-rich biotites can be produced by continued de-sulfidation of rock by a mineral reaction such as 7:

KAlS i3Or + 3FeS * 4H,O:K-feldspar pyrrhotite fluid

KFerAlSirO,o(OH), + 3 HrSbiotite fluid (7)

The K-feldspar involved in reaction 7 first appears inrocks collected from the garnet zone although the re-action by which it forms cannot be precisely specifiedfrom the few samples collected (reaction 6 is a possi-bility). Varying degrees of desulfidation would resultin varying degrees of progress of reaction 7 and con-sequently in the range of biotite compositions in Fig-

FERRY: SULFIDE GRAPHITE SCI/ISIS

ure 7. Half of the pyrite-free rocks plotted in Figure 7do not contain K-feldspar. In these samples eitherthe K-feldspar was completely consumed by desulfi-dation reaction 7 or the rocks never contained K-feldspar. If the rocks never contained K-feldspar,then the composition of biotite in them must be ex-plained by some other mineral reaction. There is in-sufficient evidence, however, to speculate what theseother mineral reactions may have been. Some of thepyrite-free rocks plotted in Figure 7, of course, mayhave not contained pyrite when biotite first crystal-lized in them. Such a situation could arise, as forsample I I I lA, if the pyrite-pyrrhotite transition wascompleted in a specimen under metamorphic condi-tions of the chlorite zone. The composition of biotitein these rocks may therefore be a relict of some bio-tite-forming reaction in pyrite-free rocks. It would beimpossible, however, to identify such rocks becausethere is no obvious way to tell at what point in themetamorphic evolution of a particular high-graderock that pyrite was consumed.

Fluid-rock ratios during metamorphism

If the above analysis of the graphitic sulfde-richschists is correct, pyrite + pyrrhotite-bearing rocksprincipally experienced reactions 4 and 4,A' duringprogressive metamorphism at garnet zone conditionsand at higher grades. Reaction 4 produces fluid withcomposition &,, : 0.67; X.o": 0.33. Reaction 4Aproduces a pure HrS fluid. There exists a large dis-crepancy between the composition of fluid generatedby reactions 4 and 4A and the composition of COr-H,O-CH4 fluid with which the graphitic sulfide-richrocks were in equilibrium (Table 6). This discrep-ancy in fluid compositions may be resolved using ar-guments developed previously from the study ofmetamorphosed carbonate rocks (Ferry, 1980a;Rumble et al., l98l). The graphitic sulfide-richschists probably equilibrated during metamorphismwith a fluid mixture. One component of the fluid wasrich in HrS and u,as generated internally within therocks by reaction 4 and 4,{. The other component ofthe fluid originated from (unknown) sources externalto the graphitic sulfide-rich schists. The externally-derived component was probably rich in those spe-cies that are consumed by reactions 4 and 4,A' as wellas those species present in the equitbrium fluid butnot produced by reactions 4 and 4A (e.9., HrO andCIt). Judging from the compositions of the fluidmixtures with which the rocks equilibrated (Table 6)the mixtures were primarily composed of the ex-ternally-derived component with only small admix-

tures of the internally-generated, HrS-rich com-ponent.

If some assumptions are made, the amount of ex-ternally-derived component that interacted with arock specimen during the metamorphic event can bequantitatively estimated (cf. Ferry, 1980a). First, itwill be assumed that the externally-derived fluidcomponent contained no HrS (and hence no totalsulfur) prior to mixing with the fluid component gen-erated by reactions 4 and 4A. This assumption is a"conservative" action because it leads to minimumestimates of the amount of the externally-derivedfluid component. The assumption is justified by thevery low sulfur content of fluids that were in equilib-rium during metamorphism with other common rocktypes in the study area (Ferry, 1976; 1979)- Let { rep-resent a measure of progress for reaction 4 or 4A,where

t : Vz (moles pyrrhotite/IO00 cm3 metamorphosedrock) (7)

The variable { is a simple measure of fluid speciesproduced or consumed by reactions 4 and 4A. Thevalue of { is equivalent to (a) half the number ofmoles H,S produced by reaction 4 and 4A; (b) halfthe number of moles HrO consumed by reaction 4;(c) the number of moles CO, produced by reaction 4;and (d) the number of moles CHo consumed by reac-tion 4A. The composition of metamorphic fluid inequilibrium with the sulfide-rich rocks is related to {and to the number of moles of the externally-derivedfluid component, N, by the following relation:

X r , , r : 2 € l ( N + $ ( 8 )

Values of { and X"," can be derived from data in Ta-bles 3 and 6, and N may be calculated from equation8. If some assumption is made about the compositionof the externally-derived fluid component, then Nmoles fluid can be converted to a certain volume offluid. A second assumption made in calculatingfluid-rock ratios during metamorphism is that the ex-ternally-derived fluid component had the same pro-portions of HrO:CHo:CO, as the fluid in equilibriumwith the schists. Such an assumption seems reason-able because the small X',. contents of the equilib-rium fluid require that the equilibrium fluid be domi-nant ly composed of the external ly-derivedcomponent. Consequently the equilibrium fluidsshould have compositions close to the composition ofthe externally-derived fluid component. Moles offluid were converted to volume of fluid using molarvolumes of HrO from Burnhan et al. (1969) and mo-

928 FERRY: SULFIDE GRAPHITE SCI//STS

lar volumes of CO, from Shmonov and Shmulovich(1974). Molar volumes of CHo were derived from thefugacity coefficients of Ryzhenko and Volkov (197l)using the following relation:

(AlnfcH,/ a P) r : Y." ^/ Af (e)Components were assumed to mix ideally with re-spect to volume in the fluid.

Fluid-rock ratios calculated for samples for whichthere are data on equilibrium fluid compositions arepresented in Table 8. Documentation that desulfida-tion reactions like 4 and 4,A, occurred during meta-morphism is significant because it demonstrates thatvery large volumes of externally-derived fluid inter-acted with the rocks during metamorphism (fluid-rocks ratios ) I on a volume basis). Results reportedin Table 8 are in harmony with recent investigationsof the volumes of fluid that interact with carbonaterocks during metamorphism (Ferry 1980a; Bowmanet al.,1980; Rumble et al.,l98l)

A model for desulfidation during metamorphismThe following sequence of events is envisioned

during the metamorphic evolution of the graphiticsulfide-rich schists. At the end of diagenesis the rockscontain pyrite, dolomite, muscovite, albite, graphiticmaterial, and a variety of accessory minerals. Essen-tially all iron in the rocks is combined with sulfur aspyrite. Grain boundaries are wetted, probably byHrO and CHo. As temperature increases duringmetamorphism, small amounts of pyrrhotite willform dominantly by reaction 4,A.. Reaction will cease.however, as soon as the amount of HrS, released bydesutfidation, accumulates along grain boundaries inconcentrations that equal those for a fluid in equilib-rium with pyrite + pyrrhotite * carbon. For furtherreaction to occur, the accumulated HrS must be re-moved from the rock. It is envisioned that the HrS isflushed out by an aliquot of H,O-CH.-CO, fluid de-rived from (unknown) sources external to the sulfide-rich schists and very low in total sulfur content. Theexternally-derived fluid is believed to flow throughthe sulfide-rich rocks in response to either small, lo-cal pressure gradients or buoyancy forces. Once theinitially-produced HrS has been flushed out by theexternally-derived fluid, a new increment of reaction4A occurs. Actually reaction and fluid flow probablydid not take place in discrete steps but in one contin-uous process. The flow of externally-derived fluid issufficiently slow that minerals in the rock are able tobuffer, through mineral-fluid reactions, the composi-tion of fluid occupying the pore space of the schists at

Table 8. Volume of externally-derived C-O-H fluid that interactedwith a reference volume of 1000 cm3 graphitic sulfide-rich schistduring metamorphism. Extensive quantities refer to 1000 cm3

metamorphoscd rock

Spec inen no les o f

Nsber pyr rho t i te

no les vo lune o f vo lmet t rc

f l u i d f l u i d ( c n r ) f l u i d / r o c k

1122C1 0 0 7 824041 1 0 3 - I

2 . r 02 4 32 . 6 66 . l 0

5 . 02 . 42 . 23 5

I 0 5 0 1 3 8 9 5 5 0 0 41 2 1 5 7 2 . 4 2 2 3 7 51 . 3 3 0 7 0 . 5 6 2 2 2 63 0 5 0 1 1 8 . 8 s 3 5 0 1

any one instant. The constant influx of externally-de-rived fluid, initially out of equilibrium with the sul-fide-rich rocks, is however, an essential driving forcebehind the desulfidation reaction. Because the influxof externally-derived fluid drives the desulfidation re-action, the reaction is not exclusively dependentupon increasing temperature and may occur at anymetamorphic grade. Consequently desulfidation re-actions 4 and 4A are observed to go to completion insome samples in all metamorphic zones.

At some point, as temperature continues to riseduring metamorphism, mineral reactions occur in thegraphitic sulfide-rich schists, that produce biotite andK-feldspar at the expense of dolomite, muscovite,and other minerals. In pyrite-bearhg rocks, the reac-tion involves an assemblage of very low variance andconsequently biotite of only a very restricted compo-sitional range forms. Continued influx of externally-derived fluid drives desulfidation reaction 4,A' caus-ing the ratio pyite/(pyrite * pyrrhotite) in the rocksto decrease and depleting the rock of sulfur. In somerocks pyrite is completely converted to pyrrhotite.Further infiltration of the externally-derived fluidcauses additional desulfidation by driving reaction 7in rocks with K-feldspar. As reaction 7 progresses,biotite in the rock becomes progressively enriched iniron components. Desulfidation of pyrrhotite-bearingrocks by reaction 7 proceeds either until one of themineral reactants is exhausted (e.g., K-feldspar) oruntil the metamorphic event terminates.

When temperatures have reached those character-istic of sillimanite zone conditions, desulfidation ofpyrite-bearing rocks occurs dominantly by reaction 4rather than 4,A'. Very few sulfide-rich schists retainpyrite. Most schists contain only pyrrhotite and withincreasingly iron-rich biotite. Fluid-rock ratios inTable 8 are an approximate measure of the volumeof externally-derived fluid that has flowed throughthe schists as desulfidation occurred during the entiremetamorphic event.

The model of desulfidation predicts how the S/Femolar whole-rock ratio should change in all graphitic

FERRI: SULFIDE GMPHITE SCI//STS

sulfide-rich rocks as a function of sutfide mineralogy.The prediction is illustrated in the inset to Figure 5by the heavy line. Arrows point in the direction ofchanges produced by progressive desulfdation. Theinclined portion of the heavy line reflects the effectsof reactions 4 and 4A; the vertical portion of the linereflects the effects of reaction '7. Data points for pyr-rhotite-bearing rocks with and without pyrite closelyfit the expected trends. The model of desulfidationalso predicts how the total number of moles of ironsulfide per reference volume of rock should changein all graphitic sulfide-rich rocks as a function of sul-fide mineralogy. The prediction is illustrated in theinset to Figure 6 by the heavy line. Arrows againpoint in the direction of changes produced by pro-gressive desulfidation. The subhorizontal portion ofthe heavy line reflects the effects of reactions 4 and4A; the vertical poftion of the line reflects the effectsof reaction 7 . Data points for the sulfide-rich schistsclosely fit expected trends.'The data points in Figure7 also are consistent with the model for desulfldation.The horizontal array of data points for pyrite-bearingrocks reflects that desulfidation of pyrite-bearingrocks does not involve sulfide-silicate reactions. Thevertical array of data points for pyrite-free rocks re-flects the effect of reaction 7 on biotite composition.

Types of externallyderived fluids

The model for desutfldation presented above isvery similar to a model for decarbonation duringmetamorphism proposed by Rumble et al. (1981).The principal difference between the desulfidationand riecarbonation models is the composition of theexternally-derived fluids. Mineral equilibria in thegraphitic sulfide-rich schists suggest that the ex-ternally-derived fluids were HrO-CH.-CO, mixtures.Mineral equilibria in calc-silicate rocks suggest thatthe externally-derived fluid was essentially pure HrO.This compositional difference appears to correlatewith the difference in composition of fluid inclusionsfound in calc-silicate rocks and in graphitic schists.Fluid inclusions found in calc-silicate rocks are eithernearly pure HrO or aqueous brines (Crawford et al.,1979; Kreulen, 1980). Neither pure HrO nor aqueousbr.ines are produced by common decarbonation reac-tions in calc-silicate rocks. The fluid in the inclusionsmay represent trapped samples of externally-derivedfluid. COr-HrO-Ctt fluids with various composi-tions have been observed as inclusions in schists andsemi-pelitic rocks (Crawford et al., 1979; Hollisterand Bumrss, 1976; Kreulen, 1980). COr-HrO-CHofluids are not produced by common dehydration re-

actions in metapelitic rocks. The fluid inclusions inpelitic rocks may represent trapped samples of ex-ternally-derived fluid.

Petrologic studies and studies of fluid inclusionsare consistent with at least two types of externally-derived fluid. One is HrO-rich and associated withcalc-silicate rocks while the other is a COr-HrO-CHomixture and is associated with graphitic metashales.The two types of externally-derived fluid appear tooccur together in at least some terrains. In the studyarea of Figure I graphitic sulfide-rich schists inter-acted during metamorphism with externally-derivedCOr-H,O-CH. fluids (this study) while calc-silicaterocks interacted with externally-derived H'O fluid(Ferry, 1980a). At Naxos some calc-silicate horizonscontain inclusions of nearly pure HrO while schists,quartzites, and marbles contain inclusions of pureCO, (Kreulen, 1980). In the Prince Rupert area, Brit-ish Columbia, calc-silicates include aqueous brines,while semi-pelitic rocks contain inclusions of COr-CHo mixtures. These observations sugg€st a highlychannelized and complicated mechanism of fluidtransfer during metamorphism.

The origin of the two types of externally-derivedfluid is not known. The fluids could be products ofmetamorphic devolatilizaticn reactions. Some ex-ternally-derived fluid, however, may originate in themantle. Externally-derived HrO could conceivablyrepresent seawater/meteoritic water that has pene-trated deep into the crust. The origin of the ex-ternally-derived fluids may be revealed by multi-disciplinary studies involving mineral equilibria,fluid inclusions, and stable isotopes.

AcknowledgmentsResearch was supported by a Cottrell Grant from Research

Corporation and NSF grants EAR 77-227'll and 80-20567 (EarthSciences Section). Electron microprobe analyseg were obtained atthe Geophysical Laboratory with the kind permissiou ofll. S. Yo-der, Jr., Director. Thanks are due to C. F. Lewis for performingwhole-rock sulfur and carbon analyses. C. V. Guidotti, M. J.Holdaway, C. Klein, and R. P. Wintsch are warmly thanked forreviews that led to substantial irnprovements in an early version ofthe manuscript.

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Manuscript received, December 5, 1980;acceptedfor publication, April 22, 1981.

petrology of graphitic sulfide-rich schists from south ... · occur with prograde metamorphism. a...

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