The role of fluids in the metamorphism of komatiites, Agnew nickel deposit, western Australia

Contrib Mineral Petrol (1987) 96:J51-162 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1987

The role of fluids in the metamorphism of komatiites, Agnew nickel deposit, western Australia

Martin J. Gole, Stephen J. Barnes, and Robin E.T. Hill C.S.I.R.O. Division of Minerals and Geochemistry, Private Bag, Wembley 6014, Western Australia

Abstract. The Agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy. Olivine + tremolite + chlorite + cummingtonite 4- enstatite assemblages occur at the margin of the ultramafic lens, giving way to olivine + anthophyllite, olivine + talc and olivine+antigorite assemblages successively inwards. These rocks are interpreted as having crystallized from komariitic lavas, and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates. The relative modal abundances of metamorphic olivine, tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith. Peak metamorphic conditions are estimated at 550 ~ C, based on garnet-biotite thermometry, at a maximum pressure of 3 kb. This temperature falls within the narrow range over which metamorphic olivine may co-exist with enstatite, anthophyllite, talc or antigorite depending upon the fugacity of water in the metamorphic fluid. The observed mineralogical zonation is therefore attributed to infiltration by COz-rich fluids, generated by decarbonation of talc-carbonate rocks formed during pre-metamorphic marginal alteration of the ultramafic lens. Metamorphic fluids were essentially binary mixtures of water and CO2, with minor HzS having a maximum partial pressure less than 1 percent of total pressure. Ensta- rite-bearing assemblages formed in the presence of CO2-rich fluids at fluid: rock volume ratios close to one, while anthophyllite, talc and antigorite bearing assemblages formed in the presence of progressively more water-rich fluids at progressively lower fluid-rock ratios.

Introduction

The Agnew (formerly known as Perseverance) nickel mine is located in the southern part of the 150 km long Agnew- Wiluna Greenstone Belt in the Archean Yilgarn Block of Western Australia (Naldrett and Turner 1977) (Fig. 1). Binns et al. (t976) have shown that the metamorphic grade increases southward along the greenstone belt from pre- hnite-pumpellylite facies around Wiluna to amphibolite facies in the Sir Samuel-Agnew area. Metamorphic olivine is stable in the Agnew area, and in rocks as far north as the Six Mile deposit. Binns and Groves (1976) estimated the peak metamorphic temperature at the Agnew deposit

Offprint requests to: M.J. Gole

at about 650 ~ C. We show in this study that the effect of CO2 in the metamorphic fluid is to lower this estimate to 550 ~ C.

Metamorphic mineral assemblages in the lenticular ultramafic body at the Agnew mine show an overall zonation from olivine-enstatite-bearing assemblages at its margin to olivine-anthophyllite, olivine-talc and olivine-antigorite assemblages progressively further towards its centre. This zonarion is typical of contact metamorhic aureoles in set-

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152

pentinites (e.g., Tromsdorff and Evans 1972; Frost 1974), but in this case has developed during regional metamorphism. The origin of the zonation is the main focus of this paper. The olivine-rich lithologies can be modelled within the M g O - S i O 2 - H 2 0 - C 0 2 system, and these rocks provide an excellent case study of application of experimental phase equilibria to the natural system in a regional metamorphic environment.

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Distribution of mineral assemblages

Accounts of the local geology are given by Martin and Allchurch (1975) and Billington (1984). The igneous precur- sors to the Agnew ultramafic rocks include spinifex-textured komatiites, olivine orthocumulates, olivine-sulfide cumulates (net-textured or matrix ore) and coarse-grained olivine adcumulates. These have been almost entirely recon- stituted mineralogically and texturally during metamorphism and penetrative deformation, although igneous textures are preserved in the olivine-sulfide cumulates and olivine adcumulates. Spinifex-textured rocks are locally preserved in strain-free areas. The ultramafic rocks formed in a volcanic environment (Gole et al. 1986 and in preparation; Hill et al. 1985) penecontemporaneous with enclosing acid to intermediate volcanics and volcaniclastic sediments. The nickel sulfide deposit is associated with a lenticular body of mostly adcumulate dunite 700 m thick and 2 km long underlain, overlain and flanked to the north and south by komatiite flows (Fig. 2). Olivine distribution in the basal flows indicates that the entire steep W-dipping sequence (Fig. 3) is overturned and youngs to the east. The distribution of the various ultramafic lithologies is complicated by folding and shearing (Barnes et al., in preparation).

A cross section through the southern end of the mine workings (Fig. 3) illustrates the mineral assemblage zoning. Tremolite-chlorite + olivine + enstatite rocks occur on the margin of the ultramafic body against felsic metasediment- ary rocks and also form tabular bodies within the olivine- sulfide rocks of the next zone inwards. Eastward from the olivine-sulfide rocks are two generally well defined zones of olivine-anthophyllite followed by olivine-talc. The olivine-talc rocks occur either in direct contact with igneous- textured olivine adcumulate (Fig. 3), or separated from it by olivine-antigorite rock. In localized low-sulfide areas within the olivine-sulfide zone, enstatite-, anthophyllite- and talc-bearing assemblages are juxtaposed, with anthophyllite-bearing assemblages always between those containing enstatite and talc. The boundaries between enstatite, anthophyllite, talc and antigorite zones are sharp and recog- nizable in hand specimen. The core of the dunite lens is composed of a coarse-grained, igneous textured olivine adcumulate, which has retained its monomineralic character during regional metamorphism. Rare examples of equilibrium intergrowths of olivine and dolomite are located in the t remol i te - ch lor i te - olivine _+ enstatite and o l iv ine- sulfide zones on the western margin of the ultramafic.

Enstatite and anthophyllite-bearing assemblages are restricted to the lower (western) portion of the dunite lens (Fig. 3). Olivine-talc assemblages are common within flows overlying the dunite lens, adjacent to the Perseverance Fault on the eastern boundary of the greenstone belt. The dominant metamorphic assemblage in olivine-rich lithologies to the north and south of the lens is olivine-antigorite, with limited pockets of olivine-talc. In rocks contaifling signifi-

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Fig. 2. Geological surface plan of the Agnew Mine area, showing the distribution of the various komatiitic rock types and metamorphic equivalents. Rock type recognition is based on relict igneous textures and mineralogy where possible, together with whole rock chemical composition. Dips are sub-vertical, and the adcumulate lens plunges steeply to the south. The cross section shown in Fig. 3 is 500 m south of the mine shaft

Fig. 3. East-west geological cross-section through the southern margin of the Agnew orebody, on the western margin of the ultramafic lens, looking north. The distribution of metamorphic mineral assemblages is shown. Top of section is approximately 480 m below present surface elevation. ~] Country rock; [ ] Olivine-sulfide cumulate; [ ] Bladed olivine-sulfide rock; [~ Tremolite + chlorite + olivine__ enstatite; [ ] Olivine + anthophyllite; [ ] Olivine + talc;

Olivine adcumulate; ~A Olivine + antigorite. 18800N-18900N composite cross section

cant Ca and A1 the dominant regional metamorphic assemblage is olivine-tremolite-cummingtonite-chlorite. High- grade olivine-bearing assemblages in the immediate vicinity of the mine are commonly well preserved, whereas else- where rocks are invariably partially to totally altered by retrograde serpentinization. In most retrogressed rocks, however, sufficient relict minerals or microstructures re- main to allow their former high-grade metamorphic assemblages to be ascertained. Post-metamorphic talc-carbonate alteration is uncommon and restricted to narrow zones around late-stage faults,

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Structure and metamorphism

A strong penetrative bedding-parallel foliation is present in the country rocks and in the tremolite-chlorite-rich ultramafic lithologies, but is weakly developed or absent in other ultramafic rock types. A crenulation and associated foliation is developed in the noses of later folds that deform this regional foliation. In the country rocks, metamorphic amphibole, biotite and outgrowths of garnet porphyroblasts cut across both foliations, indicating that peak metamorphic temperatures outlasted deformation. Within the ultramafic rocks, anthophyllite, bladed olivine, enstatite and talc have random orientations and in places cut across a weak foliation. This also indicates growth after the main regional structures were developed. Similar evidence for a post-kinematic metamorphic peak has been observed by Binns et al. (1976), Archibald et al. (1978) and numerous other workers in the greenstone belt terrain of the Yilgarn Block.

Petrography of ultramafic rock types

Dunites (olivine adcumulates). These compose the bulk of the Ag- new ultramafic lens, and are true dnnites with in excess of 98% modal olivine ranging in composition from F092.5 to F094,5. Oliv- ine grains range up to about 1 cm in diameter, and show well developed igneous adcumulate texture (Fig. 4A). This texture is identical to that preserved in serpentinized dunite bodies in low grade areas of the greenstone belt (e.g., Honeymoon Well, Fig. 1), and pre-dates metamorphic recrystallization. Cores of olivine grains show a distinct brown colouration, characteristic of relict igneous olivines which have experienced amphibolite facies metamorphic conditions (Binns et al. 1976). The brown cores show an abundance of very fine grained inclusions (Binns and Champness 1985), principally fine rods and dendrites of chrome spinel and needles of tremolite and chlorite. These are interpreted as products of exsolution during metamorphism. Outer rims are colourless, and narrow domains of colourless fine-grained recrystallized olivine are developed between the coarse relict grains. A similar microstructure in experimentally strained dunite is attributed by Zeuch and Green (1984) to dynamic recrystaltization during deformation. Lobate, intergranular chromite grains, relatively uncommon in the Agnew dunite, have very narrow ferrichromite rims and are generally associated with trace amounts of chlorite.

Olivine-sulfide rocks. These consist of olivine in a continuous matrix of pyrrhotite and pentlandite with minor chalcopyrite and pyrite, and formed as original net-textured or matrix ore, i.e., as a physical accumulation of olivine and immiscible magmatic sulfide liquid. Relict igneous textures are rarely preserved, but typically olivines show brown relict igneous cores with extensive marginal recrystallization as in the adcumulates. Towards the margin of the ultramafic, olivine is often completely recrystallized to spectacular coarse blades and sheaves several centimetres long in random orientation (Barnes et al., in preparation).

Olivine-talc rocks. These show a wide range in modal proportions of the principal minerals, and a corresponding range in texture. Rocks from the olivine-talc zone adjacent to the main dunite core are olivine rich with about 5 modal percent lath-shaped talc. Fig- ure 4 B shows neoblastic olivines, intergrown with talc, surrounding and growing into relict brown igenous cores. In many cases the brown olivine has completely disappeared, leaving a completely recrystallized and usually slightly foliated intergrowth of sugary clear olivine and talc laths. Chlorite is a common accessory, and disseminated sulfides (pentlandite with subordinate pyrite and pyrrhotite) increase in abundance towards the olivine-sulfide ore zone. Olivine compositions range from Fo91 to Fo94 and talc contains in excess of 98 mol% of the Mg end member (Fig. 5).

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Fig. 4A-F. Photomicrographs of ultramafic metamorphic assemblages. A Olivine adcumulate with relict igneous texture. Brown cores contain very fine grained inclusions of chrome spinel, magnetite, tremolite and chlorite. Opaques along grain boundaries are pentlandite and pyrite. B Olivine-talc rock, showing relict igneous olivine (dark, cloudy) overgrown by clear, inclusion free neoblastic olivine and laths of talc. Minor pyrrhotite, pentlandite, chlorite and chrome spinel. C Medium to fine grained neoblastic clear olivine with coarse sheaves of anthophyllite, minor pentlandite, pyrrhotite, chromite and chlorite. D Enstatite (upper right, showing cleavage) and highly fractured clear neoblastic olivine (lower left) with curved blades of cummingtonite cutting enstatite cleavage. Enstatite is partly retrogressed to talc, and olivine to lizardite and magnetite. E Clear neoblastic olivine (high relief, fractured) in matrix of fine, weakly foliated chlorite and randomly oriented blades of tremolite and cummingtonite, with minor pyrrhotite and pentlandite. F Pseudo-graphic intergrowth of olivine and dolomite. All photomicrographs in plane polarized light

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Fig. 5. Fe/Mg ratios of metamorphic minerals coexisting with olivine. Energy-dispersive microprobe analyses

Olivine-talc assemblages are also developed within the olivine- sulfide zone and in rocks overlying the adcumulate lens. These typically contain up to 50% talc, and show a range of textures. "Jackstraw" texture (Evans and Tromsdorff 1974a) with bladed metamorphic olivines in a matrix of fine grained talc is observed, but more typically olivine forms ragged porphyroblasts. 'Up to five percent each of chlorite and tremolite are usually found within these talc-rich rocks.

Olivine-anthophyllite rocks. These show strong microstructural and compositional affinities with the lath-textured olivine-talc rocks. They contain up to ten percent anthophyllite, which forms spectacular blades and sheaves 1 to 5 cm long, catting across olivine grain boundaries (Fig. 4C). Brown cores are occasionally seen in olivine grains, but more commonly olivine in these rocks is completely recrystallized. Accessory phases include chlorite, disseminated intergranular sulfides and talc, the latter as a replacement product of anthopbyllite.

Table 1. Compositions of coexisting minerals in olivine-enstatite- tremolite-cummingtonite assemblage. Averages of four microprobe analyses for each phase

Olivine Enstatite Tremolite Cummingtonite

S i O 2 41.09 57.60 57.50 59.70 TiO= 0.00 0.00 0.11 0.00 A1203 0.00 0.00 1.31 0.11 FeO 10.07 7.04 2.41 6.88 MnO 0.28 0.20 0.00 0.27 MgO 49.31 34.63 24.21 30.26 CaO 0.00 0.09 11.08 0.19 Na20 0.00 0.00 0.42 0.00 K20 0.00 0.00 0.00 0.00

100.75 99.56 96.97 97.40

Si 1.005 1.998 7.877 8.045 A1 0.210 Fe 0.204 0.204 0.275 0.776 Mn 0.006 0.006 0.000 0.032 Mg 1.780 1.790 4.943 6.086 Ca 0.003 1.627 0.028 Na 0.112

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Olivine-enstatite rock. These show a range in texture and modal mineralogy. Enstatite content varies from 5 to 50%, and clear recrystallized olivine from 40 to 80%. Chlorite, tremolite and cummingtonite occur in equal proportions up to a total of 20%, and disseminated sulfides up to 20%. The two amphiboles coexist stably, occurring as discrete grains and lamellar intergrowths. Simi- lar intergrowths have been interpreted as the result of exsolution (Robinson et al. 1982). Representative microprobe analyses are given in Table 1. Olivine, enstatite and cummingtonite have similar Mg/(Mg + Fe) ratios ranging from 88 to 93 tool.%, while tremolite is significantly more magnesian (Fig. 5).

Jackstraw texture is sporadically developed, with enstatite, chlorite etc. occupying space between interlocking metamorphic olivine blades. Most samples show a variably foliated annealed intergrowth of coarse (2-10 ram) grains of enstatite and clear neoblastic olivine, with randomly oriented amphiboles cutting the foliation (Fig. 4 D). Enstatite is partially altered to talc, and it is possible that some of the olivine-talc rocks described above previously contained enstatite. Minor dolomite has been observed in enstatite- bearing rocks, hut its status as part of the prograde assemblage is in doubt. In one sample, a 5 mm thick veinlet of bladed olivine and dolomite cuts an olivine-enstatite rock in which enstatite appears to be replaced by dolomite, which is in turn overgrown by cummingtonite. This relationship is indicative of metasomatism by CO2-rich fluids.

Tremolite-chlorite-olivine rocks. Modal abundance of tremolite and chlorite in Agnew ultramafics increases with increasing CaO and A12Oa contents, giving rise to a spectrum of rock types from tremolite-chlorite schists, with subordinate cummingtonite and talc, grading with increasing olivine content to foliated or (rarely) jackstraw textured olivine-rich rocks with subordinate intergranular tremolite and chlorite (Fig. 4E). These rocks are interpreted as metamorphosed komatiite flows, the variation in present olivine content reflecting an original alternation of spinifex textured A- zones and olivine-enriched cumulate B-zones. A pervasive foliation is defined by olivine augen and chlorite-rich bands. Tremolite and cummingtonite grow both as prismatic grains within the follication, and as randomly oriented post-kinematic prisms and sheaves.

Chrome spinel, often rimmed by chlorite, is a common accesso~ ry in tremolite-chlorite-bearing rocks and in the enstatite-bearing lithologies, and is strongly zoned from chromite cores to ferro- chromite margins. This feature is attributable to re-equilibration

Fig. 6. Photomicrographs (plane polarized light) of olivine-antigorite rocks. A Bladed neoblastic olivine, now almost totally replaced by bladed retrograde antigorite and brucite, with interstitial areas of finely intergrown platy olivine and prograde antigorite. Coarse grained blebby opaques are mostly intergrown pentlandite and magnetite. B Detail of area between coarse olivine blades, showing areas of aligned platy olivine intergrown with antigorite. C Equilib- rium intergrowth of olivine (highest relief), dolomite (intermediate relief) and antigorite in vein cutting olivine antigorite rock

of relict igneous chromite with the metamorphic silicate assemblage. Sulfides (pyrrhotite, pentlandite and minor chalcopyrite) occur from trace amounts up to 20 modal percent.

Olivine-antigorite rocks. This rock type is sporadically developed within the core of the olivine adcumulate lens, and constitutes the dominant assemblage in the laterally correlative olivine-rich rocks. The typical texture (Fig. 6A) consists of coarse interlocking blades up to 3 cm long of clear neoblastic olivine, and polygonal interblade areas up to 5 mm across occupied by a delicate bladed

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Table 2. Average compositions of metamorphic rock types

TCC OTC OETC OT OA OAnt Oac

SiO2 48.5 (4.7) 44.4 (2.5) 45.0 (2.7) 46.0 (2.6) 42.1 (1.4) 41.1 (1.4) 41.6 (1.2) TiO2 0.23 (0.14) 0.14 (0.13) 0.15 (0.14) 0.04 (0.02) 0.04 (0.02) 0.00 (0.00) 0.01 (0.01) A1203 4.9 (2.2) 2.8 (1.8) 2.7 (0.9) 1.2 (1.0) 0.6 (0.4) 0.06 (0.06) 0.39 (0.41) FeO 7.5 (2.4) 7.8 (2.0) 8.4 (2.1) 6.1 (1.3) 8.0 (0.5) 6.2 (0.2) 6.1 (1.2) MnO 0.18 (0.09) 0.14 (0.07) 0.18 (0.06) 0.10 (0.04) 0.14 (0.02) 0.09 (0.01) 0.09 (0.03) MgO 31.6 (3.7) 42.7 (4.0) 42.1 (2.9) 46.9 (4.0) 50.0 (1.5) 52.3 (0.6) 51.5 (1.0) CaO 6.7 (3.3) 1.8 (2.7) 1.6 (1.3) 0.5 (1.5) 0.02 (0.02) 0.19 (0.19) 0.07 (0.04) Na20 0.07 (0.08) 0.02 (0.02) 0.03 (0.02) 0.02 (0.01) 0.01 (0.01) 0.01 (0.01) 0.02 (0.04) K20 0.07 (0.24) 0.03 (0.09) 0.02 (0.02) 0.04 (0.08) 0.01 (0.01) 0.01 (0.02) 0.03 (0.03) Cr203 0.28 (0.40) 0.24 (0.15) 0.18 (0.06) 0.17 (0.05) 0.16 (0.14) 0.10 (0.05) 0.17 (0.15) M/Si 1.10 (0.17) 1.58 (0.17) 1.55 (0.14) 1.67 (0.17) 1.91 (0.09) 2.02 (0.06) 1.97 (0.07)

Samples 49 124 12 22 6 3 81

TCC tremolite-chlorite cummingtonite rock; OTC olivine-tremolite-chlorite-cummingtonite; OETC olivine-tremolite-chlorite-enstatite- cummingtonite; O T olivine-talc ( + tremolite, chlorite); OA olivine-anthophyllite; OAnt olivine-antigorite; Oac olivine adcumulate. Aver- ages of given number of analyses (XRF fused disk method), recalculated volatile and sulfide free. Standard deviations in parentheses. M/Si = molar (MgO + MnO + FeO)/SiO2

intergrowth of olivine and antigorite (Fig. 6B). The coarse bladed and interblade olivines have identical compositions (Fo92-93) within individual samples. The texture of the interblade areas shows a superficial resemblance to igneous spinifex texture, but compositionally the rock is equivalent to olivine adcumulate (Table 2), and could not have contained significant amounts of interstitial komatiite liquid. Bladed olivine-antigorite-dolomite intergrowths are occasionally developed as thin veins within the dunite (Fig. 6C), and dolomite may have been a primary component of the interblade assemblage. This rock type appears to be particularly susceptible to the growth of retrograde antigorite, forming trellis-like replacement textures after metamorphic olivine (Fig. 6A).

Olivine-carbonate rocks. Patches of olivine-dolomite rock with equilibrium prograde microstructures are developed within the marginal zone. Texture ranges from randomly-oriented neoblastic olivine blades in a matrix of granular dolomite to an unusual graphic olivine-dolomite intergrowth (Fig. 4F). The proportion of carbonate ranges from about 5 to 70%. Veinlets less than a centi- metre wide of bladed olivine and dolomite have been observed cutting olivine-enstatite rocks.

Contact-zone rocks. Contacts between ultramafics and felsic country rock are marked in many cases by a narrow zone (5-20 cm) of talc-cummingtonite rock, having coarse bent sheaves of cummingtonite in a matrix of fine grained talc. Contacts between tremolite-chlorite rock and felsic metasediment typically show a tran- sition zone 20 to 40 cm wide containing chlorite, green pleochroic actinolite with colourless tremolite cores, and phlogopite-rich bands with minor tourmaline. Thin calc-silicate selvages occupy contacts between country rock and sulfide-rich ultramafics, and contain complex assemblages of clinopyroxene (salite), actinolite, calcite, epidote, grossular (or hydrogrossular) and tourmaline.

Whole-rock chemical compositions

Whole-rock chemical compositions o f more than two hundred samples of ultramafic rock are plotted in Fig. 7, and average analyses are listed in Table 2. Analyses were carried out by X-ray fluorescence on fused disks. Full analytical data are given in Barnes et al. (in preparation). The overall compositional variance has two principal components: 1) Igneous differentiation, and physical mixing of different proport ions of komatiitic magma and cumulus olivine in the protolith; and

2) Element mobility resulting from fluid-rock interactions during alteration and metamorphism. Calcium shows considerable scatter, and has been largely stripped from olivine- rich rocks with MgO in excess of 42%. This conclusion is based on the very low Ca/A1 ratios in these rocks, compared with predicted igneous values of around 1 for typical komatiitic rocks.

Tremolite-chlorite dominated assemblages are distinctly enriched in Ca, A1 and Si relative to olivine-talc, olivine- anthophyllite and olivine-enstatite assemblages, and are interpreted as metamorphosed equivalents o f spinifex textured A-zones of komatiite flows. Olivine-tremolite-chlorite rocks are indistinguishable chemically from olivine-enstatite-tremolite-chlorite rocks, and correspond to olivine orthocumulates in composition. Talc-rich lithologies contain higher silica contents for a given MgO, indicating that talc abundance is related to metasomatic introduction of silica. Lath-textured olivine-talc rocks and olivine-anthophyllite rocks overlap chemically. The more magnesian samples are compositionally similar to monornineralic dunites, corresponding to olivine mesocumulates with small proport ions of original trapped liquid. Molar ratios o f (MgO + F e O + MnO)/SiO2 are slightly less than 2 (Table 2), compared with values of almost exactly 2 in adcumulate dunites and pure olivine-sulfide rocks. The excess silica determined the presence and abundance of talc, anthophyllite or enstatite, but not their relative stabilities. The abundance o f tremolite and chlorite is controlled by the content of Ca and A1 in the rock, which is principally determined by the original proport ion o f komatiite liquid to cumulus olivine in the protolith. Olivine-antigorite rocks have compositions identical to those of olivine adcumulates, implying that another silica-deficient phase, presumably either carbonate or magnetite, must have been present in the prograde assemblage. This is now obscured by retrograde serpentinization.

Country rock assemblages

Few of the country rock assemblages have been found use- ful in constraining metamorphic conditions. Aluminium silicates are not present in the vicinity of the mine, and the nearest known occurrence is that of andalusite in metasedi- ments 11 km to the north (P. Sauter, personal communica-

60 A g n e w - r n e t a m o r p h o s e d u i t ramaf ics

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tion 1986). Only a single specimen containing coexisting garnet and cordierite has been found, and although garnet- biotite pairs are common, many of the garnets have compositions inappropriate for geothermometery. Plagioclase and potassium feldspar in the felsic rocks are partially altered to white mica and amphibole, and metamorphic plagioclase in a metagabbro to the west of the ultramafic lens has highly variable compositions. Estimation of metamorphic conditions in the vicinity of the Agnew Mine has been based on the ultramafic assemblages and on garnet-biotite pairs.

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Garnet-biotite thermometry. Garnet in garnet-biotite bearing country rocks are strongly zoned with cores enriched in CaO, MnO and FeO relative to overgrowth rims (Ta- ble 3). All garnet core and most rim compositions were too CaO- and MnO-rich to be of use in thermometry. Some overgrowths, however, had suitable compositions, and reflect conditions of the post-kinematic metamorphic peak. These conditions are regarded as appropriate for other post-foliation minerals, notably bladed olivine, enstatite, anthophyllite and talc in the ultramafic rocks.

Data from electron probe microanalysis of garnet overgrowths and surrounding biotite (Table 3) give rise to a range of calculated temperatures, depending on the thermometer used. Average temperatures of 535 ~ C and 560 ~ C are obtained using calibrations of Ferry and Spear (1978) and Newton and Haselton (1981) respectively, excluding two samples with high CaO and MnO in garnet (Table 3).

Garnet-cordierite thermometry. Applicaiton of Holdaway and Lee's (1977) exchange thermometer to a single garnet- cordierite pair yields 557 ~ C (lnKD of 2.43), in agreement with the garnet-biotite temperatures.

Metamorphic pressure. The rare presence of cordierite is indicative of low pressure (Binns et al. 1976), but no quanti- tative estimate can be made owing to the absence of a coexisting aluminosilicate. Assuming a maximum temperature of 550 ~ C at Agnew and a gentle north-south thermal gradient along the greenstone belt, the presence of andalusite 11 km to the north indicates a maximum pressure of 3 kb (Holdaway 1971).

Theoretical phase equilibria in M g O - S i O 2 - - H 2 0 - C O 2

The Agnew ultramafic assemblages can be represented in the system MgO - C a O - SiO2 - A1203 - H 2 0 - C O 2 . The A1203 component can be eliminated by projection from chlorite, which is ubiquitous and stable over the whole range of possible P - T - X conditions, and accounts for virtually all the alumina present in the rocks. Similarly, tremolite accounts for almost all the calcium present, and coexists stably with olivine to temperatures in excess of 800~ (Jenkins 1983; Skippen and McKinstry 1985). The only other Ca-bearing phase recognised as a prograde mineral at Agnew is dolomite, which also coexists with olivine over a wide field (Slaughter et al. 1974; Skippen 1974). The relevant equilibria can therefore be expressed in the MgO - SiO2 - H20 - CO2 system.

The basic topology of this system has been worked out by Johannes (1969) and Evans and Tromsdorff (1974). Re- cent data on the stability of anthophyllite relative to talc and enstatite (Chernosky et al. 1985; Day et al. 1985) have been incorporated into a T - X plot (Fig. 8). Relevant equilibrium parameters are summarized in Table 4 for reactions between pure end-members forsterite (F), enstatite (E), anthophyllite (A), talc (T), magnesite (M) and antigorite (An).

Calculation of phase diagram. Isobaric invariant curves were calculated for a total pressure of 3 Kbars, using the formu- lation of Kerrick and Jacobs (1981) for non-ideal mixing of supercritical H 2 0 - CO/fluids. Dashed curves were calculated to model actual mineral compositions observed at Agnew, taking a typical olivine composition of F o 9 3 and

158

Table 3. Summary of garnet-biotite pair compositions

Sample No. Garnet Biotite

Xca XMn XMg X~e Mg/*o XAIVI Xx~ Mg/*o K

Temp. ~ C

FS NH

WAP149-168 R 0.063 0.043 0 .101 0.793 0.127 WAPt36A-715 R 0.632 0.008 0.127 0.833 0.153 WAP136A-700 R 0.052 0.005 0.132 0 .821 0.150 WAP136A-715.4 R 0.030 0 .011 0.135 0.824 0.164 WAP136AX-764 R 0.064 0.030 0.083 0.823 0.101

C 0.137 0.224 0.029 0 .611 0.047 WAP136A-713 R 0.024 0.014 0.128 0.834 0.153 WAP56-968 R 0.126 0.052 0.052 0.770 0.067 WAM8-348 R 0.116 0.032 0.073 0.779 0.094

C 0.120 0.259 0.035 0.586 0.060

0.097 0.026 0.883 0.136 0.025 0.985 0.099 0.027 0.984 0.122 0.028 1.053 0.121 0.035 0.618 0.109 0.040 0.561 0.128 0.027 0.926 0.093 0.047 0.405 0.073 0.038 0.560 0.095 0.045 0.572

0.144 0.155 0.153 0.156 0.163

0.165 0.166 0.168

506 527 522 529 543

547 548 552

530 540 542 541 568

557 597 597

R garnet overgrowth rim, C garnet core, Fe* total Fe as FeO X~ ar = i/(Fe + Mg + Ca + Mn), X~ i~ = i/(Fe + Mg + Mn + A1 w + Ti)

K - (Mg/Fe*)6ar;" FS Ferry and Spear (1978); NH Newton and Haselton (1981) (Mg/Fe*)Bio

Table 4. Summary of equilibrium data for linearly independent reactions used in construction of the M g O - SiO2 - H 2 0 - CO2 phase

in the equation log K = @ + B + T ( P - - 2 0 0 0 ) following Evans and Tromsdorff (1974). T in ~ P in diagram. Values are coefficients

bars. F forsterite, E enstatite, A anthophyllite, T talc, An antigorite, M magnesite

Reaction A B C

FAE F + A = 9 E + W --7717 11.30 0.111 FTA 9T + 4F = 5A + 4H20 - 38 793 55.23 0.157 FTAn An = 18F + 4T + 27H20 - 269 098 424.61 - 1.769 FMT 5M + T = 4F + H20 + 5CO2 - 29 610 54.86 0.528

Sources of data: FAE, FTA and FTAn from Day et al. "best fit" phase diagram using data of Chernosky et al. (1985) on anthophyllite stability. FMT from compilation of Evans and Tromsdorff (1974)

PTotal = 3 Kbars 700 - E

650. A ~

T~ ~ T F " . . . . ~ ~.'~'~ ~'~

550 ~ ATE

500 I I I I f I I I I H20 0.5 CO 2

XC02

Fig. 8. Isobaric T-Xco= section for system MgO--SiO 2 - CO 2 - HzO at 3 kb showing stability relations among enstatite (E), anthopbyllite (A), talc (T), forsterite (F), antigorite (An) and magnesite (M). Dashed lines show effect on equilibria of solid solution of iron, using measured compositions of Agnew minerals from Fig. 5. Metastable extensions omitted for clarity

values for K Fe/Mg between olivine, talc, anthophyl l i te and enstat i te based on microprobe analyses o f coexisting phases (Fig. 5). Ant igor i tes in prograde olivine-antigori te assemblages were found to be essentially i ron free. Activi ty-composi t ion relat ions for olivine and or thopyroxene are based on Sack (1980). In the absence o f comparab le da ta for talc and anthophyll i te , we follow the formulat ions of Evans and Tromsdor f f (1974) as follows:

(Y Anth~ (1) aMg-anthoph. ~ k ~ ~Mg t

aMg-talc = (XTgle) 4 (2)

Displacing composi t ions from the pure end members by addi t ion of ferrous i ron has minimal effect on the forster- i te-enstat i te-anthophyll i te (FAE) and antigori te-forsteri te- talc (AnFT) equilibria, but lowers the temperature of the forster i te- talc-anthophyll i te (FTA) react ion by about 20 ~ C. The effect of this, as predicted by Evans and Tromsdor f f (1974) is a change in the phase topology in the CO2 rich region (Fig. 9). In the pure Mg system, curves F A E and F T A intersect at isobaric invar iant point F A T E , which is connected to a second invariant poin t F E M T by a short segment o f the dehydra t ion react ion T + F = 5 E + H 2 0 (TFE). F o r na tura l composi t ions, F T A is displaced down- wards to intersect the dehydra t ion and decarbonat ion curve magnesite-talc-forsteri te (FTM), giving rise to two different isobaric invar iant points, F T A M and F A M E , and eliminat- ing the possibil i ty of stable coexistence of talc and enstatite. All these possible invar iant points are located in a restricted area in the CO2 rich region of T - X space, between 547 and 557 ~ C and Xco2 0.92 to 0.97 (Fig. 9).

Owing to a lack of thermodynamic data, magnesiocummingtoni te does not appear in these calculations. Rice et al. (1974) propose that magnesiocummingtoni te is a high temperature po lymorph of anthophyll i te , while Mueller (1973) suggests that cummingtoni te may be stabilized by higher F e O / M g O ratios for a given temperature. The evidence from Agnew tends to suppor t the second interpreta-

159

560 ~\\x ATE

T(~ k \x

550

000 o,,-+ \ / anthophyllite \ ~ Olivine +

enstatlte

T'~ 550 ,~" ~ '~

5401 l 0.9 0.95 1.0

Xco 2

Fig. 9. Stability relations for CO2 rich portion of MgO-SiO2- H20-CO2 at 3 kb, showing effect of incorporation of iron in solid solution. Top diagram shows relations in iron free system, bottom shows equilibria for phases in equilibrium with FO93 olivine. Heavy curves outline stability fields of olivine-enstatite and olivine- anthophyllite

tion. Cummingtonite occurs in slightly more iron rich rocks, coexisting with olivines from Foss to Fo91, as compared with anthophyllite which is not found in rocks with olivines less magnesian than about Fogo.

Stability of Agnew assemblages

The Fe/Mg distribution between olivine and coexisting fer- romagnesian silicates (Fig. 5) is similar to that reported by Evans and Tromsdorff (1974) and Frost (1974). The ratio shows a coherent pattern and justifies the treatment of these coexisting phases as equilibrium assemblages.

The theoretical phase diagram indicates that all the prograde ultramafic assemblages encountered at Agnew, i.e. olivine-enstatite, olivine-anthophyllite, olivine-talc, olivine- antigorite and olivine-carbonate, could coexist stably at 550 ~ C and 3 kb, with widely fluctuating fluid composition between pure water and pure carbon dioxide. This would require a fairly precise coincidence, since the upper stability limit of antigorite plus olivine (in pure water) and the lower stability of enstatite plus forsterite (the FEMT isobaric invariant point) are calculated to be within 2 ~ C of one another. This calculated temperature of 550 ~ C is, however, in remarkable agreement with that calculated from garnet- biotite assemblages. Taking into account uncertainties in the experimental data base, and in activity-composition re- lationships, we conclude that this agreement is significant, and that metamorphism of the Agnew ultramafic rocks took place at a peak temperature of 550_+ 20 ~ C under open system conditions, with infiltrating fluids of widely varying water/CO2 ratio imposing local equilibrium.

The most serious problem with this interpretation is the

widespread distribution of the olivine-anthophyllite assemblage, which according to the phase diagram is only stable at 550 ~ C over a very narrow range of fluid compositions. Two possible explanations are offered. Firstly, the experimental data shown by Chernosky et al. (1985) on the reaction FAEW allow for an uncertainty in temperature of about 20 ~ C, and this, combined with uncertainty in activity-composition relations, results in a large uncertainty in the size of the olivine-anthophyllite stability field for Agnew compositions. Secondly, the observed assemblages may re- cord a more complex thermal history, with anthophyllite- and enstatite-bearing assemblages forming early at a slightly higher temperature, and the olivine-antigorite assemblage forming later at about 550 ~ C, simultaneous with the final setting of the garnet-biotite geothermometer. This is consistent with observed partial retrogression of enstatite and anthophyllite to talc, alhough this may imply a late influx of water-rich fluid under peak thermal conditions. The wide distribution of olivine-talc assemblages indicates that peak metamorphic temperatures never exceeded 640 ~ C, the upper limit of stability for talc plus Fo93 olivine.

The observed zoning sequence from olivine-antigorite to olivine-enstatite from interior to margin of the ultramafic lens corresponds to a change in fluid composition from almost pure water to almost pure CO2. The source and relative volume of the infiltrating fluids will be considered below, but it is first necessary to consider whether other volatile species may have played a role.

Intensive variables and speciation in the metamorphic fluid

The presence of pyrrhotite in the Agnew ultramafics im- poses constraints on oxygen and sulfur fugacities during metamorphism. The calculations of Whitney (1984a, b) on the F e - S - O - S i system have been adapted for this pur-

550~ -2 / /

py / / / / ~',L/

=4 / / / / J-b / /.I/I /

- ~ A

- / Y / / / _ / Y •

/ I s / " @ I -lo / / ~ / , . ~ - / o l

/ +,1 i / ./ -12 b~O~'i ///// 1 -14 /

-28 -26 -24 -22 -20 -18

log f o 2

Fig. 10. Stability of minerals in the system F e - S - S i - O in terms of fugacities of 02 and $2 at 550 ~ C, after Whitney (1984a, b), Light solid lines show stability fields for pure Fe species pyrite (PY), pyrrhotite (PO), magnetite (MT) and fayalite (FA) in equilibrium with quartz. Heavy solid lines show displaced equilibria for natural mineral compositions observed at Agnew, at a silica activity defined by equilibrium between olivine and enstatite of the appropriate composition. Diagonal dashed lines show ratios of fugacities of H2S and water, calculated from Whitney (t984b)

160

pose (Fig. 10). Phase boundaries between fayalite, pyrrhotite, magnetite and pyrite are shown for quartz-saturated conditions as functions of oxygen and sulfur fugacity at 550 ~ C, our estimate of peak metamorphic temperature at Agnew. Phase boundaries have been calculated for the case of olivine (Fo93) in equilibrium with enstatite (Fs93). Faya- lite and ferrosilite activities are determined from data of Sack (1980), silica activity is defined by coexisting olivine and orthopyroxene (Morse 1979), and FeS activity in pyrrhotite is taken from Toulmin and Barton (1964). In this case the relevant sulfide phase is nickeliferous MSS, but the effect of Ni on the calculation is negligible. The oxygen fugacity of the buffer assemblage olivine (Fo93)-enstatite- magnetite has been calculated similarly from data of Myers and Eugster (1983) on the QFM buffer, and provides an upper limit on fo~, since ferric iron-bearing spinels occur as widespread accessory minerals. Considerable disequilib- rium is evident in Agnew spinels, however, and no reliable estimate of magnetite activity can be made. We conclude that the metamorphic fluid within the enstatite zone was constrained to lie on the " O L + OPX + PO" line of Fig. 10, probably close to the isobaric invariant point olivine + enstatite + pyrrhotite + magnetite, indicating a sulfur fugacity of about 10-5 and an oxygen fugacity of about 10-2o.

Isopleths of H2S/H20 fugacity ratio have been calculated from data quoted by Whitney (1984b) (Fig. 10). At 550 ~ C, the fugacity of HzS in the metamorphic fluid was approximately one hundredth that of water, indicating partial pressures of H2S ranging from about 1 bar in equilibrium with olivine plus enstatite to about 30 bars in COz-poor fluids in equilibrium with olivine plus antigorite. H2S is the only significant sulfur species under these conditions, and hence it is clear that sulfur has no significant influence on silicate mineral assemblages.

Frost (1979) shows stability relations amongst C - H - O - S species for conditions of interest here. Interpolating between his diagrams drawn for 2 and 5 kb and 500 and 600 ~ C, these calculations indicate a maximum CH4 mole fraction of the fluid of about 0.001. Graphite should be stable in equilibrium with CO2 rich fluid at or below an oxygen fugacity of 10-22 at 550 ~ C. The absence of graphite and presence of ferric iron in spinel in olivine-enstatite rocks at Agnew therefore indicates an fo~ of between 10-22 and 10 -19"6. These calculations confirm that metamorphic fluids at Agnew may be modelled as binary mixtures of H20 and CO2.

Source of metamorphic fluids

A large component of the metamorphic fluid was generated by devolatilization reactions in the country rocks. This accounts for large volumes of water, but the source of carbon dioxide is more problematical. Thin calc-silicate bands occur in the country rock, but are volumetrically insignificant. A possible source of CO2 is from carbonate introduced prior to regional metamorphism as a result of alteration. Many ultramafic bodies in the lower-grade portion of the Agnew-Wiluna belt show rinds of talc-carbonate alteration, e.g., Mt. Keith (Burt and Sheppey 1975). The former presence of talc-carbonate at Agnew is indicated by the presence of prograde olivine-dolomite lithologies as described above. Significantly, the phase diagram presented above shows that the assemblage tale-magnesite has an upper stability limit of about 550 ~ C. Under peak conditions at Agnew,

therefore, talc-carbonate rocks would have broken down to either olivine-carbonate or olivine-talc assemblages (depending upon the ratio of talc to carbonate in the protolith), releasing a fluid containing five parts CO2 to one part water.

Simultaneously, water-rich fluids would have been generated by dehydration reactions in the country rock, and also by the breakdown of pre-metamorphic serpentine in the ultramafic body. The abundance of brown cores of olivine grains in the main adcumulate mass indicates that these rocks were probably never intensely serpentinized, but the decrease in abundance of brown relative to clear neoblastic olivines towards the margins of the relative to clear neoblastic olivines towards the margins of the lens suggests that a zone of pre-metamorphic serpentinite was developed between the talc-carbonate rind and the fresh core.

The presence of a dry, unserpentinized dunite core throughout the metamorphic history, and the consequent continuous presence of a fluid activity gradient into the dunite, would enable hydration reactions to take place during prograde metamorphism, as water generated from breakdown of serpentine in the marginal zone migrated into previously anhydrous dunite. This is in contrast to the typical situation in metamorphosed serpentinites, where there is a net dehydration and expulsion of water (e.g., Frost 1974).

A model for progressive metamorphism at Agnew

We envisage pre-metamorphic alteration as following the model of Eckstrand (1975), involving an inward-migrating serpentinization front followed by a carbonation front, gen- erating concentric marginal zones of talc-carbonate and serpentinite (Fig. 11). The bulk of the dunite core remained impermeable to fluids, and retained fresh igneous mineralogy. Fluid access to the dunite core may have occurred along fracture zones, giving rise to discrete zones of serpentinization within the core.

Regional amphibolite facies metamorphism resulted in breakdown of these alteration assemblages (Fig. 11, frame 2). Breakdown of serpentine produced a continuing influx of water into the dunite core. Simultaneously, the outer zone of the ultramafic body was infiltrated by CO2- rich fluid generated by the breakdown of talc-carbonate. The resulting gradient in fluid composition, from CO2 rich at the margin to H20 rich in the core, produced the observed zonation in mineralogy. Serpentinized zones within the dunite core converted to olivine plus antigorite, which remained the stable prograde assemblage beyond the limit of COz infiltration. The situation at the margin of the ultra- marie was complicated by continuing influx of water from country-rock dehydration reactions. This produced the observed juxtaposition of enstatite, anthophyllite and talc in the marginal zone.

Variance of Agnew assemblages. The ultramafic rocks at Agnew are characterized by assemblages of high variance in terms of the four component system. The dominant assemblages are olivine-talc, olivine-anthophyllite and olivine-antigorite, which are divariant at a given total pressure. Equilibrium intergrowths of more than two of these phases are absent or at least very rare. The only univariant assemblage recognized is olivine-enstatite-cummingtonite, which is restricted in distribution. In view of the occurrence

161

Limit of fluid /ingress

~ ~'Serpentinization: < o ~ . ". : z.one : . Dry "

dunite Marginal talc-carbonate

zone 1. Pre-regional metamorphism

Country rock

CO,

Talc-carb-*-olivine + CO2-rich vapour

H20 from serp. dehydration

Enst~Anth~Talc~Anti

COI~ rich~ -- ',-120 tic

~Olivine-antigorite rock

Dry dunite

\

2. Regional prograde metamorphism

Fig. ll. Model for generation of metamorphic mineral zonation involving (1) formation of an outer talc-carbonate rind and (2) breakdown of talc- carbonate and serpentine during prograde metamorphism. See text for full discussion

of cummingtonite in lamellar intergrowth with tremolite, this assemblage should probably be regarded as divariant also, with CaO as an additional component. The only possible invariant assemblage found, allowing for the extra component, is olivine-enstatite-cummingtonite-tremolite-dolomite, but the status of this as an equilibrium prograde assemblage is in doubt. The observed dominance of high over low variance assemblages is the hallmark of a system open to fluid components, in which mineral stabilities are controlled by fluid composition (Rice and Ferry 1982).

Fluid-rock ratios

The relative volumes of fluid involved may be assessed in terms of reaction progress (Rice and Ferry 1982). Consider- ing the isobaric univariant reaction

9 Talc + 4 Forsterite = 5 Anthophyllite + 4 H20

we define a reaction progress variable ~ = 1/5 (moles anthophyllite produced per 1,000 cm 3 of rock). Progress of this reaction to the right generates pure H20, but the partial molar volume of H20 in the fluid in equilibrium with this assemblage is fixed at about 0.1 at the temperature of interest, giving

Xn~o = 4 ~/(4 ~ + nco~) = 0.1 (3)

where nco2 is the number of moles of CO2 infiltrating the rock. Typical olivine - anthophyllite rocks contain approximately 5 modal percent anthophyllite, corresponding to about 0.2 moles per 1,000 cm 3 of rock. Substituting into equation (3), and taking a value of 50 cm 3 for the molar volume of CO2 at 550 ~ C and 3 kb (Shmonov and Shmulo- vich 1974) gives an estimate of 75 cm 3 for the volume of infiltrating CO2 per 1,000 cm 3 of rock. This dehydration reaction can apparently be driven by infiltration at relatively modest fluid-rock ratios.

A similar calculation for the anthophyllite-forsterite-enstatite equilibrium gives much higher fluid-rock ratios, be- cause of the higher modal abundance of pyroxene in the olivine-enstatite rocks and the higher Xco~ of the fluid. For a rock with 30 modal percent enstatite, the corresponding version of equation (3) gives an estimate of about 1,000 cm 3 of infiltrating CO2 per 1,000 cm 3 of rock, i.e., a fluid:rock ratio of one.

These results may be compared with the volumes of fluid produced by talc-carbonate breakdown. Consider a pure olivine rock, completely converted to talc-carbonate and then metamorphosed back to olivine again by the reaction

5 Magnesite + Talc = 4 Forsterite + H20 + 5 CO2.

The dehydration/decarbonation reaction generates about 150 cm 3 of water and 1,500 cm 3 of CO2 per 1,000 cm 3 of rock. The proposed mechanism could therefore generate relatively large volumes of olivine-anthophyllite rock but smaller volumes of olivine-enstatite rock, consistent with the relative proportions of the different assemblages at Ag- new. We conclude that the olivine-enstatite assemblages developed within the original talc carbonate zone, close to the isobaric invariant point FEMT. the mineral zonation at Agnew corresponds to a gradient in fluid composition, and to a decrease in implied fluid: rock ratio from high values in the marginal enstatite-bearing zone, through to significantly lower values in the anthophyllite, talc and antigorite zones.

Application to other areas

Similar concentric zonation of olivine-enstatite, olivine-talc and olivine-anthophyllite assemblages from margin to core of dunite bodies has been reported from the Forrestania area of Western Australia (Porter and McKay 1981). The regional metamorphic grade is reported as being slightly higher than that at Agnew, with country rock assemblages recording temperatures of 665~ at 3 kb. Porter and McKay interpret the zonation as the result of Xco/XHo variations, and the mechanism proposed above for Agnew is equally applicable here.

Scotford and Williams (1983) describe a variety of metamorphic assemblages incorporating olivine, enstatite, anthophyllite, talc, antigorite, tremolite and chlorite from ultramafic bodies in the Appalachians in Virginia and North Carolina. They found no correlation between ultramafic mineral assemblages and metamorphic grade as indicated by country rocks, and interpret the ultramafic assemblages as the result of metasomatism. The Agnew model may well apply here also, with the distribution of tremolite and chlorite related to primary whole rock composition, and the presence of enstatite, anthophyllite and talc controlled by CO2 infiltration.

Acknowledgements. This work was carried out as a portion of a comprehensive study of the Agnew nickel deposit, partly funded by B.P. Minerals Australia, Mount Isa Mines, the Agnew Mining Company and the Western Australia Mining and Petroleum Re- search Institute. We gratefully acknowledge the full co-operation and assistance of the Agnew Mining Company and geological staff. Diagrams were drafted by Angelo Vartesi. The manuscript was reviewed by D.R. Hudson, R.A. Binns and W.L. Griffin, whom we thank for their constructive comments. This paper is a contribu- tion to I.G.C.P. project 161.

162

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Received December 24, 1986 / Accepted February 24, 1987

The role of fluids in the metamorphism of komatiites, Agnew nickel deposit, western Australia

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