Mineralogical Magazine, April 2002, Vol. 66(2), pp. 313–326

Hypogene violarite of exsolution origin from Mount Keith, Western Australia: field evidence for a stable pentlandite ^violarite tie line

B. A. GRGURIC* Geology and Resource Evaluation Department, WMC Resources Ltd., Mount Keith Operation, P.O. Box 238, Welshpool Delivery Centre, W.A. 6986, Australia

ABSTRACT

In most documented occurrences, violarite (FeNi2S4) occurs as a product of the supergene alteration of primary pentlandite or millerite. Earlier experimental phase relations studies predicted the possible existence of a stable violarite–pentlandite tie line, though there has been little field evidence supporting this hypothesis, and the preferred topology in the Ni-Fe-S system involves a pyrite–millerite tie line. This paper documents the occurrence of violarite-pentlandite+pyrite assemblages which, on the basis of mineral chemistry and textural evidence, appear to be hypogene. Primary cobaltian violarite (with 2.1À13.2 wt.% Co) occurs as lamellae in pentlandite in the MKD5 nickel sulphide orebody at Mount Keith, central Western Australia. These lamellae are interpreted to be of exsolution origin. Cobalt is preferentially partitioned into violarite, resulting in high Ni:Co ratios in the associated pentlandite relative to pentlandite in violarite-free assemblages. Hypogene violarite-millerite+pentlandite assemblages were also noted. In all hypogene assemblages, violarite differs in both textural and mineral chemical characteristics from supergene violarite from the upper portions of the MKD5 orebody. The implications of the assemblages for the known low-temperature phase relations in the Ni- Fe-S-(Co) system are discussed.

KEYWORDS: violarite, pentlandite, Ni-Fe-S system, Mount Keith, Western Australia.

Introduction In addition to supergene occurrences, violarite has been noted by Hudson and Groves (1974) as a VIOLARITE (FeNi2S4), is the most economically rare primary phase in apparent equilibrium with important member of the thiospinel group of vaesite, pyrite and millerite, with millerite (Keele minerals. It occurs abundantly in the supergene and Nickel, 1974), and with polydymite, chalco- alteration zones of many massive and dissemi- pyrite, pyrite and secondary violarite (Riley, nated Ni sulphide deposits where it replaces 1980). The possible occurrence of hypogene primary pentlandite or millerite (Nickel, 1973; violarite was predicted by Craig (1971) who Nickel et al., 1974; Misra and Fleet, 1974). In the obtained violarite as a synthetic exsolution Ni deposits of Western Australia the weathering product of Ni-Fe-S monosulphide solid solution profiles are generally deep and, as a consequence, (mss) annealed below 4618C. With the lower- violarite-bearing supergene ores may constitute a temperature decomposition of the mss, hypogene considerable proportion of the total ore reserve of pentlandite-violarite assemblages are a possibility a deposit. on the basis of Craig’s work. Confirmed natural occurrences of hypogene pentlandite-violarite assemblages have been lacking, however, leading to controversy as to whether pentlandite- * E-mail: [email protected] violarite or millerite-pyrite constitutes the stable DOI: 10.1180/0026461026620032 low-temperature assemblage in the Ni-Fe-S

# 2002 The Mineralogical Society B. A. GRGURIC

(+Co) system (Vaughan and Craig, 1985). Since conduits, with enveloping magnesite-antigorite millerite-pyrite assemblages have been noted in haloes. The most distal manifestation of this hypogene ores, and the commonly observed alteration process is a lizardite-brucite assemblage supergene violarite-pentlandite intergrowths are with associated hydrotalcite group minerals not considered equilibrium assemblages, most (Grguric et al., 2001). This sequence of alteration petrologists have favoured a millerite–pyrite tie assemblages is essentially identical to that line (Vaughan and Craig, 1997). described by Eckstrand (1975) in the Dumont Recently, an extensive optical and microanaly- serpentinite complex, Quebec, with the exception tical study of sulphides in the MKD5 orebody at that no original igneous olivine is preserved in Mount Keith, Western Australia was undertaken MKD5. Petrogenetic indicators suggest that in order to define the variation in sulphide mineral hydrothermal alteration at Mount Keith occurred chemistry and assemblages deposit-wide. This at temperatures below 3208C(Ro¨dsjo¨and paper documents violarite-pentlandite+pyrite Goodgame, 1999). A distinctive feature of the and violarite-millerite+pentlandite assemblages deposit is the stratigraphic zonation of sulphide noted during the course of this study, which, on assemblages. This zonation is interpreted to be a the basis of mineral chemistry and textural consequence of: (1) primary variations in sulphur evidence, appear to be hypogene. The occurrence saturation during segregation of sulphides from of violarite as oriented laths in host pentlandite the primary magma and subsequent cooling; and and the partitioning of low-level Co between the (2) modifications to sulphide/oxide sub-solidus coexisting phases would suggest that the inter- phase relations as a result of contrasting oxygen growths are of exsolution origin. The implications and sulphur fugacity conditions during later of these assemblages with respect to the proposed hydrothermal alteration. low-temperature phase relations in the Ni-Fe-S The bulk of the ore zone comprises pentlandi- (+Co) system will be examined. te+pyrrhotite assemblages in a well-defined, steeply-dipping adcumulate dunite unit known as the Pentlandite Domain (Fig. 1). The underlying Geological setting Millerite Domain adcumulate dunite unit is S- An outline of the general geology and miner- poor relative to the Pentlandite Domain, and alization style of the MKD5 orebody (27813’S contains no pyrrhotite. Sulphide assemblages in 120832’E) is given in Dowling and Hill (1993) the Millerite Domain are dominated by high-Ni and Hopf and Head (1998), and a more detailed pentlandite, millerite, godlevskite and heazlewoo- discussion of the alteration systematics is given in dite. Hypogene violarite-bearing assemblages Ro¨dsjo¨ and Goodgame (1999). MKD5 is a large, occur predominantly near the stratigraphic base low-grade (av. Ni grade 0.58%) nickel sulphide of the Pentlandite Domain, and locally within the orebody hosted by a series of Archaean komatiitic Millerite Domain (Fig. 1). dunite and peridotite units, which form part of the Supergene violarite is also common in the Agnew-Wiluna greenstone belt, in central upper zones of MKD5 and persists down to a Western Australia. Disseminated Ni-Fe-sulphide vertical depth of ~100 m. A detailed examination minerals (dominated by pentlandite, pyrrhotite, of the mineralogy and geochemistry of the upper millerite and pyrite) occur as an intercumulus weathered zone of MKD5 is presented in Butt and phase in sites interstitial to former olivine grains, Nickel (1981). Sulphide minerals associated with and the deposit falls into the Group 2B Ni supergene violarite include secondary millerite, sulphide deposit classification of Lesher (1989). pyrite, marcasite and relict pentlandite. As will be The host cumulate dunites and peridotites have demonstrated, both the textural characteristics and been completely serpentinized and partially mineral chemistry of supergene violarite are carbonate altered after deformation and meta- markedly different from those of its hypogene morphism to mid–upper greenschist facies. The equivalent in the MKD5 orebody. retrograde serpentinization/carbonation event was initiated by infiltration of H2O-CO2-rich fluids which exploited early cross-cutting (D1 or D2) Ore petrology faults/shears as conduits (Ro¨dsjo¨ and Goodgame, Hypogene violarite 1999; Widdup, 2000). The infiltrating fluids Intercumulus sulphide blebs containing hypogene reacted with the ultramafic wall rock to produce violarite in the Pentlandite Domain range in size a talc-magnesite assemblage proximal to these from 40 mm to 1.5 mm (average 0.5 mm) and

314 A STABLE PENTLANDITE^VIOLARITE TIE LINE

MKD MKD MKD MKD 137 131 184 144

MKD SUPERGENE ZONE 259

ADCUM. PENT. DOMAIN ADCUM.

MILLERITE HW ORTHO- DOMAIN CUMULATE ADCUMULATE PERIDOTITE

MESOCUMULATE PERIDOTITE

FW

Hypogene violarite assemblages 100 m

FIG. 1. Simplified geological cross-section through the MKD5 orebody (Mine grid 31760N) showing locations of hypogene violarite-bearing assemblages and diamond drill holes.

consist of aggregates of Ni-Fe sulphide grains lamellae are oriented parallel to the host lamellae, surrounded by a corona of magnetite. The outer but owing to the fine nature of the intergrowth, the margins of the magnetite coronas are partially exsolved phase could not be identified. In all replaced by iowaite, chlorian pyroaurite or ferroan samples where hypogene violarite was observed magnesite in most cases. A coarse network of in the basal Pentlandite Domain, pyrrhotite is magnetite ‘crossbars’ is typically present within absent from the assemblage. Some assemblages the blebs. Such sulphide/gangue textures are include anhedral or subhedral crystals of cobalt- representative of the bulk of the nickel miner- and nickel-bearing pyrite to 60 mm, which occur alization in MKD5. In hypogene violarite-bearing embedded in host pentlandite grains. This pyrite assemblages, violarite occurs as lath-like or does not exhibit classic ‘bravoite’ concentric arcuate, flame-like lamellae within pentlandite zoning in reflected light (e.g. Vaughan, 1969; grains (Figs 2a,b), and accounts for 3 to 20% of Ramdohr, 1969); though microanalytical data the total sulphide (by volume). The violarite indicate that the crystals are weakly zoned, with lamellae are 3 to 60 mm wide, and are generally in up to 2.57 wt.% Co and 3.76 wt.% Ni substituting parallel or orthogonal arrangement, suggesting for Fe. crystallographic control over their orientation, as Hypogene violarite in the Millerite Domain would be expected if formed by homogeneous occurs in assemblages with millerite and or high- exsolution from a cubic host (e.g. pentlandite). Ni pentlandite (Table 1). As in the Pentlandite Further support for an exsolution rather than Domain, sulphide blebs are surrounded by replacement origin is the observation that violarite magnetite coronas but, on average, the blebs are lamellae are not typically associated with grain smaller (0.4 mm), and incipient to extensive boundaries, fractures, or hairline cracks in the replacement of magnetite by ferroan magnesite host pentlandite (Figs 2a,b). Under closer exam- is commonly observed. Obvious laths are less ination at high magnification (12506), even finer evident in the case of violarite-millerite assem- (<1 mm) lamellae of a pale, higher reflectance blages, but there still appears to be some phase could be discerned within the violarite geometric relationship between the coexisting lamellae in some samples (Fig. 2c). These phases (Fig. 2d). The dearth of lath or flame

315 B. A. GRGURIC b a mt

mt ppnn

pn viol viol mt

c d pn viol

mill

viol

pn

FIG. 2. Reflected light photomicrographs of MKD5 e violarite-bearing assemblages (oil immersion). (a) Reticulated, flame-like hypogene violarite lamellae (viol) in pentlandite (pn) with surrounding magnetite (mt). Drill hole MKD149, 340.7 m; Pentlandite Domain. Field of view is 250 mm. (b) Orthogonal hypogene viol violarite laths in pentlandite. Note magnetite ‘crossbar’ between pentlandite grains, and orientation of lamellae viol relative to {111} pentlandite cleavage cracks. Drill hole MKD137, 551 m; Millerite Domain. Field of view is 600 mm. (c) Detail of hypogene violarite-pentlandite intergrowth showing finer lamellae contained within violarite lath. Drill hole MKD149, 340.7 m. Field of pn view is 100 mm. (d) Intergrown hypogene violarite and millerite (mill). Darker fracture fill is magnetite and magnesite. Drill hole MKD95, 335 m; Millerite Domain. Field of view is 250 mm. (e) Supergene violarite replacing hypogene pentlandite. Note grain-boundary and microfracture control on distribution of violarite. Supergene violarite exhibits characteristic poor polish and abundant microfractures. Magnetite surrounds sulphide grains. Drill hole MKD10, 290 m; Pentlandite Domain. Field of view is 250 mm.

316 TABLE 1. Examples of compositions of coexisting violarite, pentlandite and millerite from the MKD5 orebody (WDS microprobe analyses).

Wt.% Hole/depth Domain Mineral Ni Co Fe Cu S Total{ Assemblage Formula (norm. to 4 S)

MKD27 498 m Pent h-viol 33.36 6.23 17.70 0.00 41.90 99.27 pn, h-viol (Ni1.74 Fe0.97 Co0.32 )S3.03 S4 pn 42.60 0.30 23.96 0.01 32.94 99.86 pn, h-viol TBEPNLNIEVOAIETELINE TIE PENTLANDITE^VIOLARITE STABLE A MKD33 220 m Pent h-viol 37.62 2.29 17.02 0.00 41.44 98.47 pn, h-viol, py (Ni1.98 Fe0.94 Co0.12 )S3.04 S4 pn 42.25 0.19 24.59 0.00 32.64 99.87 pn, h-viol, py MKD144 250.4 m Pent h-viol 27.27 10.09 19.76 0.02 41.10 98.36 pn, h-viol (Ni1.45 Fe1.10 Co0.53 )S3.08 S4 pn 40.05 0.42 26.34 0.00 32.71 99.67 pn, h-viol MKD149 340.7 m Pent h-viol 37.91 3.06 15.74 0.00 42.67 99.45 pn, h-viol (Ni1.94 Fe0.85 Co0.16 )S2.95 S4 pn 42.83 0.16 23.54 0.00 33.30 99.97 pn, h-viol

MKD95 335 m Mill h-viol 42.02 5.02 11.37 0.00 40.53 98.99 mill, h-viol (Ni2.27 Fe0.64 Co0.27 )S3.18 S4 317 mill 63.44 0.16 0.82 0.00 34.99 99.48 mill, h-viol MKD137 551 m Mill h-viol 39.26 2.50 16.94 0.03 41.46 100.2 pn, h-viol (Ni2.07 Fe0.94 Co0.13 )S3.14 S4 pn 42.25 0.12 24.44 0.00 33.04 99.87 pn, h-viol MKD205 385.9 m Mill h-viol 33.68 13.27 9.08 0.04 42.13 98.28 mill, pn, h-viol (Ni1.75 Fe0.49 Co0.69 )S2.93 S4 pn 41.02 0.84 24.68 0.08 32.95 99.68 mill, pn, h-viol mill 61.24 0.46 2.06 0.01 35.45 99.29 mill, pn, h-viol

MKD10 290 m Pent s-viol 37.62 1.70 18.30 0.00 38.90 96.59 pn, py, mc, s-viol (Ni2.11 Fe1.08 Co0.10 )S3.29 S4 pn 37.05 1.47 27.86 0.00 33.54 100.07 pn, py, mc, s-viol MKD33 205 m Pent s-viol 38.35 0.64 18.15 0.02 42.27 99.50 pn, py, mc, s-viol (Ni1.98 Fe0.99 Co0.03 )S3.00 S4 pn 37.67 0.73 28.57 0.00 32.66 99.73 pn, py, mc, s-viol MKD127 138 m Pent s-viol 44.46 0.82 11.06 0.00 39.62 96.13 pn, s-viol (Ni2.45 Fe0.64 Co0.05 )S3.14 S4 pn 37.26 0.70 28.64 0.00 32.71 99.41 pn, s-viol

Abbreviations: Pent – Pentlandite Domain; Mill – Millerite Domain; h-viol – hypogene violarite, s-viol – supergene violarite, pn – pentlandite, mill – millerite, py – pyrite, mc – marcasite. { Total includes minor and trace elements in addition to major elements tabulated here. B. A. GRGURIC

textures may be due to the generally higher Se, Sb, Ag, As, Cu and Co. A natural pentlandite proportion of violarite in Millerite Domain standard (Astimex #36 sourced from Manibridge, assemblages (up to 40% by volume) and does Canada) was used for Ni, Fe and S, and Johnson- not negate the possibility of an exsolution origin. Matthey Co metal (99.997%) was used as a Co Hypogene violarite from both the Millerite and standard. Pentlandite Domains takes a high polish and shows little tendency to tarnish in air. Similar characteristics were noted in hypogene violarite Results from Black Swan, Western Australia by Hudson Hypogene violarite assemblages and Groves (1974). Examples of typical microprobe analyses of sulphides from hypogene and supergene violarite-bearing assemblages are given in Supergene violarite Table 1. Hypogene violarite consistently gave Secondary supergene violarite, which replaces analytical totals between 98 and 100%, and in all pentlandite and millerite in the upper zones of the cases contained significant Co (2.08À orebody, occurs in association with secondary 13.27 wt.%). The Cu contents of violarites were millerite, pyrite, marcasite and relict pentlandite low (<0.08 wt.%), or below detection. Natural (Butt and Nickel, 1981). In some cases, thick rims violarites are typically metal-rich relative to of secondary magnetite are developed around compositions on the FeNi2S4-Ni3S4 join (Misra sulphide blebs. In contrast to its hypogene and Fleet, 1974; Vaughan and Craig, 1985), and equivalent, supergene violarite occurs in textures the microprobe data indicate that MKD5 hypo- that testify to a replacement origin, namely the gene violarites are either close to stoichiometric, presence of violarite as fringes along late fractures or metal-rich (Table 1 and Fig. 3), with an and as well-developed replacement fronts inward average composition yielding the formula from grain margins of primary pentlandite (Ni1.94Fe0.86Co0.27)S3.07S4. The atomic Ni, Co (Fig. 2e). Identical replacement textures are and Fe contents of hypogene violarites were documented from the supergene zones of both normalized to 100% and plotted in Ni3S4-Co3S4- massive and disseminated nickel deposits world- Fe3S4 space in Fig. 4, together with the composi- wide (e.g. Misra and Fleet, 1974; Nickel et al., tions of hypogene violarites from Black Swan, 1974; Watmuff, 1974). Supergene violarite takes W.A. (Hudson and Groves, 1974) and the a poorer polish than its hypogene equivalent and Madziwa mine, Zimbabwe (Riley, 1980). As can has a distinctive mottled or pitted appearance in be seen, MKD5 and Madziwa mine hypogene reflected light (Fig. 2e). Microfractures are thiospinels plot in the violarite field with a abundant in coarser violarite aggregates, and are variable linnaeite component. Black Swan ‘violar- developed as a result of the volume reduction ites’ are mainly polydymite; however in the case associated with the replacement process (Nickel, of both Black Swan and Madziwa mine samples, 1973; Misra and Fleet, 1974). Upon exposure to similar chemistry to MKD5 violarite is not air, polished aggregates of supergene violarite expected since these thiospinels do not occur in tarnish rapidly. pentlandite-bearing assemblages. Atomic Ni and Co contents of both hypogene and supergene violarites and associated pentlan- Mineral chemistry dite are plotted in Fig. 5. The trend of the Microprobe analysis hypogene violarite data indicates that the varia- A total of twenty drill core samples containing tion in violarite composition involves systematic either hypogene or supergene violarite were substitution of Co for Ni. As can be seen in Fig. 6, selected for analysis. Electron microprobe the variation in Co content of hypogene violarite analysis of sulphides was conducted at the also varies linearly with that of coexisting Centre for Electron Microscopy and pentlandite, clearly indicating a partitioning of Microstructure Analysis (CEMMSA) at the this low-level element between the two phases. University of Adelaide. A Cameca SX51 was Pentlandite associated with hypogene violarite is used in wavelength dispersive mode, with characteristically Ni-rich (40À42 wt.%; Table 1 accelerating voltage and beam current set at and Fig. 5), however Co is preferentially parti- 20 kV and 20 nA, respectively. All samples tioned into violarite (Fig. 6) resulting in high were carbon coated and analysed for Fe, Ni, S, atomic Ni:Co ratios (75:1 to 710:1; mean 177:1)

318 A STABLE PENTLANDITE^VIOLARITE TIE LINE

FIG. 3. Compositions of sulphide phases in hypogene violarite-bearing assemblages in MKD5 shown plotted in (Ni+Co)-Fe-S space. Tie lines join coexisting phases. Abbreviations: py – pyrite, mpo – monoclinic pyrrhotite, hpo – hexagonal pyrrhotite, pn – pentlandite, viol – violarite, vs – vaesite, poly – polydymite, mill – millerite, gs – godlevskite.

FIG. 4. Compositions of MKD5 violarites shown plotted in ternary Co3S4-Ni3S4-Fe3S4 space. Hypogene violarite/ polydymite from Black Swan, Western Australia (data from Hudson and Groves, 1974) and the Madziwa mine, Zimbabwe (data from Riley, 1980) are also plotted. Diagram modified from Vokes (1967).

319 B. A. GRGURIC

FIG. 5. Co and Ni contents of pentlandite and violarite in the MKD5 orebody. Solid tie-lines link coexisting hypogene phases. Dashed tie-lines link coexisting supergene phases. Data from 20 drill core samples.

in pentlandite. In contrast, the Ni:Co ratio of observation that (supergene) violarites are often pentlandite of similar high-Ni content in violarite- extremely porous. Normalized cation composi- free assemblages (e.g. pentlandite-millerite or tions of supergene violarites are plotted in Fig. 4, pentlandite-heazlewoodite) varies from 15:1 to and in most cases can be seen to be either 87:1 (mean 43:1). The implications of Co intermediate in the violarite-polydymite solid partitioning between hypogene violarite and solution, or Fe-rich violarite. In contrast to pentlandite will be discussed later in the paper. hypogene violarite, the Co content of all samples Millerite in violarite-bearing assemblages was is low (0.64À1.70 wt.%). All supergene violarites found to be essentially stoichiometric, with minor are metal-rich (Table 1) with Me:S varying from Fe (0.82À2.06 wt.%) and Co (0.15À0.65 wt.%) 3.01À3.46; however the accuracy of this ratio has substituting for Ni (Table 1 and Fig. 3), and is to be viewed with caution given the low totals of similar in composition to millerite in violarite-free most analyses. Variation in supergene violarite assemblages. As mentioned previously, anhedral cation composition appears to be related to Fe:Ni to subhedral pyrite present in some violarite- substitution with a relatively uniform Co content pentlandite assemblages showed minor Co and Ni (Fig. 5) and, in contrast to hypogene pentlandite/ substitution for Fe. violarite pairs, the Co distribution between super- gene violarite and coexisting pentlandite is ~1:1 (Fig. 6). As proposed by Misra and Fleet (1974), it Supergene violarite assemblages is likely that the Co content of violarite is simply Supergene violarites typically gave low analytical inherited from the precursor pentlandite in the totals (~96%; Table 1) and, upon application of low-temperature replacement process, hence the the electron beam in spot mode, exuded small essentially flat tie-lines in Fig. 5. Relict pentlan- amounts of volatiles (probably water introduced dite in the supergene zone is chemically indis- during polishing), which resulted in localized tinguishable from primary pentlandite and shows blistering of the carbon coating. This behaviour variation in Ni content depending on the primary and the mottled appearance in reflected light are assemblage. In the case of primary pentlandite- consistent with Vaughan and Craig’s (1985) pyrrhotite or pentlandite-only assemblages, which

320 A STABLE PENTLANDITE^VIOLARITE TIE LINE

FIG. 6. Distribution of Co between coexisting violarite and pentlandite in the MKD5 orebody. Supergene and hypogene mineral pairs are plotted. Data from 20 drill core samples.

dominate the Pentlandite Domain, the Ni content presence of a significant Co content, in most cases of pentlandite is typically 36À38 wt.% (Table 1 between 2 and 10 wt.%. As discussed, hypogene and Fig. 5). Primary pentlandite in the Millerite violarites in MKD5 are significantly cobaltian Domain is associated with low-Fe phases such as regardless of whether they occur in violarite- millerite, godlevskite and heazlewoodite, and is pentlandite+pyrite or violarite-millerite+pen- Ni-rich (40À42 wt.% Ni). tlandite assemblages. Pentlandite is also capable of accepting significant Co in its crystal structure (Riley, 1977) and thus the violarite-pentlandite Discussion mineral pair is a suitable system in which to The textural and mineral chemistry evidence examine Co partitioning behaviour. A regular given above highlights the marked differences distribution of an element between mineral pairs, between supergene and hypogene thiospinels in asshownbyCoinhypogeneviolariteand the MKD5 deposit. The mode of occurrence of pentlandite in Fig. 6, is generally considered to hypogene violarite is, in all cases, indicative of an indicate equilibrium distribution of that element equilibrium association with pentlandite and (McIntire, 1963; Hyndman, 1972) and, by millerite, with either smooth arcuate boundaries inference, equilibrium between the two host between the coexisting phases or distinct exsolu- phases. If it is assumed that the observed Co tion textures. In addition, geochemical evidence partitioning is governed by the Berthelot-Nernst for equilibrium exists in the form of minor distribution law (see McIntire, 1963), then the element partitioning behaviour, discussed below. slope of the straight-line fit of the data in Fig. 6 is the partition coefficient (KD)ofCointhe violarite-pentlandite mineral pair: Cobalt partitioning KCo = CCo /CCo = 15.52 In documented occurrences of hypogene violarite D viol pn Co (e.g. Hudson and Groves, 1974; Keele and Nickel, Where KD is the partition coefficient of Co in Co Co 1974; Riley, 1980), a common feature is the the violarite-pentlandite pair, and Cviol and Cpn

321 B. A. GRGURIC

are the concentrations of Co (in atomic percent) in tions are of little help in themselves since violarite and pentlandite, respectively. The high identical alteration styles host diverse Ni-Fe partition coefficient indicates strong partitioning sulphide assemblages in MKD5, including of Co in the assemblage into violarite and is pentlandite-pyrrhotite, pentlandite-millerite and consistent with the unusually low Ni:Co ratios heazlewoodite-pentlandite. It is likely that there measured in the associated pentlandite. The were slight but significant differences in the possible crystal chemical reason for strong Co conditions of alteration, and in particular fluid

partitioning behaviour in violarite will not be fS2 and fO2 conditions and the level of sulphide examined here but is worthy of further investiga- assemblage ‘self-buffering’, which are not tion. Given the lack of well-defined constraints on recorded in the present alteration mineralogy but the P/T conditions of violarite exsolution in may have locally favoured the formation of MKD5, the partition coefficient given above is violarite-bearing assemblages. Hypogene of limited practical application; however the violarite-bearing assemblages were not observed marked difference in the slopes of the data in in talc-magnesite rocks in MKD5, which suggests Fig. 6 may be of use in distinguishing hypogene that alteration conditions were not oxidizing and supergene violarites in nickel deposits, where enough to form the pyrite-violarite-vaesite assem- it is associated with pentlandite. The processing blages observed in Black Swan ores (Hudson and behaviour of supergene violarite is often poor in Groves, 1974; Barnes and Hill, 2000). comparison with that of hypogene sulphides. The The protore (magmatic) bulk composition of porous nature and rapid development of oxidized the sulphide blebs is another possible contributing coatings on supergene violarite can result in factor to the formation of violarite-pentlandite inhibited flotation response and consequent assemblages, given the apparent stratigraphic reduced metal recovery in a flotation circuit. In control on their distribution (Fig. 1). Within the contrast, there is no evidence for poor flotation Pentlandite Domain a general trend of increasing response during the processing of hypogene Ni tenor of sulphide assemblages is seen from the violarite-bearing ores in the Mount Keith circuit. stratigraphic top to the base of this dunite unit The ability to distinguish readily-floating hypo- (left to right in Fig. 1). The bulk of the Pentlandite gene violarite from supergene violarite using Domain is dominated by pentlandite-pyrrhotite mineral chemistry data may therefore be advanta- assemblages with pentlandite only, pentlandite- geous when forecasting the metallurgical perfor- pyrite and violarite-pentlandite+pyrite assem- mance of nickel ores. blages becoming more common adjacent to the boundary with the Millerite Domain. Hypogene violarite-pentlandite assemblages also occur spor- Conditions of formation adically within the core of the Pentlandite Given the dearth of documented occurrences of Domain. Examination of the whole-rock chem- hypogene violarite-pentlandite assemblages, it is istry within the Pentlandite Domain reveals no highly likely that they are of rare occurrence and means of differentiating violarite-bearing zones in thus the possible factors that led to their formation drill core from other assemblages, with the in MKD5 are worthy of consideration. A exception of pentlandite-pyrrhotite assemblages characteristic feature of Group 2B deposits such containing a high modal proportion of pyrrhotite as MKD5 is the hypogene alteration of the (i.e. S-rich). In most cases, the molar (Ni+Co):S ultramafic host rock which has a strong influence ratio is centred on 0.6, with the key aspect, the on the nature and mineral chemistry of the pre- Ni:Fe ratio of the sulphide assemblages, being existing magmatic sulphide minerals, present in obscured by the fact that in addition to sulphides, volumetrically minor quantities. In all samples in several other common minerals such as magnetite, which hypogene violarite was observed, the host magnesite and hydrotalcite group minerals dunites are altered to either a lizardite-bruci- contribute to whole-rock Fe. Clearly, specific te+iowaite/pyroaurite or an antigorite-magnesite measurement of sulphide bulk compositions assemblage, indicative of alteration by H2O- and within the Domain would be required to identify CO2-bearing fluids (Eckstrand, 1975). The areas where violarite-pentlandite assemblages are common occurrence of mesh-textured lizardite likely to occur. in the former assemblage suggests formation by The overall conclusion to be gained from the single-stage hydration of olivine below 3208C observations above is that no unusual or (Ro¨dsjo¨ and Goodgame, 1999). These observa- anomalous geochemical or alteration conditions

322 A STABLE PENTLANDITE^VIOLARITE TIE LINE

are apparent (e.g. introduction of exotic compo- which violarite forms at the expense of hypogene nents other than H2O and CO2) which may have pentlandite, and as such cannot represent an contributed to the formation of hypogene equilibrium situation. The fact that pyrite and violarite-bearing assemblages in the MKD5 millerite coexist in hypogene ores is not proof of orebody. As stated earlier, the present sulphide equilibrium, however, and it is significant that assemblages reflect the combination of primary millerite-pyrite assemblages are not represented variations in magmatic sulphide compositions and in previous experimental studies. On the other subsequent modifications to sulphide/oxide subso- hand, the explanation of the formation of lidus phase relations during serpentinization and hypogene violarite-pentlandite assemblages is carbonate alteration. Locally within MKD5, the relatively straightforward in the light of previous precise combination of these conditions must experimental studies of phase relations such as have existed to allow hypogene violarite-pentlan- those of Kullerud et al. (1969), Shewman and dite +pyrite and violarite-millerite+pentlandite Clark (1969), Craig (1971, 1973) and Misra and assemblages to form and be preserved. Fleet (1973). Hypogene violarite-millerite assemblages The phase relations in the Ni-Fe-S system are appear to be more widespread in nature and are relatively simple at high temperatures. At 9928C, documented at Kambalda, Western Australia by the central portion of the Ni-Fe-S ternary system Keele and Nickel (1974) and at Black Swan by is dominated by the monosulphide solid solution Hudson and Groves (1974) and Barnes and Hill (mss), which forms a broad band extending from (2000). Recently, composite hypogene violarite- the Fe-S join to the Ni-S join (Kullerud et al., millerite grains were noted by the present author 1969). Exsolution of pentlandite from the mss in sulphide separates from the Serp Hill nickel begins at ~6108C (Kullerud, 1963) and, at 3008C, prospect, Yakabindie, located ~25 kms south of Misra and Fleet (1973) reported that the mss Mount Keith. withdraws from the Ni-S join and separates into There is a common misconception that violarite two mss phases, mss1 and mss2, with ~25 and forms only as a supergene phase, and therefore it 33 at.% Ni respectively. These phases coexist is conceivable that hypogene violarite-pentlandite with pentlandite (containing ~33.5 at.% Ni) and assemblages may well be more widespread in vapour as a univariant assemblage. They noted nickel deposits, but have been misidentified as that at 2308C, the limiting composition of the Fe- supergene. rich mss1 is ~17 at.% Ni, the solvus having withdrawn sufficiently to allow tie-lines between pyrite and Ni-rich pentlandite to be established. Implications for phase relations Although the phase relations between pyrite and The acceptance, on the basis of the textural and mss were not investigated experimentally in their mineral chemistry evidence given above, that study, the common natural association between violarite occurs in low-temperature equilibrium pyrite and pentlandite led Misra and Fleet to with pentlandite, millerite and pyrite has signifi- interpret the existence of a divariant field of cant implications for the established phase pentlandite-pyrite interposed between two univar- relations in the Ni-Fe-S system. As mentioned iant assemblages; mss1-pyrite-pentlandite+vapour in the introduction, controversy has existed as to and pyrite-mss2-pentlandite+vapour. Pentlandite- whether violarite-pentlandite or millerite-pyrite pyrite assemblages are observed in the Pentlandite constitutes the stable assemblage at low tempera- Domain in MKD5 and consist of stoichiometric ture, with most petrologists favouring the latter pyrite crystals, identical to those observed in assemblage. On the basis of experimental studies hypogene violarite assemblages, embedded in (e.g. Craig, 1971, 1973; Misra and Fleet, 1973), a pentlandite containing 38+1wt.% Ni violarite–pentlandite tie-line is tenable, but the (%30 at.% Ni). The compositions of the coex- previous lack of authenticated hypogene violarite- isting phases are consistent with Misra and Fleet’s pentlandite assemblages, combined with the proposed phase relations. At 2308C the composi- frequently observed coexistence of millerite and tion of mss2 lies approximately mid-way between pyrite in ores has led to general acceptance of a that of end-member violarite and high-Ni low-temperature millerite–pyrite tie-line pentlandite. It is proposed here that the violarite- (Vaughan and Craig, 1997). Supergene violarite- pentlandite intergrowths observed in MKD5 result pentlandite intergrowths, as illustrated in Fig. 2e, from the breakdown of Misra and Fleet’s mss2 represent an arrested replacement process in phase at temperatures below 2308C. Depending

323 B. A. GRGURIC

S

Fe Ni

py vs

gr sm viol poly mpo hpo tr mill

gs pn heaz

to Ni-Fe alloys

FIG. 7. Interpreted low-temperature phase relations in the Ni-Fe-S system, based on Kullerud et al. (1969), Craig (1973), Misra and Fleet (1973, 1974), Hudson and Groves (1974) and assemblages in MKD5. Solid tie lines pertain to assemblages confirmed in MKD5. Abbreviations additional to Fig. 3 are: tr – troilite, sm – smythite, gr – greigite, heaz – heazlewoodite.

on the bulk composition of the sulphide blebs, possibilities include polydymite developed as a either pyrite or millerite may be present in the result of the low-temperature decomposition of assemblage. As proposed by these workers, mss2 the violarite–polydymite solid solution, or coexists with the millerite-solid solution at 2308C, perhaps a more Co-rich thiospinel. which is consistent with the equilibrium millerite- At this stage, it is necessary to consider the violarite and millerite-violarite-pentlandite occurrence of hypogene millerite-pyrite bearing assemblages noted here. The presence of the assemblages, since the assumption that a violarite- aforementioned divariant pentlandite-pyrite field pentlandite tie-line represents thermodynamic separating mss1 and mss2 explains why pyrrhotite equilibrium invalidates a stable millerite-pyrite is not observed in violarite-bearing assemblages. tie-line. Millerite-pyrite bearing assemblages are In the light of these interpretations, a low- typically associated with talc-carbonate alteration temperature ternary diagram based on assem- in ultramafic rocks, examples of which include blages observed in this study, and data from Black Swan (Groves et al., 1974; Hudson and Kullerud et al. (1969), Misra and Fleet (1973, Groves, 1974; Barnes and Hill, 2000), and 1974), Craig (1973), and Hudson and Groves Kambalda (Keele and Nickel, 1974). A common (1974) is given in Fig. 7. The pentlandite-violarite feature of Ni-Fe sulphide assemblages associated tie-line implies specific compositional ranges for with talc-carbonate alteration in the Yilgarn of both of these phases, with the final topology in Western Australia is evidence of hydrothermal this case (Fig. 7) defined using the measured sulphidation of the assemblage, since the compositional data in Fig. 3. Given the lack of attendant H2O-CO2-rich hydrothermal fluids proper characterization of the fine secondary appear to have carried reduced sulphur (Ro¨dsjo¨, exsolution lamellae observed in violarite lamellae 1999). Arsenic is also commonly present in the (Fig. 2c), their nature and relationship to the fluid, resulting in the appearance of phases such as phase relations in Fig. 7 is conjectural, but gersdorffite, nickeline and maucherite.

324 A STABLE PENTLANDITE^VIOLARITE TIE LINE

Sulphidation is commonly manifested in the 1974), and calls into question the existence of the appearance of hydrothermal pyrite in the sulphide stable low-temperature millerite-pyrite tie-line assemblage, and in talc-carbonate zones in generally accepted by ore petrologists. MKD5, this (often weakly arsenian) phase (4) On the basis of previous phase relations typically occurs as subhedral crystals containing studies, it is hypothesized that hypogene violarite- micro-inclusions of galena, bornite, cadmian pentlandite assemblages form as a result of the sphalerite and chalcopyrite. If pyrite is of late decomposition of Ni-rich mss below 2308C. hydrothermal origin, the implication is that it may Depending on the precise bulk composition of not be in equilibrium with pre-existing sulphide the sulphide, pyrite or millerite may be present in phases such as millerite. Later metamorphic re- the assemblage. In high sulphidation assemblages equilibration of the S-rich assemblage can result such as occur at Black Swan, hypogene violarite in the formation of pyrite-violarite/polydymite- (or polydymite) coexists with pyrite and vaesite. vaesite assemblages such as occur at Black Swan Further work should examine experimental (Hudson and Groves, 1974; Barnes and Hill, exsolution of violarite from pentlandite in Co- 2000). Millerite occurs in association with these bearing bulk compositions and compare the Co high-sulphidation assemblages at Black Swan partitioning behaviour with the natural analogues (Hudson and Groves, 1974), but all indications reported here. The finer-scale secondary lamellae point to it being a later retrograde alteration observed in violarite lamellae should be char- product of vaesite (Barnes and Hill, 2000). A acterized, and this may require TEM work. Their detailed investigation of all known millerite- existence suggests a two-stage exsolution process pyrite-bearing assemblages is beyond the scope possibly involving the breakdown of the violarite- of this paper; however, the low-temperature phase polydymite solid solution. relations presented here highlight the necessity to interpret pyrite-bearing Ni-Fe sulphide assem- Acknowledgements blages with care, especially since pyrite has a tendency towards idiomorphism and apparent The author wishes to thank Craig Noble of equilibrium with associated phases, regardless of CEMMSA for assistance with microprobe its paragenetic position (Ramdohr, 1969). analysis and Ernie Nickel and Allan Pring for comments on a draft of the paper. David Vaughan and Rob Ixer are thanked for helpful reviews. Conclusions and suggestions for further work (1) Hypogene cobaltian violarite occurs in References apparent exsolution-derived intergrowths with pentlandite+pyrite and millerite+pentlandite in Barnes, S.J. and Hill, R.E.T. (2000) Metamorphism of the MKD5 orebody. In the case of pentlandite- komatiite hosted nickel sulfide deposits. Pp. bearing assemblages, Co is strongly partitioned 203À216 in: Metamorphosed and Metamorpho- into violarite. High Co in hypogene violarite genic Ore Deposits (P.G. Spry, B. Marshall and relative to associated pentlandite serves as a F.M. Vokes, editors). Reviews in Economic means of distinguishing this assemblage from Geology, 11. Society of Economic Geologists, supergene equivalents. Boulder, CO, USA. (2) Hypogene violarite-bearing assemblages Butt, C.R.M. and Nickel, E.H. (1981) Mineralogy and geochemistry of the weathering of the disseminated occur in lizardite-brucite+hydrotalcite group or nickel sulfide deposit at Mt. Keith, Western antigorite-magnesite-altered dunites, and no Australia. Economic Geology, 76, 1736À1751. unusual or anomalous geochemical or alteration Craig, J.R. (1971) Violarite stability relations. American conditions are apparent which may have contrib- Mineralogist, 56, 1303À1311. uted to the formation of these assemblages in the Craig, J.R. (1973) Pyrite-pentlandite assemblages and MKD5 orebody. Hypogene violarite-millerite other low temperature relations in the Fe-Ni-S assemblages have been documented from other system. American Journal of Science, 273A, deposits. 496À510. (3) The occurrence of hypogene violarite- Dowling, S.E. and Hill, R.E.T. (1993) The Mount Keith pentlandite assemblages in apparent equilibrium ultramafic complex and the Mount Keith nickel validates earlier low-temperature ternary phase deposit. Pp. 165À170 in: Crustal Evolution, diagrams which include a tie-line between Metallogeny and Exploration of the Eastern violarite and pentlandite (e.g. Misra and Fleet, Goldfields (P.R. Williams and J.A. Haldane editors).

325 B. A. GRGURIC

Australian Geological Survey Organization, Record, 391À403. 1993/54. Nickel, E.H. (1973) Violarite À a key mineral in the Eckstrand, O.R. (1975) The Dumont serpentinite: a supergene alteration of nickel sulphide ores. model for control of opaque nickeliferous mineral Australasian Institute of Mining and Metallurgy, assemblages by alteration reactions in ultramafic Perth Conference, May 1973, pp. 111À116. rocks. Economic Geology, 70, 183À201. Nickel, E.H., Ross, J.R. and Thornber, M.R. (1974) The Grguric, B.A., Madsen, I.C. and Pring, A. (2001) supergene alteration of pyrrhotite-pentlandite ore at Woodallite, a new chromium analogue of iowaite Kambalda, Western Australia. Economic Geology, from the Mount Keith nickel deposit, Western 69,93À107. Australia. Mineralogical Magazine, 65, 427À435. Ramdohr, P. (1969) The Ore Minerals and their Groves, D.I., Hudson, D.R. and Hack, T.B.C. (1974) Intergrowths. Pergamon, Oxford, UK, 1174 pp. Modification of iron-nickel sulfides during serpenti- Riley, J.F. (1977) The pentlandite group (Fe,Ni,Co)9S8: nization and talc-carbonate alteration at Black Swan, new data and an appraisal of structure-composition Western Australia. Economic Geology, 69, relationships. Mineralogical Magazine, 41, 1265À1281. 345À349. Hopf, S. and Head, D.L. (1998) Mount Keith nickel Riley, J.F. (1980) Ferroan carrollites, cobaltian violar- deposit. Pp. 307À314 in: Geology of Australian and ites, and other members of the linnaeite group: PapuaNewGuineanMineralDeposits(D.A. (Co,Ni,Fe,Cu)3S4. Mineralogical Magazine, 43, Berkman and D.H. Mackenzie, editors). The 733À739. Australasian Institute of Mining and Metallurgy, Ro¨dsjo¨, L. (1999) The alteration history of the Agnew- Melbourne. Wiluna Greenstone Belt, Western Australia, and the Hudson, D.R. and Groves, D.I. (1974) The composition impacts on nickel sulphide mineralisation. PhD of violarite coexisting with vaesite, pyrite and thesis, University of Western Australia. millerite. Economic Geology, 69, 1335À1340. Ro¨dsjo¨, L. and Goodgame, V.R. (1999) Alteration of the Hyndman, D.W. (1972) Petrology of Igneous and Mt. Keith nickel sulphide deposit. Pp. 779À782 in: Metamorphic Rocks. McGraw-Hill, New York, 533 Mineral Deposits: Processes to Processing (C.J. pp. Stanley editor). Balkema, Amsterdam. Keele, R.A. and Nickel, E.H. (1974) The geology of a Shewman, R.W. and Clark, L.A. (1969) Pentlandite primary millerite-bearing sulfide assemblage and phase relations in the Fe-Ni-S system and notes on supergene alteration at the Otter Shoot, Kambalda, the monosulfide solid solution. Canadian Journal of Western Australia. Economic Geology, 69, Earth Sciences, 7,67À85. 1102À1117. Vaughan, D.J. (1969) Zonal variation in bravoite. Kullerud, G. (1963) Thermal stability of pentlandite. American Mineralogist, 54, 1075À1083. The Canadian Mineralogist, 7, 353À366. Vaughan, D.J. and Craig, J.R. (1985) The crystal Kullerud, G., Yund, R.A. and Moh, G.H. (1969) Phase chemistry of iron-nickel thiospinels. American relations in the Cu-Fe-S, Cu-Ni-S and Fe-Ni-S Mineralogist, 70, 1036À1043. system. Pp. 323À343 in: Magmatic Ore Deposits Vaughan, D.J. and Craig, J.R. (1997) Sulfide ore mineral (H.D.B. Wilson editor). Economic Geology stabilities, morphologies and intergrowth textures. Monograph, 4. Economic Geology Publishing Pp. 367À434 in: Geochemistry of Hydrothermal Ore Company, CO, USA. Deposits (H.L. Barnes editor). Wiley, New York. Lesher, C.M. (1989) Komatiite-associated nickel sul- Vokes, F.M. (1967) Linnaeite from the Precambrian phide deposits. Pp. 44À101 in: Ore Deposition Raipas Group of Finnmark, Norway: an investigation Associated with Magmas (J.A. Whitney and A.J. with the electron microprobe. Mineralium Deposita, Naldrett, editors). Reviews in Economic Geology, 4. 2,11À25. Economic Geology Publishing Company, CO, USA. Watmuff, I.G. (1974) Supergene alteration of the Mt. MacIntire, W.L. (1963) Trace element partition coeffi- Windarra nickel sulphide ore deposit, Western cients À a review of theory and applications to Australia. Mineralium Deposita, 9, 199À221. geology. Geochimica et Cosmochimica Acta, 27, Widdup, H. (2000) Structural geology of the Mt. Keith 1209À1264. Ultramafic Complex. BSc (Hons) thesis, University Misra, K.C. and Fleet, M.E. (1973) The chemical of Melbourne, Australia. compositions of synthetic and natural pentlandite assemblages. Economic Geology, 68, 518À539. Misra, K.C. and Fleet, M.E. (1974) Chemical composi- [Manuscript received 17 July 2001: tion and stability of violarite. Economic Geology, 69, revised 18 February 2002]

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