Hypogene Violarite of Exsolution Origin from Mount Keith, Western Australia: Field Evidence for a Stable Pentlandite ^Violarite Tie Line
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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