Hydrothermal Pentlandite (Ni,Fe)9S8 from Kambalda, Western Australia: Occurrences, Formation Conditions, and Association with Orogenic Gold

Abstract

Abstract Pentlandite, (Ni,Fe)9S8, most commonly occurs in mafic or ultramafic rocks in association with other sulfide minerals, including pyrrhotite and chalcopyrite. However, at a few localities pentlandite has been found in hydrothermal settings. At Kambalda in Western Australia, hydrothermal pentlandite occurs in three different vein types: (1) sulfide veins, (2) quartz-calcite veins associated with biotite-epidote alteration, and (3) calcite veins containing arsenides and base metal sulfide minerals. In all three vein types, pentlandite (1) is restricted to veins crosscutting magmatic Ni sulfide lenses, (2) only occurs within <10 m of sulfide bodies, and (3) is associated only with pyrrhotite. In some cases, the pentlandite in the hydrothermal veins has higher Fe and lower Co contents compared to that in the magmatic Ni sulfide ores. The sulfarsenide minerals also show a bimodal distribution in terms of their chemistry: those associated with hydrothermal veins are dominated by Ni, whereas those associated with a primary magmatic origin are rich in Co. Intermediate compositions are observed, notably where hydrothermal sulfarsenide minerals directly overgrow earlier magmatic sulfide minerals. Thermodynamic calculations show that the hydrothermal pentlandite-pyrrhotite assemblages can form from highly reduced, hydrothermal fluids at near neutral pH. Pentlandite deposition can be triggered by an increase in pH and/or a decrease in temperature. Associated gangue minerals, such as biotite, feldspar, and quartz, are likely deposited as a result of concomitant fluid–rock interaction. High fluid temperatures (i.e., 400–500 °C) favor pentlandite formation due to an increase in Ni solubility. The pentlandite-pyrrhotite-biotite assemblage has a narrow stability field coinciding with a field of elevated Au solubility, which prevents coprecipitation of native gold with this mineral assemblage. In contrast, the pentlandite-arsenide mineral assemblage shows a larger stability field which overlaps with conditions allowing Au precipitation. The modeling results suggest that identification of the mineral assemblages in which pentlandite occurs is important in mineral exploration, one implication being that pentlandite-rich, hydrothermal veins will be expected to be Au-free. Conversely, pentlandite-poor or -absent veins, possibly containing Ni-arsenide minerals, may contain gold and could thus be indicative of enhanced prospectivity for gold.