Re–Os isotopic constraints on the genesis of the Sally Malay Ni–Cu–Co deposit, East Kimberley, Western Australia

Abstract

Sulphide mineralisation and a cyclic suite of unmineralised troctolite and peridotite from the Sally Malay intrusion, East Kimberley, Western Australia, a possible Voisey's Bay analogue, were analysed for their trace element and platinum group element (PGE) abundances, Sm–Nd and Re–Os isotopic compositions. Massive sulphides have low common Os concentrations and high Re/Os ratios, similar to massive sulphide ores from the Duluth Complex and Voisey's Bay, and are consistent with low R factors (∼200) ( R factor=effective mass of silicate melt with which a given mass of sulphide melt has equilibrated). All Re–Os isotopic data from massive sulphides and unmineralised sections from the Sally Malay intrusion do not form a single isochron, possibly the result of variations in the R factor within the ore system. However, samples with only disseminated or matrix sulphides yield a Model 3 isochron of 1893±57 Ma, within error of the magmatic 1844±3 Ma U-Pb zircon age, suggesting that these samples have remained isotopically closed. Massive sulphide ores yield exceptionally radiogenic initial Os isotopic compositions ( γ Os=+950 to +1300), whereas disseminated sulphide mineralisation and unmineralised troctolite and peridotite yield lower γ Os values (+60 to +470). The Os isotopic data suggest that the Sally Malay intrusion and associated Ni–Cu–Co mineralisation were derived from crustally contaminated mantle melts. The Re–Os isotopic data support a two-stage model for the development of the Sally Malay intrusion ores. The first stage required crustal contamination of a basaltic melt at depth which promoted sulphide-saturation and the formation of an immiscible sulphide melt phase. The second stage involved transport of the sulphides in the host basaltic magma, resulting in a low R factor and a low metal tenor. Passive deposition of the magma produced a basal sulphide layer with a very radiogenic crustal Re–Os isotopic composition. The variable initial Os isotopic composition is likely to reflect a variable R factor, which renders isochron systematics difficult to interpret.