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

Sulphide mineralisation and a cyclic suite of unmineralised troctolite and peridotite from the Sally Malay intrusion, East Kimberley, Western Australi a, a possible Voisey's Bay analogue, were analysed for their trace element and platinum group element (PGE) abundances, Sm-Nd and Re-Os isotopic compo sitions. Massive sulphides have low common Os concentrations and high Re/Os ratios, similar to massive sulphide ores from the Duluth Complex and Voise y's Bay, and are consistent with low R factors (similar to 200) (R factor = effective mass of silicate melt with which a given mass of sulphide melt h as equilibrated). All Re-Os isotopic data from massive sulphides and unmine ralised sections from the Sally Malay intrusion do not form a single isochr on, 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 cl osed. Massive sulphide ores yield exceptionally radiogenic initial Os isoto pic compositions (gamma(Os) = + 950 to + 1300), whereas disseminated sulphi de mineralisation and unmineralised troctolite and peridotite yield lower g amma(Os) values (+ 60 to + 470). The Os isotopic data suggest that the Sall y 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 firs t stage required crustal contamination of a basaltic melt at depth which pr omoted sulphide-saturation and the formation of an immiscible sulphide melt phase. The second stage involved transport of the sulphides in the host ba saltic magma, resulting in a low R factor and a low metal tenor. Passive de position of the magma produced a basal sulphide layer with a very radiogeni c crustal Re-Os isotopic composition. The variable initial Os isotopic comp osition is likely to reflect a variable R factor, which renders isochron sy stematics difficult to interpret. (C) 1999 Published by Elsevier Science B. V. All rights reserved.