Abstract
The Moran komatiite-hosted Ni sulfide deposit at Kambalda (Australia) is one of the better preserved orebodies at Kambalda. Its geochemical signature is used to investigate the evolution of the sulfide mineralization. The orebody has several parts, including a flanking segment where massive sulfides formed relatively early and a central portion in a 40-m-deep erosional embayment representing a later generation of massive and net-textured sulfides. Basal massive sulfides within the deep embayment vary systematically in their chalcophile element contents (Ni, PGE, Au, Te, As, Bi). Elements compatible in monosulfide solid solution (MSS) exhibit the highest concentration at the edge of the orebody (up to 4.3 ppm Ir + Os + Ru + Rh), whereas incompatible elements are most concentrated in the centre (up to 11.2 ppm Pt + Pd + Au). This difference in element distributions is explained by fractional crystallization of sulfide melt from the edge towards the centre. To explain the vertical movement of the residual fractionated melt, a new model of sulfide crystallization is proposed. A low-viscosity boundary layer containing incompatible elements is formed between MSS and sulfide melt. This melt propagates with the crystallization front towards the centre of the sulfide melt pool. Trace element variations in pentlandite (e.g. Co) and composite Co- and Bi-bearing arsenide-telluride grains suggest that during the final stages of crystallization, an immiscible Co-As-Te-Bi melt is formed.
Highlights
Archaean komatiite-hosted Ni-Cu-Co sulfide deposits are an important source for sulfide Ni (Gole and Barnes 2020)
We investigate geochemical variations over the Moran orebody and consider how this variation resulted from the crystallization of Cu-poor sulfide melt pools
The amount of sulfides in basal sulfide-matrix breccia ore was added to massive sulfides as the breccia sulfides represent part of the massive sulfide pool (Barnes et al 2018)
Summary
Archaean komatiite-hosted Ni-Cu-Co sulfide deposits are an important source for sulfide Ni (Gole and Barnes 2020). The East Yilgarn komatiite Ni province is the third largest Ni producer in the world after Sudbury and Noril’sk (Hronsky and Schodde 2006; Naldrett 2010). The Yilgarn deposits are always metamorphically and tectonically overprinted to various degrees, and igneous textures in sulfides are commonly modified or destroyed (Barnes et al 1988a, b; Barnes 2006a, b). This is contrary to many mafic intrusion-hosted sulfide deposits where igneous textures in sulfides tend to be better.
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