Nickel tenor variations between Archaean komatiite-associated nickel sulphide deposits, Kambalda ore field, Western Australia: the metamorphic modification model revisited

Summary Geochemical data for 870 ore samples of 14 nickel sulphide (NiS) deposits from throughout the major Archaean Kambalda ore field, Western Australia, reveal highly heterogeneous Ni tenor (wt% Ni in 100wt% sulphide) variation that is difficult to explain solely by magmatic processes. The Ni tenor values for the deposits range from 6.2wt% Ni (Helmut deposit) to 19.7wt% Ni (Carnilya Hill deposit), close to the range for within single deposits (9.7–19.3wt% Ni). Contents of Ni and platinum-group elements (PGE) broadly increase with decreasing Fe and with increasing abundance of metamorphic pyrite+magnetite±silicates. In turn, the abundance of the metamorphic phases appears to be complexly related to structural setting, metamorphic grade, alteration type, and proximity to felsic intrusion. Chondrite-normalised multi-element plots of deposit compositions reveal relative depletions in Au, As, Bi, and Te.The relationship of increasing Ni content with secondary phase abundance indicates a strong role for metamorphic modification in the tenor variation. Replacement of pyrrhotite by pyrite+magnetite±chlorite during oxidation reduced the abundance of Fe sulphide relative to Ni sulphide and increased the Ni tenor of the residual sulphide. The extent of the oxidation reflects the extent of alteration fluid ingress along deformation structures and fabrics during talc-carbonate alteration, regional metamorphism, and felsic intrusion related to D3. The relative depletions of Au, As, Bi and Te combined with relative enrichments of these metals in nearby orogenic gold deposits mean that NiS deposits could represent metal reservoirs for Archaean gold hydrothermal systems.