The association between Ni-Cu-PGE sulfide and Ni-Co lateritic ores and volcanic facies within the komatiites of the 2.7 Ga East Yilgarn Craton Large Igneous Province, Western Australia

Abstract

A critical factor in the generation of komatiite-hosted Ni-Cu-PGE sulfide deposits, the addition of external sulfur via assimilation of S-bearing substrate beneath channelised lava flows, is a function of the komatiite flow field architecture. This architecture is also critical in the development of magnesium-rich bedrock lithologies that are prerequisites for the formation of komatiite-hosted Ni-Co laterite deposits. To better understand the controls on the spatial distribution of both these deposit types a GIS-referenced map of komatiite flow regimes within the komatiitic component of the c. 2.7 Ga East Yilgarn Large Igneous Province and from parts of adjacent older greenstone belts in the Yilgarn Craton has been constructed using data from the field and the literature. The regimes essentially divide komatiite sequences into those composed of thin (0.5–100 m) laterally restricted (50 m–3 km) cooling units and those composed dominantly of thick (100 m–1 km) and laterally extensive (3–100 km) cooling units. The regimes are subdivided into six facies and five sub-facies based on both stratigraphic and lateral variations in lithologies. The facies define regional-scale variations reflecting markedly different spatial and temporal crystallisation conditions within komatiite flow fields. There is a marked spatial association between known magmatic nickel sulfides and particular komatiite facies. Facies favourable for hosting magmatic sulfides are those that contain early-formed, very magnesian cumulate sequences requiring high effusion rates of very hot, turbulently flowing lava to form and thus have the potential to be contaminated by S-bearing substrate rocks. Sulfide-mineralised komatiites make up about 9% of the area of the current outcrop of the 2705 Ma komatiite LIP. Facies that have a low potential for nickel sulfide mineralisation are sheet-like bodies in which basal rocks were emplaced under relatively quiescent conditions and where hot, turbulently flowing lava and crystallisation of olivine-rich cumulates developed only after the build-up of a pile of high-porosity olivine cumulates allowing no interaction with the substrate. Nickel-cobalt laterite deposits are associated with thick, originally laterally extensive high magnesian sequences. Some such sequences are also favourable for magmatic sulfide mineralisation but others occur in sheet-like sequences.