Contrasting komatiite belts, associated Ni-Cu-(PGE) deposit styles and assimilation histories

摘要

The Agnew-Wiluna greenstone belt in the Yilgarn Craton of Western Australia is the most nickel-sulfide-endowed komatiite belt in the world. The Agnew-Wiluna greenstone belt contains two mineralised units/horizons that display very different volcanological and geochemical features. The Mt Keith unit comprises >500 m-thick spinifex-free adcumulate-textured lenses, which are flanked by laterally extensive orthocumulate-textured units. Spinifex texture is absent from this unit. Disseminated nickel sulfides, interstitial to former olivine crystals, are concentrated in the lensoidal areas. Massive sulfides are locally present along the base or margins of the lenses or channels. The Cliffs unit is locally >150 m thick and comprises a sequence of differentiated spinifex-textured flow units. The basal unit is the thickest, and contains basal massive nickel-sulfide mineralisation. The Mt Keith and Cliffs units display important common features: (i) 疝gO contents of 25-30% in inferred parental magmas; and (ii) Al/Ti ratios of similar to 20 (Munro-type). However, the Mt Keith unit is highly crustally contaminated (e. g. LREE-enriched, high HFSEs), whereas the Cliffs unit does not display evidence of significant crustal assimilation. We argue that the distinct trace-element concentrations and profiles of the two komatiite units reflect their different emplacement style and country rocks: the Mt Keith unit is interpreted to have been emplaced as an intrusive sill into dacitic volcanic units whereas the Cliffs unit was extruded as lava flow onto tholeiitic basalts in a subaqueous environment. The mode of emplacement and nature of country rock is the single biggest factor in controlling mineralisation styles in komatiites. On the other hand, evidence of crustal contamination does not necessarily provide information of the prospectivity of komatiites to host Ni-Cu-(PGE) mineralisation, despite being a good proxy for the style of komatiite emplacement and the nature of country rocks.