Geochemistry of mineralised and barren komatiites from the Perseverance nickel deposit, Western Australia

Abstract

The Perseverance (formerly known as Agnew) and the neighbouring Rocky's Reward nickel deposits are associated with metamorphosed komatiite flows erupted onto a substrate of felsic crystal tuffs. The deposits are overlain by the Perseverance Ultramafic Complex, which consists of a central dunite lens, 700 m thick by 3 km wide, flanked by olivine orthocumulates and spinifex textured komatiites. A suite of samples of A-zones of komatiite flows from the area has been analysed for major and trace elements, rare earth elements and platinum-group elements. The samples fall into two distinct geochemical groups on the basis of major and rare earth elements. (1) Samples from A-zones of spinifex-textured flows flanking the central dunite show well constrained, linear trends of major and trace elements typical of komatiite suites related by simple olivine fractionation. (2) Samples from the Perseverance Mine (mineralised and basal flows), from Rocky's Reward and from unmineralised flows at the base of the Perseverance Ultramafic Complex all show evidence for LREE enrichment relative to the first group. They are also generally higher in SiO2 and lower in FeO for a given MgO content. Geochemical features of the second group are consistent with the effects of crustal assimilation. This is in line with current models for nickel sulphide genesis, based largely on Kambalda, which emphasise the role of thermal erosion of sulphidic footwall sediments. The chalcophile element (Ni, Ir, Ru, Pt, Pd, Au and Cu) concentrations in A-zone samples from the study area are typical of komatiites. Komatiites from the Perseverance mineralised units and from Rocky's Reward show evidence of depletion of the more strongly chalcophile PGE relative to Ni and Cu. This feature is also evident in massive and matrix sulphide ore samples, although less evident in the main Perseverance cloud sulphides. Quantitative geochemical computer modelling indicates that the major and rare earth element chemistry of the samples analysed is consistent with derivation by combined assimilation of felsic material and crystallisation of olivine from a primitive komatiite magma. The parental composition is represented by the spinifex textured units flanking the dunite lens. Compositions of komatiite liquids, sulphides and cumulus olivines can be modelled successfully by varying degrees of batch equilibration between komatiite magma, olivine and assimilated sulphidic sediment. Models involving combined assimilation and fractional crystallisation, in which olivine and sulphide are fractionally segregated as they form, are much less successful at matching the data. In physical terms, this suggests that the mineralisation is due to wholesale assimilation of floor rocks close to the site of sulphide accumulation. Comparison with quantitative computer model trends suggests an average ratio of assimilation to olivine fractionation in the order of 0.5 to 1 in order to generate the observed degrees of LREE enrichment in flowtops, Ni depletion in olivine and PGE depletion in sulphides and flow tops.