Platinum-group element alloy inclusions in chromites from Archaean mafic-ultramafic units: Evidence from the Abitibi and the Agnew-Wiluna Greenstone Belts

The paper investigates the role of primary magmatic phases in the fractionation and concentration of PGE in Archaean mafic and ultramafic systems. The composition of chromites and olivines in sulphur-poor (S<0.6 wt%) komatiites from the Agnew-Wiluna Belt (Western Australia), and of chromite concentrated from komatiitic basalt, ferropicritic basalt and tholeiitic basalt from the Abitibi Belt (Canada) were analysed. The results of laser ablation ICP-MS analyses show that PGE-bearing alloys are not stable in crystallising komatiite and that ruthenium is soluble in chromite during crystallisation. Conversely, analyses of chromites separated from Theo's Flow tholeiitic basalt indicate that Ir-Os-(&PLUSMN;Pt) enrichments (&GT;200 ppb) reflect the presence of PGM. Chromites from Fred's Flow komatiitic basalt contain Ir-rich clusters, whereas Pt enrichments (&GT;370 ppb) in Boston Creek ferropicritic basalt reflect the presence of Pt-rich compounds. The presence of PGE-bearing alloys in Theo's Flow and Fred's Flow is due to late S-supersaturation, whereas the presence of Pt-rich compounds in Boston Creek Flow reflects high state of melt oxidisation. The lack of PGE-bearing alloys in the olivines and chromites of komatiites can be explained by thermal instability of PGM, depletion in PGE at the mantle source, early S-supersaturation, the oxidisation conditions of the melt, or a combination of these factors.