The distribution of PGE in fractionated Archaean komatiites, Western and Central Ultramafic Units, Mt Keith region, Western Australia

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Abstract
The Western and Central Ultramafic Units are two of three major komatiite units that characterize the Archaean greenstone stratigraphy in the Mt Keith region of Western Australia. The units include a complex array of thin flows up to 25 m thick. Bladed olivine and acicular pyroxene spinifex‐textured varieties are common, with the former more predominant. The flows exhibit variably developed textural layering and chemical differentiation, particularly the thinner units. Olivine‐bearing spinifex‐textured flows from the Western Ultramafic are consistently more magnesian than their counterparts from the Central Unit; (≥32 wt% MgO vs 26–30 wt% MgO) and contain significantly lower amounts of all PGE. The olivine‐bearing flows exhibit irregular and complex PGE distribution patterns. Pt, Pd and Rh exhibit incompatibility with respect to olivine, and Ir and Ru behave in both a compatible and incompatible manner. The distribution patterns are in accordance with processes of layered komatiite flow formation proposed by Arndt (1986) and Turner et al. (1986). Higher incompatible PGE concentrations in pyroxene spinifex‐textured flows, over their olivine‐rich counterparts is consistent with a genetic link by olivine fractionation. Metal ratio plots of Pd/Ir vs Ni/Cu and Ni/Pd vs Cu/Ir are typical of komatiites (Barnes et al. 1988), and support this genetic relationship. Negative Pt anomalies (relative to abundances of Rh and Pd) characterize all olivine‐bearing flows. The pyroxene‐bearing flows do not exhibit this anomaly, suggesting the possible involvement of a Pt metal phase.