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Trace and platinum-group element geochemistry and the development of the Merensky unit of the Western Bushveld complex

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Abstract

Whole-rock samples of the Merensky Unit were analysed for major elements, Cu, Ni, Cr, Co, Eu, Th, U, Cs, and the platinum-group elements Pt, Pd, Rh, Ir and Au. There is a sympathetic variation of Cu, Ni, Co, and S throughout the sequence. Th and U abundances are highest around the Merensky Reef, and their distribution in the rest of the unit reflects the amount of mesostasis. Eu distribution is governed by primary plagioclase. Pt, Pd, Rh, Ir and Au (PM) are concentrated in the Merensky Reef. The relative proportions throughout the unit are Ir<Rh<Pd⪕Au<Pt. Au, Pd, Pt are depleted upwards in the plagioclase-rich part of the unit. Except for Ir, the PM correlate with Th and U at lower PM levels. The PM are highly intercorrelated and there is considerable variation in the individual ratios. The distribution of trace elements in the Merensky Unit was governed by the growth rates of the primary silicates. At low abundances the PM behave as incompatible elements, together with late S. At higher concentrations the primary S content controls PM abundances. Residual liquid compositions computed from a marginal-facies analysis show parallel FeO and SiO2 enrichment trends; interaction of these trends resulted in the Merensky Reef and its attendant mineralization.

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  1. Fundamental Research Unit, Johannesburg Consolidated Investment Co., Randfontein, South Africa

    C. A. Lee

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Lee, C.A. Trace and platinum-group element geochemistry and the development of the Merensky unit of the Western Bushveld complex. Mineral. Deposita 18, 173–190 (1983). https://doi.org/10.1007/BF00206207

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