Abstract

Forty whole-rock samples representing the principal silicate rock types of the lower and the lower critical zones of the Bushveld Complex were analyzed for Ir, Ru, Rh, Pt, Pd, and Au. Ir concentration ranges from 0.1 to 2.2 ppb, with sporadic values greater than 4 ppb. There is no readily evident trend of Ir enrichment or depletion up the sequence. Dunites (85-98% olivine) have less Ir (<1 ppb) than the pyroxenites. Ru ranges from 2 to 15 ppb, with the lower part of the sequence having on average less than 5 ppb and the upper part more than 5 ppb. Rh ranges from 1.5 to 5 ppb with sporadic values exceeding 5 ppb. The majority of the Pt values are less than 30 ppb but range from 10 to 130 ppb. Pd was detected in seven of the samples and in these it correlates with the higher Au levels. Au is generally less than 2 ppb but ranges up 13 ppb.The platinum-group element contents of the Bushveld layered rocks normalized to C1 chondrite exhibit trends that parallel the Merensky reef (except for Pt) and the trends of the magnesian basaltic Bushveld related sills. Interelement relationships are consistent with platinum-group mineral control on platinum-group element distribution in the nonmineralized layered rocks. The parallel trends for the contrasting sets of data show that, in the absence of abundant sulfides which concentrate Pt and Pd (due to their greatly enhanced chalcophile character over the other platinum-group elements), the ratios are constant throughout the development of the complex to the stage of the Merensky reef.

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