Abstract

Previous work has defined three suites of sills and marginal rocks adjacent to and below the Rustenburg Layered Suite of the Bushveld Complex, namely an ultramafic (MgO approximately 30%), a magnesian basaltic (MgO approximately 12%), and a tholeiitic suite (MgO approximately 7%). These three suites are thought to represent magmas which have intruded and been incorporated into the Rustenburg Layered Suite at various times during its formation. The elements Ir, Ru, Rh, Pd, Pt, and Au have been analyzed in 17 samples representative of the three suites. The ultramafic sills have the highest total noble metal content (46 ppb) followed by the magnesian basaltic suite (37 ppb) and the tholeiitic suite (21 ppb). The ultramafic sills are characterized by a relatively fiat chondrite-normalized platinum-group element pattern with an average Pt/Ir ratio of 3.52 and a pronounced Rh anomaly. The magnesian basaltic and tholeiitic suites both have steep positive chondrite-normalized patterns with average Pt/Ir ratios of 16.14 and 40.5, respectively, and no Rh anomaly. Modeling done using incompatible trace elements and petrographic mixing suggests the ultramafic sills represent crystal mushes (of cumulate olivine and chromite) that were intruded into the floor during the crystallization of the lower portion of the Rustenburg Layered Suite. The enrichment of Ir, Ru, and Rh in the ultramafic sills appears to be associated with the cumulate enrichment of olivine or chromite in these sills.The chondrite-normalized patterns found for the Merensky Reef and the Platreef are similar to those of the magnesian basaltic magma. The pattern found for the UG-2 is much flatter and in some respects very similar to that found for the ultramafic sills.On a worldwide basis the platinum-group element contents of the magnesian basaltic and tholeiitic magmas generally fall in the range of basalts and chill zones of other layered suites. The magnesian basaltic magma does have one of the highest Pt contents measured for a magma.

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