Ru-Os-Ir-Pt AND Pt-Fe ALLOYS FROM THE EVANDER GOLDFIELD, WITWATERSRAND BASIN, SOUTH AFRICA: DETRITAL ORIGIN INFERRED FROM COMPOSITIONAL AND OSMIUM-ISOTOPE DATA

摘要

The compositionally diverse platinum-group minerals (PGM) from the Evander Goldfield, in the eastern part of the Witwatersrand Basin, South Africa, have been studied for the first time by a number of modern techniques. The characteristic feature of PGM from Evander is an extensive presence of Ru-rich alloys (i.e., Ru-Os-Ir, Ru-Os-Ir-Pt, Ru-Ir-Pt, Ru-Pt), which prevail over osmium, iridium, rutheniridosmine, Pt-Fe, Pt-Ru-Fe and Pt-Ir-Os alloys and other PGM. The ~(187)Os/~(188)Os value, measured by N-TIMS in PGM that contain Os in the range 18-53 wt. percent (i.e., Pt-Ir-Os alloy, ruthenium, rutheniridosmine and osmium), varies from 0.0987 to 0.1068, revealing the lowest three ~(187)Os/~(188)Os values (0.0987-0.1024) reported so far in terrestrial PGM. The ~(187)Os/~(188)Os value measured by LA MC-ICP-MS in PGM with Os contents between 2 and 10 wt. percent (Pt-Fe, Ru-Ir-Pt and Ru-Pt alloys) was found to range from 0.1053 to 0.1095. The model ~(187)Os/~(188)Os ages obtained for the main set of PGM (4104-3020 Ma, n = 12) imply that the PGM are detrital and were thus not deposited by later hydrothermal fluids. They also favor a scenario in which the majority of PGM were incorporated into the Witwatersrand basin by their release during weathering of ultramafic or mafic source-rocks.