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

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 187Os/188Os value, measured by N–TIMS in PGM that contain Os in the range 18–53 wt.% ( i.e., Pt–Ir–Os alloy, ruthenium, rutheniridosmine and osmium), varies from 0.0987 to 0.1068, revealing the lowest three 187Os/188Os values (0.0987–0.1024) reported so far in terrestrial PGM. The 187Os/188Os value measured by LA MC–ICP–MS in PGM with Os contents between 2 and 10 wt.% (Pt–Fe, Ru–Ir–Pt and Ru–Pt alloys) was found to range from 0.1053 to 0.1095. The model 187Os/188Os 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.