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 O.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 hydrotherrmal 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.