Ru-Os-Ir-Pt and Pt-Fe alloys from the Evander goldfield, Witwatersrand Basin, South Africa; detrital origin inferred from compositional osmium-isotope data

• Published in: Canadian mineralogist Vol. 42; no. 2; pp. 631 - 650
• Main Authors: Malitch, Kreshimir N, Merkle, Roland K. W
• Format: Journal Article
• Language: English
• Published: Mineralogical Association of Canada 01-04-2004
• Subjects: absolute age; Africa; alloys; Archean; crystal form; detrital deposits; Economic geology; electron probe data; Evander gold field; gold ores; heavy mineral deposits; ICP mass spectra; iridium; iron; isotope ratios; isotopes; laser ablation; laser methods; mass spectra; metal ores; metals; Mineralogy; nonsilicates; Os-188/Os-187; osmium; platinum; platinum group; platinum minerals; Precambrian; ruthenium; South Africa; Southern Africa; spectra; stable isotopes; Witwatersrand
• ISSN: 0008-4476; 1499-1261
• DOI: 10.2113/gscanmin.42.2.631

The characteristic feature of PGM from the Evander Goldfield is the extensive occurrence of Ru-rich alloys; these prevail over osmium, iridium, rutheniridosmine, Pt-Fe, Pt-Ru-Fe and Pt-Ir-Os alloys and other PGM. The 187Os/188Os value (measured in PGM with Os 18-53 wt %) varies from 0.0987 to 0.1068, revealing the lowest values so far reported in terrestrial PGM. The 187Os/188Os value in PGM with Os 2-10 wt % 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) imply that the PGM are detrital and not deposited by later hydrothermal fluids.They also favour a scenario in which most of the PGM were incorporated into the Witwatersrand basin by their release during weathering of mafic or ultramafic source rocks.