Mineral composition and osmium isotopes of PGE mineralization of eastern Witwatersrand (South Africa)

The composition and isotope-geochemical features of platinoid mineralizatio n of the Evander goldfield in the east of the Witwatersrand Basin (South Af rica) were characterized for the first time. According to zirconometry data , this mineralization was formed at an interval of 3074-2714 Ma. For studyi ng platinum-group minerals (PGM), we used a complex of methods including th e scanning electron microscopy (SEM), roentgen spectral microanalysis (RSMA ), and negative thermal ionization mass-spectrometry (NTIMS). The Os-187/Os -188 ratio was used for identifying source of ore substance and establishin g the model Re-Os age of PGM from the paleoplacers of the Kimberley reef of the Central Rand Group at the Evander goldfield. It was established that t he Os-187/Os-188 value, determined by the NTIMS in individual PGM grains (i ridous osmium, osmious iridium, rutheniridosmine, and other minerals of the system Ru-Os-Ir-Pt), varied in the range of 0.1050-0.1091. Independently o f the PGM chemical composition, two heterogeneous isotope groups were disti nguished for a dominating PGM batch (n = 10). The first group is characteri zed by the average Os-187/Os-188 value of 0.1053 +/- 0.0004; the second gro up, of 0.1065 +/- 0.0003. As the obtained values are lower than those obtai ned for the contemporary mantle (0.12736 +/- 0.00016), we calculated a mode l Re-Os PGM age, which turned out to be 3220 +/- 70 and 3040 +/- 40 Ma, cor respondingly. In line with a wide development of polycomponent solid soluti ons of the system Ru-Os-Ir-Pt, the obtained model ages allow us also to sug gest that the source of the PGM substance was Archean mantle slightly diffe rentiated relative to platinum-group elements (PGE). A bedrock source of th e PGM heavy concentrates of the Witwatersrand Basin could be, most probably , Archean ultramafites of an unspecified type. According to their compositi on, these rocks are, probably, the most close to Paleozoic mantle residual ultramafites (restites) of the dunite-harzburgite complex or clinopyroxenit e-dunite complex (of the Gulin type), which are also specialized to refract ory PGE.