The composition and isotope-geochemical features of platinoid mineralization of the Evander goldfield in the east of the Witwatersrand Basin (South Africa) were characterized for the first time. According to zirconometry data, this mineralization was formed at an interval of 3074-2714 Ma. For studying platinum-group minerals (PGM), we used a complex of methods including the scanning electron microscopy (SEM), roentgen spectral microanalysis (RSMA), and negative thermal ionization mass-spectrometry (NTIMS). The 187Os/188Os ratio was used for identifying source of ore substance and establishing 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 the 187Os/188Os value, determined by the NTIMS in individual PGM grains (iridous osmium, osmious iridium, rutheniridosmine, and other minerals of the system Ru-Os-Ir-Pt), varied in the range of 0.1050-0.1091. Independently of the PGM chemical composition, two heterogeneous isotope groups were distinguished for a dominating PGM batch (n = 10). The first group is characterized by the average 187Os/188Os value of 0.1053 ± 0.0004; the second group, of 0.1065 ± 0.0003. As the obtained values are lower than those obtained for the contemporary mantle (0.12736 ± 0.00016), we calculated a model Re-Os PGM age, which turned out to be 3220 ± 70 and 3040 ± 40 Ma, correspondingly. In line with a wide development of polycomponent solid solutions of the system Ru-Os-Ir-Pt, the obtained model ages allow us also to suggest that the source of the PGM substance was Archean mantle slightly differentiated relative to platinum-group elements (PGE). A bedrock source of the PGM heavy concentrates of the Witwatersrand Basin could be, most probably, Archean ultramafites of an unspecified type. According to their composition, these rocks are, probably, the most close to Paleozoic mantle residual ultramafites (restites) of the dunite-harzburgite complex or clinopyroxenitedunite complex (of the Gulin type), which are also specialized to refractory PGE.
|Number of pages||14|
|Journal||Geology of Ore Deposits|
|Publication status||Published - May 2000|