Platinum-group elements in Cu-sulphide ores at Carolusberg and East Okiep, Namaqualand, South Africa

Abstract

Cu-sulphide ores at Carolusberg and East Okiep have Cu/Ni ratios of up to 80, an order of magnitude higher than most magmatic sulphide ores elsewhere. In contrast, Se/S ratios (500–1700 × 10−6) and PGE tenors (up to 5 ppm) of the sulphides are in the range of more typical magmatic sulphide ores. The observed metal patterns may be explained by a process of monosulphide solid solution (mss) fractionation of a magmatic sulphide melt at depth, but this model is currently considered unlikely, due to the paucity of refractory ores in the district. Assimilation of Cu-rich country rocks during ascent of the Koperberg magmas proved difficult to test with the available data, but this provides no explanation for the common high-grade metamorphic setting of similar ores elsewhere. A restitic origin of the pyroxenites appears to explain many of the observed ore features and is presently favoured here. Desulphidization of a primary magmatic sulphide ore could not have yielded the observed metal patterns and is therefore considered to be of relatively minor importance in ore genesis.