The Geology of Prieska Copper Mines Limited

The discovery of the Prieska Copper Mines Ltd. copper-zinc orebody in South Africa in 1968 focused the attention of economic geologists on the little-known Bushlnanland gneiss area to the west of the KaMen quartzites, where intensive exploration recently has resulted in at least two additional major discoveries. The main Prieska orebody, with a probable age of 1,250 m.y., and the Volgelstruisbult Annex massive sulfide body occur in a series of quartz-feldspar rocks, quartz-biotite gneisses, and quartz-plagioclase-amphibole gneisses which are interpreted as meta-morphosed volcanic rocks of calc-alkaline affinity, called the Copperton Volcanic Pile, which have been infolded into the basement complex. A major crustal fracture, the Copperton fault, bounds the Copperton Volcanic Pile and an assemblage of more basic volcanic rocks, the Van Wvks Pan Volcanic Pile, in the east and plays a role in the preservation of the KaMen quartzites. Metamorphism of upper amphibolite rank has destroyed many of the volcanic textures in the Copperton Volcanic Pile but the preservation of lithic and crystal pyroclastic fragments, amygdales, flow banding, and shards is nevertheless spectacular. Inclusions are also common in the massive sulfide bodies. Wall-rock alteration of the phyllic and i)ropylitic type is present around both the sul-fide bodies but is not well understood. The main Prieska orebody comprises four main textural types--porphyroblastic, granular, hackly, and disseminated, and there is a tendency for zinc to occur preferenti-ally with the granular type. Mineralogical zoning occurs over the areal extent of the orebody but not through the width of the massive sulfides, and there is a tendency for zinc to occupy the lateral edges of the orebody while copper is relatively evenly spread. Textural and stratigraphic evidence suggests that the main Prieska orebody represents a pre-existing sulfide mass, possibly a stockwork ore, which was redeposited by volcanic action