The genesis of sulfide mineralization in a portion of the Potgietersrus Limb of the Bushveld Complex

Abstract

In the Potgietersrus limb of the Bushveld Complex, South Africa, sulfide mineralization is associated with the intrusive contact between Bushveld pyroxenitic gabbros and floor rocks consisting of Malmani dolomite, the overlying Penge banded ironstone, and sedimentary rocks of the Pretoria series. Whole-rock values of up to 0.5 and 1.8 weight percent copper and nickel, respectively, were obtained in borehole intersections through the contact zone and values of up to 28 ppm platinum group metals have been reported from the area.Petrographic and isotope geochemical studies were undertaken on a representative suite of rocks. It was shown that the pyroxenitic gabbro in contact with the floor rocks was equivalent in composition to the Merensky Reef. Xenoliths of Malmani dolomite incorporated into the Bushveld rocks reacted with the magma and were metamorphosed to calc-silicate and carbonate silicate hornfelses. Increased sulfide concentrations are found in association with the xenoliths. These rocks, together with the Bushveld pyroxenitic gabbros and recrystallized argillaceous sediments of the Pretoria series and the pre-Bushveld noritic sills, constitute a 120-m-thick mineralized zone known as the Platreef.The results of carbon and oxygen isotope determinations on unaltered Malmani dolomite are consistent with values expected for an evaporitic environment of deposition during the Precambrian. Anhydrite normally associated with evaporitic sequences is likely to have been preserved in the Malmani dolomite when it was incorporated as xenoliths into the Bushveld magma. Sulfur isotope studies on sulfides from the Platreef zone give a range of delta 34 S values of from 6.8 to 9.2 per mil. As the delta 34 S values for primary magmatic Bushveld sulfides fall in the range -0.6 to +3.5 per mil, the isotope geochemistry clearly indicates a contribution of sedimentary-derived sulfur to the original Bushveld magma. Sulfide mineralization in the Platreef is considered, therefore, to have been derived from the combined influence of a sulfur-rich Merensky Reef-type magma and the incorporation of additional sulfur from the Malmani dolomite, either through direct diffusion or in association with a carbonate melt.Between the banded ironstone and the Platreef zone there is a 50-m-thick zone of coarsegrained iron- and sulfide-rich pegmatoidal pyroxenite. Pyrrhotite from these rocks gave delta 34 S values of 2.7 and 8.7 per mil which are within the possible range of magmatic sulfur. These rocks may be related to the intrusive pipes found elsewhere in the Bushveld Complex.