A geochemical profile through the Uitkomst Complex on the farm Slaaihoek, with special reference to the platinum-group elements and Sm-Nd isotopes

Abstract

The Uitkomst Complex is a mineralized, layered basic to ultrabasic intrusion, hosted by sedimentary rocks of the lower part of the Transvaal Supergroup. It is situated on the farms Uitkomst 541JT and Slaaihoek 540JT, about 25 km north of Badplaas and 50 km east of the eastern limb of the Bushveld Complex in the Mpumalanga province of South Africa. The intrusion plunges between 8 to 10º to the northwest with an established length of 12 km and a total thickness of 850 m. It is divided into seven lithological Units (from base to top), the Basal Gabbro (BGAB), Lower Harzburgite (LHZBG), Chromitiferous Harzburgite (PCR), Main Harzburgite (MHZBG), Pyroxenite (PXT), Gabbronorite (GN) and Upper Gabbro units (UGAB). A detailed petrographic and geochemical investigation of borehole core SH176, which provided a complete intersection of the Uitkomst Complex was carried out. The study shows that the Complex may have crystallized in a dynamic magma conduit setting. The whole rock geochemical trends indicate that there is a reversed fractionation in the basal portion of the Complex and a lack of fractionation in much of the MHZBG. Trace and REE variations show a decrease in concentration with height, contrary to what is expected of a progressively differentiating magma in a close system. Further, the platinum-group element concentration of the four basal units show no depletion with increasing height, suggesting that the individual units are not related to each other by means of in situ fractionation. Instead, a model whereby the individual units crystallized from distinct pulses of magma best explains the data. By comparing Nd isotopes and ratios of highly incompatible trace elements like [Th/La]n and [Sm/Ta]n from the Uitkomst Complex and Bushveld Complex it is seen that the Uitkomst magmas are of a similar lineage as the B1 magma of the Bushveld Complex, supporting a genetic link between the two complexes. The upper portion of the Uitkomst Complex shows values more akin to B3 magmas indicating the possible presence of more than one type of magma. Based on the available S isotope and trace element data, the sulphides of the Complex appear to have formed within the Complex, probably in response to contamination of the magma with dolomite. Entrainment of sulphides from depth is considered unlikely. The relatively low Cu/Ni ratios of the sulphides in the LHZBG, PCR and MHZBG (Cu/Ni 0.03 to 0.8) may be modelled by sulphide segregation from B1 magma and not from fractionation of sulphides that were later entrained in the streaming magma.