A GEOCHEMICAL PROFILE THROUGH THE UITKOMST COMPLEX ON THE FARM SLAAIHOEK, SOUTH AFRICA, WITH REFERENCE TO THE PLATINUM GROUP ELEMENTS AND SM-ND ISOTOPES

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 farm Uitkomst 541JT and Slaaihoek 540JT, about 25km north of Badplaas and 50km east of the eastern limb of the Bushveld Complex in the Mpumalanga Province of South Africa. The intrusion has an established length of 12 km and a total thickness of 850m. It is divided into 7 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 shows that the Complex may have crystallized in a dynamic 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 Rare Earth Element variations show a decrease in concentration with height, contrary to what is expected of a progressively differentiating magma in a closed system. Further, the PGE 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 and the 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. 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 segregation from B1 magma and not from fractionation of sulphides that were later entrained in the streaming magma.