Sulphide and oxide mineralisation in the Uitkomst Complex, South Africa: origin in a magma conduit

Abstract

In the magmatic system of the Uitkomst Complex, several factors may influence the occurrence of major sulphide segregation: (i) a mixing of compositionality contrasting magmas; (ii) a rapid decrease in temperature at the fast cooling margins of the intrusion; (iii) a decrease in FeO content as a result of chromite and magnetite precipitation; and (iv) the assimilation of dolomitic, or siliceous, sulphide-bearing country rocks. The original Uitkomst magma had a typical primary magmatic S isotope signature (0ℵ δ 34S) and on emplacement became contaminated to a variable degree with 32S-enriched sedimentary S (<− 10ℵ δ 34S). The sulphide mineralisation occurs as: (i) a massive sulphide body in the quartzite and granite basement; and (ii) disseminated sulphides within the BGAB and LHZBG Units. Geochemical variations throughout the complex and the combined thickness of the chromitite seams and the associated massive sulphide bodies strongly underline the argument for the conduit-type nature of the intrusion similar to Voisey's Bay and Noril'sk. The over large quantity of sulphides for the complex's thickness is mainly explained by separation from a larger magma pool at depth, entrapment by the ascending magma and deposition in the conduit. The fractionation of magma probably occurred at depth and involved olivine, as indicated by the distinct Ni depletion of the chills, at undepleted Cu contents. The Uitkomst ores probably represent a mss cumulate with Cu-Pt-Pd-enriched fractionated sulphide melt remaining unexposed. The origin of mineralisation due to country rock assimilation at Uitkomst is comparable with that of the Platreef portion of the Potgietersrus Limb of the Bushveld Complex. The S isotope signature (6.3–9.2ℵ δ 34S) of the Platreef mineralisation is also indicative of a sedimentary contribution, although by evaporitic layers. The similar Pt/Pd ratio of Uitkomst and the Platreef, however, implies compositional variations within the Bushveld magma rather than fundamentally different magma sources of both complexes.