Olivine and sulfur isotopic compositions of the Uitkomst Ni–Cu sulfide ore-bearing complex, South Africa: evidence for sulfur contamination and multiple magma emplacements

Abstract

The Uitkomst Complex in northern South Africa is host to a Ni–Cu deposit containing a minerable reserve of 2.9 Mt massive ore with grades of 2% Ni, 1% Cu and 6 ppm Pt+Pd and an indicated plus inferred resource of 98 Mt disseminated ore with grades of 0.6% Ni, 0.2% Cu and 1 ppm Pt+Pd. Based on similar age and composition to that of the Bushveld Complex, the Uitkomst Complex is believed by some investigators to be a satellite body of the Bushveld Complex. It has a tubular shape with an exposed surface area of about 0.8×8 km and a thickness of up to 1000 m, apparently exploiting the bedding planes of gently dipping footwall sediments that include shale, quartzite and dolomite. The Uikomst Complex comprised of stratiform mafic and ultramafic rock units that from base to top include gabbro, sulfide-mineralized harzburgite, unmineralized harzburgite, pyroxenite, gabbronorite and gabbro. The compositions of olivine from different rock units are significantly different. Olivines from the gabbronorite unit are highly fractionated, containing <30 mol% Fo (forsterite, Mg 2SiO 4) and <300 ppm Ni. Olivines from the underlying pyroxenite and harzburgite units are much more primitive, containing 86–91 mol% Fo. Abrupt changes in the contents of Ni in olivine occur within the pyroxenite unit, and between the mineralized and unmineralized harzburgite units. The contents of Ni in olivine from the unmineralized harzburgite unit are between 2900 and 3600 ppm. The contents of Ni in olivine from the underlying mineralized harzburgite units are ∼1500 ppm lower. Olivines from the lower part of the pyroxenite unit are similar to olivines from the underlying harzburgite unit, whereas olivines from the upper part of the pyroxenite unit contain much lower Ni (<600 ppm). Numerical modelling suggests that the strikingly different compositions of olivine are related to different parental magmas with different MgO/FeO ratios and/or Ni contents. The δ 34S values of the basal gabbro and unmineralized rock units range from −0.9‰ to 2.6‰, which are similar to typical mantle values (∼0±3‰). In contrast, the δ 34S values of the sulfide ores in the harzburgite units are significantly lower, varying between −2.6‰ and −7.1‰. The low δ 34S values of the sulfide ores indicate addition of 34S-depleted crustal sulfur. The obvious sources of such crustal sulfur are the Malmani dolomite and the Timebal Hill shale that contain pyrite with negative δ 34S values up to −11‰ and −18‰, respectively. Sulfide saturation in the magma of the mineralized harzburgite units is thought to have resulted from addition of the crustal sulfur. The resulting immiscible sulfide liquid droplets were then concentrated in the base of the magma channel. Some of the sulfide liquid was expelled into the basal gabbro that separated the active magma channel from the footwall quartzite. New magma then entered the channel, displacing most of the resident magma to form the unmineralized harzburgite. The overlying pyroxenite, gabbronorite and gabbro units formed either by in situ differentiation of the same magma giving rise to the underlying unmineralized harzburgite or by subsequent emplacement of a more evolved magma.