The petrogenesis of the Uitkomst complex, Mpumalanga Province, South Africa

Abstract

The Uitkomst Complex is a Ni-Cu-PGE-Cr-mineralised layered basic intrusion, situated approximately 250 km due east of Pretoria and 20 km north of Badplaas in the Mpumalanga Province, South Africa. The Complex hosts the first principal nickel mine in South Africa. It began operation in 1996. The elongated trough-shape of the Complex suggests that it may be a magma conduit. Being of appproximately similar age as the Bushveld Complex, it has been suggested in the past that it represents a feeder zone to the Bushveld Complex. The present study was initiated to examine this model and provide guidelines for the exploration of further sulphide bodies that may remain unexposed. This is in view of the fact that feeder zones of large magmatic bodies and volcanic flows elsewhere may also host important sulphide mineralization. The Complex consists of six lithological units (from bottom to top): Basal Gabbro (BGAB), Lower Harzburgite (LHZBG), Chromitiferous Harzburgite (PCR), Main Harzburgite (MHZBG), Pyroxenite (PXT) and Gabbronorite (GN) Units. The 6 m thick BGAB Unit, developed at the base of the intrusion, shows a narrow chilled margin against the floor rocks. It grades upwards into the sulphide-rich, pyroxenitic and xenolith-bearing 50 m thick LHZBG Unit. The overlying PCR Unit (thickness 60 m) is overlain by the 330 m thick MHZBG Unit. The PXT and GN Units (total combined thickness of 310 m) form the uppermost portions of the intrusion. Large parts of the intrusion are highly altered by late magmatic, hydrothermal processes causing widespread serpentinization, talc-carbonate alteration, saussuritization and uralitization. Whole-rock compositional variations reveal a broadly symmetrical pattern, in that the base and the top of the intrusion are relatively evolved, whereas the centre is relatively primitive. The LHZBG and PXT Units show forsterite contents of olivines from Fo80 (NiO 0.28 wt.%) to Fo.i2 , whereas the MHZBG Unit has olivine compositions around Fo89 with a maximum of F~1 (NiO 0.59 wt.%) at about 400 m elevation above base (e.a.b.). The Mg# of the Ca-poor pyroxenes varies, irrespective of the mineral morphology, from a maximum of 91.9 in the MHZBG Unit to a minimum of 49.3 in the lower part of the Gabbronorite Unit. Chromite compositions are highly variable within single samples throughout the stratigraphy. Plagioclase shows extreme compositional variation of over 30 mol. % with cumulus plagioclase in the BGAB Unit having anorthite contents between An70 and An80, interstitial plagioclase in the harzburgitic units An90 and cumulus plagioclase at the base of the Gabbronorite Unit An92. Thus, the Mg#, Cr and Ni levels are also highest in the central harzburgitic portions of the concentrated in the gabbroic rocks at the base and the top. The knowledge of the local stratigraphy and the findings about the geometry of the magmatic body from borehole intersections led to the postulation that the Uitkomst magma chamber formed at a depth of at least 8 km along a dilational jog. This magma intrusion initially created a tubular chamber roughly 750 m deep and 1 km wide with a proven length of more than 12 km. Subsequent flow of magma through this conduit eroded the sidewalls to their present shape. Geochemical modelling of the different chill zones of the Complex reveals that at least 3 parental magmas were involved in the Complex's formation. The first magmas entering the chamber were gabbroic and represent a mixture of Bushveld 82 and an evolved Bushveld 81 magma. This magma formed the chill and marginal zones, and the phaneritic gabbroic cumulate of the Basal Gabbro Unit (Integration stage). The major and trace element pattern above 10 m above base of intrusion suggests that this magma was later replaced by an unfractionated 81 magma (F0), which introduced a period of more intense magma flow through the chamber (Conduit stage). The BGAB and LHZBG Units also show evidence of contamination with quartzitic and dolomitic country rock. Trace element modelling revealed that during the conduit stage of the Complex, three successive cycles, each consisting of fractional crystallization under stagnated flow conditions followed by a period of increased flow, yielded 370 m of olivine-chromite cumulates (LHZBG, PCR and MHZBG Units). The homogeneous cumulate rocks are reflected in the very constant major element chemistry of this interval. Geochemical reversals between 400 and 500 m elevation above base indicate that the conduit-stage came to an end when the flow of magma ceased and closed system conditions prevailed, yielding olivine-chromite dominated, orthopyroxene-dominated and plagioclase-dominated cumulates, respectively, with increasing height. The top 100 m of rock are interpreted as a transition between chilled gabbroic magma and the cumulate rocks, implying some roof crystallization. The mineralization is concentrated in the lower three rock units and massive sulphide lenses in the floor. Based on experience with other mineralized magmatic bodies, these massive sulphide occurrences and the disseminated sulphides are thought to be most probably caused by a sulphide liquid segregating within the conduit as a result of magma mixing and contamination with country rock material. i5 34S isotope ratios show a bi-modal distribution pattern with welldeveloped modes at 0 and -6 per mil. This suggests that part of the magmatic sulphur (i5 34S close to 0 per mil) became contaminated by sedimentary sulphur (less than -10 per mil i5 34S). Mass balance calculations, assuming a magma of an evolved Bushveld 81 composition as initial liquid, indicate that only approximately 10 % assimilation of dolomite and pyritic shale is necessary to explain the bulk average sulphur isotope ratio of -5 to -6 per mil observed at Uitkomst. Cu/Ni ratios of the massive and disseminated ore are around 0.5. Since the chilled margins have Cu/Ni ratios of approximately 1, one would expect the sulphides to have Cu/Ni ratios of 3. The low Cu/Ni ratios of the Uitkomst sulphides raises the possibility that the ores represent a mss cumulate with the Cu(as well as pt and Pd)-rich fractionated sulphide being entrapped and removed by the streaming magma and deposited elsewhere in the conduit. Alternatively, the sulphides may have segregated from a less evolved magma than the chilled margin. The massive chromite mineralization is exclusively associated with rocks that crystallized from the primitive 81 magma. Since the observed cumulate thicknesses are not large enough to produce the observed chromitite thicknesses in situ, it is concluded that magma of at least five times the volume of the exposed intrusion pulsed through the conduit, precipitating chromite. intrusion, but the incompatible trace elements Rb, Zr, Y and P, as well as V and Ti are ii