Downdip development of the Ni‐Cu‐PGE‐bearing mafic to ultramafic uitkomst complex, Mpumalanga province, South Africa

The about 2055-Ma-old mafic to ultramafic Uitkomst Complex in the Mpumalanga Province of South Africa hosts the low-grade-large-tonnage Ni-Cu-PGE deposit, Nkomati. The complex is regarded to represent a satellite to the Bushveld Complex and a feeder to an eroded magmatic reservoir in the southeast. Aeromagnetic surveys and previous drilling indicated an overall northwestern-downdip extension of the complex, but the question is to what extent and in which expression can the complete intrusion be found under cover in the northwest? Answering this, a mineralogical, geochemical and geochronological investigation of a borehole intersection of the whole complex at Little Mamre was carried out, using petrography, XRF, EPMA and LA-ICP-MS U–Pb analyses of zircons for age determination. Although the total thickness of the rock units is larger than to the southeast, emplacement, litho- and mineral chemistry trends, expression of alteration mineralogy and style of sulphide mineralisation are similar. The amount of sulphide mineralisation is on average less than in the southeast. The upper ultramafic unit contains, more frequently, pegmatoidal sections, and the Chromitiferous Harzburgite unit has less massive chromitite layers than the southeastern parts of the complex, whereas the gabbro(-norite) units contain more interstitial liquid with late-stage minerals. The findings confirm that the anvil-shaped intrusion in cross section continues with increased thickness towards northwest at a shallow dip; although approaching the deeper part of the igneous reservoir, mineral compositions are partially more evolved. The overall mineralogical consistency downdip supports a situation of multiple magma replenishment along a flat-lying, northwest–southeast trending conduit, resulting in an evolved cumulus mineral assemblage in the upper part

Downdip Development of the Ni-Cu-PGE-Bearing Mafic to Ultramafic Uitkomst Complex, Mpumalanga Province, South Africa

The about 2055-Ma-old mafic to ultramafic Uitkomst Complex in the Mpumalanga Province of South Africa hosts the low-grade-large-tonnage Ni-Cu-PGE deposit, Nkomati. The complex is regarded to represent a satellite to the Bushveld Complex and a feeder to an eroded magmatic reservoir in the southeast. Aeromagnetic surveys and previous drilling indicated an overall northwestern-downdip extension of the complex, but the question is to what extent and in which expression can the complete intrusion be found under cover in the northwest? Answering this, a mineralogical, geochemical and geochronological investigation of a borehole intersection of the whole complex at Little Mamre was carried out, using petrography, XRF, EPMA and LA-ICP-MS U–Pb analyses of zircons for age determination. Although the total thickness of the rock units is larger than to the southeast, emplacement, litho- and mineral chemistry trends, expression of alteration mineralogy and style of sulphide mineralisation are similar. The amount of sulphide mineralisation is on average less than in the southeast. The upper ultramafic unit contains, more frequently, pegmatoidal sections, and the Chromitiferous Harzburgite unit has less massive chromitite layers than the southeastern parts of the complex, whereas the gabbro(-norite) units contain more interstitial liquid with late-stage minerals. The findings confirm that the anvil-shaped intrusion in cross section continues with increased thickness towards northwest at a shallow dip; although approaching the deeper part of the igneous reservoir, mineral compositions are partially more evolved. The overall mineralogical consistency downdip supports a situation of multiple magma replenishment along a flat-lying, northwest–southeast trending conduit, resulting in an evolved cumulus mineral assemblage in the upper part

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

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 iiThesis (PhD)--University of Pretoria, 1998.GeologyPhDUnrestricte

Spaceborne Mine Waste Mineralogy Monitoring in South Africa, Applications for Modern Push-Broom Missions: Hyperion/OLI and EnMAP/Sentinel-2

Remote sensing analysis is a crucial tool for monitoring the extent of mine waste surfaces and their mineralogy in countries with a long mining history, such as South Africa, where gold and platinum have been produced for over 90 years. These mine waste sites have the potential to contain problematic trace element species (e.g., U, Pb, Cr). In our research, we aim to combine the mapping and monitoring capacities of multispectral and hyperspectral spaceborne sensors. This is done to assess the potential of existing multispectral and hyperspectral spaceborne sensors (OLI and Hyperion) and future missions, such as Sentinel-2 and EnMAP (Environmental Mapping and Analysis Program), for mapping the spatial extent of these mine waste surfaces. For this task we propose a new index, termed the iron feature depth (IFD), derived from Landsat-8 OLI data to map the 900-nm absorption feature as a potential proxy for monitoring the spatial extent of mine waste. OLI was chosen, because it represents the most suitable sensor to map the IFD over large areas in a multi-temporal manner due to its spectral band layout; its (183 km × 170 km) scene size and its revisiting time of 16 days. The IFD is in good agreement with primary and secondary iron-bearing minerals mapped by the Material Identification and Characterization Algorithm (MICA) from EO-1 Hyperion data and illustrates that a combination of hyperspectral data (EnMAP) for mineral identification with multispectral data (Sentinel-2) for repetitive area-wide mapping and monitoring of the IFD as mine waste proxy is a promising application for future spaceborne sensors. A maximum, absolute model error is used to assess the ability of existing and future multispectral sensors to characterize mine waste via its 900-nm iron absorption feature. The following sensor-signal similarity ranking can be established for spectra from gold mining material: EnMAP 100% similarity to the reference, ALI 97.5%, Sentinel-2 97%, OLI and ASTER 95% and ETM+ 91% similarity

A Bushveld-related high-Ti igneous suite (HITIS) derived from an alkali to transitional basaltic magma, South Africa

A suite of high-ti mafic to felsic igneous rock (acronym:HITIS) was emplaced at -2055 Ma as km-sized bodies to the South of the Bushveld Complex, South Africa, stretching from Parys and Potchefstroom in the south-west to Marble Hall in the north and Badplaas in the east. Currently known members of the suite include the Marble Hall sills and related intrusives and breccia, the Basal Gabbro Unit of the Uitkomst Complex, the Lindeques Drift and Heidelberg intrusions, the volcanic Roodekraal Complex, the Rietfontien Complex, the Koedoesfontein Complex, the Schoogezïcht (previously Kaffirskraal) Complex, the Vredefort alkali granite, as well as the high-Ti basalt and FeTip-enriched basaltic lava of the Rooiberg Group. The volcanic rocks extruded on mildly folded and denuded metasedimentary rocks and lava of the Pretoria Group, whereas the plutonic rocks intruded at a level between the upper Witwatersrand Supergroup and the lower Pretoria Group, but largely within the dolomite of the Chuniespoort Group. The mafic rocks in this suite are dominated by clinopyroxene (salite to augite), FeTi-oxide (magnetite-ilmenite), amphibole (largely edinitic and hastingsitic), olivine (FO44 to FO80), orthopyroxene and plagioclase (An<50), forming an array of dioritic and subordinate gabbroic rocks with associated clinopyroxene±magnetite(-ilmenite)±olivine±plagioclase cumulates. Locally, hybrid calc-silicate rocks derived from reaction of the magma with dolomite of the Chuniespoort Group are developed. The felsic rocks are syenodiorite and alkali granite with K-feldspar, albite, alkali pyriboles and biotite. Trace elements systematics suggests that the HITIS rocks are derived from an alkali to transitional basaltic parent. The most primitive rocks representing liquid fractions are developed in the chill zone of the Basal Gabbro Unit of the Uitkonmst Complex, (Mg# 0.55 to 0.68) and in the Marble Hall diorite sills (Mg# = 0.53 to 0.65). These are followed by the more evolved high-Ti basalt (Mg# = 0.45 to 0.50) of the Rooiberg Group, the mugearite lavas (Mg# = 0.29 to 0.47) of the volcanic Roodekraal Complex, the Fe-Tip lava (Mg# = 0.17 to 0.30) of the Rooiberg Group, and finally the Vredefort alkali granite (Mg# = 0.26 to 0.72). This rock series resulted from deep-seated (lower crustal?) amphibole fractionation, followed by shallow level clinopyroxene±magnetite(-ilmenitel±olivine±plagioclase fractionation with the FeTi-oxide phases becoming more dominant in the later stages. The Vredefort alkali granite represents the final liquid in the liquid line of descent. The HITIS magma marks an early stage in the Bushveld magmatic event and has a close temporal relationship with the boninitic B1-type magma, regarded as parental to the Lower Zone of the Bushveld Complex. It also appears to have been derived from the same mantle source that yielded the tholeiitic B3-type magma, which is in part responsible for the Main Zone of the Bushveld Complex. Minor copper and PGE concentrations are associated with cumulate rocks of the HITIS