Uitkomst Complex Research Articles

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.

A siliceous komatiitic source of Bushveld magmas revealed by primary melt inclusions in olivine

Our experimental thermobarogeochemical study of the primary melt inclusions in olivine (Fo91) from poikilitic harzburgite to dunite of the Uitkomst Complex in the Bushveld Igneous Province revealed a series of melt compositions that are the result of combined fractional crystallization and assimilation. The primary melt inclusions up to 50 μm in size are fully crystallized and composed of predominantly olivine and orthopyroxene with highly subordinate chromite, sulfide, amphibole and plagioclase accompanied by a fluid phase in shrinkage voids. The most primitive melt within the series has a re-homogenization temperature of >1450 °C and contains 22.47 wt% MgO, 12.57 wt% FeO, 51.90 wt% SiO2, 2.50 wt% CaO, 8.27 wt% Al2O3 and 1.69 wt% Na2O recalculated to equilibrium with the host olivine. This is the first direct measurement of the komatiitic parental magma of the Bushveld magmas, inferred in previous studies. The estimated maximum PT parameters of the melt corresponds to 2.5 GPa and 1590 °C implying that the primitive melt inclusions in the olivine were trapped while still in the upper mantle and before further crustal assimilation. This primitive melt shows the steepest slope (La/Yb)PM, the highest Ni and is depleted in volatile components (168 ppm H2O, 264 ppm Cl, 258 ppm F and 114 ppm S according to secondary ion mass spectrometry measurements) compared to more evolved compositions, which were progressively enriched in volatiles up to 7665 ppm Н2О, 1728 ppm Cl, 2288 ppm S, and 549 ppm F as the rehomogenization temperature fell to 1335 °C. The progressive assimilation of crustal material responsible for magma modification is also manifested by the occurrence of CaNa volatile-rich minerals (such as amphibole, phlogopite, albite, sodalite, natrolite, pectolite, Cl-apatite and carbonate) in the interstitial assemblage and in “exotic” polyphase inclusions in cumulus olivine.The concentrations and ratios of the incompatible elements in the melt inclusions in Uitkomst olivine are similar to those in the model compositions of the ultramafic Bushveld parental magma and support a common source of ultramafic melts in the Bushveld Province. The main components of the melts, however, are not in accord with either boninitic or komatiitic lineages indicating a unique nature for the Bushveld magmas.

A 2058 Ma paleopole for the Kaapvaal Craton: Implications for late Rhyacian plate motion and duration of the Bushveld Complex

The Rustenburg Layered Suite (RLS) of the Bushveld Complex, the largest ultramafic–mafic-layered intrusion on Earth, is associated with satellite intrusions and marginal sills with which it shares a common parental heritage (i.e. the B1, B2 and B3 parental magmas). The duration of the RLS emplacement is regarded as a focused event of less than 1 million years. Some of the more primitive satellite intrusions and marginal sills have been interpreted to predate the main Bushveld Complex. The precision and extent of existing age data, however, have not allowed confirmation of the relative timing of their emplacement. This study provides paleomagnetic support of a longer period of emplacement for the RLS and its marginal sills and satellite intrusions. Stepwise demagnetization of marginal sills with B1-geochemical affinity sampled regionally and associated with the exposed western and eastern limbs of the RLS reveals stable near antipodal components with improved grouping upon structural restoration of sills to their paleo-horizontal. 40Ar/39Ar dating of one of the studied sills indicates a crystallization age of 2058.5 ± 7.9 Ma. Although this age overlaps with the precise 2056–2055 Ma ages from the RLS, it is similar to seemly older ages reported from the lowermost part of the RLS and from other Bushveld-related units (e.g. pyroxenitic sills and Uitkomst Complex) with B1 geochemical affinity. A mean paleopole (λ = 26.63°N; ϕ = 036.02°E; A95 = 8.80; 20 sites) was calculated from tilt-corrected new and previous data from the 2058 Ma B1-marginal sills and Uitkomst Complex. This pole, which is assumed to be primary, is statistically distinguishable from the existing 2056–2055 Ma Bushveld Complex paleopole. This suggests a small, but appreciable amount of plate motion separating punctuated events of magma emplacement in the Kaapvaal Craton during the lateRhyacian Period. Our results support at least two episodes of magma emplacement in the formation of the RLS and its associated marginal intrusions. These are separated in time by a few million years (i.e. a B1-type magma pulse around 2058 Ma and the B2 and B3-type magma pulses following closely on each other around 2056–2055 Ma).

Platinum-Group Elements and Gold in Base Metal Sulfides, Platinum-Group Minerals, and Re-Os Isotope Compositions of the Uitkomst Complex, South Africa

Platinum-Group Elements and Gold in Base Metal Sulfides, Platinum-Group Minerals, and Re-Os Isotope Compositions of the Uitkomst Complex, South Africa

New U–Pb age and paleomagnetic constraints from the Uitkomst Complex, South Africa: clues to the timing of intrusion

The Uitkomst Complex is considered to be coeval with, and genetically linked to the Rustenburg Layered Suite (RLS) of the Bushveld Large Igneous Province. This study reports new paleomagnetic, geochemical and geochronological results from the Uitkomst Complex and a crosscutting dyke at the Nkomati Mine. Primary magnetisations for the complex and the dyke are statistically indistinguishable. This, together with the geochemical signature of the dyke, suggests a late-magmatic link with the Uitkomst Complex. A Virtual Geomagnetic Pole calculated for the complex at Longitude 28.7°N and Latitude 58.5°E (dp = 6.2; dm = 9.4; N = 3) differs from the 1900 Ma and younger-aged poles for the Kaapvaal Craton. It, however, shares similarities with the poles from the 2058 to 2054 Ma RLS and other ~2.0 Ga paleopoles for the Kaapvaal Craton. Moreover, a new U–Pb baddeleyite age of 2054 ± 7 Ma given by a coarse-grained gabbroic sample from Nkomati’s underground mine provides a minimum age constraint on the crystallisation of the Uitkomst Complex. This date is nominally older than the 207Pb/206Pb zircon age of 2044 ± 8 Ma previously reported for this complex, and also near identical to the new age of 2054.89 ± 0.37 Ma from the Critical Zone of the RLS. Here, it is suggested that the Uitkomst Complex was emplaced at the same time as, if not before the Critical Zone. Data presented here have bearings on the location of the Uitkomst Complex in the time frame model recently proposed for the Bushveld magmatism.

Stable isotope, fluid inclusion, and mineral chemistry constraints on contamination and hydrothermal alteration in the Uitkomst Complex, South Africa

The Uitkomst Complex is a satellite intrusion of the Bushveld Complex and contains the only mine currently producing Ni as the main product in South Africa. Previously reported sulfur isotopic data suggest that externally derived sulfur was important for the generation of sulfide mineralization in harzburgite units that comprise the lower portion of the Complex. We have determined oxygen isotopic compositions of minerals and variations in pyroxenes and plagioclase compositions, and analyzed fluid inclusions in quartz from highly altered zones in order to evaluate the importance of country rock assimilation and hydrothermal processes in the evolution of the complex. Oxygen isotopic compositions of pyroxene and olivine from the sulfide-bearing harzburgite in the Complex range from 5.2 to 5.9‰ and suggest that bulk assimilation of high- 18O country rocks by parental magmas did not occur or was minimal. Sulfur transfer to magma was accomplished via a fluid which did not perturb the oxygen isotope composition of the magma. Harzburgite, pyroxenite, and gabbronorite that overlie the ore-bearing harzburgites contain minor amounts of sulfide minerals that are characterized by mantle-like δ 34S values of 0 ± 2‰. Low abundances of sulfides in these units are consistent with segregation of cotectic proportions. Hydrothermal alteration is pronounced in the upper and lower marginal zones of the Complex. Secondary quartz and albite are common in the upper portion of the Main Gabbronorite unit. Measurements of fluid inclusion in quartz indicate that alteration took place at ~ 500 °C and involved a moderately saline fluid of 20 to 25 eq. wt.% NaCl. Oxygen and hydrogen isotopes of secondary silicate minerals are most consistent with the involvement of an evolved, long path-length meteoric water that infiltrated the upper and lower contacts zones of the Complex.

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 Rietfontein Complex, the Koedoesfontein Complex, the Schoongezicht (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 (Fo 44 to Fo 80 ), orthopyroxene and plagioclase (An ), 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 element 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 Uitkomst 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 FeTiP 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(-ilmenite)±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.

Platinum Group Elements in the Uitkomst Complex, South Africa

The mafic-ultramafic Uitkomst Complex of South Africa is a tubular intrusion that is believed to be coeval and cogenetic with the 2054 Ma Bushveld Complex. It consists of a thin (3.5-m) basal gabbronoritic phase that is overlain by ca. 450 m of ultramafic rocks and 250 m of gabbroic and dioritic rocks. The basal 300 m of the intrusion hosts disseminated Ni-Cu-platinum group element (PGE) sulfides (100 Mt at ca. 0.55% Ni, 0.17% Cu). In addition, several lenses of massive sulfides (2.9 Mt at 2.04% Ni, 1.13% Cu, and 6 ppm total PGE) are situated in the immediate floor of the intrusion. We determined the PGE concentrations in 86 samples from the Uitkomst Complex. The rocks containing disseminated sulfides have up to 3 ppm total PGE and the massive sulfides have up to 7 ppm total PGE. Sulfide segregation was triggered by assimilation of external sulfur from the country rocks adjacent to the intrusion. The most important contaminant was probably dolomite of the Malmani Subgroup that includes sulfidic shale interlayers. The composition of the magmatic sulfides can be modeled by applying an R factor (mass ratio of silicate melt to sulfide melt) of between 800 and 1,000. The sulfides in the ultramafic rocks show little tendency of metal depletion with height and thus appear to have segregated from successive surges of fertile magma. This suggests that the basal portion of the Uitkomst Complex crystallized in an open magmatic system, e.g., a magma conduit. The gabbroic and dioritic rocks in the upper portion of the complex are relatively PGE depleted (Cu/Pd mostly higher than in primitive mantle), suggesting that they crystallized from the residual magmas to the ultramafic rocks in a closed magmatic system. Some of the ultramafic rocks in the central portion of the complex are S poor (mostly <700 ppb S) but contain relatively high amounts (30–250 ppb) of total PGE. These rocks have unusually low Pd/Ir (0.6–ca. 10), Cu/Zr (typically <0.5), and high Pt/Pd (up to 19), suggesting that either Pd, Cu, and S were remobilized or that the magma was undersaturated in sulfide liquid at the time the rocks formed and that platinum group minerals (PGM) crystallized. The present study suggests that sill-like bodies of Bushveld affinity that intruded dolomitic rocks may generally have an enhanced potential to host massive magmatic Ni-Cu-PGE sulfides.

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

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.

The Platinum Group Minerals in two parts of the Massive Sulphide Body of the Uitkomst Complex, Mpumalanga, South Africa

Platinum Group Elements (dominated by palladium) are present in the Ni-Cu-Co bearing massive sulphide ore of the Uitkomst Complex, Mpumalanga Province, South Africa. Mineralogical investigations were conducted in two different parts of the ore body and it was found that these elements are contained in fine-grained mineral phases that are largely attributed to five mineral types based on composition and optical properties. Most of these phases fall within the quaternary system Pd-Bi-Te-Sb. Mineral assemblages of this composition are generally regarded as indicative of relatively low temperature (probably less than 490°C) conditions for the formation of the associated massive sulphide ores. Although the mineral phases reported are similar, there are significant differences in the relative abundances of individual mineral phases between the two areas. The distribution of the individual phases recorded for the deeper part of the ore body indicates an increase in Type II (michenerite) grains towards the base of the zone sampled, an antipathetic relationship between Type I (merenskyite) and Type II (michenerite) phases, and a concentration of Type III (testibiopalladite) grains at the top and the base of the zone. The Platinum Group Mineral grains are preferentially associated with pyrrhotite and generally located along grain boundaries. The relative abundances of the Platinum Group Elements in the massive sulphide body are similar to that of other Ni-sulphide deposits such as Sudbury and Noril’sk, but differ significantly from that of the Merensky Reef, UG-2 Reef, and Platreef ore bodies of the Bushveld Complex.

PGE-bearing mafic-ultramafic sills in the floor of the eastern Bushveld Complex on the farms Blaauwboschkraal, Zwartkopje, and Waterval

Mafic-ultramafic sills of up to 450m in thickness occur near the contact between sedimentary and volcanic rocks of the Silverton Formation, Transvaal Supergroup, on the farms Blaauwboschkraal, Zwartkopje and Waterval, some 10km north of Waterval Boven in the Mpumalanga Province. The sills consist of peridotite, harzburgite, pyroxenite and gabbro, and may locally contain up to about 10% Ni-Cu-PGE sulphides. Metal contents of the rocks reach 0.7% Cu, 0.8% Ni, and 2 ppm PGE. Sulphides are found at variable levels within the intrusions and are interpreted to have precipitated from distinct surges of magma streaming through a conduit. This model is analogous to that proposed for the Uitkomst Complex, which hosts sulphides of broadly similar composition to the present bodies and which consists of broadly similar lithologies, apart from the presence of chromitite in the latter. The lithological and compositional similarities between the intrusions raise the possibility of undiscovered economic sulphide concentrations in the present bodies. However, olivines in most of the ultramafic rocks are undepleted in Ni, suggesting that either sulphide segregation was minor and localized, or that any metal-depleted magmas were flushed out of the conduit by undepleted magma.

The Platinum Group Minerals in two parts of the Massive Sulphide Body of the Uitkomst Complex, Mpumalanga, South Africa

Research Article| December 01, 2001 The Platinum Group Minerals in two parts of the Massive Sulphide Body of the Uitkomst Complex, Mpumalanga, South Africa H.F.J. Theart; H.F.J. Theart Department of Earth Sciences, University of Pretoria, Pretoria, South Africa, E-mail: htheart@postino.up.ac.za Search for other works by this author on: GSW Google Scholar C.D. de Nooy C.D. de Nooy Anglovaal Mining Limited, P O Box 1212, Florida, 1710, South Africa, E-mail: dorritn@avmin.co.za Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2001) 104 (4): 287–300. https://doi.org/10.2113/gssajg.104.4.287 Article history first online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation H.F.J. Theart, C.D. de Nooy; The Platinum Group Minerals in two parts of the Massive Sulphide Body of the Uitkomst Complex, Mpumalanga, South Africa. South African Journal of Geology 2001;; 104 (4): 287–300. doi: https://doi.org/10.2113/gssajg.104.4.287 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Platinum Group Elements (dominated by palladium) are present in the Ni-Cu-Co bearing massive sulphide ore of the Uitkomst Complex, Mpumalanga Province, South Africa. Mineralogical investigations were conducted in two different parts of the ore body and it was found that these elements are contained in fine-grained mineral phases that are largely attributed to five mineral types based on composition and optical properties. Most of these phases fall within the quaternary system Pd-Bi-Te-Sb. Mineral assemblages of this composition are generally regarded as indicative of relatively low temperature (probably less than 490°C) conditions for the formation of the associated massive sulphide ores. Although the mineral phases reported are similar, there are significant differences in the relative abundances of individual mineral phases between the two areas. The distribution of the individual phases recorded for the deeper part of the ore body indicates an increase in Type II (michenerite) grains towards the base of the zone sampled, an antipathetic relationship between Type I (merenskyite) and Type II (michenerite) phases, and a concentration of Type III (testibiopalladite) grains at the top and the base of the zone. The Platinum Group Mineral grains are preferentially associated with pyrrhotite and generally located along grain boundaries. The relative abundances of the Platinum Group Elements in the massive sulphide body are similar to that of other Ni-sulphide deposits such as Sudbury and Noril’sk, but differ significantly from that of the Merensky Reef, UG-2 Reef, and Platreef ore bodies of the Bushveld Complex. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

PARENTAL MAGMA AND EMPLACEMENT OF THE STRATIFORM UITKOMST COMPLEX, SOUTH AFRICA

The Uitkomst Complex, a Ni–Cu–PGE–Cr-mineralized layered basic intrusion situated approximately 200 km due east of Pretoria, South Africa, hosts commercially viable magmatic base-metal sulfide deposits. The Complex has a concordant 207Pb/206Pb zircon age of 2044 ± 8 Ma, suggesting that it is coeval with the Rustenburg Layered Suite (RLS) of the Bushveld Complex. Chemical modeling, using summation and least-squares optimization techniques, provides evidence that the boninitic Bushveld B1 magma is parental to both the lower ultrabasic and upper basic layered series of the Uitkomst Complex. The layered series crystallized in two stages, i.e., a lower conduit and an upper closed-system stage. The tubular shape of the Uitkomst Complex is the result of the intersection of a near-horizontal bedding plane fault with an existing vertical fracture zone under tensional conditions. During the conduit stage, a combination of magma mixing, contamination and flow dynamics may have facilitated sulfide formation and segregation. The identification of Bushveld B1 magma as the major parental magma of the Uitkomst Complex has significance in the exploration for similarly mineralized sub-RLS intrusive bodies.

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

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.

The age of the Marble Hall diorite, its relationship to the Uitkomst Complex, and evidence for a new magma type associated with the Bushveld igneous event

Mesocratic to leucocratic diorite intruded as sills in the deformed Marble Hall Fragment in the vicinity of Marble Hall, Mpumalanga Province, South Africa. A mesocratic variety gives a 207Pb/206Pb SHRIMP age on zircon of 2055.6 ±3.1 Ma. This age similar to that of the Bushveld Complex and the conduit-like Uitkomst Complex. Because the more primitive variety of Marble Hall diorite is chemically comparable with the chill-zone rocks of the Basal Gabbro of the Uitkomst Complex, a new magma type (Bu-type magma), associated with the Bushveld igneous event, at c. 2056 Ma, is postulated. The emplacement of this magma preceded the emplacement of the Bushveld B 1-type magma, and may also be parental or genetically related to the Lindequesdrift intrusion (in Malmani dolomite), the Roodekraal Complex, and the Rietfontein Complex in the Potchefstroom area. These findings reinstate the economic potential of all these sub-Rusteriburg Layered Suite intrusives.

Geophysical surveys at the Nkomati Mine, Mpumalanga, South Africa

Nkomati Mine, a joint venture between Anglovaal Mining and Anglo American, exploits a magmatic Ni-Cu-Co-PGE massivesulphide deposit associated with the Uitkomst Complex, inMpumalanga, South Africa. The mineralisation consists of flatlyingmassive sulphides, which are magnetic, dense andconductive. Geophysics has been used to assist the explorationprogram for further resources. Methods that were tried includesurface magnetic, gravity and controlled source audiomagnetotelluric (CSAMT) surveys, plus downhole electromagnetic(DHEM) and downhole magnetometric resistivity (DHMMR)surveys.The downhole methods were the most effective for deep targets. Because of the highly conductive nature of the sulphides, standardimpulse type time-domain DHEM did not give good results untiltransformed into step-type data. However, strong responses wereobtained from frequency-domain measurements. Since all of theholes were vertical, no useful cross component data could becollected and the location of the sources suffered from radialsymmetry. In contrast, the DHMMR was able to resolve on whichside of the hole the conductors lay.CSAMT successfully defined the disseminated and massivemineralisation at depth. Surface magnetic and gravity surveys wereuseful for outlining relatively shallow mineralisation outside of themain complex. Qualitative interpretation, and modelling of thegravity and magnetic data, showed a good correlation with theCSAMT results.

Geology of the ultrabasic to basic Uitkomst complex, eastern Transvaal, South Africa: an overview

The Uitkomst complex in eastern Transvaal, South Africa, is a mineralized, layered ultrabasic to basic intrusion of Bushveld complex age (2.05–2.06 Ga) that intruded into the sedimentary rocks of the Lower Transvaal Supergroup. The complex is situated 20 km north of Badplaas. It is elongated in a northwesterly direction and is exposed over a total distance of 9 km. The intrusion is interpreted to have an anvil-shaped cross-section with a true thickness of approximately 800 m and is enveloped by metamorphosed and, in places, brecciated country rocks. Post-Bushveld diabase intrusions caused considerable vertical dilation of teh complex. The complex consists of six lithological units (from bottom to top): Basal Gabbro, Lower Harzburgite, Chromitiferous Harzburgite, Main Harzburgite, Pyroxenite and Gabbronorite. The Basal Gabbro Unit, developed at the base of the intrusion and showing a narrow chilled margin of 0.2 to 1.5 m against the floor rocks, has an average thickness of 6 m and grades upwards into the sulphide-rich and xenolith-bearing sequence of the Lower Harzburgite Unit. The latter unit averages 50 m in thickness and is gradationally overlain by the chromite-rich harzburgite of the Chromitiferous Harzburgite Unit (average thickness 60 m). Following on from the Chromitiferous Harzburgite Unit is the 330 m thick Main Harzburgite Unit. The Pyroxenite and Gabbronorite Units (total combined thickness of 310 m) form the uppermost formations of the intrusion. The three lower lithological units, Basal Gabbro to Chromitiferous Harzburgite, are highly altered by late magmatic, hydrothermal processes causing widespread serpentinization, steatitization, saussuritization and uralitization. Field relations, petrography and mineral and whole rock chemistry suggest the following sequence of events, The original emplacement of magma took place from northwest to southeast. The intrusion was bounded between two major fracture zones that gave rise to an elongated body, which acted as a conduit for later magma heaves. The first magma pulses, forming the chilled margin of the intrusion, show chemical affinities to a micropyroxenite described from the Bushveld complex. The Lower Harzburgite and Chromitiferous Harzburgite Units, judged from the abundance of xenoliths, originated by crystal settling from a contaminated basic magma. The Main Harzburgite crystallized from a magma of constant, probably also basic, composition, which flowed through the conduit after formation of the lower three lithological units. At a late stage of emplacement, after replenishment in the conduit came to a standstill, closed system conditions developed in the upper part of the complex, resulting in a magma fractionation trend of increasing incompatible elements contents towards the top of the intrusion. The mineralization in the lower three rock units and at the base was most probably caused by a segregating sulphide liquid forced to precipitate by the oxidative degassing of dolomite. Sulphur isotope ratios indicate various degrees of contamination of the magma by the enveloping sedimentary rocks, which provided the necessary amounts of S to reach S saturation.