R. Grant Cawthorn

Sills associated with the Bushveld Complex, South Africa: an estimate of the parental magma composition

Abstract Four distinct ages of sills intrude the floor rocks under the Bushveld Complex. The first is a metadolerite and pre-dates the Bushveld Complex. The middle two are generally related to the Complex itself. One has quench-textured olivine and orthopyroxene crystals and chemically equates to the parental magma of the Bushveld Complex. The second suite is a hypersthene microgabbro, representative of the second major injection of magma into the Complex. The last is doleritic and does not obviously correlate with any portion of the layered sequence. The sills genetically related to the Complex display several features 1. -lack of chilled margins, coarse grain size, ability to melt country rock xenoliths, extreme heterogeneity 2. -atypical of normal sills, but explicable in their present context.

Gravity modeling of Bushveld Complex connectivity supported by Southern African Seismic Experiment results

Recent gravity modelling of the Bushveld Complex indicates that the western and eastern limbs of the Bushveld Complex are connected at depth. The model predicts a downwarp in the Moho beneath the Bushveld Complex, ensuring observed Airy isostatic balance is achieved. By constraining a new Bouguer gravity model with published Vibroseis results, crustal thicknesses determined using the receiver function method, and seismic velocity modelling of the crust from the Southern African Seismic Experiment; we demonstrate that the connected model of the Bushveld Complex is consistent with all available data. Crustal thicknesses determined from receiver functions indicate that the depth to the Moho thickens from a value of ~35 to ~40km in the southern Kaapvaal craton to ~50km beneath the central region of the Bushveld Complex. This seismologically determined Moho varies significantly from that calculated from Airy isostatic balance based solely on topography as a load in this region. The corresponding crustal velocity model, determined from inverting the receiver function results for Bushveld Complex stations, also indicates a thick crust and delimits a ~6km thick high velocity zone in the upper 10km of crust attributed to the presence of the Bushveld Complex. Comparison of the seismic crustal model with drill core data on the mafic rocks of the Bushveld Complex suggests a correspondence between high seismic velocities and high densities in the upper crust. Both the gravity model and the seismological results imply a density contrast of about 0.30mg/m3 at the crust/mantle boundary beneath the Bushveld Complex. We also find that the Moho transition beneath the Bushveld Complex is significantly broader than that beneath the rest of the Kaapvaal craton, outside of the Limpopo Belt. By constraining the modelling of the gravity data with these seismological results, outcropping geology and published Vibroseis profiles, we show that the dense mafic units of the western and eastern Bushveld Complex can be interpreted as having originally been emplaced as a connected sheet (or series of connected sheets), which has subsequently been deformed and faulted. The seismological results of the Kaapvaal project support the interpretation of a connected Bushveld Complex.

The Platinum Group Element Deposits of the Bushveld Complex in South Africa

School of Geosciences, University of the Witwatersrand, Private Bag 3, Johannesburg 2050, South Africa; E-mail: grant.cawthorn@wits.ac.za There are enough platinum group element deposits in the Bushveld Complex in South Africa to supply world demands for many decades or even a century using current mining techniques. Demonstrated reserves and resources published by mining companies make detailed calculations up to a maximum of about twenty years ahead, but there is abundant and adequate geological evidence that these deposits continue far beyond where mining companies have proven according to rigorous international reporting codes. For each 1 km of depth into the Earth in the Bushveld Complex there is in the order of 350 million oz of platinum. For comparison, annual production of platinum from the Bushveld Complex currently is only around 5 million oz. The distinction between ‘reserves’,‘resources’ and ‘deposits’ is also explained in this article.

The Bushveld Complex, South Africa

The mafic rocks of the Bushveld Complex, South Africa, were emplaced into a stable cratonic shield some 2.06 b.y. ago, and have remained remarkably well preserved from deformation, metamorphism and low temperature alteration, at least partly by its isostatic impact on the entire crustal thickness. The generation and emplacement of possibly 1 million km3 of magma within 65,000 years and the lateral continuity of layering (for up to 100 km) remain intriguing challenges to understanding the evolution of this igneous body. The intrusion is exposed as a three-lobed body, up to 7 km thick, with inward dipping layers that range from dunite to monzonite. Platinum-group element-rich orthopyroxenite, chromitite and vanadiferous magnetitite layers contain vast proportions of the World’s deposits of these commodities. Modal layering, on scales from mm to tens of m, ranges from well-developed in some vertical sections to virtually absent in others. Distinctive layers (ranging from mm to many tens of m) can be identified in two or even three lobes testifying to their connectivity. Feeders to the intrusion cannot be identified, and the exact compositions (and numbers) of the parental magmas are still debated. Rates and effectiveness of their mixing also require resolution. Models of magma additions and extents of mixing lead to very conflicting interpretations in terms of rapidity and vertical extents of the sequence affected. As the largest known mafic intrusion it represents an end-member in terms of magmatic chamber processes.

Evidence for the trapped liquid shift effect in the Mount Ayliff Intrusion, South Africa

Postcumulus trapped liquid shift in layered complexes produces cumulate minerals with more fractionated compositions than the original primary phases. This effect is shown by olivine compositions from the base of the Mount Ayliff Intrusion, where varying proportions of olivine to interstitial liquid produce a suite of rocks which define a tight linear trend on a binary whole-rock plot of MgO versus FeO. Extrapolation of this trend constrains the composition of the primary cumulus olivine to the range Fo84–86, whereas olivine actually present have compositions Fo77–83. The magnitude of the discrepancy between the theoretical and observed olivine compositions correlates directly with the weight fraction of interstitial liquid. These observations are quantitatively predicted by the trapped liquid shift model. They also argue against significant migration of residual liquid. Trapped liquid shift is documented over a vertical interval of 60 m. It occurred in rocks lying only 1 m above the basal contact of the intrusion and hence must be a comparatively rapid process.

Crystallization of titaniferous chromite, magnesian ilmenite and armalcolite in tholeiitic suites in the Karoo Igneous Province

Titaniferous chromite (up to 8 wt% TiO2) and magnesian ilmenite (up to 10 wt% MgO) coexist at the base of the differentiated tholeiitic Mount Ayliff Intrusion in the Karoo Province of southern Africa, suggesting that the original magma was TiO2-rich. Picritic lavas with 3% TiO2 from the Lebombo monocline of the Karoo Province also contain microphenocrysts of magnesian ilmenite (up to 6 wt% mgO) and armalcolite (up to 7 wt% MgO). These oxide mineral associations and compositions are atypical of tholeiitic magmas, in which chromite usually has less than 1 wt% TiO2, ilmenite less than 3 wt% MgO and armalcolite is rarely a primary mineral. Experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of TiO2 on the crystallization and composition of chromite, ilmenite and armalcolite. The results indicate that increasing the TiO2 content of picritic magmas increases the TiO2 content of the spinel, mainly at the expense of Al2O3, whereas Cr2O3 is not affected. Spinel compositions in the Mount Ayliff Intrusion (with over 45 wt% Cr2O3, less than 10 wt% Al2O3 and 8 wt% TiO2) were duplicated in experiments on a picrite at temperatures of about 1,200°C at the Ni/NiO buffer. Increasing fO2 from fayalite-magnetite-quartz to Ni/NiO buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite. The total FeO content of the liquid has little influence on the crystallization temperature of these phases. The TiO2 content of the liquid, when either ilmenite or armalcolite crystallizes, varies inversely with SiO2 content. The MgO content of the liquid at which ilmenite or armalcolite crystallizes depends upon the TiO2 content of the starting composition, with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt% MgO and 5.5 wt% TiO2. The partition coefficent for MgO between armalcolite or ilmenite and liquid is about 1.5. Observed magnesian armalcolite and ilmenite compositions in picrite lavas (both minerals) and in the Mount Ayliff Intrusion (ilmenite only) are consistent with crystallization from a TiO2-rich magma with approximately 5 wt% MgO. The Fe23+TiO5 component of armalcolite in the picrite lavas matches those formed experimentally at temperatures of 1,150–1,110°C and fO2 of the Ni/NiO to Ni/NiO+1 log unit. Similarities also exist between the compositions of chromite, ilmenite and armalcolite and liquid fraction-ation trends of some Hawaiian high-TiO2 lavas and the experimental studies presented here.

The role of intercumulus residua in the formation of pegmatoid associated with the UG2 chromitite, Bushveld Complex

The UG2 chromitite in the Critical Zone of the Bushveld Complex is underlain by pyroxenite, the uppermost 50 cm of which is a pegmatoidal pyroxenite. Overlying it is an olivine pyroxenite. Concentrations of K2O, Ba, Zr and P2O5 show no enrichment in pegmatoid relative to over‐ and underlying silicate rocks. Whole rock Mg/(Mg+Fe) ratios and normative plagioclase compositions show no systematic differences between coarsegrained and adjacent finer‐grained lithologies. Potholes are associated with the UG2 but no geographic or structural relationships exist between UG2 and Merensky potholes. Comparison of geochemical profiles through normal and potholed UG2 chromitite show no evidence of fluid channelling as has been documented through Merensky potholes. These observations do not support the hypotheses of preferential trapping of, or reaction with, residual liquid or fluid at the magmatic stage in these pegmatoidal horizons or in the formation of potholes. Injection of hot primitive magma is indicated by the f...

The geochemistry of vanadiferous magnetite in the bushveld complex: Implications for crystallization mechanisms in layered complexes

Detailed trace-element analyses of pure magnetite from four continuous borehole intersections through the main magnetitite layer from the upper zone of the Rustenburg Layered Suite of the Bushveld Complex are presented. One section has been analysed at one centimetre intervals. Rapid depletion of Cr occurs over short, vertical sequences near the base of the layer, which is due to bottom-crystallization and the resulting chemical depletion of a thin layer of liquid. Sudden increases in Cr content of magnetite are attributed to convection cells which bring undepleted magma into the zone or crystallization. We suggest that these cells individually have lateral extents no greater than hundreds of metres, but collectively may be traced at a specific stratigraphic horizon for several tens of km. This lateral traceability of the effect of convection cells at approximately uniform stratigraphic height demonstrates the long-held implicit assumption that time-planes are in general parallel to the layering, and does not support the hypothesis that layers in the Bushveld Complex grew laterally. The activity of these cells is highly variable, with long periods of quiescence interspersed with periods of rapid, small-scale overturn. Most convection cells do not impinge upon the floor, and the abruptness of the resulting chemical reversal is largely a function of the thickness of the layer of depleted liquid trapped between the cumulates and the sole of the convection cell. Occasionally, these cells do touch the cumulate pile and may even cause erosion. This material may be redeposited elsewhere in the magnetitite layer either as mineralogically distinct fragments if erosion penetrated below the layer or, in the present instance, as a chemically chaotic pile of magnetite. The abruptness of the chemical reversals severely restricts the extent to which post-cumulus redistribution of elements or re-equilibration with percolating trapped liquid (infiltration metasomatism) may have occurred. The appearance of disseminated plagioclase in magnetite layers in variable proportions and in a non-systematic manner in the four profiles is attributed to fluctuations in pressure.

Magma addition and possible decoupling of major- and trace-element behaviour in the Bushveld Complex, South Africa

Abstract Incompatible trace-element abundances and ratios in cumulate rocks from the Bushveld Complex may be used to predict the levels at which new magmas may be injected. Sudden and sustained changes in absolute abundances suggest new magma addition while gradual increases are consistent with fractionation of a large body of magma. The present data suggest input above the Merensky Reef at the top of the critical zone. Near the top of the lower and main zones there may be other additions. However, in the latter case, the chemical effect is spread through a large vertical interval and suggests very slow diffusive mixing between new and residual liquids. The different behavioral patterns may be related to whether new magma intrudes above or below the residual magma, the former situation resulting in delays between timing of injection and significant influence in the chemistry of the cumulates.

Cooling of the Bushveld Complex, South Africa: Implications for paleomagnetic reversals

Igneous rocks record the direction of the Earth’s magnetic field as they cool through their Curie temperature. The mafic magmas of the 8-km-thick Bushveld Complex of South Africa took 65 k.y. to be emplaced, 180 k.y. to solidify (to 900 °C), and a further 500 k.y. for the entire intrusion to cool below 580 °C, the Curie temperature of magnetite. Once solid, the cooling of this intrusion occurred mainly from the top downward, with slower cooling through its floor. As a result, the upper rocks cooled through their Curie temperature before those at the base; the portion 6 km below the upper contact was the last to reach the Curie temperature. Thus, the intrusion records a mainly top-down sequence of three paleomagnetic reversals starting with N (normal direction). The last two are also recorded from the base of the mafic sequence upward as it cooled through 580 °C later than the top. The lateral variations in thickness of the Bushveld Complex are important in this interpretation, because thinner sections cooled more quickly. Hence, reversals do not always correlate with stratigraphy. Specific reversals provide a cooling marker horizon that may crosscut the stratigraphic layering. The interpretation of the order and number of paleomagnetic reversals presented here differs from previous interpretations that envisage the oldest paleomagnetic directions to be recorded sequentially from the base upward, and has implications for the interpretation of paleomagnetic results from all thick intrusions, mafic and felsic.