Browsing by Author "Koegelenberg, Corne"
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- ItemExperimental evidence for sulphide magma percolation and evolution : relevant to the chromite bearing reefs of the Bushveld Complex(Stellenbosch : Stellenbosch University, 2012-03) Koegelenberg, Corne; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Pt mineralization within the Bushveld Complex is strikingly focused on the chromitite reefs, despite these horizons being associated with low volumes of base metal sulphide relative to Pt grade. Partitioning of Pt (Dsil/sulp) from silicate magma into immiscible sulphide liquid appears unable to explain Pt concentrations in chromitite horizons, due to the mismatch that exists between very large R factor required and the relevant silicate rock volume. Consequently, in this experimental study we attempt to gain better insight into possible Pt grade enhancement processes that may occur with the Bushveld Complex (BC) sulphide magma. We investigate the wetting properties of sulphide melt relevant to chromite and silicate minerals, as this is a key parameter controlling sulphide liquid percolation through the cumulate pile. Additionally, we have investigated how fractionation of the sulphide liquid from mono-sulphide-solid-solution (Mss) crystals formed within the overlying melanorite might affect sulphide composition and Pt grades within the evolved sulphide melt. Two sets of experiments were conducted: Firstly, at 1 atm to investigate the phase relations between 900OC and 1150OC, within Pt-bearing sulphide magma relevant to the BC; Secondly, at 4 kbar, between 900OC to 1050OC, which investigated the downwards percolation of sulphide magma through several layers of silicate (melanorite) and chromitite. In addition, 1atm experiments were conducted within a chromite dominated chromite-sulphide mixture to test if interaction with chromite affects the sulphide system by ether adding or removing Fe2+. Primary observations are as follows: We found sulphide liquid to be extremely mobile, the median dihedral angles between sulphide melt and the minerals of chromitite and silicate layers are 11O and 33O respectively. This is far below the percolation threshold of 60O for natural geological systems. In silicate layers sulphide liquid forms vertical melt networks promoting percolation. In contrast, the extremely effective wetting of sulphide liquid in chromitites restricts sulphide percolation. Inter-granular capillary forces increase melt retention, thus chromitites serve as a reservoir for sulphide melt. Sulphide liquid preferentially leaches Fe2+ from chromite, increasing the Fe concentration of the sulphide liquid. The reacted chromite rims are enriched in spinel end-member. This addition of Fe2+ to the sulphide magma prompts crystallization Fe-rich Mss, decreasing the S-content of sulphide melt. This lowers Pt solubility and leads to the formation of Pt alloys within the chromitite layer. Eventually, Cu-rich sulphide melt escapes through the bottom of the chromitite layer. These observations appear directly applicable to the mineralized chromitite reefs of the Bushveld complex. We propose that sulphide magma, potentially injected from the mantle with new silicate magma injections, percolated through the silicate cumulate overlying the chromitite and crystallized a significant volume of Fe-Mss. Chromitite layers functioned as traps for percolating, evolved, Cu-, Ni- and Pt-rich sulphide liquids. This is supported by the common phenomenon that chromitites contain higher percentages of Ni, Cu and Pt relative to hanging wall silicate layers. When in contact with chromite, sulphide melt is forced to crystallize Mss as it leaches Fe2+ from the chromite, thereby further lowering the S-content of the melt. This results in precipitation, as Pt alloys, of a large proportion of the Pt dissolved in the sulphide melt. In combination, these processes explain why chromitite reefs in the Bushveld Complex have Pt/S ratios are up to an order of magnitude higher that adjacent melanorite layers.
- ItemGeology, structural evolution and controls of hydrothermal gold mineralization in the Eastern Karagwe-Ankole fold belt, North Western Tanzania(Stellenbosch : Stellenbosch University, 2016-03) Koegelenberg, Corne; Kisters, Alexander F. M.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: In central-east Africa, the north-western margin of the Archaean Tanzania Craton (TC) is overlain by imbricated, low grade, volcano-sedimentary rocks of the Karagwe-Ankole belt’s (KAB) Eastern Domain (ED). Centred in the ED, vast stretches of a sheared and Au mineralized basement-cover contact are exposed along margins of the Mugera-Nyakahura (MN) inlier. To date, no detail research has been done in the area and the regional geology has been described from only broad reconnaissance studies. As part of an exploration project the first high resolution geological maps of key prospects and larger, more encompassing, scale maps of the basement-cover region was compiled. Mapping was supplemented by a regional scale structural traverse of the ED and selected sampling for analysis of micro-structures, geochronology and oxygen isotopes. The collective structural data has indicated that basement gneisses of the MN inlier may be considered as part of a forethrusted tectonic wedge caused by regional top-to-the-SE thick skinned thrusting. Above and in front of the wedge diagnostic back-thrusts and the reversal of kinematic fabrics in weak, often graphitic, metapelitic rocks of the Muyaga Group depict top-to-the-NW, hinterland-directed, tectonic transport along the main “roof” detachment. To the east under-thrusting of the coarse clastic Bukoba Group by the Muyaga Group has also created a distinct triangle zone at the frontal termination of the KAB. 39Ar-40Ar muscovite ages of detachment mylonites in the easternmost, and latest developing, parts of the KAB has constrained timing of D2 to at least 1326 ± 10 Ma. This age corresponds with the youngest end of the main phase of granite plutonism and mafic dyke emplacement in the KAB (1380 – 1328 Ma) and may point towards a Mesoproterozoic collisional event between the Congo- and Tanzania Cratons. U-Pb detrital zircon ages of the Muyaga- and Bukoba Groups have indicated uplift, erosion and subsequent reworking of Muyaga Group sediments and layered volcanics into the Bukoba basin after 1780 Ma, but before 1568 Ma. The Bukoba Group thus correlates with the Bwezigoro Group in SW Uganda, indicating the presence of an extensive Paleo- and/or Mesoproterozoic foreland basin overlying the western margin of the TC and Uganda Block. Lastly, controls of D2 fluid flow and mineralization along the low angle phyllonitic detachment are linked to NE trending ramp structures that were most favourable for the initiation of slip and development auriferous quartz vein networks. Upwards into the Muyaga Group progressive fold amplification and eventual fold-lock of second order anticlines, cored by competent and chemically re-active ferruginous mafic sills, are responsible for the late-kinematic development of auriferous quartz veins. Oxygen isotope values of D2 quartz veins and host rocks have indicated that fluids are derived from dehydrated clastic sediments of the Kagera Supergroup and, as such, may suggest that gold associated with greenstones of the TC have not been remobilized during D2 fold-and-thrust development in the ED. Collectively these findings greatly enhance the understanding of the geological evolution of the KAB’s easternmost parts and provides future research and exploration with a much improved geological background.