3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
7 results
Search Results
Item The role of sulphur in siderophile and chalcophile element enrichment of the lithospheric mantle: constraints from Kaapvaal mantle xenoliths and high-pressure partial melting experiments of S±C-bearing eclogite(2019) Burness, SaraSulphur- and carbon-bearing melts play an important role as metasomatic agents within the cratonic mantle lithosphere. While carbon-bearing melt metasomatism has been linked to various lithosphere enrichment processes including, diamond formation and kimberlite magmatism as part of the deep carbon cycle, information about the behaviour of sulphur during metasomatism and partial melting is less well constrained. Sulphide minerals predominantly host the mantle platinum group element (PGE), siderophile and chalcophile element budgets, and their behaviour during melting controls the movement of these economically important metals. Here I present a case study of eclogite and peridotite metasomatism by sulphur±carbon-bearing fluids/melts by comparing new experimental work with geochemical, stable isotopic and petrologic investigations of sulphide-bearing eclogite and peridotite xenoliths from several key kimberlite occurrences on the Kaapvaal craton, South Africa. The experiments conducted from 2 to 3.5 GPa and 1050 to 1300 ºC on sulphur±carbon-bearing eclogite compositions demonstrate that the critical factors concerning the mobility of sulphur in upper mantle eclogite are the degree of partial melting and the compositions of the resulting melts. Pure sulphide melt is shown to be largely immobile within the MORB-like eclogite system unless aided by more fluid-mobile carbonate±silicate melts at these upper mantle conditions. Sulphur is effectively mobilised as immiscible sulphide liquid melt pools within CO2-free basaltic-andesite melts and as dissolved elemental sulphur within intermediate carbonate-silicate melts at >15 % partial melting of the mixed volatile-bearing eclogite compositions. Comparative investigations of eclogite and peridotite xenoliths show that the metasomatic precipitation of secondary sulphide is intimately linked to metasomatism of the host silicate assemblage. The sulphide minerals are exceptionally redox sensitive and contain variable PGE contents with characteristic I-PGE/P-PGE fractionations, which are controlled by the compositional character of the metasomatic melt/fluid. Specifically, the xenoliths investigated here show evidence for several contrasting depth-dependent, and sometimes location-specific, metasomatic events which may have affected both the Kaapvaal cratonic lithosphere as well as the craton above. Moreover, this study demonstrates that metasomatic enrichment of the cratonic lithosphere by sulphur±carbon-bearing fluids/melts has resulted in isotopic heterogeneities and the addition of new minerals such as sulphide and potentially also diamond since the Archean.Item Application of 3D seismics to enhance mapping of potholes in the western Bushveld Complex, South Africa(2019) Sehoole, Lebogang Tshepiso CharmaineThe ~2.06 Ga Bushveld Complex in South Africa is widely known as the world’s largest layered igneous intrusion. The Bushveld Complex hosts the world's largest platinum and chromium deposits. The two of the major economic platinum-bearing ore bodies (known as reefs) are the Merensky and the Upper Group-2 (UG-2) reefs, which are located at depths between 500 m and 1.5 km below surface. Mining operations on these horizons are often complicated by the presence of geological features such as faults, dykes, potholes and iron-rich ultramafic pegmatite (IRUP) bodies. This emphasises the importance of characterising these features and predicting their occurrence. In this study, the 3D reflection seismic method is employed to delineate these structures (mainly potholes) within the Western Bushveld Complex, South Africa. Potholes are slump structures that disturb mining processes and ultimately decrease the mining productivity. Various state-of-the-art techniques were used to identify and analyse potholes, including (i) horizon-based seismic attributes, (ii) complex-trace attributes, (iii) 3D volumetric attributes, (iv) difference-of-two-surfaces, and (v) geostatistical methods. The seismic techniques complimented each other in detecting and identifying approximately 43 potholes, which were then used for geostatistical analysis. The results suggest that pothole structures are asymmetric and are often associated with faults. This study also reveals that a majority of the potholes imaged in the seismic data have propagated between the Merensky Reef and the UG-2 levels, i.e., MR and UG-2 are affected by the same potholes. These potholes are randomly distributed and exhibit some clustering. A k-mean cluster analysis was implemented and revealed two clusters that are present in the study area. The various hypotheses on the formation of potholes are investigated. This study rules out some of the hypotheses for the formation of potholes and proposed that the potholes might have formed from multiple mechanisms as opposed to just one. Statistical analyses suggest a positive linear relationship between the pothole depth and the pothole diameter, indicating that the pothole may have grown in width and depth at the same time.Item Neoproterozoic metasediments and base metal mineralisation in an area north of the Mpande Dome in southern Zambia: an alternative lithostratigraphy classification(2018) King, Heather LeslieProspecting in 2013 to 2015 identified rift-related metasediments in the Mpande Dome area, south of Lusaka, Zambia. The metasediments were thought to be comparable to the Roan Group of the Katanga Supergroup within the Zambian Copperbelt and therefore prospective as a target for sediment-hosted, stratiform copper mineralisation. The metasediments of this area, known as Shantumbu, were last actively explored in the 1930 to 1960s’. Shantumbu is approximately 30km south southeast of Lusaka, Zambia and lies on the northern margin of the Mpande Dome which is south of the Mwembeshi Zone. The Shantumbu area is a highly tectonised region as it is the convergence point of the Zambezi Belt, the Lufilian Arc and Irumide Belt. Simpson et al. (1963), Smith (1964), and Mallick (1966), classified the metasediments as belonging to the Chunga and Cheta groups (Smith, 1964) or the Shamazio, Kafue and Cheta groups (Mallick, 1966), of the Katanga Supergroup. In 2000, Porada and Berhorst proposed correlation of the stratigraphy with the Katanga Supergroup in the Zambian Copperbelt. For many decades, the lack of outcrop and exploration information and the effect of Pan African deformation and high-grade metamorphism, impeded prospecting. The recent exploration activities on Shantumbu, which included logging of cores, petrography, geochemical and mineralogy, highlighted a fresh examination of the metasediments and the copper mineralisation in this area were required. This is the first detailed information in recent years on the metasediments around the Mpande Dome. The result of the research has showed the adoption of the Roan Group stratigraphy of the Katanga Supergroup, as it occurs in the Zambian Copperbelt, is proposed rather than the nomenclature for metasediments forming part of the Zambezi Supracrustal Sequence. An alternative stratigraphic classification to the nomenclature of the Chunga and Cheta formation has been raised, indicating the metasediments north of the Mpande Dome are the southernmost limit of the Roan Group in Zambia. It therefore appears the Proterozoic rift related terrigenous and marine sediments of the Katanga Supergroup are continuous from the Central African Copperbelt southward to the Kafue area and Shantumbu. The metasediments at Shantumbu progress from arenites and siltstones above what is interpreted to be Kafue Rhyolite and Nazingwe metavolcanics. The metavolcanics cap the alkali granites and gneisses of the Basement. The proportion of arenites deposited diminished and gave way to siltstone-dominated units, which in turn gave way to carbonate and lesser siltstone units. A marine transgression surface terminated the detrital units. The siliciclastic units below the transgression surface correlate to the Mindola Formation. The dolomitic carbonate and arenite units correlate with the Copperbelt Orebody Member of the Kitwe Formation, Roan Group. Carbonates, and lesser siltstone and arenite units, deposited in a near-shore marine environment overlie the Copperbelt Orebody Member, and correlated with the Kitwe Formation. The youngest succession identified on Shantumbu comprised calcitic carbonates deposited in a marine basin progressively starved of terrestrial sediments. The carbonates are correlated with the Bancroft Member of the Kirilabombwe Formation. Petrographic examination of the drill core, together with mineralogy studies and geochemical analyses, have resulted in a contemporary account of the rift basin architecture in the Shantumbu area to be presented. New insight into the depositional environments related to rift initiation, climax and cessation, sulphide mineralisation, and oxic and anoxic conditions during deposition, were obtained. The geochemical characteristics and mineralogy indicate the metasediments were subjected to a complex range of alteration and metasomatism. The alteration mineralogy and presence of minerals such as glaucophane, epidote, albite and sphene, indicated the area was subjected to low-temperature (250 - 400oC) and high-pressure metamorphism, that of epidote-amphibolite facies, and retrograde metamorphosed to greenschist facies. Potassic and sodic alteration is characteristic of the detrital lower successions and the upper carbonate successions respectively. Alteration caused by diagenesis-compaction, dewatering of the terrestrial sediments, and later stage remobilisation of potassic and sodium-bearing fluids. Deformation of the area was related to the Pan African Orogeny. Mineralisation on Shantumbu is associated with arenites capped by siltstone near the first occurrences of dolomitic carbonate in the stratigraphy. Most copper deposits in the Zambian Copperbelt are found within 200m of a marine transgression surface, within and adjacent to the Copperbelt Orebody Member. The formation of stratabound base metal sulphides in the lower detrital sequences versus the lack of base metal sulphides in the marine-dominated sequences, suggests the base metals were sourced from the Basement units and transported via basinal brines, rather than introduced from a magmatic origin. The copper sulphides intersected in drill core at Shantumbu, and the copper oxides which crops out, are comparable to the sediment-hosted, stratiform copper deposits of the Zambian Copperbelt, and other sediment-hosted, stratiform copper occurrences globally, such as White Pine in Michigan, the Zechstein Basin in Europe, Spar Lake in Montana, the copper occurrences in Namibia and Botswana on the Kalahari Craton, and the Mangula and Mhangura deposits in Zimbabwe. The continuation of the metasediments from Shantumbu north through the Chongwe Copperbelt and the southerly termination of the Roan Group on the northern margin of the Mwembeshi Zone, strengthens the argument that Shantumbu is contiguous with the Roan Group of the Katanga Basin. The age of the Kafue Rhyolite and Nazingwe formations in the Mpande Dome region, have been age dated at 879 ± 19 Ma (Hanson et al., 1994; Selley et al., 2005; Johnson et al., 2007), which has provided further support that Shantumbu is part of the Neoproterozoic Katanga Basin system, rather than the older rift systems to the south of Zambia, such as the Zambezi Belt or the Magondi Mobile Belt. The examination of the metasediments on Shantumbu has shown that further research would advance the understanding of the rift basin stratigraphy and related sediment-hosted, stratiform copper mineralisation. Such research topics include the investigation of the presence of metavolcanics on the northern margin of the Mpande Dome and the Kafue Rhyolite and Nazingwe Formation hosting copper sulphides, and a comparison and correlation, or otherwise, of the metasediments comprising the Chongwe Copperbelt to the Shantumbu metasediments.Item The Waterberg platinum-group element deposit, South Africa(2018) McCreesh, Matthew James GerardThe Waterberg platinum-group element deposit of the Bushveld Complex is located within the Southern Marginal Zone of the Limpopo Mobile Belt, north of the Hout River Shear Zone. The Bushveld succession is approximately 1.2 km thick, and dips 34o to 38o towards the northwest. The succession is composed of basal Marginal sills, an Ultramafic Sequence, a troctolite-gabbronorite-anorthosite sequence, and an Upper Zone. The overlying Waterberg Group was deposited between ~2.05 to ~1.95 Ma, after extensive erosion of the roof rocks and the Upper Zone and before the intrusion of a dolerite sill associated with the ~1.1 Ga Umkondo Igneous Province. The magmatic history of the Waterberg deposit stratigraphy is believed to have started with the intrusion of the basal Marginal sill composed of fine-grained pyroxenite and gabbronorite similar to the Marginal Zone lithologies elsewhere in the Bushveld Complex. These sill-like intrusions interacted with the granitic gneiss footwall of the Southern Marginal Zone to form a basal granofels unit. The overlying Ultramafic Sequence is the most primitive unit of the Waterberg stratigraphy, and hosts the lower F zone mineralisation. This sequence is dominated by a basal orthopyroxenite, overlain by serpentinised harzburgite. Chromite occurs as disseminated clusters with rare discontinuous chromitite stringers. The overlying troctolite-gabbronorite-anorthosite sequence forms a magmatic erosional contact with the underlying Ultramafic Sequence. The basal troctolite is interpreted to have formed from magma mixing between the residual melt of the harzburgite and the influx of a gabbroic melts. The troctolite grades upwards into gabbronorite and anorthosite with sporadic occurrences of inverted pigeonite, which are typical characteristics of the Main Zone elsewhere in the Bushveld Complex. The top contact of this sequence is associated with the upper T zone mineralisation and the occurrence of cumulus magnetite, an indicator mineral of the Upper Zone elsewhere in the Bushveld Complex. The Upper Zone is composed of disseminated cumulus magnetite, however it lacks the magnetitite layers seen in the rest of the Bushveld Complex. The two PGE-Cu-Ni-Au mineralised intervals vary from 3 m to 60 m thick. The upper T zone is restricted to the southern portion of the project area, whereas the lower F zone extends for a known 17 km along strike. The lower F zone mineralisation is dominated by sperrylite and Pt-Pd bismuthotellurides and these platinum-group metals are mainly associated with primary magmatic base-metal sulphides, which have undergone alteration during serpentinisation followed by the formation of secondary sulphide assemblages. The sulphur isotopic signatures suggest that the F zone mineralisation is mantle-derived with no contamination involved in the formation of the mineralisation. The upper T zone mineralisation hosted by more felsic-rich lithologies is dominated by Pt-Pd bismuthotellurides. The mineralised zone contains relics of primary magmatic sulphides and is characterised by the development of a chalcopyrite-millerite-pyrite assemblage, associated with hydrothermal quartz and hydrous silicates. The fluid–induced style of platinum-group metal remobilisation, high Au/PGE ratios, and high proportion of native gold in the high-grade T zone mineralisation in cumulus magnetite-bearing lithologies is unique to the Bushveld Complex. The genesis of the T zone is interpreted to result from the prolonged fractionation of the troctolite-gabbronorite-anorthosite sequence with the residual melt interacting with the first influx of Upper Zone fertile melt. The sulphur isotopic results indicate that the mineralisation is mantle-derived and that no significant contamination is involved in the formation of the T zone ores. Although there are similarities with the Bushveld succession south of the Hout River Shear Zone and with the rest of the Bushveld Complex, significant differences in the thickness and type of lithologies, mineral and whole-rock geochemistry and mineralisation of the arcuate intrusion of the Waterberg segment, compared with the eastern, western and northern limbs of the Bushveld Complex, suggests that the Waterberg Bushveld body may represent a structurally controlled separate compartment wedge between the Hout River Shear Zone to the south and the Palala Shear Zone to the northItem The geology, petrology and controls on copper mineralisation in Exploration Prospecting Licence (EPL) 5836 along the matchless belt, Windhoek, Namibia(2017) Ekandjo, Halleluya NaantuThe Kariam Prospect mineralisation is a quartz-carbonate vein hosted Cu (Mo-Au) deposit associated with quartz-carbonate schist and biotite-chlorite schist of the Kuiseb Formation in the Southern Zone or Khomas Trough of the Damara Orogen in Namibia. Geological mapping within the ‘Northern Part’ of the Kariam Prospect has identified two different types of schists which are different in composition and different in outcrop appearance, namely the fine-grained mica-rich and medium-grained quartz-rich schist. They dip shallowly between 15° - 55° towards 220° - 320° and strike ENE-WSW. They were originally sediments deposited around 630 Ma during the Neoproterozoic. Two different sets of veins have been noted around the schists reflecting different hydrothermal activities which have taken place in the area. The veins are i) Quartz-carbonate magnetic veins dipping steeply at 80° SE, estimated to have formed after deformation and ii) Quartz non-magnetic veins parallel to the schistosity with a steep dip inferred to have formed during deformation. A petrographic and borehole core investigation revealed that the different types of schists exhibit a different mineralogy attributed to alteration. Mineralisation in the form of chalcopyrite, pyrite, pyrrhotite and molybdenite occurs as disseminated ore, patches, blebs, and veinlets within the veins and matrix of the host rocks proximal to the mineralised veins. Adjacent alteration comprises proximal quartz-carbonate and early/distal biotite chlorite zoned clusters.Item Palynostratigraphy and correlation of three wells in the deep offshore Cenozoic Niger Delta(2017) Oláyíwolá, Moshood AdégbőyègaRecently hydrocarbon exploration and production in the Niger Delta Basin have shifted to offshore areas and this has required risk appraisal for their successful implementation. In order to improve our understanding of the petroleum system of these areas ditch-cuttings samples selected from three deep offshore wells A, B and C are subjected to palynological and sedimentological analyses to create a palynomorph biostratigraphic framework and microfloral zones. Palynostratigraphic data and lithofacies data are integrated for the first time (in this study area) with those of the wireline well log data for the better understanding of the sequence stratigraphy of this region. In general, the palynological results revealed abundant and diverse palynomorph occurrences. Although land-derived palynomorphs represented by angiosperm pollen, gymnosperm pollen, pteridophyte/bryophyte spores, fungal spores and freshwater algae dominated the observed palynomorph assemblages, the forms of marine origin comprising dinoflagellate cysts, microforaminiferal wall-linings, silicoflagellates, diatom frustules and scolecodonts, were rarely represented. Angiosperm pollen comprised 79-85 % of the recovered palynomorphs in the wells. They are dominated by Zonocostites ramonae, Z. duquei, Monoporites annulatus, Arecipites spp., Cyperaceaepollis spp., Psilastephanocolporites spp., Psilatricolporites crassus, Chenopodipollis spp., Retistephanocolpites gracilis, Nymphaeapollis clarus, Peregrinipollis nigericus, Canthiumidites spp., Gemmamonoporites sp., Retibrevitricolporites obodoensis, Retitricolporites irregularis, Pachydermites diederixi, Echitricolporites spinosus and Elaeis guineensis. The pteridophyte/bryophyte spores comprised 9-10% of the total palynomorph assemblages and among these, Verrucatosporites spp., Laevigatosporites spp., Stereisporites sp. and Acrostichum aureum were prominent. Based on these taxa the strata penetrated by these wells ranged in age from Late Miocene (P860 Subzone) to Pleistocene (P900 Subzone). The contrasting relative abundances of mangrove and Poaceae pollen during the Late Miocene to Pleistocene indicated fluctuations in the climatic conditions in this area during these epochs. These unstable climatic conditions are interpreted to have resulted from rapid changes in the sea level. The palynofloral assemblages revealed that the sediments were deposited in three main iv depositional environments: nearshore, shallow and marine. They are further subdivided into deltaic distributary channel, tidal channel, submarine channel, regressive marine and prograding delta sub-environments. The sand units of these sub-environments have good reservoir quality, while the shales provide good quality source and sealing rocks in the Niger Delta. Integration of the palynostratigraphy, lithofacies and well log data reflects some cycles of sequence systems tracts and sedimentary surfaces- with similar patterns and occur within almost the same depth interval ranges. The major contribution of this research has been to show that there is some degree of correlation between microfloral zones and depositional sequences based on the various methods employed. These relationships imply that climatic conditions, coupled with other factors like localized subsidence, played an important role in the sea level changes in the Niger Delta area. Four major depositional sequences of Late Miocene-Pleistocene age have been delineated. The sequences are bounded chronologically by type-1 sequence boundaries (SBs) 5.5 Ma SB, 4.1 Ma SB and 3.0 Ma SB. These depositional sequences are interpreted to have experienced four major flooding events marked by four maximum flooding surfaces (MFS), namely the 5.8 Ma MFS, 5.0 Ma MFS, 3.9 Ma and 2.0 Ma MFS in ascending order. This suggests that the combined analyses of palynostratigraphy, lithofacies and well log data can be used as a means to predict the location of the system tracts and sedimentary surfaces in order to raise the confidence in the correlation and interpretation of depositional sequences.Item The stratigraphy, structure, and gold mineralization of the Jamestown and Sheba Hills areas of the Barberton Mountain Land(2015-05-28) Anhaeusser, C. R. (Carl Robert)