3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
4 results
Search Results
Item A study of geomagnetic field time variations over southern Africa using a regional harmonic spline core magnetic field model derived from CHAMP satellite and ground magnetic observations(2019) Nahayo, EmmanuelThe geomagnetic field spatial variation in Southern Africa is characterised by a high horizontal gradient, and its study requires high spatial resolution data. Regional field models are more suitable than global models to study small scale features of the geomagnetic field variation. They use more dense data, hence a good data coverage allows a detailed description of the geomagnetic variation in the area of investigation. In Southern Africa, the ground recording stations are limited (4 magnetic observatories and 38 geomagnetic repeat stations) and satellite data are needed for studies where high spatial and temporal resolution data are required. The southern African region borders the South Atlantic Anomaly (SAA) where the strength of the magnetic field is approximately 30% weaker compared to other regions of the same latitudes. In an attempt of supporting geomagnetic field model data users and investigating the evolution of the South Atlantic Anomaly, a study to develop and derive a southern African regional field model combining CHAMP satellite data with ground-based data showed that the regional model over Southern Africa can be improved by combining both satellite and ground-based data. The internal Earth processes generating the main field can be studied by modelling the core field using inversion of geomagnetic field components measured above the surface of the Earth, and they assist in monitoring the long-term variation of the geomagnetic field on time scales of 1 year and more, therefore giving valuable information on the evolution of the SAA in southern Africa. The study of the geomagnetic field time variation in the southern Africa region was carried out applying the harmonic splines technique on CHAMP satellite and ground data that have been recorded between 2001 and 2010. A Southern Africa Regional Model (SARM) was derived and evaluated using global models, such as International Geomagnetic Reference Field (IGRF-11) and GFZ Reference Internal Magnetic Model (GRIMM), by calculating the difference between SARM and global models. The results of this study suggest that the 200 km gridded SARM model developed from only satellite data is shown to match monthly averaged ground data to within 1.5%, 1.6% and 0.7% for X, Y and Z, respectively, suggesting the temporal variations are valid where ground data are not available. In addition, these results also confirm the earlier findings of the 2007 magnetic jerk and rapid secular variation fluctuations of 2003 and 2004 on the eastern edge of the SAA which extends 1000 km into South Africa. CHAMP satellite and relatively densely spaced (300 km) ground-based data measured over southern African region between 2005 and 2010 are incorporated in a new regional, harmonic spline based, core field model. Our new SACFM-2 model is compared to our previously developed SARM model, a regional model based only on satellite data, and the global CHAOS-6 model developed by Finlay et al. (2016). The results agree well with the CHAOS-6 model (within 0.4%) in the vertical (Z) component and total field (F) that are used to investigate the evolution of the South Atlantic Anomaly in the region. The computed maps of main field of the Z component and total intensity F show a steady decrease of the field between 2005.5 and 2010.5, reaching an average of 40 nT and 50 nT per year, respectively, in the southwest of South Africa, indicating evolution of the South Atlantic Anomaly under Southern Africa and an increase of the geographical area of this feature. In addition, a southern African lithospheric magnetic model (SALMM) over Southern Africa was developed based on the Spherical Cap Harmonic Analysis method using CHAMP satellite measurements at an altitude between 332 km and 365 km, and between 2007.0 and 2009.0 epochs. This lithospheric magnetic model SALMM was compared to a regional model developed by Vervelidou et al. (2018) and the global model MF7. Our lithospheric magnetic model SALMM was interpreted in terms of regional (100s km) geological features and long-wavelength geological magnetic anomalies.Item Developing an advanced understanding of Au-U-C associations in the Witwatersrand Basin : using traditional and experimental techniques(2017) Woods, TremainThe Witwatersrand Basin is the single largest gold source ever found. Gold exhibits a strong association with carbon and as much as 40% of the gold mined from the metasedimentary basin is found in carbon-bearing reefs. Carbon also contains high concentrations of uranium minerals. Although the carbon, gold and uranium association is well-documented, there is little consensus about the formation of these associations. Thus, a detailed geological investigation was undertaken on material from 12 carbon-bearing reefs throughout the Central Rand Group from across the Witwatersrand Basin. Concurrently a novel experimental component of the investigation was conducted in an attempt to simulate, for the first time ever, the Au-U-C associations in the Witwatersrand Basin. Petrography revealed that in more than 60 samples from carbon-bearing Witwatersrand Reefs, gold most often occurred between carbon spindles, as minor inclusions within carbon spindles and as coatings to the carbon forms. Furthermore, gold within the conglomerates occurred as veinlets, rims on oxide grains, as micro-particles in phyllosilicates and secondary quartz and as inclusions in secondary pyrite grains. These textures were interpreted as secondary features formed during metamorphism and alteration of the Basin. Hydrocarbons displayed textures that were also interpreted as being secondary; these included nodules that infilled interstices within the reef matrix and multiple phases of hydrocarbon enclosed within spindles comprising carbon seams. Electron microscope images were taken of a hydrothermal carbon nodule enclosed in cubic pyrite. The images revealed sheet-like hydrocarbons and fibrous/tube-like forms. Critically, the hydrothermal carbon nodule also contained micro-particulate crystalline gold and pyrite within vesicles or gas bubbles within in the nodule. In the carbon seams, uraninite occurred within the spindles. The uraninite appeared fragmented and displaced by hydrocarbon growth structures. Therefore, it was suggested that uraninite was the precursor to secondary hydrocarbon and gold precipitation. Detailed petrography revealed the textures of particulate Au-U-C associations in the Witwatersrand Basin but elemental micro-mapping was necessary to determine the distribution of metals, major elements, trace elements and rare earth elements within carbon seams and carbon nodules. Carbonaceous materials were closely associated with disseminated elemental sulphur, mobile elements and rare earth elements. The occurrence of mobile elements and sulphur disseminated through the hydrocarbon suggested that the carbonaceous matter was formed from a fluid phase. Organic sulphur compounds in fluids derived from sedimentary organic matter are theorised to enhance the solubility of various metals in hydrocarbon phases. The occurrence of Au, As, Ag, Ti, V, U, Hg, Fe, Co, Cu, Cr, Mn and other metals in the hydrocarbons in the Basin indicated that a process other than radiolytic polymerisation may have been involved in the concentration of certain metals from a liquid phase. The similarity of metals concentrated in Witwatersrand carbonaceous matter to those of modern day crude oils and petroleum liquids derived from the degradation of Type I kerogens provided a potential mechanism for remobilisation in the Basin. These observed Au-U-C associations laid the foundation for experimental simulation. Catalytic Chemical Vapour Deposition experiments used acetylene and hydrogen gas to precipitate solid nanocarbon materials onto uranium-bearing powders. Results showed that for increasing uranium concentration, greater masses of carbonaceous materials were deposited onto the uranium-bearing powders. Therefore, carbonaceous product deposition was confirmed to be correlated with uranium concentrations. When uranium was leached out of the samples, the mass of carbon deposited was significantly decreased. The growth of carbon started at 300°C, was physically visible at 450°C and completely encapsulated the powders forming a product similar to Witwatersrand carbon seams at 600°C. Raman spectra indicated that the experimentally formed carbonaceous products had similar signatures to those of Witwatersrand carbon. The simulation of C-U associations was confirmed by electron microscopy, which showed that carbon nanotubes, carbon nanofibers and sheet-like materials precipitated onto uraninite grains under experimental conditions. These experimentally formed structures were comparable to those observed in Witwatersrand seam carbon, especially the sheet-like and fibrous/tube-like materials. It was suspected that gold distribution in carbon seams could provide useful data for hydrocarbon formation processes. Consequently micro-computed tomography, in conjunction with automated electron dispersive spectra, was used to examine the distribution of gold in three dimensions. The sample analysed was a carbon seam from the Carbon Leader Reef that contained exceptionally high gold grades. Gold in this carbon seam was found to occur most concentrated between carbon spindles in addition to small particles within spindles. Furthermore gold concentrations were highest at the footwall and hanging wall contacts and decreased towards the centre of the seam. The hydrocarbon spindle form also changed from bulbous at the contacts to more uniform and elongated in the centre of the carbon seam. It was therefore suggested that hydrocarbon growth and gold crystallisation in carbon seams was akin to crystal growth in quartz veins, where spindles are elongated parallel to the principal stress direction. In order to experimentally simulate Au-U-C associations, gold in solution at high pressures was required. Solvothermal experiments that used an autoclave and sucrose in solution were undertaken. Sucrose represented a common component found in petroleum liquids derived from Type I kerogens, similar to what is postulated for the Witwatersrand Basin. The experiments showed that hydrocarbons could precipitate at lower temperatures compared to temperatures observed in the Basin – c.a. 180°C. In addition, the effect of pressure was to enhance the breakdown of sucrose into more sheet-like carbonaceous products when 800 kPa of nitrogen gas was added to the reaction vessel. Finally, a solution of 0.01 M aurichloric acid was added to the experiments to determine if gold in solution would be precipitated during hydrocarbon precipitation. Results showed that almost all the gold was precipitated from solution as crystalline and nano-particulate gold. Strikingly, gold that precipitated was enclosed by hydrocarbons and was also seen to adhere onto the surfaces of sheet-like carbonaceous materials, indicating that the gold and hydrocarbon are intimately associated and that hydrocarbons facilitate the precipitation of gold from solution. Although experimental conditions could not match geological conditions exactly, the experimental products were comparable to the Witwatersrand Au-U-C associations. The results from this research show that catalytic electron promotion of uranium and other metals play an important role in hydrocarbon structuring and precipitation. The model of fluid remobilisation of hydrocarbons and gold is further enhanced by the evidence presented in this study. In conclusion, remobilisation textures were seen in the Au-U-C associations from the Witwatersrand Basin. Similar associations were experimentally precipitated using a hydrocarbon and gold-rich fluidItem Seismological and mineralogical studies of the world’s deepest gold-bearing horizon, the Carbon Leader Reef, West Wits Line goldfields (South Africa): implications for its poor seismic reflective character(2016) Nkosi, Nomqhele ZamaswaziThe measurements of physical rock properties, seismic velocities in particular, associated with ore deposits and their host rocks are crucial in interpreting seismic data collected at the surface for mineral exploration purposes. The understanding of the seismic velocities and densities of rock units can help to improve the understanding of seismic reflections and thus lead to accurate interpretations of the subsurface geology and structures. This study aims to determine the basic acoustic properties and to better understand the nature of the seismic reflectivity of the world’s deepest gold-bearing reef, the Carbon Leader Reef (CLR). This was done by measuring the physical properties (ultrasonic velocities and bulk densities) as well as conducting mineralogical analyses on drill-core samples. Ultrasonic measurements of P- and S-wave velocities were determined at ambient and elevated stresses, up to 65 MPa. The results show that the quartzite samples overlying and underlying the CLR exhibit similar velocities (~ 5028 m/s-5480 m/s and ~ 4777 m/s-5211 m/s, respectively) and bulk densities (~ 2.68 g/cm3 and 2.66 g/cm3). This is due to similar mineralogy and chemical compositions observed within the units. However, the CLR has slightly higher velocity (~ 5070 m/s-5468 m/s) and bulk density (~ 2.78 g/cm3) than the surrounding quartzite units probably due to higher pyrite content in the reef, which increases the velocity. The hangingwall Green Bar shale exhibits higher velocity (5124 m/s-5914 m/s) and density values (~ 2.89 g/cm3-3.15 g/cm3) compared to all the quartzite units (including the CLR), as a result of its finer grain size and higher iron and magnesium content. In the data set it is found that seismic velocities are influence by silica, iron and pyrite content as well as the grain size of the samples, i.e., seismic velocities increase with (1) decreasing silica content, (2) increasing iron and pyrite content and (3) decreasing grain size. Reflection coefficients calculated using the seismic velocities and densities at the boundaries between the CLR and its hangingwall and footwall units range between ~0.02 and 0.05, which is below the suggested minimum of 0.06 required to produce a strong reflection between two lithological units. This suggests that reflection seismic methods might not be able to directly image the CLR as a prominent reflector, as observed from the seismic data. The influence of micro-cracks is observed in the unconfined uniaxial compressive stress tests where two regimes can be identified: (1) From 0 - 25 MPa the P-wave velocities increase with progressive loading, but at different rates in shale and quartzite rocks owing to the presence of micro-cracks and (2) above stresses of ~20 - 25 MPa, the velocity stress relationship becomes constant, possibly indicating total closure of micro-cracks. The second part of the study integrates 3D reflection seismic data, seismic attributes and information from borehole logs and underground mapping to better image and model important fault systems that might have a direct effect on mining in the West Wits Line goldfields. 3D seismic data have delineated first-, second- and third-order scale faults that crosscut key gold-bearing horizons by tens to hundreds of metres. Applying the modified seismic attribute has improved the imaging of the CLR by sharpening the seismic traces. Conventional interpretation of the seismic data shows that faults with throws greater than 25 m can be clearly seen. Faults with throws less than 25 m were identified through volumetric (edge enhancement and ant-tracking seismic attributes) and horizon-based (dip, dip-azimuth and edge detection seismic attributes) seismic attribute analysis. These attributes provided more accurate mapping of the depths, dip and strikes of the key seismic horizon (Roodepoort shale), yielding a better understanding of the relationship between fault activity, methane migration and relative chronology of tectonic events in the goldfield. The strato-structural model derived for the West Wits Line gold mines can be used to guide future mine planning and designs to (1) reduce the risks posed by mining activities and (2) improve the resource evaluation of the goldbearing reefs in the West Wits Line goldfields.Item Stratigraphy and sedimentary environments of the Late Permian Dicynodon Assemblage Zone (Karoo Supergroup, South Africa) and implications for basin development(2016) Viglietti, Pia AlexaThe Dicynodon Assemblage Zone (DiAZ) spans the last three million years of the Late Permian (Lopingian) Beaufort Group (Karoo Supergroup). Fluvio-lacustrine conditions covered the entire Karoo Basin during this period, preserved as the rocks of the Balfour, Teekloof, and Normandien formations. However widely separated exposures and few dateable horizons make correlating between lithostratigraphic subdivisions difficult. Here a revised litho- and biostratigraphic framework is provided for the Upper Permian DiAZ. The Balfour Formation’s Barberskrans Member (BM) is renamed due to identifying the Oudeberg Member and not the BM at the current type locality (Barberskrans Cliffs). It is renamed Ripplemead member (RM) after Ripplemead farm 20 km north of Nieu Bethesda where it outcrops. The Teekloof Formation’s Javanerskop member and Musgrave Grit unit in the central Free State Province are regarded mappable units whereas the Boomplaas sandstone (BS) may represent a unit that is a lateral equivalent to the Oudeberg Member. Palaeontological and detrital zircon data suggest none of these locally persistent sandstone horizons correlate temporally. Three index fossils that currently define the DiAZ (Dicynodon lacerticeps, Theriognathus microps, and Procynosuchus delaharpeae) appear below its lower boundary and disappear below the Permo-Triassic Boundary (PTB), coincidentally with the appearance of Lystrosaurus maccaigi. The base of the DiAZ is redefined, with the revived Daptocephalus leoniceps and T. microps re-established as the index fossil for the newly proposed Daptocephalus Assemblage Zone (DaAZ), and is subdivided into two subzones. Da. leoniceps and T. microps’ appearance define the lower and L. maccaigi defines the base of the upper subzone. The same patterns of disappearance are observed at the same stratigraphic interval throughout the basin, despite the thinning of strata northward. Additionally wetter floodplain conditions prevailed in the Lower DaAZ than in the Upper DaAZ which likely reflects climatic changes associated with the Permo-Triassic mass extinction (PTME). Palaeocurrent and detrital zircon data demonstrate a southerly source area, and recycled orogen petrography indicates the Cape Supergroup is the source of Upper Permian strata. Dominant late Permian zircon population supports the foreland nature of the Karoo Basin. Orogenic loading/unloading events are identified by two fining-upward cycles, separated by a diachronous third-order subaerial unconformity at the base of the RM and Javanerskop members. Sediment progradation northwards was out-of-phase with the south and wedge-shaped. Distributive fluvial systems depositing sediment within a retroarc foreland basin best explains these observations. Lithostratigraphic beds and members are recommended for use as local marker horizons only in conjunction with other proxies, such as index fossils or radiometric dates in future studies.