3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Characterisation of mafic crustal xenoliths from the Wyoming Craton, Montana (USA), using accessory mineral geochronology and geochemistry, with implications for lower crustal evolution(2019) Thakurdin, YashirvadCharacterization of mafic crustal xenoliths from the Wyoming Craton, Montana (USA), using accessory mineral geochronology and geochemistry, with implications for lower crustal evolutionItem Hydrogeological study of geological features in the groundwater system of Tweefontein 360KT farm, Limpopo Province(2018) Dalasile, Piwo-kuhleA hydrogeological study was conducted to investigate the possible influence of geological structures on the groundwater flow (regime) and dynamics at the Tweefontein farm 360KT, near Steelpoort, Limpopo Province. The aim of the investigation was to understand the groundwater system, as well as to devise appropriate measures that promote proper groundwater resource management, which will allow for prediction and mitigation of possible groundwater ingress into underground workings. Field observations, cross sections, borehole data, chemical analyses and environmental stable isotopes were used to understand the influence of geological features on flow dynamics in Tweefontein farm. Geological features, faults and dykes were found to enhance groundwater flow due to the presence of interconnected cross-cutting joints. Furthermore, weathering was also found to enhance groundwater flow within these structures. Calcrete and ferricrete within the weathered zone can act as a barrier or limit vertical flow of groundwater to the fractured zone, and this can enhance the formation of later flows which may contribute to formation of springs. There is no major ingress of groundwater into existing underground workings near a prominent NE-striking faulted shear zone that is partly overlain by a river. It can be postulated, based on the documentary evidence collected in this study that the inability of water to ingress underground workings overlain by rivers is attributed to depth, as well as the infilling within the prominent NE-striking geological features. Groundwater within the study area shows a Ca-Mg-HCO3-dominated water type indicative of fresh, shallow circulating groundwater. However, there also is a chloride-dominated facies showing the strong effect of evaporation within the shallow weathered zone aquifer. The enrichment in Ca and Mg ions may be attributed to weathering of ferromagnesian silicate minerals of the Bushveld Igneous Complex. Highly enriched stable isotope (δ2H and δ18O) signatures on surface-water and shallow groundwater suggest the presence of evaporation prior to infiltration. Water from mine fissures plot on the local meteoric water line suggesting direct and preferential recharge through geological structures during periods of rainfall in the summer and winter months. Groundwater in deep mine fissures shows a highly depleted isotopic signature compared to water in the shallow weathered aquifer, which suggests limited vertical hydraulic connection.Item Structure of the fada-gourma shear zone Eastern Burkina Faso(2018) Sehloho, Pulane RelebohileThe Baoulé-Mossi Domain in eastern Burkina Faso is a greenstone belt-granitoid terrane, formed in a volcanic arc tectonic environment. Volcanism and magmatism were coeval at around 2239 Ma, and this formed the basement rocks of the Fada-Gourma Shear Zone (FGSZ) region. Crosscutting relations observed in the field, aided by zircon U-Pb age data reveal that pluton emplacement continued to post-2167±12 Ma. The youngest units are NW-trending dolerite dykes, dated at 1814±26 Ma in eastern Burkina Faso. The study area has five defined deformation events. D1 is responsible for the remnant NW-trending S1 foliation in some of the granite gneisses and the foliated and gneissic granodiorites outside of the shear zones. The σ1 principal compressive stress orientation was NE-SW. The metamorphic grade associated with D1 is amphibolite facies. D2 had a NW-SE oriented compressive stress direction, and this resulted in thrust faulting and the NE-trending dominant structural grain in the study area. The associated metamorphic grade for D2 is lower amphibolite facies. D3 involved a dextral-reverse slip along faults with a NE trend, with southeast-block-up displacement. The principal compressive stress axis was ENE-WSW to ESE-WNW directed. Metamorphic grade associated with D3 is greenschist facies to lower amphibolite facies. D4 caused the sinistral reactivation of the NE-trending D3 structures. The sinistral shear overprint the dextral displacement and a sigmoidal foliation related to D4 is discernible on the aeromagnetic imagery data. The principal compressive stress axis during D4 was NNE-SSW to N-S oriented. The metamorphic grade during D4 was greenschist facies. D5 is a late-stage brittle deformation that led the formation of NW-trending extensional fracture cleavage planes. The imagery data was integrated with field data to aid in establishing the orientation of foliations, and the kinematic and geometric information at map scale and at outcrop scale. Petrographic descriptions provided information at a micro-scale where available. The study area is host to gold mineralization, and there is a network of active artisanal mines in the northern domains of the study area. Tambiga Hill is one of the larger actively mined quartz stockwork vein deposits in the Gourma region. Gold mineralization is concomitant with quartz veins, and is hosted either adjacent to the quartz veins, or within the veins. Gold is either invisible, or occurs as nuggets. Pyrite, tourmaline, ankerite and minor malachite are also present as accessory minerals within the quartz veins. Gold mineralization is associated with the late stages of D2, and D3.Item Modelling the Witwatersrand basin: a window into neoarchaean-palaeoproterozoic crustal-scale tectonics(2017) Molezzi, MarcelloThe aim of this study was to investigate and evaluate the 3D structural architecture around the Vredefort dome in the Witwatersrand basin, in particular the unexposed southern portion. This was done in order to establish strato-tectonic relationships, first order deformation structures, and basement architecture. The outcomes provide a more detailed architecture around the central uplift that may be used in future work aimed at examining the nature of giant terrestrial impacts. In summary, the integration of borehole, surface mapping, and 2D reflection seismic data provides a well constrained 3D geological model of the dome, central uplift, and adjacent areas (covering approximately 11600 km2). Seven structural features are discussed from the 3D modelling results. These include, (1) a normal fault in the lower West Rand Group, (2) an undulate, normal faulted truncation plane, constrained as post-West Rand Group and pre or early-Central Rand Group, (3) a truncation plane and local enhanced uplift constrained as pre to syn-VCF, (4) a listric fault system, constrained as post-Klipriviersberg Group and syn-Platberg Group, (5) a truncation plane, constrained as syn-Black Reef Formation, (6) folds, including a large asymmetric, gentle anticline here named the Vaal Dam Anticline, constrained as post-Magaliesberg Formation and pre-Vredefort impact, and (7) a listric fault across the southeastern margin of the Vredefort dome, constrained as late to post-central uplift formation. The findings support previous work by Tinker et al. (2002), Ivanov (2005), Alexandre et al. (2006), Dankert and Hein (2010), Manzi et al. (2013), Jahn and Riller (2015), and Reimold and Hoffmann (2016). However the findings oppose various parts of previous work by Friese et al. (1995), Henkel and Reimold (1998), and Reimold and Koeberl (2014). A new term is also proposed for the periclinal folds located around the central uplift, i.e., impact-type curvature-accommodation folds. This study demonstrates the importance of integrating multiple sources of data into a single 3D spatial environment in order to better refine and distinguish impact-related deformation from the pre-existing basement architecture.Item The tectono-chronological evolution of the Bushveld complex(1995) Coetzee, HendrikDetailed high precision geochronological studies have been performed on the 2054 Ma old Bushveld Complex, in an attempt to unravel its tectonic and thermal evolution in the period immediately following intrusion and crystallisation. The geochronological techniques used have been specifically chosen to sample specific temperature episodes in the cooling of the Complex, rather than to necessarily provide an accurate emplacement age, The Bushveld Complex is seen in this study as part of the Bushveld Magmatic Province, rather than as an isolated intrusion, The geochronological data are therefore interpreted in the context of the current understanding of the Proterozoic tectonic and thermal history of the Kaapvaal Craton. The development of clean chemical methods and accurate geochronological methods are essential to this type of study. The reduction of laboratory blanks, especially for lead and the development of laboratory techniques for the analysis of small samples therefore played an important part in this study. It has been possible to lower analytical blanks, especially lead blanks to levels where the analysis of small samples is possible. In addition, the zircon evaporation technique was attempted. Phlogopite micas from the Critical Zone of the Bushveld Complex give a wlde range of Rb-Sr model ages, some almost 100Ma older than the preferred age. This indicates a period of hydrothermal alteration of considerable duration at the same time as the intrusion. The slightly young Rb-Sr age recorded for all the mica and whole rock data collected for this study indicates the alteration of the micas which is evident from petrographic and electron microprobe studies. U-Pb and Pb-Pb zircon ages are also Significantly younger than the preferred age, indicating a degree of alteration. This is also seen in the discordance of the zircons seen in the U-Pb data.Item Geophysical investigation into the geology, geometry and geochronology of the South African Pilanesberg Complex and the Pilanesberg dyke system(2016) Lee, Sally-AnneThe Mesoproterozoic Pilanesberg Complex, South Africa, is the world’s largest alkaline intrusive complex. Mapped geological field relationships suggest the Complex has circular inward dipping layers. However, it is unclear how the dipping layers extend at depth. As a result, the 3D geometry of the Pilanesberg Complex is unknown. Modelling of the Pilanesberg Complex uses 2D forward models as well as 3D forward and inversion, gravity and magnetic data models, to set limits on the 3D geometry of the Pilanesberg Complex. The 2D Bouguer gravity models and geology maps indicate that some of the Bushveld Complex Main Zone shifted to the west of the Pilanesberg Complex during emplacement. This, and a highly faulted country rock, accounts for a portion of how the host rock was able to accommodate the Pilanesberg Complex intrusion. The geometry of the Complex is explored with test gravity models where the model of outward dipping and vertically dipping cylinders are unable to match the Bouguer gravity signal over the Complex, but the inward dipping model matched the data to provide a possible solution for the geometry of the Complex. The Pilanesberg Complex geometry is modelled with 3D magnetic inversion, 3D forward gravity models and 2.5D gravity test profiles that were all constrained by the surface geology. The different models correlate so that best data fit for the Complex is represented by an overall inward dipping structure. Surface geological measurements indicate that the northern edge of the Complex dip out to the north. The 3D forward modelling was able to produce a positive solution that matched the gravity data with a northward dipping northern edge. The dipping northern edge is also observed on the University of British Columbia, UBC, 3D gravity inversion and the Euler deconvolution gravity profile solutions. The depth of the Pilanesberg Complex from 3D forward gravity modelling is estimated to be between 5 and 6 km. The Complex is suggested to have undergone block movement where the northern block and southern block are separated by the 30 km long Vlakfontein fault, which bisects the Complex from the north-east to the south-west. The image processing contact depth, Euler deconvolution solutions and the 3D Voxi inversion model suggest that the fresh bedrock is closer to surface in the north, while the southern block appears to be approximately 1km deeper than the northern block. The northern dip and block movement are explained by complicated structural events that include trap door graben settling which hinged on the northern edge as well as faulting and external block movement during a regional lateral extensional event. The Pilanesberg Complex intruded during a larger system of alkaline intrusions, known as the Pilanesberg Alkaline Province. The intrusions are associated with the Province due to their ages and chemical affinity. This Province includes two dyke swarms that radiate to the north-west and south of the Pilanesberg Complex, as well as smaller circular clinopyroxenite intrusions throughout the Bushveld Complex. The Pilanesberg dyke system and the circular clinopyroxenite intrusions are reversely magnetised with IGRF corrected values ranging between -150 to -320 nT compared to the normally magnetised 166 to 330 nT values of the Pilanesberg Complex. This suggests that a magnetic reversal occurred between the emplacement of the Pilanesberg Complex and the dyke System. The age data of the Complex and dyke Swarm suggest a magnetic reversal could have occurred between the emplacement of the Pilanesberg Complex and the Pilanesberg dyke System. The Complex is dated at 1602 ± 38 Ma and 1583 ± 10 Ma, from two white foyaite samples from the southern edge (using 40Ar/39Ar amphibole spectrum analysis). These ages are vastly different from previously reported ages, which ranged between 1200 Ma and 1450 Ma (Harmer R., 1992; Hansen et al., 2006). The error analysis has improved considerably from the published dates making the proposed dates plausible for the intrusion of the Pilanesberg Complex as the first and main intrusion of the Pilanesberg Alkaline Province. The Pilanesberg dyke System intruded much later between 1219 ± 6 Ma to 1268 ± 10 Ma for the red syenite dyke samples (using 40Ar/39Ar on feldspars spectrum analysis) and 1139 ± 18 Ma obtained for the grey syenite dyke (using 40Ar/39Ar on amphiboles inverse isochronal analysis). The dyke Swarm dates are significantly younger than the previously published ages for the dykes, which were between 1290 Ma and 1330 Ma (Van Niekerk, 1962; Emerman, 1991).