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 Petrogenesis of the LG-6 chromitite at Ruighoek mine western limb of the Bushveld Complex, South Africa(2017) McIntosh, RyanThe LG-6 chromitite layer is the thickest (0.90 to 1.20 m thick) chromitite layer in the Lower Group chromitites of the Bushveld Complex and is of economic significance owing to the relatively high Cr-content. It can be traced across the entirety of the western limb and is mined in both the western limb and the eastern limb. This study evaluates previously published models of chromitite formation using data from the LG-6 chromitite at Ruighoek Mine, western Bushveld Complex. Data includes petrographic studies of the reef and host rocks, whole rock analysis of the silicate host rocks and reef, and mineral chemistry for orthopyroxene, olivine and Cr-spinel using electron probe microanalysis (EPMA). In the Ruighoek region the LG-6 chromitite comprises up to 95 vol. % chromite and is typically hosted by orthopyroxenite. Borehole data indicated an area (about 250 m2 in size) where the LG-6 is entirely hosted by harzburgite (42% orthopyroxene, 11% Cr-spinel, 14% olivine, 32% serpentine and 1% other) rather than orthopyroxenite. The whole rock and mineral chemistry revealed that the LG-6 chromitite in this area has an exceptionally high Cr/Fe ratio, up to 2.1. The whole rock data also indicated several compositional reversals in terms of MgO, Al2O3, Cr2O3, FeO, and Mg# (Mg/ [Mg+Fe2+]) for the unenriched borehole 13R-3, and compositional reversals in Cr/Fe and Cr# (Cr/ [Cr+Al]) for the enriched borehole 13R-9 upwards through the chromitite layer. The hanging wall harzburgites are characterized by an increase in Mg# for the mineral chemistry of the Cr-spinel, orthopyroxene and olivine compared to those in the footwall harzburgite. Importantly, spatial 3D modelling of borehole data at Ruighoek mine (19 drill-cores) indicates that the elevated Cr/Fe ratio in LG-6 chromitite is coincident with a depression in the topography of the chamber floor at the time of formation of the LG-6 chromitite. These data are difficult to reconcile with existing models for chromitite formation in layered intrusions, such as the models for gravity settling, addition of a Cr-spinel crystal-laden magma, or a pressure increase. Thus, this work has developed a new model for formation of the LG-6 chromitite at Ruighoek Mine. The exceptionally high Cr/Fe ratio of LG-6 chromitite and its close association with harzburgite is attributed to multiple replenishments of the chamber by relatively primitive magmas. These are inferred to either be saturated in olivine and chromite, or chromite alone. The occurrence of relatively primitive rocks within the depression is suggested to be related to a local feeder situated within the depression. Injection of new, relatively dense magma pulses from the feeder are inferred to spread out across the chamber floor as basal flows owing to compositional stratification of the resident magma at the time of development of the LG-6 chromitite. The replenishing magmas contributed to the existing compositional stratification in the chamber, resulting in the most primitive composition within the depression of the chamber floor. Subsequent crystallisation of the most primitive magmas within the depression resulted in local development of LG-6 chromitite with exceptionally high Cr/Fe ratios together with the enclosing harzburgitic rocks. The thickness of the LG-6 chromitite is attributed to continuous replenishment by large volumes of new, chromite-saturated, magmas via the feeder channel located in the depression. This study suggests that magma stratification and the replenishment of the chamber by chromite-saturated magmas played an important role in the development of the chromitite layers of the Bushveld Complex.Item Ground water pollution at sanitary landfill sites: geohydrological, environmental isotope and hydrochemical studies(1998) Butler, Michael JohnThis study determines the potential of prerncting pollution to ground water by sanitary landfills. The tracing capabilities of both stable and radioactive environmental isotopes are also evaluated. Four landfills were selected, the Linbro Park and Waterval sites in Johannesburg, and the Bloemfontein northern and southern landfill sites. The sites all differ in geological environment, size. age and physiographic setting. [Abbreviated Abstract. Open document to view full version]Item Geochemistry of single diamond crystals by instrumental neutron activation analysis(2015-02-20) Damarupurshad, Ashok KumarItem Mineralogical - Geochemical Investigation of two sections across the Permian-Triassic Boundary in the Continental Realm of the Southern Karoo Basin, South Africa(2006-11-17T06:21:34Z) Coney, LouiseThe Late Permian (251.0 ± 0.4 Ma) mass extinction is universally acknowledged as the most consequential of the five major Phanerozoic mass extinctions. More than 90% of marine species, ~70% of terrestrial vertebrates, and ~90% of plant life were lost in a very short interval. The nature of the Permian-Triassic (P-Tr) boundary and the cause of the mass extinction associated with it have been the subject of extensive international debate. Possible causes for the P-Tr extinction include asteroid/comet impact, oceanic anoxia, volcanism, methane clathrate dissociation, or combinations of these causes. Geochemical studies of the P-Tr boundary have traditionally been focused on the marine realm, as the boundary in continental sections is typically difficult to pinpoint. One continental setting of the P-Tr boundary that has, however, received much attention is that in the main Karoo Basin, South Africa. The Karoo Basin is a large retro-arc foreland basin which accumulated sediment from the Carboniferous (300 Ma) through to the Early Jurassic (180 Ma) in southwestern Gondwana. Mineralogical and geochemical investigations across two palaeontologically well-constrained continental P-Tr boundary sections at Commando Drift Dam and Wapadsberg in the southern Karoo Basin of South Africa have been undertaken in order to aid in our understanding of this extinction event. The Commando Drift Dam section is also constrained palaeomagnetically. There is a change in paleosol colour across the P-Tr boundary from green-grey to red-brown, which is believed to reflect a change of oxidizing conditions at the P-Tr boundary. Quartz grains were examined for possibly impact-produced microdeformation features, but these were not found. Iridium concentrations are below the detection limit (by instrumental neutron activation analysis) and the sections could not be evaluated as to whether any significant enrichment has taken place at the P-Tr boundary. Major element chemical profiles are dominated by the signatures of carbonate nodular horizons in both sequences. Iron contents (and accompanying siderophile element abundances) increase across the palaeontologically-defined P-Tr boundary, followed by a decrease thereafter. The major element concentrations, together with the effects of weathering, largely control trace element distribution. Carbon isotopic results from the Commando Drift Dam section show a gradual decrease in values before the P-Tr boundary, with a larger negative excursion at the P-Tr boundary. Above the boundary, gradual recovery to initial ratios is observed, followed by another gradual decrease in values to the palaeomagnetically defined boundary. No evidence supporting an extraterrestrial impact extinction mechanism has been found. Rather, the carbon isotope data from this study support two gradual palaeoclimatic changes separated by a sudden change in the carbon isotopic content of the atmosphere. The size and nature of these excursions support the addition of large amounts of anoxic material into the atmosphere. This is proposed to have been caused by the multiple influx of carbon dioxide, methane and other greenhouse gases at various times and by different mechanisms. Such a release of carbon dioxide, methane and other greenhouse gases could have been caused by the coincident volcanic event (the formation of the Siberian Traps) and the episodic release of methane clathrates.