3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Geochemistry of magnetite layers in the upper zone of the Bushveld Complex, South Africa(2015-05) Maila, Ramphelane PrinceThe Upper Zone (UZ) of the Bushveld Complex (BC) comprises several magnetitite layers throughout the entire sequence with the most prominent layer, the 2 m thick Main Magnetitite Layer (MML), located towards the base of the sequence. Magnetite mineral separates have been obtained from the UZ with particular focus on the MML in vertical profiles through the MML, Layer 1 and bifurcations of the MML, as well as profiles along the base of the MML and bifurcations. Magnetite mineral separates were also collected from Bierkraal and UCAR mine drill cores. The magnetite mineral separates were analyzed primarily for Cr and V as these two elements have the highest partition coefficients (D>200 and D=20-25 respectively) in magnetite and can be used as magmatic tracers. Electron microprobe data from the Bellevue drill core are also included. The gradational upper contacts of magnetitite layers with overlying anorthosite could be interpreted to suggest that the magnetitite layers accumulated through crystal settling. However, vertical profiles through 1 m of the MML all show an upward exponential decrease in Cr content (12 000-580 ppm) which is inconsistent with crystal settling but better explained by diffusion controlled bottom crystallization. The sharp base of the MML with the underlying anorthosite may suggest that the MML crystallized due to an abrupt event. The MML is not entirely homogeneous as evidenced by lateral heterogeneity along the base of the MML, identified by irregular Cr concentrations along the base of the MML and magnetitite bifurcations. This heterogeneity further supports the contention that the magnetitite layers are a product of diffusion controlled bottom crystallization. Reversals in Cr content, of differing magnitudes, in 3 of 4 vertical profiles above a dome structure interrupting the MML and in 2 of 4 vertical profiles through the MML, are attributed to intermittent convection on various scales bringing primitive undepleted magma into the crystallization zone. The magnitude of the reversals depends on the level to which the convection descends. The feldspar parting, a 10 cm thick horizon with cumulus plagioclase 1 m above the base of the MML, appears at a fairly constant Cr content in magnetite. The lack of a chemical break immediately above the feldspar parting suggests a physical process, such as pressure change, as a mechanism to account for the mineralogical change from the feldspar parting into massive magnetite in the upper portion of the MML. Vanadium, unlike Cr shows no systematic trends. Vanadium content of magnetitite layers is found to be comparable to that of the disseminated magnetite thus ruling out the possibility of a change in fo2 as a mechanism to induce magnetite crystallization. Disseminated magnetite in the UZ is suggested to have re-equilibrated with pyroxene and/or olivine during subsolidus ii cooling resulting in lower MgO contents of the disseminated magnetite compared to that of massive magnetitite layers. Similarities between magnetitite layers in Magnet Heights (eastern lobe); UCAR mine drill core, east of Brits (western lobe); Bierkraal drill core, north of Rustenburg (western lobe) and Bellevue drill core (northern limb) suggest that the different lobes of the BC may be connected.Item Neutron activation analysis of samples from the Kimberley Reef Conglomerate(2015) Rasmussen, Stephen EricThe technique of instrumental neutron activation analysis as applied to the analysis cf geological material has beer siudiel with particular emphasis on methods of reducing or eliminating analytical errors. The neutron flux gradients in the reactor irradiation facilities used were found to introduce errors of up to 20%. The use of iron foils to monitor both the thermal and fast neutron flux received by individual samples has been shown to reduce iiradiation errors to approximately 1%. The contribution of epithermal neutron resonances to (n, >’ ) reaction cross sections has been recognised. Estimated epithermal neutron flux factors have been shown to reduce the associated errors by as much as 7%.Item Integrated strato-tectonic, U-Pb geochronology and metallogenic studies of the Oudalan-Gorouol volcano-sedimentary Belt ( OGB) and the Gorom-Gorom granitoid terrane (GGGT), Burkina Faso and Niger, West Africa(2015-05-06) Tshibubudze, AsinneThe Palaeoproterozoic Baoulé-Mossi domain of the West African Craton in northeastern Burkina Faso hosts numerous gold deposits such as Essakane and Tarpako. Integrated strato-tectonic, geophysical, geochemical, geochronological, regional stratigraphic framework and metallogenic studies of the Oudalan-Gorouol volcano-sedimentary Belt and the Gorom-Gorom Granitoid Terrane have provided new insight into the geotectonic evolution of the northeastern part of Burkina Faso. This work outlines the structural context and architecture necessary for forming these deposits. In this work, a new strato-tectonic model is proposed for the area by integrating field data and geophysical, geochemical, and geochronological data. The integrated data highlights and characterizes the setting of the Essakane gold mine and gold camp relative to the location of other regional gold deposits, metamorphosed Birimian Supergroup, intrusive rocks and shear zones. Structural, geochemical and geochronological analyses have helped to clarify the geological evolution of the Oudalan-Gorouol volcano-sedimentary Belt and the Gorom-Gorom Granitoid Terrane during the Tangaean (D1) and Eburnean (D2) orogenies through to the Wabo Tampelse Event (D3). Further to these, zircon U-Pb geochronology data have demonstrated that the Oudalan-Gorouol volcano-sedimentary Belt and the Gorom-Gorom Granitoid Terrane represent some of the oldest outcropping geology in the Palaeoproterozoic Baoulé-Mossi domain recognised to date. The geochronology and geology suggest that the basement or a pre- Birimian crust to the Birimian Supergroup may be found in the northeast of Burkina Faso. The Eburnean Orogeny in northeastern Burkina Faso is preceded by two phases of deformation (D1-x and D1), and two phases of magmatism. The first, D1-x, is associated with the emplacement of the Dori Batholith at the onset of D1 (2164 – 2141 Ma). D1 ductile-brittle deformation formed F1 folds and discrete high-strain mylonite zones that deformed the Oudalan- Gorouol volcano-sedimentary Belt and the Gorom-Gorom Granitoid Terrane during a southwestdirected palaeo-principal compressive stress. The pre-Birimian to Birimian supracrustal rocks and intrusions were regionally metamorphosed during D1 to greenschist to amphibolite facies with development of mineral assemblage of quartz-chlorite-muscovite ± chloritoid to biotite-potash feldspar ± hornblende. D1 is also associated with volcanic arc type calc-alkaline magmatism, producing TTGs enriched in heavy rare earth elements. The Eburnean Orogeny (2130 – 1980 Ma) is characterised by northwest-southeast shortening; it was followed by north-northwest - south-southeast shortening with development of northeast trending sinistral strike-slip faults and shears. D2 brittle-(ductile) deformation is manifested by refolding of F1 by northeast-trending F2, and development of a pervasive northeast-trending S2 to S2-C foliation. Metamorphic grade attained greenschist facies during D2, with development of mineral assemblage of quartz-chlorite-muscovite ± actinolite. The Wabo Tampelse (D3) deformation event is brittle in character and does not significantly affect the regional geological architecture in the study area.Item Petrology and geochemistry of the Tjakastad (Barberton) ICDP cores(2014-07-21) Coetzee, GraceThe Komati Formation of the Barberton Greenstone Belt is the type locality of the rock type known as Komatiites. Komatiites are ultramafic lavas that were generated and erupted mainly during the Archaean. They give insight into volcanism on the early Earth as well as the nature of the mantle and melting processes. During 2010-2011 the International Continental Drilling Programme (ICDP) Barberton Project drilled two cores (BARB 1 and BARB 2) into the Komati Formation to obtain continuous sections of the komatiite strata. These cores were drilled to gain knowledge about the structure, textures, compositions, processes and contact relationships of komatiite flows, which could not be obtained from surface outcrop because of lack of continuity and relatively poor exposure. The drill holes also intersected a volcanic tumulus unit, the first of its type recognized in komatiite lava flows, allowing insight into the processes that created the tumulus and the processes responsible for creating the differentiated komatiite flows. The core was logged in detail, revealing a variety of rock-types and styles of volcanism. The rock-types encountered range from massive and differentiated ultramafic komatiites, through komatiitic basalts to mafic basalts. Some minor later intrusions of gabbros and dolerite are also present. The komatiites and komatiitic basalts are extrusive lavas and represent continuous eruptive sequences. The gabbros are typically intrusive, but can represent late stage crystallization. Contact relationships are evident in the core, where they have not been eroded by overlying flows, and are used to distinguish 85 individual flows in BARB 1 and 65 in BARB 2. Chill margins are typically between 5 and 50 cm thick and brecciated contacts are usually 5- 15 cm thick where present. Only rare examples of original mineralogy (olivine and pyroxene) are preserved because of pervasive alteration of the rocks. Alteration minerals are serpentine, chlorite, tremolite and magnetite. Early serpentine veining was followed by later stage magnesite veining. The opaque minerals – chromite with secondary magnetite overgrowth – are mostly located at olivine grain boundaries. The tumulus unit in the BARB 1 core was created by upward doming of the upper skin of a lava tube. The unit is 90 m thick and consists of five textural sections: basal cumulates, harrisite, pyroxene spinifex, gabbro-pyroxenite and a hyaloclastite unit that caps the sequence. The cumulates contain macrocrystic olivine grains that reach 15 mm in length; they are elongated and rounded, aligned in certain horizons and are tightly packed, with a maximum of 20 % matrix. The harrisite is a form of skeletal olivine (with crystals up to 5 cm in length) that grew upwards from the underlying cumulate layer. Between the skeletal olivine crystals are small (< 2 mm) crescent-shaped pyroxenes contained within the melt residue. Pyroxene grains in the spinifex lava reach 20 cm in length and are surrounded by a fine-grained matrix. The gabbro and pyroxenite layers contained within the spinifex layer are interpreted to represent the last stage of crystallization within the structure since they are chemically related to the tumulus and no chill margins are present between the gabbro and surrounding pyroxene spinifex. Both the spinifex and gabbro contain unaltered pyroxene crystals and the gabbro also contains relatively fresh plagioclase. The hyaloclastite breccia consists of fragmented fine-grained chill margin blocks derived from the upper crust of the lava tube. The fragments are surrounded and supported by a glassy shard-like matrix. Inflation processes are evident in the tumulus and gave rise to multiple layers of large elongated olivine cumulates together with the upward and outward bulging of the upper crust to form a hyaloclastite breccia. Chemically the tumulus exhibits a Fo93 olivine control. Fractionation processes are clear in MgO vs. depth, binary diagrams and REE plots. Element concentrations are between 1 and 11 times chondrite with a very small LREE enrichment. Differentiated komatiite flows are composed of three lithologies: basal cumulates, olivine spinifex and chill margin zones. The cumulus olivines have a crystal size of 0.5-1 mm, are euhedral and enclosed by a 30 to 60 % melt component. The spinifex olivine forms random or parallel sheets on a centimetre scale and is completely altered to serpentine. Between the olivine spinifex are chemically more evolved pyroxene spinifex blades, which are smaller (millimetre to centimetre scale) and altered to a combination of serpentine-chlorite-tremolite. The fine-grained chill margins of the flows are typically 1-10 cm wide and in some cases contain contact breccias or hyaloclastites. Three packages of differentiated komatiite flows occurring at several stratigraphic intervals where sampled in detail. The lowest package BARB 1 (89-118 m) exhibits chemical trends that are interpreted to indicate a combined crystallization control by olivine and pyroxene. This is evident in the rock compositions and by petrographic studies that reveal the presence of two cumulus phases. The chemical compositions of the other two differentiated packages, BARB1 (378-420 m) and BARB 2 (252-274 m) are controlled by crystallization or accumulation of Fo93 and Fo94, respectively. These packages have olivine cumulates which are surrounded in some cases by pyroxene oikocrysts. In some samples the pyroxenes have unaltered cores. The REE plots are well constrained between 2 and 9 times chondrite values, and have slight LREE enrichment. The tumulus structure and differentiated flow packages BARB 1 (378-420 m) and BARB 2 (252-274 m) have similar mineralogies and compositions but contain different textures. This is attributed to the size of the tumulus in comparison to the differentiated flows and implies similar magma processes and origins for all three units. The BARB 1 (89-118 m) interval appears to have undergone slightly different processes, as indicated by the presence of the two cumulus phases. This package is also altered to a greater extent than the others and no unaltered pyroxene is present.Item The volcanology, geochemistry and metallogenic potential of the goren volcano-sedimentary belt, northeast Burkina Faso, West Africa(2014-07-01) Peters, Luke Fred HorstThe Palaeoproterozoic southern Goren volcano-sedimentary belt of northeast Burkina Faso represents a sequence/package of rocks formed in a back-arc basin environment. Evidence is based on a conformable sequence of basalts that have geochemical affinities of normal mid-oceanic ridge style basalts (N-MORB) as well as arc-related magmas. Tholeiitic compositions, determined by major and trace element geochemistry, are exclusive to the study area, which is unique to northeast Burkina Faso. Flat, chondrite-normalised REE patterns suggest dominantly N-MORB compositions, with the occasional elevated Th and Al2O3 values, suggestive of arc basalts. Depleted chondrite-normalised concentrations of phosphorus (0.5-0.03 times lower than chondrite) and Ti (1-5 times chondrite) as well as anomalous concentrations of N-MORB-normalised Ta (80-120 times greater) are characteristic of the tholeiitic basalts of the Goren belt. The volcanic rocks are intercalated with a succession of volcanogenic manganese deposits, siltstone, volcaniclastite, volcanogenic greywacke and a discrete pyroclastic breccia deposit composed of lapilli- to bomb-sized volcanic debris, indicative of bimodal volcanism. The volcanic facies were established through field and petrographic interpretations. The subaqueous facies architecture consists of proximal, coherent and autoclastic basaltic flow units containing porphyritic, aphyric, amygdaloidal and hyaloclastic textures as well as pillowed, jointed and flow-banded basaltic lithofacies. Proximal to distal volcanogenic manganese deposits intercalated with basalt flows indicate a syn-volcanic genesis. A discrete pyroclastic interbed (~300 m thick) within a volcanogenic, marginal marine to offshore greywacke succession occupies a distal facies relative to the volcanic source and indicates a bimodal source of volcanism, i.e., mafic and intermediate. Volcanogenic manganese deposits are characterized by quartz-ankerite stockwork breccia, a braunite-ilmenite-magnetite-chalcopyrite-pyrite-(gold) mineral assemblage and increased Ba concentrations, indicating syn-volcanic seafloor-related hydrothermal activity. The Goren volcano-sedimentary belt presents potential for economic mineralisation in volcanogenic primary manganese and base metal deposits.