Masters Degrees (Geology)

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Browsing Masters Degrees (Geology) by Advisor "Magson, Justine"

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    The Sr isotopic stratigraphy of the LCZ-UCZ transition in the Western Limb of the Bushveld Complex
    (University of the Free State, 2021) Malatji, Mafete; Roelofse, Frederick; Magson, Justine
    Data on the modal mineralogy, whole-rock geochemistry, plagioclase mineral chemistry and Sr isotopic compositions in lithologies covering an interval of ~100 m across the Upper Critical Zone (UCZ) and Lower Critical Zone (LCZ) transition in the Western Limb of the Bushveld Complex are presented in this study. The aims of this study were to (1) investigate the presence or absence of isotopic disequilibrium in plagioclase (2) to investigate differences between the LCZ and UCZ from a geochemical, petrological and Sr isotopic perspective and (3) to refine chromitite formation models using the data obtained over the course of the study. Samples were obtained from the BH7929 drill core donated by Impala platinum to the University of the Free State. Samples were analysed using transmitted light microscopy, X-Ray Fluorescence Spectrometry (XRF), Inductively Coupled Plasma Mass Spectrometry (ICPMS), Electron Probe Micro-Analyzer (EPMA) and Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA MC-ICP-MS) to produce whole-rock major and trace element geochemistry and plagioclase elemental and isotopic compositional profiles across the UCZ-LCZ transition. Results reveal that the LCZ is composed of orthopyroxene-dominated lithologies that display cryptic layering, hosting two chromitite layers (MG1-2), with plagioclase predominantly existing as an intercumulus phase. Plagioclase is predominantly cumulus in the UCZ, dominating the UCZ lithologies that display modal layering. Two chromitite layers were investigated in the UCZ (MG3-4). Compositional breaks in whole-rock major and trace elements are detected at the UCZ-LCZ transition and at the level of the chromitite layers, reflecting variations in the dominant mineral phases. Fractionation indices including whole-rock Mg# and Cr/V ratio reveal little variation throughout the study interval in silicate-dominated lithologies, with variations mostly detected at the level of chromitite layers. Plagioclase An% averages 82.10 ± 1.90% in the UCZ, whereas it averages 73.58 ± 2.60% in the LCZ. Chromitite layers in the LCZ reveal lower An% values in comparison with adjacent silicate lithologies, whereas the UCZ reveals very little to no variations between chromitites and silicate lithologies. Sri values in the UCZ average 0.7059 ± 0.0003, whereas Sri in the LCZ averages 0.7054 ± 0.0004. Decreases in the Sri value of plagioclase are observed at the level of the chromitite layers in the LCZ, whereas the UCZ reveals a constant Sri up the stratigraphy. The data provide credence to the importance of magma mixing (i.e. Irvine, 1977) as a process operational in the formation of chromitite layers within the LCZ and UCZ and argue against models suggesting variations in intensive parameters or in-situ crystallization as dominant processes in the formation of chromitite layers. It is proposed that the UCZ-LCZ transition displays credible evidence for the repeated intrusion of batches of isotopically distinct magmas, with chromitite layers in the LCZ forming in response to the mixing of newly introduced and resident magma in a manner analogous to that envisaged by Irvine. The MG3 layer in the UCZ also appears to have formed as a direct consequence of mixing between newly intruded UCZ magma and the residual LCZ magma. The MG4 layer does not preserve Sr-isotopic evidence for magma mixing as it has similar Sri as that of adjacent silicate lithologies. In order to account for the mass balance of Cr, it is argued that at the level of chromitite layers, intruded magma pulses were chromite-laden, with additional chromite formation occurring in response to magma mixing. The MG1 chromitite layer provides potential evidence in support of such an argument in the form of multiple isotopically distinct populations of plagioclase that may have been intruded along with suspended chromite crystals.
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    ItemOpen Access
    A Sr-isotopic investigation of bifurcating chromitite layers of the UG1 at the Impala Platinum Mine, Rustenburg
    (University of the Free State, 2023) Nyakane, Tshepo Felix; Magson, Justine; Roelofse, Frederick
    Chromitite bifurcations hosted within, but not limited to, the Upper Group 1 (UG1) chromitite layer in the Critical Zone of the Bushveld Complex are one of the most enigmatic geological features encountered. Several researchers have attempted to develop models explaining how these bifurcations could have been formed. Most of these studies were heavily based on field observation with little to no geochemical data to support their findings. In this study, samples of an exposure of chromitite bifurcations from the UG1 chromitite at Shaft No.11 of Impala Platinum Limited in the Western Limb of the Bushveld Complex were utilised to perform petrographic and geochemical work including Sr-isotopic determinations on plagioclase. The geochemical data collected, along with field observations, were used to develop a conceptual model explaining the development of the bifurcations. Four sample cuts (D, C, B, and A) across the anorthositic footwall of the UG1 chromitite, each with a width of 10 cm and varying lengths were sampled from the study area using a diamond saw. The sample cuts represent vertical transects across a set of bifurcating chromitite layers, taken approximately 1 m apart, on the northern side of the approximately 40 m section. Thirty-four polished thin sections were made representing all the sample cuts. The polished thin sections were studied petrographically with an Olympus BX51 microscope. Electron microprobe analyses were carried out to obtain the compositions of chromite and plagioclase crystals from the samples, and Laser Ablation Multi-Collector Inductively Coupled Mass Spectrometry was used to obtain in-situ isotopic compositions of the plagioclase crystals. Plagioclase in the anorthosite layers exhibits very little variation in An% with average values of 75.10 ± 3.27, 74.26 ± 1.93, 75.10 ± 3.27 and 73.85 ± 1.89 for sample cut D to A, respectively. Plagioclase in the chromitite layers reveals much more significant variation in An% with average values of 70.69 ± 14.15, 78.16 ± 15.26, 56.49 ± 33.13 and 55.50 ± 36.68 for sample cuts D to A, respectively. The in-situ plagioclase isotopic composition reveals that the initial ⁸⁷Sr/⁸⁶Sr ratios of plagioclase in anorthosite show very little variation both vertically and laterally through sample cuts A to D, with an average value of 0.7062 and individual layers that are generally within error compared to adjacent layers. Most chromitite layers also display ⁸⁷Sr/⁸⁶Sr ratios that are comparable to those observed in the anorthosite, although some of the thicker layers returned values that are more radiogenic, e.g., the bottom thick layer in sample cut A, which returned values on the order of 0.709 – 0.710. Taking into account the field relations along with the petrography and geochemistry of the study area, it is envisaged that the chromitite bifurcations in the study area were formed in the following stages: (1) Development of an irregular floor through the thermo-chemical erosion of the underlying anorthosite footwall. (2) The intrusion of chromite-rich slurry (mass balance requirement) as a basal flow resulted in thick chromitite layer deposition on an uneven surface. (3) Development of cyclic anorthosite and chromitite forming bifurcations. Pressure fluctuations (magma influxes, roof rupturing events, shock waves) permitted rapid transitions between the system's chromitite and plagioclase stability fields. A large reservoir of melt likely buffered compositional and isotopic changes. (4) The intrusion of a chromite-rich slurry led to renewed erosion and formation of the thick upper chromitite layer, with the thin chromitite layers now appearing as offshoots from the base of this layer. (5) Downward intrusion of slurry into rheologically weak zones led to the development of additional bifurcations.