Doctoral Degrees (Earth Sciences)
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- ItemGully dynamics evolution under environmental change pressures(Stellenbosch : Stellenbosch University, 2024-03) Olivier, George; De Clercq, W. P. ; Van De Wiel, Marco; Fried, Jana; Mashimbye, Eric; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Gully erosion is a severe land degradation process, primarily impacting land resources on-site and water resources off-site. When active in a catchment, it can be the dominant driver of soil loss, causing significant environmental and socio-economic consequences. However, other soil erosion mechanisms remain at the forefront of research, which contributed to our inability to assess gully erosion on a catchment to regional scale. The current capability to model gully erosion on larger geographic extents remains limited due to the complexity of interactions of control factors and various sub-processes driving gully expansion. In this study, an approach to apply local case studies to inform on regional gully severity is introduced to address modelling shortcomings, and an initial scaled framework is provided, which could be implemented for future regional scale investigations and monitoring. South Africa has a long history of erosion problems and has been considered an area with high gully incidence. The “hotspot” perception, coupled with the diverse climatic and geo-environmental attributes exhibited in South Africa, motivated the use as the focal region for this study. Local case study sites were used to extract physiographic properties and gully severity to produce a susceptibility map for South Africa. Additional local sites were selected across the E-W climate gradient of South Africa to assess gully severity and to isolate climate and land use controls of gully erosion to provide clues on how environmental change may influence future gully erosion. The findings from the susceptibility map, which used secondary data from the literature, converged with the findings from primary data derived from sites located across the climate gradient of South Africa. Gully erosion severity increases eastwards towards the Grassland biome, in which gullying is most severe. Here, gully erosion resulted in soil losses of up to 17 t ha⁻¹ y⁻¹, which exceeds the baseline limit (27 times more) and is almost twice the sustainable limit calculated for South Africa when the upper thresholds for both these limits are used. Perceptions from landowners/ -users/ and -managers mostly align with gully concerns from the field sites, showing that their appraisals are concurrent with local gully severity. Remediation efforts are ongoing at several sites; however, measures focus on gully headcuts and do not consider vegetation establishment. Vegetation is considered critical, especially for long-term success rates of mitigation, and could be a reason for the lack of successful mitigation. The poor success rate is also disconcerting, as climate change will likely exacerbate gully erosion in South Africa. Although climate change is predicted to increase gully erosion due to larger storm magnitudes, the data presented here indicates that rainfall intensity is likely to play a secondary role in exacerbating gully erosion. Rainfall variability may be the principal driver of gully erosion. If climate change increases the frequency of El Niño Southern Oscillation events, gully erosion severity may increase and even reactivate previously stabilised gullies due to more intense rainfalls after periodic droughts. Continuous assessment and monitoring of gully extents are crucial to assessing where gullies are of concern and whether there is a change in severity. Manually digitising gullies or solely relying on fieldwork will not sufficiently address a need for monitoring via temporal data. Semi-automated detection strategies which are scaleable and transferrable would enable the extraction of gully dimensions unbiasedly and would allow to quantitively assess gully expansion (or contraction) by subtracting polygon- or raster-based output. A semi-automated approach that uses gully morphology to extract gully dimensions is developed and tested with datasets from South Africa, Namibia, Spain, and Australia. Initial assessment shows positive results, accurately predicting > 75.4% of the gullied area when scaling between small gullies (planimetric area of 1619 m²) to large gullies (planimetric area of 70246 m²). Regarding transferability to benchmark areas where other land uses were practised and where different spatial resolution data were used as input, the variance between 1.4% and 14.8% was determined, with producer accuracies above 84.5% and 70.6%. The semi-automated method has some shortcomings, with the requirement for manually digitising gully headcuts being the most pertinent. As a framework, regional assessments and monitoring should implement a scaled approach. The initial step should produce a susceptibility map using key variables associated with gullying. Following that, more computationally intensive detection strategies could be implemented, constrained to areas of most concern defined by susceptibility. Lastly, representative field sites can be identified from the detected gullies, where primary data can be retrieved to quantify gully processes, severity, and implications. Continued work is required to refine this framework, for example, refining semi-automated approaches to increase accuracy and increasing localised field sites in different geo-environments to improve trend analysis and better our understanding of how various controls interact to steer gully evolution. Lastly, this new information should yield data that can be used to build and calibrate models; such gully evolution modelling currently needs to be improved and is pivotal to further our understanding of how gully networks will react to climate and land-use changes.
- ItemGold deportment and ore characterisation of the historical Witwatersrand tailings dams with emphasis placed on the sulphides(Stellenbosch : Stellenbosch University, 2024-03) Chingwaru, Steve Jason; Von der Heyden, Bjorn; Tadie, Margreth; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: The Witwatersrand Basin, discovered in 1881, has yielded over 53,000 tons of native gold hosted in quartz pebble conglomerates. Present gold extraction methods involve comminution and direct cyanidation, resulting in the retention of 5-10 % of residual gold in tailings. Re-mining primarily targets native gold recovery, achieving an average of 30-50 % gold recovery through direct cyanidation. This implies that the remaining 50-70 % of unrecovered gold, along with approximately 30 million tons of residual sulphide waste, is re-dumped to the tailing stream. The Witwatersrand tailings significantly contribute to South Africa's pollution, causing acid mine drainage and deleterious element effluent due to sulphide waste oxidation. The mineralogical distribution of unrecovered gold during tailings reprocessing is inadequately characterized and absent from existing literature. Furthermore, the current body of literature lacks a comprehensive environmental assessment specifically addressing the movement and dispersion of deleterious elements during sulphide oxidation within the Witwatersrand tailings. This PhD research focuses on the ore characterization and mineralogical deportment of unrecovered gold within the Witwatersrand tailings from Klerksdorp, Carletonville, Evander, and Central Rand goldfields. Simultaneously, an environmental assessment explores deleterious element behaviour, retention, and release mechanisms during sulphide oxidation in Witwatersrand tailings dumps. The methodology utilized employs the multi-element, high- resolution, and low-detection capabilities of laser ablation inductively coupled plasma mass spectrometry analysis, combined with an automated mineralogical technique to analyse bulk tailings samples and their operationally defined mineral fractions. Additional analytical methods include aqua regia digestion combined with ICP-MS analysis and fire assay. Mineralogical analysis using optical microscopy, electron microscopy, and X-ray diffraction analysis provides comprehensive insights into the composition and physical properties of minerals. Metallurgical test work, incorporating gravity separation, direct cyanidation, and diagnostic leaching, validates Witwatersrand tailings behaviour in processing scenarios. Results indicate cyanide amenable gold recovery at 14- 61 %, primarily in the form of micron to nano-sized native gold particles. Non-cyanide amenable phases contributing to the refractory behaviour of gold include sulphides (mainly pyrite) and silicates at 4-38 % and 11- 32 %, respectively. In situ pyrite analysis reveals 'invisible' gold within detrital pyrite and arsenian pyrite. These Archean detrital pyrites exhibit grades comparable to auriferous sulphides from surrounding Archean granite- greenstone gold ores on the Kaapvaal craton (up to 2700 ppm). Pyrite also hosts a substantial concentration of deleterious elements, with Co, Au, and Pb strongly leached during pyrite oxidation. However, a significant proportion of As, Ni, Cu, and Zn is retained in the iron oxyhydroxide alteration rims through incorporation and absorption. The study underscores the necessity of addressing 'invisible gold' during beneficiation, proposing tailored leaching parameters and a pretreatment strategy to potentially recover up to 420 tons of gold, thereby contributing to the local economy. Furthermore, the recovery of metals can not only reclaim valuable 'sweetener' by-product metals such as Cu, Co, and Ni but also directly mitigate issues related to deleterious element pollution and acid mine drainage associated with surface tailings dumps. The findings presented in this dissertation represent significant and original contributions to multiple scientific disciplines, notably including geometallurgy, geology, mineral processing, and environmental sciences.
- ItemA phase equilibrium modelling investigation of the consequences of entrainment of components of the source on the crystallisation of mantle-derived magmas in the upper crust, with specific relevance to the petrogenesis of chromitite layers in the Rustenburg Layered Suite of the Bushveld Complex, South Africa(Stellenbosch : Stellenbosch University, 2023-03) Otto, Tahnee; Stevens, Gary; Moyen, Jean-Francois; Mayne, Matt; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Thermodynamic modelling techniques offer several advantages over experimental studies in investigations of the partial melting of mafic and ultramafic rocks under upper mantle conditions, but may not be as reliable as experimental studies in accurately predicting rock behaviour. The two main thermodynamic datasets that are in common use for such thermodynamic investigations have different levels of coverage. The Berman (1988) dataset is utilised by the MELTS family of software (Asimow & Ghiorso, 1998; Ghiorso & Sack, 1995; Ghiorso et al., 2002), and along with routines for handling activity-composition relations for solid and liquid solution phases, is embedded within the software. In contrast, the Holland & Powell (2011) dataset is used by a wide range of different software with different computational strategies. Activity-composition relations are handled by published models that are selectable and transparent. One of the programs using the Holland & Powell (2011) dataset, Rcrust (Mayne et al., 2016), has been developed to allow modelling of phase stabilities with changing bulk composition, which makes it particularly powerful for studying processes involving fractionation. In this study, the results of sets of experiments on ultramafic and mafic compositions under upper mantle conditions were compared with outputs from MELTS and Rcrust. Differences between the modelling and experimental results were quantified, and the Holland & Powell (2011) dataset combined with appropriately selected activity-composition models produced the best match with experimental results when melt fraction, melt composition, and the assemblage of coexisting phases were considered. The results demonstrate that modelling is a viable alternative to experimental techniques in investigating melting of the upper mantle. Thermodynamic modelling was used to map the envelope of uncertainty that exists around experimental results due to often unavoidable experimental pitfalls, and demonstrated that factoring in these uncertainties produced a better fit between modelling and experimental results. Next, this study developed a thermodynamic modelling methodology to investigate the stoichiometry of the melting reactions in an eclogite and peridotite as a function of pressure, temperature, and bulk compositional change due to melt loss. Pressure-temperature regions where the partial melting reactions produce peritectic crystals that increased in abundance with melt were identified. The entrainment of these crystals to the magma on segregation produces distinct shifts in the chemistry of the extracted magmas, for both sources. The results illustrate that variable amounts of peritectic crystal entrainment represents a previously unrecognised source of compositional heterogeneity in basaltic and intermediate magmas. The modelled results highlight how peritectic crystal entrainment may be a principal mechanism for creating the magmas that produce layered mafic and ultramafic complexes, because the entrained phases react with melt on ascent to produce magmas with a high crystal load once they reach the upper crust. During the partial melting of peridotite, the most common entrainable peritectic mineral is orthopyroxene, which is Cr2O3- bearing. On ascent, entrained peritectic orthopyroxene reacts to form olivine and chromite, and such magmas will enter high-level magma chambers as a melt-olivine-chromite slurry. Density segregation on intrusion allows the formation of magmatic layering if the magma body has appropriate geometry. Thus, these results provide an explanation for the presence of chromitite seams in layered mafic complexes, as well as average bulk rock Cr2O3 contents well above mantle values in some layered intrusions, such as the Bushveld Complex. Chromitite layers are a consequence of peritectic orthopyroxene entrainment and do not require any complex phase equilibrium magma chamber processes.
- ItemPhytoplankton and trace metal dynamics in the Southern Ocean(Stellenbosch : Stellenbosch University, 2023-03) Viljoen, Johannes Jacobus; Fietz, Susanne; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Marine phytoplankton in the Southern Ocean are essential for the Antarctic food web and help regulate the global ocean biogeochemistry, thereby mediating the warming effect of carbon dioxide (CO2). Yet, spatially and temporally, phytoplankton distribution and their controlling factors are still poorly studied in the Southern Ocean. Owing to low sampling resolution, features such as frontal and island regions have been less sampled. Moreover, the dynamics of trace metals which serve as essential micronutrients has been the least studied in the Southern Ocean. Most studies focussed on iron (Fe) and total chlorophyll-a, few on extended suites of metals and phytoplankton groups. There are still uncertainties how the trace metals Fe, copper (Cu), zinc (Zn), nickel (Ni), cobalt (Co), manganese (Mn), and cadmium (Cd), will affect phytoplankton abundance and community composition, especially in the understudied Southern Ocean during winter. In this thesis Southern Ocean phytoplankton dynamics and the complex interplay with various parameters were elucidated. Through three research papers, phytoplankton distribution was studied utilizing multi-parameter datasets, collected during two research cruises to the Southern Ocean one during summer in the Atlantic sector (0 - 8°E) and another during winter in the Indian sector (30°E). Summer phytoplankton distribution were characterised in the Atlantic Southern Ocean at high resolution across the major zones, within fronts, in sea ice influenced zones (e.g., polynya) and close to Subantarctic islands using a suite of HPLC phytoplankton pigments. Cyanobacteria dominated the Subtropical zone; haptophytes the Subantarctic and Polar Frontal zones while diatoms and haptophytes dominated the Antarctic zone south of the Polar Front. Additionally, distinct communities were observed in frontal, sea ice and near-island regions. For example, phytoplankton abundance in fronts were up to 10-fold higher than in the major zones while haptophytes dominated communities close to the Subtropical Front. Diatoms dominated in the areas affected by recent sea ice melt while Phaeocystis and coccolithophores dominated post-bloom communities. Close to Subantarctic islands, cryptophyte blooms were observed whereas diatoms were dominant further downstream. Results suggest that a combination of nutrient, including trace element supply and mixing regime are essential in controlling the magnitude and composition of blooms close to fronts, sea ice and islands, and in turn, affect Southern Ocean food web activity and potential carbon export. Having noticed the importance of zonal changes and small-scale features, as well as the likely importance of trace element supply, the second and third study included these dynamics. The relationship between phytoplankton dynamics (abundance and community structure) and micronutrients (i.e., trace metals) were investigated for the first time during winter in the Indian sector of the Southern Ocean. This was achieved through two studies using a unique dataset of HPLC chlorophyll-a and accessory pigment concentrations with parallel sampled macronutrients and a suite of dissolved and particulate trace metals and phosphorous concentrations. Results suggest phytoplankton were still active and a dominant contributor to the uptake and remineralisation of trace metals even though their abundance were lower than during summer. Through a suite of metal* calculations, based on the macro- and micronutrient concentrations and the estimated requirements of Southern Ocean phytoplankton, varying degrees of deficiency and potential for co-limiting conditions were proposed. Accordingly, micronutrients are suggested as a major driver of winter phytoplankton abundance and community structure across the Southern Ocean. Some trace metals had stronger relationships with specific phytoplankton groups compared to total phytoplankton abundance. In turn, specific groups, such as diatoms, were confirmed to be major drivers of trace metal dynamics across the transect through preferential uptake. For example, cyanobacteria, a group not considered by previous trace metal studies in this region, was suggested to be dependent and responsible for major uptake of Co and Mn while diatoms were strongly associated with Zn. Ultimately, the preferential uptake by specific phytoplankton groups in deficient conditions, such as diatoms, can aggravate limiting conditions and lead to a change in composition. Therefore, the strong association of specific phytoplankton groups, with different remineralisation lengths, to specific micronutrients during winter would affect the release of these micronutrients for the utilisation by phytoplankton during the following spring and summer seasons. Hence, these two studies yielded new knowledge on phytoplankton-micronutrient dynamics that contributes critical seasonal information for biogeochemical models. Collectively, the research in this thesis demonstrates the importance of understanding not only phytoplankton abundance, but also its community composition and how small- and large-scale changes in the chemical environment, including the availability of trace metals, can influence phytoplankton dynamics.
- ItemAn investigation of the molecular-level mineral chemistry of metal-bearing pyrite and its electrochemical behaviour under flotation related conditions(Stellenbosch : Stellenbosch University, 2023-03) Babedi, Lebogang; Von der Heyden, Bjorn; Tadie, M.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Pyrite (FeS₂) is an iron disulphide mineral found in hydrothermal ore deposits, including gold ore deposits, each having its unique physicochemical conditions (e.g., pH, temperature, salinity) at the emplacement site. Pyrite can incorporate several trace elements into lattice sites, making it an n- or p-type semiconductor in nature. Due to this semiconducting variance, numerous investigations have observed diverse flotation responses for pyrite from different ore sources. This study investigates how lattice-incorporated metals (As, Au, Co, and Ni) affect electronic structure and flotation collector (xanthate) response in an alkaline media. This is done by compiling a global dataset of pyrite trace element data to understand its trace element signatures and then utilizing these signatures to guide chemical vapour transport synthesis of high-purity crystals. This work uses X-ray photoelectron spectroscopy and rest potential analysis to investigate the impact of metal geochemical type and concentration on pyrite valence bands and reactivity. Valence band assessments demonstrate that metals in the pyrite lattice shift orbital contributions and Fermi levels depending on geochemical origin and concentration. The metal's nature dictates whether it effects valence band contributions around the Fermi level (Au, Co, Ni) or deeper ones (As). Pyrite's changing chemistry affects its oxidation and interaction with xanthate collector under alkaline conditions. Pyrites (pure, Co, Ni, and Au + Co-bearing) are noble and do not induce mineral surface oxidation, while As- bearing pyrite is the least noble and promotes oxidation. Noble pyrites (pure, Co, and Ni) associated with n-type semiconducting have a weaker collector interaction than the least noble (As-bearing) associated with p-type. The lack of dixanthogen on As-bearing pyrite compared to Co- and Ni-bearing pyrite shows that a greater collector-mineral interaction does not oxidize the collector on the mineral surface. Dixanthogen is present at low Ni concentrations but absent at higher concentrations, showing that collector oxidation on the mineral surface depends on metal concentration. Reactivity and electronic structural trends are correlated. The potency of collector-mineral interactions and the size of Fermi level variations as a function of metal concentration and geochemical nature are comparable. This thesis shows how synthetic minerals grown experimentally may answer important questions regarding the molecular chemistry and reactivity of sulphide minerals like pyrite. This study shows how metals impact pyrite's valence band contributions and how they affect the collector interaction. The behavior of pyrite with a xanthate collector gives important knowledge that may be used to adjust flotation collectors to best show minerals' selectivity and reactivity independent of their semiconducting qualities controlled by chemistry.