Faculty of Science (ETDs)

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Cost-effective and novel seismic methods for mineral and coal exploration: Examples from Witwatersrand goldfields and Bushveld Complex
(University of the Witwatersrand, Johannesburg, 2023) Sihoyiya, Mpofana; Manz, Musa S. D.
The reflection seismic method has its origin from oil and gas exploration in the land and offshore sedimentary soft rock environment. Since the inception of the reflection seismic technology in hardrock environment, more advanced processing methods such as migration algorithms have been established to revamp the quality of hardrock seismic data for deep mineral targeting. This study shows the value of recovering and reprocessing legacy reflection seismic data using advanced processing techniques that were not available at the time of acquisition. This is achieved through different novel processing workflows that incorporate iterative static corrections, Kirchhoff pre-stack depth migration (KPreSDM), Kirchhoff pre-stack time migration (KPreSTM), and the newly developed Fresnel-volume (FV) and coherency migration (CM) techniques. This research also shows the value of novel processing of the seismic data acquired in-mine challenging environments. The legacy data from the Kaapvaal Craton in South Africa have been processed to improve the delineation of the deep-seated mineral deposits such as the gold-bearing horizons (termed reefs) in the Witwatersrand Basin and Platinum Group Element (PGE) horizons in the Bushveld Complex. Furthermore, the shallow coal seams and associated geological structures of the Karoo Supergroup in the Evander Basin, an arcuate basin characterized by the Witwatersrand Basin towards the south and the Bushveld Complex towards the north, have been delineated for future mine planning and designs. To better image and understand the geometry of the gold-bearing reefs and crosscutting geological structures in the South Rand goldfield, an 18 km long legacy two-dimensional (2D) reflection seismic line was reprocessed using today’s standard processing workflow which employed iterative static corrections. Improved structural imaging of the steeply dipping faults (mostly normal and reverse), as well as dolerite intrusions, was mostly achieved by using KPreSTM and KPreSDM, with KPreSDM providing better structural imaging than other techniques. Moreover, reprocessing of this profile using an improved velocity model and numerical simulations assisted in delineating near surface stratigraphic units and deep-seated (> 1 km depth) geological structures that are associated with the gold-bearing reef but missed by the legacy post-stack time migrated section. In the Bushveld Complex, the PGE deposits (known as platinum reefs) are delineated using a newly developed depth migration technique called coherency migration. Imaging through CM workflow shows evidence of the complex structural architecture that controls the platinum deposits in the study area. The 50 km long legacy 2D profile traverses towards the center of the complex and provides a better understanding of the tectonic evolution in the region. Additionally, magnetic data were utilised to constrain seismic interpretation and delineate the highly magnetic major geological structures such as the Chaneng structure and strongly magnetized rocks of the Rustenburg Layered Suite. The Chaneng structure, in particular, was poorly defined by the seismic data due to its steeply dipping nature. In 2020, in-mine seismic experiments were conducted to delineate the PGE-bearing horizons such as the Upper Group-2 (UG2) and Merensky Reef. Three reflection seismic profiles from these experiments were cautiously processed to attenuate the infrastructure-generated noise. The experiments were successful and delineated the Merensky Reef and UG2 mineralisations at depths between 55 m and 124 m beneath the developmental tunnel (~550 m below the surface). Six legacy 2D reflection seismic profiles acquired for gold exploration in 1986 in the Evander Basin are processed for imaging both the Witwatersrand Basin and the Bushveld Complex, as well as the coal seams of the Highveld and Witbank coalfields. The profiles were acquired perpendicular to each other providing pseudo-three-dimensional (3D) imaging of the subsurface and better delineation of the geological structures. The degree of faulting and folding that affect the mineralisation observed on the processed profiles in all sites would not have been achieved if advanced processing workflows were not implemented. Processing of the legacy data provides a more cost‐effective way to explore the mineral deposits than reacquiring new data, which could be costly and limited by surface conditions. The processing approaches used in this thesis can be used in other brownfield mining regions where the legacy data exist, and they can also be used to revamp the quality of the data acquired in noisy mining environment
Quantitative analysis of gold in low-grade tailings from different matrices, coupled with a study into the associated uncertainties
(University of the Witwatersrand, Johannesburg, 2023) Mashale, Kedibone Nicholine; Tshilongo, James; Chimuka, Luke
Gold is one of the precious group elements that is used for various purposes, such as jewellery, auto catalysts and as a form of investment. Various countries have gold reserves, with South Africa being the leading gold producer between 1980 and 2007. However, as of 2022, it is ranked as the eighth largest producer of gold, contributing 3% to the global contribution. The majority of gold is mainly mined from the Witwatersrand Basin in Johannesburg. It is well known that mining has been ongoing for decades, which means that a significant amount of land has been mined across the country. During gold mining, a large proportion of the ore material from which the gold is extracted is waste, together with the chemicals that were used, and this waste is termed mine tailings. This implies that based on the years that gold mining has occurred for and the depth of mining, a significant amount of the tailings have been deposited into free land around the mines, some of which are close to communities. The tailings consist of traces of gold that were left due to inefficient extraction processes and other components, such as base metals. The disadvantage of this is that due to the other chemical composition of these tailings, they have the potential to be dangerous to the environment. Some tailings contain minerals such as jarosite (KFe2(SO4)2(OH)6) that cause acid mine drainage, while heavy metals such as lead, mercury, arsenic and chromium can leach into surface and ground waters, causing pollution. Furthermore, they pose a danger if the dams that they are stored in collapse, which was recently witnessed in South Africa. Because of these factors, there have been various advances made towards the beneficiation of tailings, such as utilizing them to make glass or bricks for construction. A major advancement was the reprocessing of these mine tailings to recover or extract the remaining gold, which benefits both the environment and the mining houses. Therefore, in a move to support this initiative, scientists have taken to the laboratory to develop new or optimize existing methods for the extraction and quantification of gold, which is expected to be of a low grade over time. Various methods can be used for the quantification of gold, including the conventional fire assay, wet and dry chlorination and acid digestion. Most of these are suitable for medium- to high-grade gold ores but are known to experience challenges in regard to low-grade ores. The aim of this research was therefore to find the optimum method for the quantification of gold from mine tailings emanating from the Ventersdorp Contact Reef (VCR) and Barberton Greenstone Belt (GBS). Subsequent to chemical analysis, the samples were characterized for mineralogy using X-ray diffraction (XRD) and Brunauer‒Emmett‒Teller (BET) surface area
The effects of elevated carbon dioxide concentrations on the physiological and biological characteristics of Portulacaria afra
(University of the Witwatersrand, Johannesburg, 2024) Basson, Domonique Courtney; Risenga, Ida
There is a concern that rising atmospheric carbon dioxide (CO₂) concentrations may affect the medicinal or nutritional profile of medicinal plants. Portulacaria afra (P. afra) is a medicinal plant used by traditional healers to treat various skin conditions. The aim of this study was to determine whether elevated CO2 concentrations will affect the physiological and medicinal properties of the leaves, stems, and roots of P. afra. This was achieved by measuring the physiological, phytochemical, antioxidant, and antimicrobial activity of the various plant parts, which were exposed to ambient (420 ppm) and elevated (600 ppm) CO₂ concentrations and comparing them to samples grown in greenhouse conditions. The plant samples were placed in a Conviron climate stimulator for three months. Three harvests were completed during thistime. The physiological properties measured include the weight of the various plant parts as well as the chlorophyll content within the leaves. The phytochemical profile of the plant was examined through a set of standard colour tests (qualitative analysis) and the determination of the total flavonoid content (quantitative analysis) in all three plant parts exposed to the various treatments. The antioxidant activity was determined by analysing the scavenging activity of the extracts against 1,1 diphenyl-2- picrylhydrazel (DPPH) and hydrogen peroxide (H2O2) with the use of a spectrophotometer. The antimicrobial activity determined the extracts’ ability to inhibit the growth of six microorganisms related to skin conditions. The results of this study revealed that there was a 65% and 39% increase in the weight of the leaves and roots, which were exposed to 600 ppm, respectively. A higher phytochemical presence was recorded in plants exposed to 600 ppm. There was a significant increase in flavonoid presence and flavonoid content in the methanolic and hot water plant samples that were exposed to elevated CO2.The leaves, stems, and roots of P. afra exhibit strong scavenging activity against DPPH and even more so towards H2O2. The strongest antioxidant activity was 13 exhibited by the methanolic leaf extracts which were exposed to 600 ppm, against H2O2 (0.2±0.7 mg/ml). The antimicrobial activity of all three plant parts was relatively constant between the different treatments. Despite the relatively weak antimicrobial activity of P. afra (MIC values > 1000 (µg/ml), a notable increase in the antimicrobial activity of the leaves against Cutibacterium. acnes were observed in samples exposed to 600 ppm of CO₂. Portulacaria afra is a medicinal plant that exhibits great resilience towards elevated atmospheric CO2 concentrations. The phytochemical and biological properties in this study displayed either no change or an increase in activity, which suggests that P. afra may continue to provide relief against certain ailments in the future despite rising atmospheric CO2 concentrations
The reproductive system of Campuloclinium macrocephalum and its implications for biocontrol implementation
(University of the Witwatersrand, Johannesburg, 2022) Moodley, Saness; Glennon, Kelsey; Goodman-Cron, Glynis
Invasive species are a threat to biodiversity therefore it is imperative to determine the factors that facilitate the invasion potential of a species. Campuloclinium macrocephalum Less. (DC), the ‘pompom weed’, is an alien invasive species in South Africa and is currently threatening the persistence of the grassland, wetland, and savanna biomes. The species is also significantly contributing to a decline in plant diversity by outcompeting native vegetation in these areas. Various integrated approaches using combinations of chemical, mechanical and biocontrol management programs have been developed to manage the spread of the species in its invaded range, however the species has still been able to persist. The persistence of the species was hypothesised to be a consequence of the co-occurrence of apomixis and polyploidy, however despite the identification of triploid and tetraploid cytotypes in South African populations of the pompom weed, the reproductive strategy of the species has not yet been determined. The aim of this study was therefore to infer whether populations of C. macrocephalum (pompom weed) reproduces via vector-mediated crosses, self-pollination or apomixis (either facultative or obligate) and examine the relationship of the mode of reproduction with ploidy level. Male fertility was also assessed to ascertain if interploidy gene flow was possible. The collated information was then used to infer the potential impact of reproductive strategies and polyploidy on biocontrol. All examined populations were shown to have high mean pollen viability percentages of 90% and 98% with no significant differences in pollen viability amongst the four populations. The high pollen viability percentages were supported by prolific pollen grain germination on the stigmatic surfaces (margins of style at base of style branches) and the sides of the style. This suggested that the pollen grains can fertilize and interploidy mating is likely possible in South African populations of the pompom weed. It is plausible that the high pollen viability is enabling triploids to act as a ‘triploid bridge’. However, the high pollen viability was confounded by the pollen tube analyses revealing that pollen tube growth is being arrested on the stigmatic surface suggesting that overall male fertility is low. The arrested pollen tube growth is typically associated with a ‘triploid block’. Nevertheless, the production of viable gametes can reduce the triploid block and facilitate gene flow between populations. The predominant mode of reproduction was determined by assessing the contribution of insects to pollen transfer, pollinator exclusion experiments, germination trials, pollen tube growth to the ovules and genetic analyses. We found that the African Monarch butterfly (Danaus chrysippus) and the honeybee (Apis mellifera) contributed most to pollen transfer in comparison to the other insects visiting C. macrocephalum. A pollinator exclusion experiment showed that the pompom weed can set seed in the absence of pollinators, albeit at lower quantities than in the open treatments. Nevertheless, germination percentages showed that reproductive success was similar between open and bagged treatments in each population. The Modderfontein population showed lower reproductive success and seedling establishment in comparison to the other populations, presumably due to the severity of the biocontrol infestation on this population. Genetic analyses revealed low genetic variation within and amongst populations. Pollen tube analyses showed no pollen tube growth to the ovules in all samples, which suggests that seed set is independent of fertilization. The lack of pollen tube growth is a strong indicator of obligate autonomous apomixis which is further corroborated by the low genetic differentiation between maternal plants and their respective offspring. The co-occurrence of apomixis and polyploidy made it difficult to discern which factor contributes more to the invasiveness of the species, however, we hypothesise that autonomous apomixis provides the pompom weed with the competitive advantage to persist in its invaded range. However, further studies on the reproductive strategies of tetraploid cytotypes are needed to confirm this hypothesis. The low genetic variation suggests that all populations should be equally susceptible to biocontrol agents, however that this may be affected by other factors such as environmental conditions or phenotypic plasticity. Phenotype plasticity refers to a single genotype producing different phenotypes in response to environmental conditions. This could reduce the efficacy of biocontrol agents as they may exhibit differential responses on different phenotypes

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