Electronic Theses and Dissertations (PhDs)

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Late Triassic to Early Jurassic ecology: An insight into diet and trophic levels using non-traditional Ca isotopes
(University of the Witwatersrand, Johannesburg, 2023-07) Davechand, Priyanka; Bybee, Grant; Choiniere, Jonah
The diet and trophic structuring of organisms in deep time is poorly understood, making comparison of ancient and modern ecosystems challenging. Proxy data (e.g., dental morphology, jaw muscle reconstruction) remain the most common mode of palaeodietary inference, but the correlative strength of these proxies remains untested due to a lack of direct evidence and an incomplete sampling of palaeobiodiversity. These major challenges in palaeodietary reconstruction can be overcome using novel geochemical markers in fossilised tooth enamel, which provide direct evidence of palaeodiet and trophic relationships. Traditional stable isotopes of carbon, oxygen, and nitrogen have been used in the past to infer palaeodiets, but these elements are susceptible to diagenetic alteration during fossil preservation and require large sample amounts for assays. In contrast, non-traditional calcium (δ44/42Ca) isotopes are less susceptible to diagenesis and require significantly smaller sample amounts. This, together with the fractionation that Ca isotopes undergo as a bio-essential element, allows δ44/42Ca to be utilised on a broad range of palaeontological questions including assessing dietary range and trophic level. The diverse ecosystems of the Elliot Formation (Karoo Supergroup) in South Africa are represented by abundant fossils of a variety of reptilian and mammalian stem lineages that coexisted during the latest Triassic–earliest Jurassic interval (218–190 Ma). The broad range of body sizes, inferred dietary preferences, and phylogenetic positions make the Elliot palaeoecosystems an ideal natural laboratory in which to apply palaeodietary isotopic tools. This dissertation aims to assess the palaeotrophic divisions of the Elliot Formation vertebrates using non-traditional δ 44/42Ca isotopes. This research uses ion-exchange chromatography on vertebrate tooth enamel to assess the palaeodietary preferences of Elliot Formation reptilian and mammalian lineages. To obtain these data, existing techniques for sample preparation of non-traditional δ 44/42Ca isotopes were modified and optimised at the Wits Isotope Geoscience Laboratory (WIGL) at the University of the Witwatersrand. δ 44/42Ca analysis was conducted on a variety of specimens across a broad range of amniote lineages, ranging from: dinosaurs such as presumed herbivorous sauropodomorphs Massospondylus and Aardonyx, the presumed omnivorous ornithischian (Lesothosaurus and Heterodontosaurus), and the presumed carnivorous theropod Megapnosaurus; to cynodont therapsids (Tritylodon, Pachygenelus and Scalenodontoides); to pseudosuchians such as the crocodylomorphs Protosuchus and Orthosuchus and earlier branching taxa (‘rauisuchians’ and poposauroids). A leaching procedure was also tested to ensure that the results produced were not influenced by diagenetic biases. Once consistent and reproducible methods were finalised, column chemistry and Multicollector-Inductively Coupled Plasma Mass Spectrometery (MC-ICPMS) analysis was conducted on the different Karoo-aged specimens. There are various outcomes from this dissertation. One important outcome was the optimisation of time for Ca separation using ion-exchange chromatography. This allowed for a shorter chemical preparation time and increased the number of analyses completed per session. Another improvement of the method was that the leaching procedure can be used to control for any diagenetic biases by removing secondary calcite in samples as old as those from the Triassic–Jurassic period. Elliot taxa were then analysed, and significant differences were found between δ 44/42Ca values of large carnivorous pseudosuchians (‘rauisuchians’; -0.45 ‰ to -1.17 ‰) and co occurring herbivorous sauropodomorph genera (-0.26 ‰ to -0.69 ‰). These results indicated that non-traditional δ 44/42Ca isotopes can be used to understand trophic structures and palaeodiets in ecosystems at least 210 million years old. We also found that while some taxa had δ44/42Ca isotope values in-line with their presumed diets, other taxa had more diverse diets than initially presumed. δ 44/42Ca-enriched values in this study provide evidence for herbivory in crocodylomorph and the oldest theropod. There is also a possibility of an omnivorous diet for presumed herbivorous Lesothosaurus as the δ 44/42Ca values are relatively depleted to other herbivores. In addition to diet, calcium plays a major role in the formation of reptilian eggs and there are documented changes in δ 44/42Ca values during the reproductive cycle. To assess this in a living system, Crocodylus niloticus, was analysed to understand if δ 44/42Ca could be used to identify the difference in sex based on the δ 44/42Ca values. No isotopic differences were found between the juvenile male and female Crocodylus niloticus samples. Testing these important ecological principles in temporally constrained formations allows us to understand the historical nature of biodiversity changes, especially across periods when environments on Earth were experiencing extreme conditions. The ability to determine factors such as palaeodiet and palaeotrophic range will enable the development and improvement of palaeoecological analysis. This research presents the first ever δ 44/42Ca values on Karoo-aged vertebrate fossils and will have a large impact on how palaeoecological reconstruction is conducted in the future of palaeosciences.
Naturally occurring asbestos (NOA) and asbestos contamination of the environment: Implications for in-situ risk assessment and rehabilitation
(University of the Witwatersrand, Johannesburg, 2023-01) Schapira, Jessica Shaye; Bolhar, Robert; Master, Sharad; Rumbold, Karl
The risk associated with asbestos minerals is an enduring global concern, especially with regards to exposure at the sources. The risks connected to exposure in their natural contexts (i.e., derelict mine sites and naturally occurring asbestos) form the subject of this study. These sites are plentiful throughout Southern Africa (and Africa) and are considered as unconfined, thereby constituting large environmental and human health risks. Asbestos in these settings is not inherently hazardous unless dispersed from its sources into environmental systems, such as the atmosphere, where it may be inhaled. Compared to occupational asbestos exposure, environmental sources present unique challenges with respect to their potential risks. Literature focused on these sites is lacking and without extensive knowledge the risks remain greatly unknown, and thus asbestos site assessment frameworks are imperative. South Africa, with its geological richness, allows for all aspects of environmental asbestos to be studied in its natural context. In this research, mineral fibres from derelict asbestos mine sites were characterised mineralogically and geochemically to identify and assess their human health hazard potential, to define the degree of toxicity and to determine the potential negative environmental effects. High concentrations of heavy metals, including copper, iron, magnesium, manganese and zinc detrimental to human health and environmental functioning were measured in these fibrous minerals using X-ray fluorescence (XRF) . The chemical stability of four asbestos minerals was studied using batch isothermal dissolution tests in acidic solutions and their stability determined as follows: chrysotile < crocidolite < amosite < anthophyllite. Significant inferences can be made regarding the persistence of asbestos particles in their natural environments from dissolution kinetics mechanisms. The mineralogical, geochemical, and microbiological characterisation of solid asbestos mine waste rock substrates indicate that their properties, such as low macro and micronutrients and lack of properties of true soils, strongly alkaline pH and low to zero microbial abundance and diversity present significant challenges to rehabilitation strategies. Such parameters are identified as important baseline conditions that need to be considered prior to rehabilitation implementation, if long-term, self-sustaining ecological restoration is to be achieved on these sites. Present bio-solutions to ensure asbestos mine land rehabilitation success are discussed and the potential of using available agronomic bio-fertilisers (microbial strain Pseudomonas fluorescens) is examined. The growth-based assays indicate that this micro-organism is suitable for an environmental biotechnology applied to ecosystem restoration of asbestos-mining lands. Naturally Occurring Asbestos (NOA) occurrences are highly dependent upon the geological conditions, and understanding these in a large variety of settings may allow the development of predictive strategies necessary for in situ identification and hazard assessment required for health risk mitigation in the context of mining and other geologically disruptive activities. A variety of natural asbestos rock samples was examined, showing how geological databases relevant for predicting natural occurrences of asbestos provide baseline data required for mitigating asbestos exposure risks.
Late Quaternary Palynological Studies at Lake St Lucia, KwaZulu-Natal
(University of the Witwatersrand, Johannesburg, 2023-08) Effiom, Angela Charles; Neumann, Frank; Bamford, Marion
Palynological studies were done on lacustrine sediments deposited during the last ~6300 and about ~2000 cal yrs BP in Mkhuze Swamp, which drains into the most northern part of Lake St Lucia located in the Indian Ocean Coastal Belt Biome of KwaZulu-Natal, eastern South Africa. The aim was to reconstruct the past vegetation and to infer past climate fluctuations as well as human disturbances to complement growing evidence from other disciplines about these questions in the area. Following standard palynological methods, samples from two cores Mkhuze River Delta (MKD-1, long core) and Mkhuze swamp (MK24-1, short core) were extracted and analyzed. Palynological results for both cores show a dominance of Poaceae (20-90%) suggesting a strong influence of grassy woodland savanna. Spirostachys pollen dominated the arboreal spectra of the pollen profile in MK24-1, but it dominated the arboreal pollen spectra for the last 2600 cal yrs BP in MKD-1 as Podocarpus pollen dominated the arboreal spectra from 6300-2600 cal yrs BP indicating a change from a forested environment to a more open woodland environment. The results also show that the mid Holocene was humid with high precipitation and high sea level while the late Holocene was warm, dry with Marine influence (more sea water flowing into the lake due to high evaporation and low fresh water supply caused by drought). Similar pollen fluctuations were observed in records from Lakes Eteza and Sibaya also located within the Indian Ocean Coastal Belt Biome. The presence of Pinus pollen at the top of the profiles suggests the onset of European settlement when pines were introduced for timber production. Other published pollen records from the region in the last ~7000 cal BP show a general trend from a mesic forested environment towards an open woodland environment with grasses which is due to climate change.
Reassessment of the Phylogeny of Basal Therapsida Using Micro-Tomography and Bayesian Phylogeny
(University of the Witwatersrand, Johannesburg, 2023) Duhamel, Alienor; Rubidge, Bruce; Benoit, Julien
This PhD research is focussed on the evolution and cranial anatomy of the earliest therapsids. The Therapsida is a paraphyletic group of land tetrapods that lived mostly during the Permian and Triassic Periods. They emerged quite suddenly in the fossil record during the middle Permian, already showing diversification into five distinct groups: Biarmosuchia, Dinocephalia, Anomodontia, Gorgonopsia and Therocephalia. A sixth group, the Cynodontia, appeared during the late Permian. Of these, the anomodonts, therocephalians, and cynodonts survived the devastating Permo-Triassic mass extinction and continued to be the dominant tetrapods during the Early and Middle Triassic Periods. Ultimately, the cynodonts would evolve into the earliest mammals. Chapter 1 introduces the scientific questions tackled in this thesis. Chapter 2 provides the necessary explanations about the material and methods used for this work. The use of CT-scanning technology allows for the detailed examination of cranial internal morphology, with the goal of incorporating inner cranial characters into a phylogenetic framework. Chapters 3 and 6 focus on the fossilisation gap at the early to middle Permian transition, which has led to debates over the timing of the early diversification of the Therapsida. In Chapter 3, the geologic position and anatomy of one of the oldest known therapsids, Raranimus dashankouensis is reviewed. The chapter concludes with a reassessment of the basal phylogenetic position of Raranimus and suggests a Roadian origin for the taxon. Chapter 6 presents a comprehensive analysis of cranial morphological characters based on the descriptions and findings from Chapters 3, 4, and 5. Using both traditional maximum parsimony and clock-based Bayesian inference of phylogeny for the first time on the Therapsida, the results suggest that therapsids originated around 280.5 million years ago and rapidly diversified into several distinct clades during the Kungurian and Roadian Epochs. The phylogenetic relationships of several taxa are reevaluated and the findings suggest that Biseridens is more closely related to the Dinocephalia and Biarmosuchia, rather than the Anomodontia. Furthermore, the results suggest that Sinophoneus may represent the basal-most dinocephalian, and that Therocephalia might be paraphyletic. Therapsids were characterised by a wide range of cranial morphologies, with the Biarmosuchia and Anomod ontia exhibiting distinct cranial features. Both groups are considered basal among therapsids. Chapter 4 of this PhD research focuses on the postnatal ontogenetic development of cranial ornamentation in Biarmosuchia and possible impact on phylogeny. The results suggests that cranial bosses and ridges are ontogenetic features and the parietal bone originates from multiple centres of ossification. Chapter 5 is a comprehensive cranial description of several basal anomodont specimens. The Chapter concludes with a review of the taxonomy of the genus Eodicynodon and proposes the creation of a new taxonomical unit for specimen NMQR 2913.
Trans-crustal and temporal perspectives of Palaeoproterozoic porphyry copper deposit formation (Haib deposit, Namibia)
(University of the Witwatersrand, Johannesburg, 2023) Jogee, Bibi Ayesha
Porphyryِcopperِdepositsِareِtheِworld’sِprimaryِsourceِofِcopperِwhichِisِanِessentialِcriticalِ metal in the green energy sector. These deposits are most often temporally found within the Phanerozoic Eon and occur principally at convergent plate boundaries. The Haib porphyry copper deposit is a rare example of a Palaeoproterozoic porphyry copper deposit. The Haib porphyry copper deposit is hosted by the rock types of the Richtersveld Subprovince. The Richtersveld Subprovince consists of an older volcano-sedimentary sequence, the Orange River Group (ORG), which was intruded by an extensive composite cogenetic suite of plutonic rocks, the Vioolsdrif Intrusive Suite (VIS). The Vioolsdrif Intrusive Suite is subdivided into several subunits, known as the Vuurdood Subsuite (mafic-ultramafic), Gaobis Diorite, Khoromus porphyry, Goodhouse Subsuite (Granodiorite) and the Ramansdrif Alkali Granite. These subsuites are variably composed of diorites, granodiorites, monzogranites, quartz- feldspar porphyry granodiorites and leucogranites as well as poorly-described clinopyroxenites and gabbros (named the Kokerboom Intrusion in this study). The age-range for the Richtersveld Subprovince is between approximately 1910 and 1865 Ma. The mineralization occurs as disseminations, blebs and veins in the feldspar porphyry andesites and quartz-feldspar porphyry granodiorites. The age of mineralization is bracketed at 1884ِ ±ِ 2ِMa. The Haib porphyry copper deposit is in close vicinity to clinopyroxene and amphibole-rich cumulates, which host novel sulphide mineralization, and present lines of evidence suggesting these are the deeper parts of the porphyry copper system. The mineralization is characterized into two main phases based on drill-core sample descriptions, petrographic observations, mineral liberation maps and isotopic analyses. Previous studies show that garnet and/or amphibole fractionation is related to the petrogenesis of porphyry copper deposits. The Haib porphyry copper deposit shows evidence for amphibole fractionation as opposed to garnet fractionation, which is supported by petrological and geochemical observations in this study. The Haib porphyry copper deposit also has limited ranges in δ65Cuِ andِ δ 56Fe signatures supporting exposure at deeper crustal levels of an arc system. Typical cumulates from deep crustal arc sections (arclogites) are coarse-grained, show cumulate textures and are composed of garnet-clinopyroxene-amphibole-iron-titanium oxides and in some cases plagioclase. The mafic-ultramafic cumulates such as the clinopyroxenites observed in the Kokerboom Intrusion, in close proximity to the Haib porphyry copper deposit, are coarse-grained, are composed of clinopyroxene-amphibole-iron-titanium oxides and consist of non-cotectic proportion of mineral phases. They also comprise a high modal abundance of pyroxene (>70 %) and show distinct cumulate textures. This study has demonstrated that the clinopyroxene and amphibole-rich cumulates which belong to the ultramafic end member of the Kokerboom Intrusion form part of the Vuurdood Subsuite. This conclusion was made through the analysis of lithological characteristics, mineral and alteration assemblages, major and trace element geochemistry and U-Pb age data. Various lines of evidence from this study indicate that the Haib porphyry copper deposit represents the deeper levels of the porphyry system. These lines of evidence include the lack of shallow alteration assemblages (propylitic and argillic alteration), the presence of early dark micaceous veins, the coarse-grained nature of the Vioolsdrif Intrusive Suite (clinopyroxenites, granodiorites, diorites, gabbros), which is indicative of slow cooling at mid-crustal batholith depths, and the presence of clinopyroxene and amphibole rich cumulates of the Kokerboom Intrusion. The clinopyroxenites are mafic, dense, and usually occur at mid- to lower-crustal levels. Geothermobarometry conducted on rocks of the Vioolsdrif Intrusive Suite yield anapproximate depth of crystallization between ~4-12 km which forms part of the mid-crustal levels of an arc system.
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
Utilizing legacy seismics and non-seismic geophysical methods for deep mineral targeting and near-surface characterization: implications for mine development planning
(University of the Witwatersrand, Johannesburg, 2023) Mutshafa, Ndamulelo; Manzi, Musa
This thesis demonstrates through several case studies how the reprocessing of legacy reflection seismic data using advanced algorithms can be of value to mineral exploration and mine development, especially in hard-rock environments. The thesis also showcases how the integration of seismic and non-seismic geophysical datasets can assist in delineating near- surface geological structures (e.g., boulders and fractures) for mine planning and designs. Papers I and II demonstrate how the reprocessed legacy reflection seismic data have been used to delineate and image the world-class gold deposits such as the Black Reef (BLR) and Ventersdorp Contact Reef (VCR) of the Witwatersrand goldfields in South Africa. The two legacy reflection seismic profiles (Paper I) were acquired in 1988 for deep mineral exploration and mine planning over an area that is dominated by dolomitic outcrops that cause scattering of seismic energy at the near-surface, preventing energy propagation into the subsurface. Various migration approaches, namely, pre-stack time, pre-stack depth, and post-stack time migration were applied to test their capabilities in improving structural imaging. Reprocessing results from the pre-stack depth migration using the Kirchhoff algorithm provided the most improved subsurface images, especially the deeper targets due to its ability to honour complex lateral variations in the velocity field. In addition, Kirchhoff's pre-stack and post-stack time migration techniques improved the continuity of the near-surface reflections below the dolomitic rocks. Paper II presents the results from the recovered and processed 25.3 km long legacy seismic survey that was acquired in 1983 by the Gold Division of Anglo-American as part of the Witwatersrand goldfields exploration program. The reprocessing of the data improved the imaging of the gold-bearing horizon termed Ventersdorp Contact Reef (VCR), which is situated at depths between ~2400 and ~4100 m below the ground surface near the South Deep mine in Fochville, South Africa. The pre-stack time and phase-shift migration approaches were tested during processing, and both revealed a dipping reflection associated with the gold-bearing horizon and major steeply dipping faults that crosscut and displace the deposit. The interpretation of the results was constrained using borehole logs and surface geology. This is encouraging and motivates the use of legacy seismic data in the exploration of deep-seated targets. Papers III, IV and V present the results from the use of multi-geophysical methods (resistivity, magnetics, seismic, ground penetrating radar and multichannel analysis of surface waves) at Tharisa platinum mine to provide a comprehensive understanding of the subsurface geology by accurately delineating and locating boulders, mapping fractures and groundwater aquifers to improve Platinum Group Elements (PGEs) mining efficiency and reduce risks. The results from these integrated geophysical methods were successful in complementing each other in terms of providing a clear picture of the near-surface geological structures to help the mine plan better for future operations.
Constraints on the genesis of orbicular granites and sulphide mineralization in the Koperberg Suite, South Africa and the Diana’s Pool area, Zimbabwe
(University of the Witwatersrand, Johannesburg, 2024) Dumisa, Senamile Siyaya
This work tackles a long-standing problem in petrology, the formation of orbicular granitoids. These bodies occur as entire facies or as areas enriched in orbicules within distinct facies of plutons. Some European examples are highly prized for their decorative qualities. However, their origin is a matter of much debate going back to times when the origins of granitoids themselves were not obvious and are confused by topics such as ‘granitization’ and over-exaggeration of metasomatic effects. Here, poorly studied outcrops of orbicular rocks from the Matopos granite batholith in the Diana’s Pool area, Zimbabwe and the Koperberg Suite, South Africa are tackeld. Enzman (1953) and Garvie (1969; 1971) studied the Koperberg Suite and the Diana’s Pool orbicules, respectively and employed field observations and petrography to examine and characterize the genesis of these rocks. However, there is little evidence presented to confirm their theories on how these rocks formed as previous work lacks detailed geochemical, mineral chemistry and isotope data to support Enzman and Gravie’s conclusions on the genesis of these rocks. Furthermore, there is no connection between the metallogenesis of the sulphide mineralization and the origin of orbicular rocks, which is a unique characteristic of particular orbicule sites in the Koperberg Suite. This study focuses on four different orbicular bodies (Orbicule Koppie, Henderson South, Henderson North and Hoogkraal Lease) from the Koperberg Suite, hosted in lithologies ranging from diorite to pyroxenite compositions. The orbicules from different orbicular share similar characteristics. Coarse- grained felsic cores, fine-grained and alternating ferromagnesian and feldspathic shells, and coarse- grained to pegmatitic matrices characterize them. The orbicules are generally spherical to ellipsoidal in shape, however, some appear to be abraded and deformed (e.g., Orbicule Koppie). The compositions and grain sizes of cores and the matrix are comparable in all localities. Both the matrix and the cores are medium- to coarse-grained and dominated by plagioclase (the matrix probably in slightly lesser proportions), microcline (in variable proportions, and seemingly absent in some cores), quartz, biotite, magnetite and orthopyroxene (in the case of Hoogkraal Lese, Henderson North and Henderson South). Contrary to the cores and matrix, shells are fine-grained and exhibit polygonal textures. In addition to this, the shells are dominated by biotite and orthopyroxene. Plagioclase in the Henderson North and Henderson South orbicules is more calcic than plagioclase in Hoogkraal Lease and Orbicule Koppie orbicules. Biotite in the Henderson North and the Orbicule Koppie orbicules is more magnesian than those at Henderson South and Hoogkraal Lease orbicules. Plagioclase in the Henderson North orbicules are characterized by elevated and radiogenic initial 87Sr/86Sr ratios than the other orbicule localities. There is also a variation in 87Sr/86Sr ratios within the individual orbicular structures (cores, shells and matrices) in individual localities. The orbicules at Henderson South, Orbicule Koppie and the Jubilee Pit host a bornite-dominated sulphide assemblage where disseminated chalcopyrite and bornite grains are characterized by granular textures and chalcopyrite is replaced by magnetite and bornite. Bismuth, Ni and Se in these sulphides appear to partition into bornite while Ag, In, Cd, Sn, Mn, Ge and Co partition into chalcopyrite. Diana’s Pool orbicular samples exhibit closely packed orbicules in a granitic matrix characterized by different types of orbicules containing coarse-grained felsic cores, fine-grained and alternating ferromagnesian and feldspathic shells, and a coarse-grained to pegmatitic matrix. The orbicules are generally spherical to ellipsoidal in shape, however, some appear to be abraded and deformed. The compositions and grain sizes of cores and the matrix are comparable. Both the matrix and the cores are medium- to coarse-grained and dominated by plagioclase (the matrix probably in slightly lesser proportions), microcline (in variable proportions, and seemingly absent in some cores), quartz, biotite and accessory hornblende and magnetite. Contrary to the cores and matrix, shells are fine-grained and exhibit polygonal textures. In addition to this, the shells are dominated by biotite and magnetite; however, they do not contain hornblende. Plagioclase in cores, shells and matrices shows an almost complete overlap of An contents. Biotite composition in the shells is significantly less magnesian than in core and matrix, whose compositions overlap. Initial 87Sr/86Sr ratios from plagioclase in shells are slightly more radiogenic than in the matrix and cores. Formation of both the Koperberg Suite and Diana’s Pool orbicules has been attributed to metasomatic processes. However, modelling of the Koperberg Suite orbicules using the Magma Chamber Simulator and a variety of textural and geochemical constraints rules out a metasomatic origin. The quartz + biotite-dominated diorite (Orbicule Koppie) and the pyroxene-diorites (Hoogkraal Lease, Henderson South, Henderson North), together with more felsic anorthosite and the more mafic pyroxenites, are all produced by progressive AFC + recharge processes, where the magma was most likely anatectic melts of the country rocks that were at granulite grade. In both Diana’s Pool and the Koperberg Suite, cores are autoliths, which are plagioclase-rich, cumulate, or rim fragments reworked by new magma inputs or injections. Heterogeneous nucleation leading to the formation of orbicular shells around the cores is attributed to adiabatic decompression of magma pulses ascending in dykes leading to superheating and resorption of early solids, and volatile exsolution, inducing undercooling, supersaturation, and shell crystallization. An alternative process that triggered superheating is magma mixing (e.g., Henderson South and Henderson North). The coarse-grained matrix crystallized later, after the orbicules formed, creating the groundmass, and locking the orbicules in place. The deformation of shells and cores (e.g., Orbicule Koppie and Diana’s Pool) suggests that the orbicules continued to evolve in the presence of a melt (matrix material) until they were emplaced at their present setting. The in situ sulphide assemblages in the Koperberg Suite have been reassessed in view of recent research on sulphide behavior. The bornite-chalcopyrite assemblage is atypical of the intermediate solid solution (iss) assemblage (chalcopyrite and pyrrhotite) observed in most Cu-Ni magmatic sulphide deposits. The high concentrations of trace elements that are incompatible with monosulphide solid solution and the depletion of Ni and Co in the sulphides are consistent with the derivation of sulphides from a Cu-rich sulphide melt that separated from a Ni-rich sulphide melt prior to magma emplacement and orbicule formation. The oxidation and Cu-enrichment of a sulphide melt that formed the mineralization in the orbicules and Koperberg Suite more generally began prior to the emplacement of the silicate Koperberg Suite magmas that formed the orbicules, suggesting no link between orbicule genesis and sulphide metallogenesis.
Innovative surface, tunnel, and in-pit geophysical methods for mineral exploration and mine planning: case studies from the Bushveld Complex mines, South Africa
(University of the Witwatersrand, Johannesburg, 2023) Rapetsoa, Moyagabo Kenneth
Innovative geophysical methods were used to study the platinum group element mineralisation and their associated geological structures at Maseve and Tharisa mines, western Bushveld Complex. Four case studies are presented in this thesis that incorporate the use of in-mine or near mine geophysical methods for mineral exploration. The first one being in-mine seismic data acquired in 2020 at Maseve mine using cost-effective seismic source and sensors, followed by innovative seismic experiments acquired in 2022 at Maseve mine to evaluate the viability of using tunnel and surface experiments for mineral exploration in a noisy, logistically difficult mine environment. Thirdly, the 2021 integrated geophysical surveys conducted at Tharisa mine to image fractures that act as water pathways into the pit. Finally, integrated geophysical techniques are used to delineate boulders to enhance future mine planning and designs at Tharisa mine. The acquired geophysical data were processed using modern processing algorithms to enhance the target mineralization and complex geological structures in all the sites. In-mine reflection seismic datasets acquired in 2020 at Maseve mine proved useful as they provided optimum imaging of the economic Platinum Group Elements (PGEs) such as the Merensky Reef and Upper Group 2 chromitite layers (known as reefs). This is one of the few in-mine seismic experiments to have been conducted in South Africa for mineral exploration. In 2022, 2D reflection seismic profiles were acquired on surface above the Merensky Reef and Upper Group 2 chromitite, together with four 2D reflection seismic profiles acquired along the mine tunnel at ~ 550 m below the surface and tens of meters above known mineralisation: Merensky Reef and Upper Group 2 chromitite layer. Interpretation of the in-mine and surface seismic data were complemented by the use of 3D ray tracing numerical simulations to understand the distribution and out-of-plane reflectivity from the target mineralization. The 2022 Maseve reflection seismic data improved the imaging of geological structures and mineral deposits. The geophysical data acquired in 2021 at Tharisa mine demonstrated the importance of using near-surface integrated geophysical methods (magnetics, seismics, and electrical resistivity) with other datasets such as borehole logs and physical property measurements to understand the geophysical response of the mineral deposits. Ground magnetic data delineated a major dyke that was identified on the aeromagnetic data and geological mapping. Electrical resistivity tomography, on the other hand, identified linear low resistivity zones that differentiateiii fractured and undisturbed hard rock. Seismic methods were important for depth to bedrock imaging. Integration of geophysical methods was encouraged by the need to understand geological structures (e.g., faults, dykes, iron-rich ultramafic pegmatites, boulders) that can have impact on the efficiency, safety and costs of mining in South Africa. Moreover, this approach encourages the implementation of innovative geophysical surveys in brownfield sites for better mine design and planning, and to increase a life of mine (LoM)
3D seismic constraints on the strato-structural evolution of the deep-water Orange Basin, South Africa
(University of the Witwatersrand, Johannesburg, 2023) Maduna, Nombuso Gladys; Jinna, Zubair; Manzi, Musa
This research utilizes seismic attributes and advanced machine learning methodologies to analyse high-resolution 3D reflection seismic data from the deep-water Orange Basin, located offshore western South Africa. The primary goal is to gain valuable insights into the basin's tectonic setting, depositional environment, and hydrocarbon potential. Significant features are delineated within the basin including (1) a gravitational collapse system in the Mesozoic Late Cretaceous, (2) mass flow features in the Cenozoic, (3) natural gas and fluid escape structures, (4) a large slope-perpendicular submarine canyon cutting Oligocene strata, and (5) multiple slope-parallel, sinusoidal channel features in the Miocene. The Late Cretaceous succession exhibits a gravitational collapse system with a translational and compressional domain detaching on seaward-dipping Turonian shales. Gravitational collapse during margin uplift formed fold-and-thrust belts along the slope characterizing the compressional domain. As they are commonly linked to hydrocarbons, the compressional domain of these systems has been extensively studied, while the translational domain has been poorly constrained due to its structural complexity. In this research, the translational domain is shown to contain a mixture of extensional tectonics (normal faults) up-dip and compressional tectonics (thrusts) down-dip, with extensive oblique-slip faults cutting thrusts perpendicularly during the translation of sediment. Variance and chaos, conventional seismic attributes, were used to manually pick and interpret the >500 regional-scale faults arising from the gravitational collapse system. Fault-net, a convolutional neural network (CNN), was compared with these edge-enhancing seismic attributes for extracting faults from the seismic volume. The CNN offers several notable advantages over conventional seismic attributes, such as automation, accelerated analysis, and improved time-efficiency on large datasets. Analysing the distribution, type, and geometry of faults within the basin gave valuable insights into the potential hydrocarbon system at work. Numerous natural gas and fluid escape features are identified in the seismic volume including an elongated mud volcano, pockmarked surfaces, and polygonal faults. The stability of the evolving margin is influenced by the underlying structure of a Late Cretaceous gravitational collapse system, also referred to as a deep-water fold and thrust belt (DWFTB) system. The fault framework within provides primary migration pathways for hydrocarbons. Major seafloor slumping occurs directly above a syncline of the Late Cretaceous DWFTB system. This slumping surrounds a structurally controlled, 4.2 km long elongated mud volcano situated between the translational and compressional domains of the underlying DWFTB system. The late Campanian has the largest accumulation of hydrocarbons evidenced by (1) an anticline with a positive high amplitude anomaly situated at the intersection of the two domains, and (2) >950 pockmarks preserved on the palaeo-surface compared to the 85 pockmarks observed on the seafloor. In addition to tectonics, the onset of stratified oceanographic circulation patterns and climate played a large role in changing depositional trends since the mid-Cenozoic. The Oligocene is characterized by a ~2.3 km wide, >13 km long, slope-perpendicular canyon formed at ~30 Ma during a major sea-level fall by a turbidity current. The Miocene is characterized by a ~14 km wide zone of slope-parallel, sinusoidal channels between water depths of 1 200–1 500 m. The formation and preservation of these features during the Miocene are attributed to the erosive interaction between two distinct water currents: (1) the Antarctic Intermediate Water flowing northwards, and (2) the deep North Atlantic Deep Water bottom currents flowing southwards; and the effects of the Benguela Upwelling System and a dry climate prevailing in southwest Africa all intensifying around 11 Ma. While pre-Miocene hydrocarbons originate from Turonian and Aptian source rocks, the origin of hydrocarbons on the seafloor is likely biogenic, arising from organic-rich sediment in the Miocene
Application of integrated methods to assess and characterise the hydrogeology of coastal aquifers in parts of Lagos, Southwest, Nigeria
(2020) Yusuf, Mumeen Adebayo
The scope of this Thesis was to apply integrated methods to characterise the groundwater systems of the Lagos Coastal Basin. Like every coastal area in the world, saline intrusion has been the major challenge threatening the fresh groundwater aquifers of the study area over the last couple of decades, and thus, necessitating its assessment. Environmental isotopes, being a reliable and standard tool in hydrological investigation, was employed in combination with geophysical and hydrogeochemical methods to study the coastal aquifer systems. Geophysical probing of the subsurface revealed an alternating sequence of clay and sand, constituting the major lithological units in the study area. The basin aquifers are hosted essentially by sands and clayey sand, while the modes of aquifer occurrences are unconfined to semi-confined and confined for shallow and deep aquifers, respectively. Hydrochemical interpretation identified a surficial thin layer of fresh groundwater overlying the main zone of saline intrusion, which essentially comprises Ca-HCO3 and Ca-Mg-HCO3, Ca-Mg-HCO3 and Ca-Mg-Cl-SO4 hydrochemical facies for both dry and wet seasons, whereas the surface waters are characterised by Mg-Cl and Na-Cl water types for the lagoon and the ocean, respectively. The evaluation of the chemical processes revealed the dominance of carbonate weathering in the shallow aquifer. Hydrochemical, statistical and geochemical model analyses identified that the groundwater chemistry is significantly controlled by water–rock interaction and ion exchange processes as well as anthropogenic activities. Stable isotopes revealed precipitation as the main source of recharge into the basin aquifer systems. Analyses of the 3 H and 14 C activities were in agreement, revealing an interesting fact about the increase in the groundwater residence time from the surface through deeper depths deducible from 3H values range between 0.1 TU and 2.8TU; 0.0 TU and 0.3 TU; and 14 C age range from 4350±10 to 1050±10 years and between 12030±69 and 7400±50 years for the shallow and deep aquifers, respectively. The mean residence time was supported by the aquifer systems‟ recharge which took place in Holocene for the shallow aquifer and Late Pleistocene–early Holocene for the deep aquifers evident from the calculated ambient temperature, 18 O and 14 C plots. The hydrogeological conceptual models showed that saline incursion severely impacted the second aquifer from a depth ≥20m to 170m in the western and central parts of the study. However, the observed local saline occurrence in places <20m was attributed to groundwater overexploitation. Conclusively, the hydrological systems of the Lagos coastal basin is continually being modified by both anthropogenic and natural activities that constitute not only a major threat to the groundwater sustainability of the Lagos coastal basin but can also consume the entire study area

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