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A 10 km x 10 km study area in the western Bushveld Complex, south of the Pilanesberg Complex, was selected for testing the inversion of vertical component gravity (Gz) data to determine the geometry of the Bushveld Complex/Transvaal Supergroup contact. This contact has a density contrast of ~0.350 g.cm-3 making it a suitable target for gravity inversion. The resulting 3D gravity model agrees well with the 3D seismic interpretation, indicating that the depths determined from the seismic data are appropriate. The gravity inversion could be extended laterally to investigate regions without seismic data coverage. This methodology may prove useful where upwellings in the floor of the Bushveld Complex distort seismic data, but can be imaged by gravity inversions. The Gz dataset was created from converted Airborne Gradient Gravity (AGG) data, combined with upward continued ground Gz gravity data, providing extensive coverage. This combined dataset was used in an interactive, iterative 3D gravity inversion methodology used to model the geometry of the Bushveld Complex/Transvaal Supergroup contact and densities of the Bushveld Complex, Transvaal Supergroup and Iron-Rich Ultramafic Pegmatoids (IRUPs). The resulting 3D gravity model provides an acceptable first-pass model of the Bushveld Complex/Transvaal Supergroup contact. In the shallow south-west region of the study area, the steeply dipping contact was determined from borehole intersections. 3D seismic data was the only constraint towards the north-east, where the contact flattens out to a sub-parallel contact, at ~2 000 m depth. In the north-western section, the Bushveld Complex/Transvaal Supergroup contact is fault-bounded by a conjugate set of the Rustenburg Fault, causing the Bushveld to onlap the Transvaal sediments. In the southern region, the contact changes as the conjugate fault dies out, and the Bushveld Complex becomes layered/sub-parallel to Transvaal sediments. This, and other geological features (e.g. faulting, folding, dykes), can be explained in relation to the regional tectonic history, relating to motion along the Thabazimbi-Murchison Lineament (TML). Pre-Bushveld emplacement NW-SE far-field stress caused NW trending extensional features in the region (e.g. Rustenburg Fault). Re-orientation of the compressive force to NE-SW, in syn- to post-emplacement, caused compressive features in the region (e.g. open folds with axes trending NW). Ground gravity data (100 m x 100 m station- and line-spacing) were also inverted to obtain a 3D model of the overburden, constrained by borehole data. However, the inversion failed to satisfy the gravity data and borehole data simultaneously, relating to difficulties in modelling the regional gravity field and the gradational nature of the weathered contact. Several rapid variations in overburden thickness were mapped, with particular success in the high frequency ground gravity survey (30 m x 30 m station- and line-spacing) with the identification of a deeply weathered (~10 m deep) channel relating to an mapped fault.

MSc, Faculty of Science, University of the Witwatersrand, 1999

Several disequilibrium textures are found to occur within the hanging wall and footwall of the UG2 chromitite layer of the Bushveld Complex, South Africa. These textures include plagioclase chadacrysts found included within orthopyroxene and clinopyroxene as well as the orthopyroxenes exhibiting round crystal boundaries that appear to be resorbed. Textures found within the UG2 stratigraphy such as linear boundaries and 120° triple junctions at interfaces of adjacent plagioclase or pyroxene grains also suggest that recrystallization has taken place. The presence of both disequilibrium textures and recrystallization textures would suggest that a complex emplacement history has occurred. Ideally, this would be expected to be manifested by minerals of the same type but which are texturally distinct showing different composition. However this has been found not to be the case; minerals that suggest disequilibrium textures show similar compositions to the minerals which appear to have formed in equilibrium. This is also the same for recrystallized crystals which show the same compositions as crystals that have not been recrystallized. For example tabular clinopyroxene, which has a compositional range of En 44.6 to En 50.5, is indistinguishable from clinopyroxene occuring as discontinuous rims, En 44.3-48.2, and as intergranular necking connecting primocrysts of orthopyroxene ( En 44.3-50.4). Similarly, plagioclase occurring as inclusions with An 66.3-76.0 is indistinguishable from plagioclase occurring as zoned or recrystallized interstitial grains ( An 69.0- An 77.4). Compositional variation has however, been found to be controlled to an extent by stratigraphy in that minerals show different compositions within one layer to the same minerals within another layer, consistent with an evolving magma composition. It is concluded therefore that while composition is not texturally controlled it is to an extent stratigraphy controlled and that the evidence collected within the study supports two models for the formation of chromite within the Bushveld complex. The evidence is consistent with a combination of the magma mixing model and magma injection model to account for the textures and compositional variations found within the study. The evidence may also show support for models involving late modification of minerals by magmatic fluids but not as prominently as for the models mentioned above / Microsoft� Word 2010 / Adobe Acrobat 9.53 Paper Capture Plug-in

Mineral detection and geological mapping through conventional ground survey methods based on field observation and other geological techniques are tedious, time-consuming and expensive. Hence, the use of remote sensing in mineral detection and lithological mapping has become a generally accepted augmentative tool in exploration. With the advent of multispectral sensors (e.g. ASTER, Landsat and PlanetScope) having suitable wavelength coverage and bands in the Shortwave Infrared (SWIR) and Thermal Infrared (TIR) regions, multispectral sensors, along with common and advanced algorithms, have become efficient tools for routine lithological discrimination and mineral potential mapping. It is with this paradigm in mind that this project sought to evaluate and discuss the detection and mapping of magnetite on the Eastern Limb of the Bushveld Complex, using specialized common traditional and machine learning algorithms. Given the wide distribution of magnetite, its economic importance, and its potential as an indicator of many important geological processes, the delineation of magnetite is warranted. Before this study, few studies had looked at the detection and exploration of magnetite using remote sensing, although remote sensing tools have been regularly applied to diverse aspects of geosciences. Maximum Likelihood, Minimum Distance to Means, Artificial Neural Networks, Support Vector Machine classification algorithms were assessed for their respective ability to detect and map magnetite using the PlanetScope Analytic Ortho Tiles in ENVI, QGIS, and Python. For each classification algorithm, a thematic landcover map was attained and an error matrix, depicting the user's and producer's accuracies, as well as kappa statistics, was derived, which was used as a comparative measure of the accuracy of the four classification algorithms. The Maximum Likelihood Classifier significantly outperformed the other techniques, achieving an overall classification accuracy of 84.58% and an overall kappa value of 0.79. Magnetite was accurately discriminated from the other thematic landcover classes with a user’s accuracy of 76.41% and a producer’s accuracy of 88.66%. Despite the Maximum Likelihood classification algorithm illustrating better class categorization, a large proportion of the mining activity pixels were erroneously classified as magnetite. However, this observation was not merely limited to the Maximum Likelihood classification algorithm, but all image classifications algorithms. The overall results of this study illustrated that remote sensing techniques are effective instruments for geological mapping and mineral investigation, especially in iron oxide mineralization in the Eastern Limb of Bushveld Complex. / Dissertation (MSc)--University of Pretoria, 2019. / Geology / MSc / Unrestricted

D.Phil. / This study which describes rocks of the Upper Critical Zone of the Bushveld Complex is subdivided into three parts. The main rock type of the Upper Critical Zone, the noriteanorthosite, is the subject of the first part. Inclusions in chromite and plagioclase were studied. The inclusions' in chromite were observed at different stages of their formation. The chromite crystals usually overgrow plagioclase, pyroxene and hydrous minerals (biotite, amphibole and clinozoisite) trapping them at grain boundaries or triple junctions of chromite host grains. With 'continuous growth of the host minerals the inclusion starts changing its shape from elongated to circular and the hydrous mineral proportion of the inclusion increases. Simultaneously amphibole changes its composition from pargasitic to tremolitic...

The Platreef has been the site of platinum mining since the 1920’s. The reef itself comprises a series of pyroxenites, gabbronorites and norites that contain xenoliths/rafts of footwall rocks. The Platreef is irregularly mineralised with PGE, Cu and Ni, and has a greater abundance of sulphides than the Merensky Reef. The main base metal sulphides within the Platreef are pyrrhotite, pyrite, pentlandite, and chalcopyrite. Extremely varied platinum group minerals occur as tellurides, bismuthotellurides, antimonides and arsenides. This study aimed to gain a clearer understanding of the formation of sulphides within the Platreef. In order to do this, cores from both the northern and southern sectors of the Platreef were sampled. A detailed study of the sulphides within these cores was conducted to identify different styles of mineralisation and their occurrences. Four different styles of mineralisation were identified: massive, net-textured, blebby and interstitial. In general, sulphides in the southern sector of the Platreef are concentrated in the lower portion of the package, whereas in the northern sector they are concentrated in the upper part although in both sectors the sulphide occurrences are associated with metasedimentary xenoliths. Conventional and multiple sulphur isotope analyses were undertaken on sulphides from cores from both the southern and the northern sectors. This was done in order to determine the source of the sulphur. These analyses were also conducted to examine sulphur isotope variations with changing footwall. Previous sulphur isotope data predominantly obtained from the central sector of the Platreef indicated a crustal contribution to the sulphur budget but did not provide much data on footwall sulphides so the nature of the crustal component was only implied. In this thesis sulphur from an external source was identified as having contributed to the formation of sulphides in both the southern and the northern sectors of the Platreef, especially for sulphides in proximity to metasedimentary xenoliths. In the southern sector of the Platreef this source was identified as most likely being pyritic shales of the Lower Duitschland Formation. In the northern sector, Malmani dolomites, which are suggested to have collapsed from the roof of the Platreef, are the most likely source of sulphur. Importantly, in the northern sector no sulphur is thought to have come from the Archaean granite footwall. Oxygen isotope analyses were conducted on samples from the southern sector of the Platreef to verify the presence of crustal contamination. Data collected indicated that there had been a crustal oxygen component involved in the formation of silicates that led to their partial recrystallisation. When compared to oxygen isotope data from the central sector of the Platreef it appears that there are variations along strike that most likely result due to the changing footwall. This data indicates a major contribution of oxygen-, sulphur- and other volatile-rich fluids to the Platreef. This led to the partial re-crystallisation of silicates, and in areas in close proximity to sulphur-bearing metasedimentary xenoliths aided in the formation of sulphides. These volatile-rich fluids most likely originated from metasedimentary xenoliths during metamorphism that then migrated through the Platreef package. When the observations from both the southern and northern sectors of the Platreef are compared and combined with pre-existing data for the central sector, several general observations can be made. 1. The entire length of the Platreef has been affected by contamination from crustal sulphur sources to some degree. This contamination is suggested to be from volatile-rich fluids which were released from metasedimentary crustal xenoliths and footwall during metamorphism. 2. The proximity between sulphide enrichment and sulphur-bearing sediments (as footwall or xenoliths) is important and indicates the source of the sulphur which led to sulphide formation. 3. Contamination occurred on a localised scale, depending on the composition of the sedimentary lithologies and the proximity of the contaminant to the magma. In the southern sector of the Platreef the source of the sulphur is almost certainly pyritic shales of the Lower Duitschland Formation. In the central sector, sulphur has most likely come from sulphur-rich dolomites and evaporites from the Malmani dolomites. In the northern sector, sulphur-rich fluids were released from Malmani dolomite rafts that collapsed from the roof into the magma during the emplacement of the Platreef. The Archaean footwall in this area has had little or no control on the formation of the sulphides within the Platreef.

A dissertation submitted to the Faculty of Science, University of Witwatersrand in the fulfillment of the requirements for the degree of Master of Science (Geology) 2017 / The LG-6 chromitite layer is the thickest (0.90 to 1.20 m thick) chromitite layer in the Lower Group chromitites of the Bushveld Complex and is of economic significance owing to the relatively high Cr-content. It can be traced across the entirety of the western limb and is mined in both the western limb and the eastern limb. This study evaluates previously published models of chromitite formation using data from the LG-6 chromitite at Ruighoek Mine, western Bushveld Complex. Data includes petrographic studies of the reef and host rocks, whole rock analysis of the silicate host rocks and reef, and mineral chemistry for orthopyroxene, olivine and Cr-spinel using electron probe microanalysis (EPMA). In the Ruighoek region the LG-6 chromitite comprises up to 95 vol. % chromite and is typically hosted by orthopyroxenite. Borehole data indicated an area (about 250 m2 in size) where the LG-6 is entirely hosted by harzburgite (42% orthopyroxene, 11% Cr-spinel, 14% olivine, 32% serpentine and 1% other) rather than orthopyroxenite. The whole rock and mineral chemistry revealed that the LG-6 chromitite in this area has an exceptionally high Cr/Fe ratio, up to 2.1. The whole rock data also indicated several compositional reversals in terms of MgO, Al2O3, Cr2O3, FeO, and Mg# (Mg/ [Mg+Fe2+]) for the unenriched borehole 13R-3, and compositional reversals in Cr/Fe and Cr# (Cr/ [Cr+Al]) for the enriched borehole 13R-9 upwards through the chromitite layer. The hanging wall harzburgites are characterized by an increase in Mg# for the mineral chemistry of the Cr-spinel, orthopyroxene and olivine compared to those in the footwall harzburgite. Importantly, spatial 3D modelling of borehole data at Ruighoek mine (19 drill-cores) indicates that the elevated Cr/Fe ratio in LG-6 chromitite is coincident with a depression in the topography of the chamber floor at the time of formation of the LG-6 chromitite. These data are difficult to reconcile with existing models for chromitite formation in layered intrusions, such as the models for gravity settling, addition of a Cr-spinel crystal-laden magma, or a pressure increase. Thus, this work has developed a new model for formation of the LG-6 chromitite at Ruighoek Mine. The exceptionally high Cr/Fe ratio of LG-6 chromitite and its close association with harzburgite is attributed to multiple replenishments of the chamber by relatively primitive magmas. These are inferred to either be saturated in olivine and chromite, or chromite alone. The occurrence of relatively primitive rocks within the depression is suggested to be related to a local feeder situated within the depression. Injection of new, relatively dense magma pulses from the feeder are inferred to spread out across the chamber floor as basal flows owing to compositional stratification of the resident magma at the time of development of the LG-6 chromitite. The replenishing magmas contributed to the existing compositional stratification in the chamber, resulting in the most primitive composition within the depression of the chamber floor. Subsequent crystallisation of the most primitive magmas within the depression resulted in local development of LG-6 chromitite with exceptionally high Cr/Fe ratios together with the enclosing harzburgitic rocks. The thickness of the LG-6 chromitite is attributed to continuous replenishment by large volumes of new, chromite-saturated, magmas via the feeder channel located in the depression. This study suggests that magma stratification and the replenishment of the chamber by chromite-saturated magmas played an important role in the development of the chromitite layers of the Bushveld Complex. / MT 2018

The Kaapvaal Craton was the scene of two major magmatic events around 2.0 Ga, namely the Bushveld Complex and the Phalaborwa Complex. Both complexes have been the subject of numerous palaeomagnetic studies during and prior to the 1980s. Despite these studies, systematic inconstancies for emplacement ages, in particular for the Bushveld Complex, have been found between the palaeomagnetic findings and well constrained ages. The greatest concern with the Bushveld Complex results are the large spread in pole positions previously determined for the different zones. This has been interpreted in prior studies to indicate that the Bushveld Complex was emplaced and cooled below the Curie temperature of magnetite over a time span of 50 my. The results obtained previously for the Phalaborwa Complex appear to be out of position (~16°) with respect to those for the Bushveld Complex. This is of concern because new geochronological data show that the Rustenburg Layered Suite of the Bushveld Complex was emplaced approximately 1 my after the Phalaborwa Complex. These inconsistencies have prompted the current re-investigation of the palaeomagnetic results for both the Bushveld Complex and the Phalaborwa Complex. New palaeomagnetic data collected from all zones of the Rustenburg Layered Suite from the Eastern, Northern and Western Lobes of the Bushveld Complex, yielded palaeomagnetic poles that eliminated the spread in the apparent polar wander path. This observation is in agreement with precise age data, constraining the time period of emplacement of the complex to ~ 6 my. Resulting beddingcorrected high blocking components from all zones produced better groupings, thereby supporting a primary magnetic signature and indicating that the complex was intruded in a near-horizontal position. Dual polarities identified within each zone of the complex and positive reversal tests have identified one of the oldest known reversals of the Earth’s magnetic field. iii Palaeomagnetic data from the Phalaborwa Complex have produced a pole position that is in close proximity to those obtained from the coeval Bushveld Complex. In an attempt to achieve a better understanding of tectonic events occurring in the Kaapvaal Craton a number of dual polarity dykes within the Bushvled and Phalaborwa Complexes were palaeomagnetic analysed. Results revealed that the acquired pole positions are in agreement with ~1.9 Ga dykes, indicating the possibility that the dykes occurring in both complexes are part of the same magmatic event. Palaeopoles generated during this study were used in refining the Kaapvaal Craton apparent polar wander path around 2.0 Ga, and in conjunction with other welldefined 2.0 Ga poles for the Kaapvaal Craton, a robust cratonic pole was produced that was used in Precambrian palaeographic reconstructions with emphasis on the postulated Vaalbara continent and the Columbia supercontinent. Palaeomagnetic reconstruction derived in this study has cast doubt on the existence of the Vaalbara continent at 2.0 Ga. Although, some support is given to the existence of the Columbia supercontinent at the same period.

A Dissertation submitted to the School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa in fulfilment of the requirements for the degree of Master of Science. May 2015 / The Upper Zone (UZ) of the Bushveld Complex (BC) comprises several magnetitite layers throughout the entire sequence with the most prominent layer, the 2 m thick Main Magnetitite Layer (MML), located towards the base of the sequence. Magnetite mineral separates have been obtained from the UZ with particular focus on the MML in vertical profiles through the MML, Layer 1 and bifurcations of the MML, as well as profiles along the base of the MML and bifurcations. Magnetite mineral separates were also collected from Bierkraal and UCAR mine drill cores. The magnetite mineral separates were analyzed primarily for Cr and V as these two elements have the highest partition coefficients (D>200 and D=20-25 respectively) in magnetite and can be used as magmatic tracers. Electron microprobe data from the Bellevue drill core are also included. The gradational upper contacts of magnetitite layers with overlying anorthosite could be interpreted to suggest that the magnetitite layers accumulated through crystal settling. However, vertical profiles through 1 m of the MML all show an upward exponential decrease in Cr content (12 000-580 ppm) which is inconsistent with crystal settling but better explained by diffusion controlled bottom crystallization. The sharp base of the MML with the underlying anorthosite may suggest that the MML crystallized due to an abrupt event. The MML is not entirely homogeneous as evidenced by lateral heterogeneity along the base of the MML, identified by irregular Cr concentrations along the base of the MML and magnetitite bifurcations. This heterogeneity further supports the contention that the magnetitite layers are a product of diffusion controlled bottom crystallization. Reversals in Cr content, of differing magnitudes, in 3 of 4 vertical profiles above a dome structure interrupting the MML and in 2 of 4 vertical profiles through the MML, are attributed to intermittent convection on various scales bringing primitive undepleted magma into the crystallization zone. The magnitude of the reversals depends on the level to which the convection descends. The feldspar parting, a 10 cm thick horizon with cumulus plagioclase 1 m above the base of the MML, appears at a fairly constant Cr content in magnetite. The lack of a chemical break immediately above the feldspar parting suggests a physical process, such as pressure change, as a mechanism to account for the mineralogical change from the feldspar parting into massive magnetite in the upper portion of the MML. Vanadium, unlike Cr shows no systematic trends. Vanadium content of magnetitite layers is found to be comparable to that of the disseminated magnetite thus ruling out the possibility of a change in fo2 as a mechanism to induce magnetite crystallization. Disseminated magnetite in the UZ is suggested to have re-equilibrated with pyroxene and/or olivine during subsolidus ii cooling resulting in lower MgO contents of the disseminated magnetite compared to that of massive magnetitite layers. Similarities between magnetitite layers in Magnet Heights (eastern lobe); UCAR mine drill core, east of Brits (western lobe); Bierkraal drill core, north of Rustenburg (western lobe) and Bellevue drill core (northern limb) suggest that the different lobes of the BC may be connected.