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

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...

M.Sc. (Geology) / Please refer to full text to view abstract

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2015 / The Bushveld Complex (BC) is the largest known layered intrusion. This suite of rock crop out in northern South Africa to form the Western, Eastern and Northern Limbs. Most research carried out focuses on the mineralized horizons in the Rustenburg Layered Suite (RLS) of the BC. This study presents a large database of wireline geophysical logs across a substantive part of the stratigraphy of the RLS. These consist of density and magnetic susceptibility datasets sampled at 1 cm. The major lithologies of the RLS intersected in the boreholes presented are gabbro, gabbronorite, norite and anorthosite whose density histograms reveal that they are predominantly normally distributed, with density averages of 2.86-2.91 g/cm3. The lithologies consist of mainly two minerals, pyroxene and plagioclase. In general, the average density increases with an increase in pyroxene. The distribution of the magnetic susceptibility for these lithologies has a large variation from SI to 13.2 SI, which is typical of layered intrusions. Susceptibility distributions are also multi-modal, asymmetric and not normally distributed, which makes the average magnetic susceptibilities less representative of the lithologies. Cross-correlation plots between density and magnetic susceptibility for several boreholes show that the above-mentioned lithologies form clusters (circular to elliptical), which typically overlap. This has been further investigated using k-means classification, to automatically detect these clusters in the cross-correlation plots and to compare these with those created by lithologies. The comparison shows some degree of correlation, implying that physical properties can be used to identify lithologies. This is particularly true for the Eastern Limb. However the classification has not been effective in all of the boreholes and often becomes complicated and an inaccurate representation of lithology log. This occurs in boreholes in which there is an overlap in the physical properties of the abovementioned lithologies. Analysis on the density and magnetic susceptibility data has also been carried out using wavelet analysis at individual locations across the BC. This has revealed multi-scale cyclicity in all of the boreholes studied, which is attributed to subtle layering created by variations in modal proportions between plagioclase and pyroxene. In addition to this, since layering is generally ubiquitous across layered intrusions, this cyclicity can be assumed to be present across the entire BC. This technique may become increasingly important should the cyclicity in physical property data correlate with reversals in fractionation trends since this may suggest zones of magma addition, whose thickness or III volumes can be quantified using wavelet analysis. This could be an important contribution since the current perspective on magma addition in the RLS is that four major additions have formed this 8 km thick suite of rocks, as opposed to smaller periodic influxes of magma. Wavelet-based semblance analysis has been used to compare the wavelengths at which the cyclicity occurs across boreholes. A comparison of wavelengths of this cyclicity shows that boreholes in the northern Western Limb show positive correlation in the density data at wavelengths >160 m and 20-60 m, while those further south show correlations at wavelengths of 120-200 m and 60-80 m. Boreholes of the Eastern Limb show positive correlation in the density and magnetic susceptibility data at wavelengths of 10-20 m, 20-30 m and 5m. These positive correlations across boreholes in density and magnetic susceptibility respectively, may imply that cyclicity may be produced by a chamber-wide process for several kilometres of the BC.

Ph.D. (Geology) / Please refer to full text to view abstract

The main magnetite seam of the Upper Zone of the Rustenburg Layered Suite (SACS, 1980) on the Bushveld Complex is known to host the world‘s largest vanadium bearing titaniferous iron ores. The vanadiferous titanomagnetites, contain vanadium in sufficient concentrations (1.2 - 2.2 per cent V₂O₅) to be considered as resources and vanadium has been mined historically by a number of companies among them Anglo-American, Highveld Steel and Vanadium and VanMag Resources as well as currently by Evraz Highveld Steel and Vanadium Limited of South Africa. The titanomagnetites contain iron ore in the form of magnetite and titanium with concentrations averaging 50-75 per cent FeO and 12-21 per cent TiO₂. The titaniferous iron ores have been historically dismissed as a source of iron and titanium, due to the known difficulties of using iron ore with high titania content in blast furnaces. The economic potential for the extractability of the titaniferous magnetites lies in the capacity of the ores to be separated into iron rich and titanium rich concentrates usually through, crushing, grinding and magnetic separation. The separatability of iron oxides and titanium oxides, is dependent on the nature in which the titanium oxide occurs, with granular ilmenite being the most favourable since it can be separated from magnetite via magnetic separation. Titanium that occurs as finely exsolved lamellae or as iron-titanium oxides with low titania content such as ulvospinel render the potential recoverability of titanium poor. The Upper Zone vanadiferous titanomagnetites contain titanium in various forms varying from discrete granular ilmenite to finely exsolved lamellae as well as occurring as part of the minerals ulvospinel (Fe₂TiO₄) and titanomagnetite (a solid solution series between ulvospinel and magnetite) . Discrete ilmenite constitutes between 3-5 per cent by volume of the massive titanomagnetite ores, and between 5-10 per cent by volume of the magnetite-plagioclase cumulates with more than 50 per cent opaque oxide minerals. The purpose of this research was to investigate the mineralogical setting and distribution of the iron and titanium oxides within the magnetitite layers from top to bottom as well as spatially along a strike length of 2 000m to determine the potential for the titanium to be extracted from the titanomagnetite ores. The titanomagnetites of the Upper Zone of the Bushveld Complex with particular reference to the Northern Limb where this research was conducted contains titanium oxides as discrete ilmenite grains but in low concentrations whose potential for separate economic extraction will be challenging. The highest concentration of titanium in the magnetite ores is not contained in the granular ilmenite, but rather in ulvospinel and titanomagnetite as illustrated by the marked higher concentration of TiO₂ in the massive ores which contain less granular ilmenite in comparison to the disseminated ores which contain 3 to 8 percentage points higher granular ilmenite than the massive ores. On the scale of the main magnetite seam, the TiO₂ content increases with increasing stratigraphic height from being completely absent in the footwall anorthosite. The V₂2O₅ content also increases with stratigraphic height except for in one of the 3 boreholes where it drops with increasing height. The decrease or increase patterns are repeated in every seam. The titanomagnetites of the main magnetite seam display a variety of textures from coarse granular magnetite and ilmenite, to trellis ilmenite lamellae, intergranular ilmenite and magnesian spinels and fine exsolution lamellae of ulvospinel and ferro-magnesian spinels parallel to the magnetite cleavage. The bottom contact of the main magnetite seam is very sharp and there is no titanium or vanadium in the footwall barely 10cm below the contact. Chromium is present in the bottom of the 4 layers that constitute the main magnetite seam and it upwards decreases rapidly. In boreholes P21 and P55, there are slight reversals in the TiO₂ and V₂O₅ content towards the top of the magnetite seams.