A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. 23 September, 2014, Johannesburg. / The Witwatersrand Basin has been mined for the last 125 years and is still one of the world’s largest producers of gold and has produced over 50 000 tonnes. However, uranium has also been mined as a by-product of gold from the Witwatersrand reefs, and over 150 000 tonnes have been produced. Over the past decades, the origin of this world class gold and uranium deposit has been debated and still remains controversial. Three main hypotheses were developed, and these are the placer, modified placer and hydrothermal models. In this study, the aims are: to evaluate how many generations of uranium-bearing minerals are in the Vaal Reef samples analysed from Great Noligwa, Moab Khotsong and Kopanang mines and to determine which model among the placer, modified place and hydrothermal best supports the emplacement of the uranium-bearing minerals in the reef. The Vaal Reef occurs in the lower parts of the Strathmore Formation of the Johannesburg Subgroup in the Central Rand Group of the Klerksdorp Goldfield in the Witwatersrand Supergroup. The Vaal Reef is split into three facies, namely the C, B and A Facies; the C and A Facies are the most economic facies at the three mines. The C Facies is well developed at Kopanang mine and the A Facies is well developed at both Moab Khotsong and Great Noligwa mines. Geochemical analyses revealed that the C Facies is enriched in uranium, carbon, sulphur and aluminium; this is due to the presence of uraninite, carbonaceous matter, pyrite and sheet silicate minerals, respectively. The A Facies, however, is more enriched in gold and quartz content, although high uranium, carbon and sulphur concentrations are found, they do not exceed the C Facies concentrations.
Mineralogical investigations showed that uraninite, brannerite and uraniferous leucoxene are the uranium-bearing minerals present in the Vaal Reef samples. Uraninite is the main mineral and occurs firstly with detrital minerals such as pyrite, zircon and chromite in the quartz matrices; the second occurrence of uraninite is with the carbonaceous matter. Brannerite and uraniferous leucoxene are suggested to be formed from the breakdown of detrital uraninite grains interacting with Ti-rich minerals such as rutile. Unlike uraninite, brannerite and uraniferous leucoxene occur mainly in the C Facies matrix and occur as patchy or irregular-shaped minerals. The uraninite grains associated with the detrital minerals are mainly round in shape with sizes up to ~150 to 200 μm. This association with the detrital minerals suggests that uraninite was deposited together with the detrital minerals at the same time and that they were in hydraulic equilibrium with one another. Therefore, uraninite is also detrital in origin.
The second generation of uraninite grains in the carbonaceous matter mainly show replacement and breakdown of uraninite by the latter, in many observations, uraninite grains are penetrated by the carbonaceous matter through cracks and are further fragmented into smaller grains. The sizes of these fragmented grains vary between 5 – 80 μm and have angular shapes, suggesting that they were first rounded and later broken down and replaced by the carbonaceous matter. A four-staged paragenetic sequence of the Vaal Reef samples was developed, and more importantly the paragenesis showed that the carbonaceous matter post-dates the deposition of uraninite.
The three-dimensional microfocus X-Ray computed tomography (3D μXCT) was applied to the Vaal Reef samples and the main objectives were to visualise and analyse the uranium-bearing minerals in the Vaal Reef samples for their sizes, shapes and distribution with respect to other mineral components in the samples in 3D. The technique is currently unable to distinguish individual minerals from one another, especially when minerals have similar grey values as a result of close attenuation coefficients, mineral compositions and density. Mineral groups were identified following this similarity, include quartz and sheet silicates as one mineral group, all sulphides as another group and uranium-bearing minerals with gold as a third mineral group. The analysed uraninite with gold mineral group in the matrix, exhibited grains up to 200 μm in size which were round in shape, as observed in 2D mineralogical techniques. These observations support mineralogical observations acquired by conventional mineralogical techniques suggesting that 3D μXCT can be used to complement other mineralogical techniques in obtaining 3D information on minerals. However, 3D μXCT has limitations such as spatial resolution, partial volume effect and overlapping of mineral grey values. It is therefore, suggested that the technique not be used as an independent tool for mineral characterisation, but rather in support of the existing mineralogical techniques.
The source area of the uraninite in the Vaal Reef of the Klerksdorp Goldfield is suggested to have been the hydrothermally altered Archaean basement granite bodies of the Witwatersrand Basin hinterland, from the Hartebeesfontein Dome northwest of the goldfield in particular. High UO2/ThO2 ratios, as determined by electron microprobe analyses (EMPA), support the notion that the uraninite grains are not a product of hydrothermal fluids, and furthermore high Pb contents showing that the uraninite grains are older than the age of the Witwatersrand deposition. In conclusion, the emplacement of uranium-bearing minerals in the Vaal Reef samples analysed in this study is best explained by the modified placer model.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/16820 |
| Date | 30 January 2015 |
| Creators | Sebola, Tlou Piet |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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