Structural-stratigraphic controls on carbon and related mineralization in the Witwatersrand basin
Joubert, Arnoldus Jacobus
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The Witwatersrand basin is loaded with carbon. The carbon deposits locations are not site specific. Many localities where deposits occur are related to the structure and the sedimentology of the particular area. The purpose of this thesis is to document and describe the distribution of carbon in the Witwatersrand basin and to establish the mechanisms controlling emplacement. The approach used was a multidisciplinary one incorporating aspects of sedimentology and macro to mesoscopic tectonic structures and their relationships with carbon distribution patterns to establish the controls on carbon emplacement. One of the major controls on carbon deposition is structural geology. Bedding parallel fractures that cut pebbles with carbon fill is indicative of the influence of bigger and more forceful movements within the ore body. The thesis begins with a back ground study of all the major theories and ends with a possible explanation for the influence of structure and sedimentology on the deposition of carbon, and a new catalyst for the dehydration reaction that could lead to the deposition of carbon. The various types of carbon have been classified and grouped to specific sedimentological and tectonic structures. These types are deposited in lithofacies horizons that are not related in space to one another. They also differ in texture. Type A is observed in reefs and bedding planes, bedding parallel fractures and on fault planes. Type B consists of massive carbon and vug type carbon. Carbon on the reef contacts in most cases is developed on intersections with fluid pathways (phylonites or shear zones) which are characterised by the alignment of the minerals within the pathway. Carbon precipitation is controlled by the type of footwall and the amount of fluid pathways. The higher the occurrence of bedding parallel fractures the more consistent the emplacement of carbon. Phylonites are classified as follows: Type 1 exhibits a low degree of deformation and the minerals show a low degree of orientation. Type 2 exhibits a medium degree of deformation and the minerals show a larger degree of orientation. Type 3 phylonite is where all the original sedimentary character of the original rock has been sheared and the deformation gives rise to a foliated rock with a distinctive foliation. It is suggested that the large extensional faults in the Free State Gold Fields and the Master Bedding plane fault in the West Rand Gold Fields are conduits for the fluids into the Basin. The in-flow of fluids is from below the reef horizons. It is further speculated that in the Free State, the fluids had a north-easterly transport direction. The SEM analyses showed new mineral associations. The mineral phases are shown in three dimensions and the order of precipitation can be deduced. The element tantalum was prominent in one of the high grade samples. The most prominent mineral in the fluid pathways within the matrix of the various reefs is pyrophyllite. The carbon is emplaced within the pyrophyllite within a fluid pathway and this is indicative of the sequence of mineralization. The uranium replaced the pyrophyllite and the pyrite crystallized in a fracture within the pyrophyllite. It is concluded that the three main minerals: carbon, uranium and gold all came in at the same time into the basin. A hydrothermal origin for carbon and associated minerals is supported by the study. The proposed hypothesis to explain the timing and origin of carbon and gold into the Witwatersrand Basin is that of the 2.02 Ga Vredefort Impact event. Gold and uranium are inferred to have been transported by the carbon plasma that originates from the mantle during the Vredefort Impact event.