The relationship between mineralisation and structure in the Pilgrim's Rest - Sabie gold-field
Zietsman, Andries Louis
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The area that was investigated, covers the metallogenetic province of the Sabie-Pilgrim's Rest Gold-field in the Eastern Transvaal. The geological formations include the Basement Granite, Godwan Formation, Wolkberg Formation, and the Transvaal System. Diabasic and pyroxenitic sills, pre-mineralisation in age, and probably genetically related to the Bushveld Complex, are present also. Three generations of dykes can be distinguished, one before, one contemporaneous with, and the other after the mineralisation. Three types of folding are present, i.e., tectonic folds that are due to magmatic intrusion. At least two periods of tectonic folding can be distinguished, the one superimposed upon the other. The axes of the earliest folds trend approximately east, while those of the later folds trend north and northeast. Both periods of folding are due to compressional forces which can be ascribed to the intrusion of the Bushveld Complex. Both tectonic and non-tectonic faults are encountered. The tectonic faults consist of low-angle thrust-faults, low-angle gravity-faults, and high-angle normal faults. The thrusts are probably due to the same compressional forces that were responsible for the nortn- and northeast-trending folds, i.e., they are probably related to the intrusion of the Bushveld Complex as well. Evidence of gravitational gliding-tectonics is provided by the presence of low-angle gravity-faults. These structures postdate the thrust-faults. The high-angle normal faults are due to tensional stresses and they are mostly post-mineralisation in age. A remarkable directional relationship between the linear structures and the folds is noticeable. The linear features, as well as the axes of the folds are orientated in three directions, namely, north, east and northeast. The gold-bearing reefs consist of interbedded as well as, transgressive, epigenetic ore-bodies. The interbedded reefs are represented by bedding-thrusts. The localisation and development of these reefs were controlled by the presence of favourable horizons and the intrusion of sills, folding, especially superimposed folding, as well as the formation of the bedding-thrusts. The conditions necessary for the emplacement of the transgressive reefs were the presence of early planes of weakness and/or dykes which are pre-reef in age. The low-angle gravity-faults resulted in the opening of pre-existing fractures such as early high-angle faults, joints and dyke-contacts. Two distinctly different periods of mineralisation are visualised, one preceding and the other postdating the bedding-thrusts. The first period is seen as a process of contact-metasomatism, caused by the intrusion of the basic sills. This process was probably localised and controlled by the folding. The second period of mineralisation was one of fissure-filling by solutions which were introduced during two phases: an early pyrite-rich phase, which was followed by a later copper- and bismuth-rich phase. The bedding-thrusts, as well as the low-angle gravity-faults, acted as channel-ways for the mineralising solutions. The second period of mineralisation, which produced ore-minerals that are typical of mesothermal deposits, is visualised as a metal-liferous front originating from the "root" of the Bushveld Complex. The ore-minerals, including gold, are considered to have been derived from the magma-chamber itself, as well as from the adjacent sedimentary rocks. The deposits of chrysotile in the area are considered to have been localised due to a similar structural control as to that which influenced the localisation of the interbedded reefs, i.e., folding.
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