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Comparison of the mineralogy and ore genesis of the Tarkwaian and Witwatersrand conglomerates

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dc.contributor.advisor Robb LJ en
dc.contributor.advisor Anhaeusser CR en
dc.contributor.author Tongu EL en
dc.date.accessioned 2016-09-22T11:18:12Z
dc.date.available 2016-09-22T11:18:12Z
dc.date.created 1994 en
dc.date.submitted 2002 en
dc.identifier.uri http://hdl.handle.net/20.500.11892/109508
dc.description.abstract The metamorphosed, auriferous quartz-pebble metaconglomerates from the Proterozoic Tarkwaian Group of Ghana and the Archaean Witwatersrand Supergroup of South Africa have been investigated with a view to determining similarities or differences between them, chiefly with respect to their mineralogical compositions and ore genesis. The Tarkwaian detritus was derived from weakly weathered source rocks. The Tarkwaian metaconglomerates comprise detrital magnetite (MT1), which is variably altered to secondary hematite, magnetite (MT2) and sulphides, mainly pyrite. Other detrital Tarkwaian minerals include monazite, zircon, quartz pebbles, lithic fragments and rare chromite. Uraninite and carbonaceous (i.e. polymerized hydrocarbon) matter, which are prevalent in the Witwatersrand, are absent in the Tarkwaian. Tarkwaian gold is entirely authigenic and associated mainly with hematite, carbonate and tourmaline. The authigenic nature of the noble metal was verified using HF acid-liberated gold morphologies and textures. Carbonic fluids originating from a deep-seated or metamorphic source transported the gold. The external source of the mineralizing carbonic fluids has been demonstrated using carbonate carbon isotopic compositions (&delta;<sup>13</sup>C values of - 2.54 to -7.15 %<sub>OPDB</sub>)- Fluid-rock interactions destabilized the carbonic fluids, causing the alteration of MT1 to hematite and the deposition of gold and carbonate. The heat source that drove the hydrothermal activity included peraluminous, S-type rhyolitic porphyry suites, which intruded the Tarkwaian deposit. By comparison, the Witwatersrand source rocks were generally strongly weathered, reflecting prevailing warm and moist atmospheric conditions during the Archaean. The Witwatersrand metaconglomerates are mineralogically complex, consisting of multiple generations of phyllosilicates, pyrite and gold, plus detrital chromite, uraninite, zircon quartz and chert pebbles amongst others. Of the phyllosilicates, at leastthree generations of chlorite occur, consisting of non-end member clinochlore and sudoite in addition to pyrophyllite, white mica, kaolinite and illite. Pyrite and gold include varieties that reflect detrital/syngenetic, diagenetic and epigenetic phases. Evidence fromHF acid-liberated gold morphologies and surface textures confirm the range of the gold population. Syngenetic and diagenetic gold occurs associated with syngenetic/diagenetic pyrites while the secondary gold is found typically restricted in and around the carbonaceous seams and accompanied by paragenetically late sphalerite-chalcopyrite-arsenianpyrite-kaolinite-illte&#177; assemblage. Arsenian pyrite is characterised by uniform, elevated Ni, Co and As contents. The Ni-Co-As contents generally distinguish post-diagenetic pyrites from the detrital and diagenetic varieties. Effectively, the multiple, genetically diverse ore minerals primarily distinguish the Witwatersrand from the Tarkwaian and provide a highly probable explanation for the prodigious gold concentration in the Archaean deposit. en
dc.language English en
dc.title Comparison of the mineralogy and ore genesis of the Tarkwaian and Witwatersrand conglomerates en
dc.type Masters degree en
dc.description.degree MSc (Med) en

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