Geology

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Browsing Geology by Advisor "Gauert, Christoph. D. K."

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    Mineralogical and geochemichal investigation of Uranium conglomerate ores of the dominion reef group, Western Witswatersrand, South Africa
    (University of the Free State, 2009-02) Rantzsch, Ulrike; Gauert, Christoph. D. K.; Van der Westhuizen, Willem. A.
    English: Throughout the thesis, the genesis of the Dominion Reefs as Au-U placer deposits with particular focus of their paragenetic sequences whose detrital and/or hydrothermal origin was investigated. Dominion conglomerate Reefs are composed of quartz pebbles and detrital heavy minerals supported by an argillaceous matrix of illitic-chloritic composition. Geochemistry and mineralogy of two distinct Reefs has been characterized using petrographic studies, various whole rock geochemical analyses, and electron microprobe analyses.XRF analyses yield numerous results. Dominion conglomerates could be classified as sublitharenite, litharenite, and Fe-sands by major element geochemistry. The heavy mineral paragenesis (garnet, pyrite, monazite, zircon, chromite, magnetite, ilmenite, and phases containing uranium and thorium) suggests that the source of sediments is most likely similar to Archean granite-greenstone terrains. For tectonic setting of the sediments a passive margin and continental rift setting can be considered. Provenance investigation based on REE analysis suggests sedimentary recycling of felsic- and upper continental crust material as source rocks. Furthermore, the evaluation of geochemical data using the Pearson correlation coefficient suggests relationships of identified minerals. Elements, hosted in detrital minerals show a good correlation as well as elements hosted in hydrothermal- or hydrothermally altered minerals.Petrographic and SEM observations provided the opportunity to discriminate different generations of U-bearing minerals present in the Reefs. The chemical composition of U-bearing minerals can be correlated with their origin. Preserved uraninite sub-rounded grains occur associated with the primary detrital heavy mineral paragenesis. Moderate to high Th content (up to 9.30 wt% ThO2) of uraninite grains provided by EMPA support their magmatic origin. All uraninite identified are of detrital origin. Moreover, the chemical results allow distinguishing two individual groups and suggest that uraninite grains have been extracted from at least two distinct granitic or pegmatitic sources. However, the majority of U mineralization is represented by re-crystallized phases as secondary uranium bearing phases (pitchblende without Th), coffinite, U-Ti and U-Th phases occurring with the uranium redistribution by later hydrothermal processes.LA-ICP-MS analyses revealed a distinct age of 3100 ± 90 Ma for the primary mineralisation. The initial stage involved the placer development with deposition of detrital uraninite and related minerals. The younger age of 2700 ± 100 Ma as hydrothermal fluid event is recognised in the geochronological data, which correspond in time to the extrusion of the Ventersdorp lavas at ~2720 Ma. In this stage hydrothermally growth of mineral phases including secondary U bearing phases, galena and other minerals occurred.