Mineralogical and geochemichal investigation of Uranium conglomerate ores of the dominion reef group, Western Witswatersrand, South Africa

[摘要] English: Throughout the thesis, the genesis of the Dominion Reefs as Au-U placer deposits with particularfocus of their paragenetic sequences whose detrital and/or hydrothermal origin was investigated.Dominion conglomerate Reefs are composed of quartz pebbles and detrital heavy minerals supportedby an argillaceous matrix of illitic-chloritic composition. Geochemistry and mineralogy of two distinctReefs 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. Fortectonic 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 uppercontinental crust material as source rocks. Furthermore, the evaluation of geochemical data using thePearson correlation coefficient suggests relationships of identified minerals. Elements, hosted indetrital minerals show a good correlation as well as elements hosted in hydrothermal- orhydrothermally altered minerals.Petrographic and SEM observations provided the opportunity to discriminate different generations ofU-bearing minerals present in the Reefs. The chemical composition of U-bearing minerals can becorrelated with their origin. Preserved uraninite sub-rounded grains occur associated with the primarydetrital heavy mineral paragenesis. Moderate to high Th content (up to 9.30 wt% ThO2) of uraninitegrains 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 uraninitegrains have been extracted from at least two distinct granitic or pegmatitic sources. However, themajority of U mineralization is represented by re-crystallized phases as secondary uranium bearingphases (pitchblende without Th), coffinite, U-Ti and U-Th phases occurring with the uraniumredistribution by later hydrothermal processes.LA-ICP-MS analyses revealed a distinct age of 3100 ± 90 Ma for the primary mineralisation. Theinitial 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 geochronologicaldata, which correspond in time to the extrusion of the Ventersdorp lavas at ~2720 Ma. In this stagehydrothermally growth of mineral phases including secondary U bearing phases, galena and otherminerals occurred.

[发布机构] University of the Free State