Abstract
The Dominion Group (DG) is a well-preserved, but comparatively little studied Mesoarchaean volcanosedimentary succession of the Kaapvaal Craton, situated unconformably below the well-studied and well-known Witwatersrand Supergroup. This study provides an overview of the DG with special attention to rock alteration and their origin in the Syferfontein Formation (SF). The DG was studied in the Ottosdal area, where the SF has the most extensive exposures, including in quarries of pyrophyllite deposits. The characteristic features and relative timing of the alterations help to understand early Earth processes and the formation of Archaean pyrophyllite deposits. A combination of field observations, petrographic, geochemical and geochronological analyses were applied to investigate the different aspects. The lowermost unit of the DG in the Ottosdal area is the Rhenosterspruit Formation (RsF), a ~80 m thick sedimentary sequence of sandstone and conglomerate with sub-economic gold and uranium enrichment. The sediments are overlain by the ~750 m thick pile of basaltic-andesitic volcanic rocks with intercalated porphyries of the Rhenosterhoek Formation (RhF). The latest stage of volcanism is represented by the ~3-km-thick SF, which is composed of subaerially emplaced rhyolite and dacite with scarce ignimbrite and agglomerate, and intercalated sedimentary rocks (known as ‘wonderstone’). The alteration assemblages observed in the SF are pyrophyllite-muscovite-rutile-quartz-aluminiumphosphate (argillic) and chlorite-sericite±albite±carbonate (propylitic). The argillic alteration domains appear as variably thick conformable horizons stratigraphically below ‘wonderstone’. The key process that resulted in argillic assemblages is interpreted to be palaeoweathering. Leached profiles formed on the exposed surface of the lavas, where kaolinite, the likely precursor mineral of pyrophyllite, was developed. Weathering resulted in the enrichment of Al, Ti and K, and depletion of Fe, Mg and Ca from the upper portion of the palaeosol. The propylitic domain is distributed regionally with a few local variations in the mineralogy and bulk-rock geochemistry...
Ph.D. (Geology)