Stratigraphic Control of Chemistry and Mineralogy in Metamorphosed Witwatersrand Quartzites

Abstract

Within metamorphosed quartzites of the Witwatersrand Supergroup bulk chemistry is the major control on the distribution of metamorphic mineral phases and on their chemistry. Bulk chemistry is in turn controlled by stratigraphic position. Pyrophyllite distribution varies directly with, whereas feldspar distribution varies inversely with, bulk Al. Chloritoid distribution is more complex and varies with both bulk Al and bulk Fe/Mg. Despite the overprint of low-grade metamorphism and, possibly, of other alteration processes, both detrital and metamorphic phyllosilicates can be recognized and characterized. Al in metamorphic white mica and metamorphic chlorite varies with bulk Al; whereas the composition of detrital white mica is unrelated to bulk chemistry. Similarly, Mg in metamorphic chlorite varies with bulk Mg. Bulk chemical variation, particularly in Al, Mg, and Na, and thus metamorphic mineral chemistry, is controlled by stratigraphic position, both within individual stratigraphie units and in the stratigraphie succession as a whole. K content increases upward and varies more strongly with position in the entire section than with position in individual units. The quartzites display a high degree of chemical maturity, as measured by the Chemical Index of Alteration, which increases upward both within individual stratigraphie units and in the section as a whole. The strong stratigraphie control on bulk chemistry suggests that intense weathering occurred in the source area and that alteration events associated with faults and other structural features play no more than a minor role in controlling variations in bulk chemistry. The observation that these rocks are strongly leached, yet retain detrital pyrite and uraninite, indicates that the source area weathering occurred under reducing conditions.