The occurrence of the pegmatites of the Mbeta area, Lower Gwanda greenstone belt in Matabeleland South Province, Zimbabwe

Musinga, Rutendo A
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The study aimed to use geological and geochemical approaches to understand the occurrence of pegmatites in the Mbeta area. Several methodologies including field mapping, laboratory work (thin section microscope studies, modal mineralogy composition analysis), X-RAY Fluorescence (XRF) and X-RAY Diffraction (XRD) analysis were used to carry out the study. The pegmatites are hosted in the Archaean Lower Gwanda greenstone belt which consists of metamorphosed and deformed mafic volcanics with lenses of intrusive serpentinite, near its regional contact with basement granites and gneisses. The mapping of the area showed that the greenstones are well exposed, consisting of finely textured epidiorites and amphibolites. Exposures of contacts in surface workings show the greenstone are fractured and schistose with a regional strikeofN800E and dip towards the south at around 450.A swarm of approximately parallel coarsely-crystalline pegmatite veins and dykes occur in the area. They are hosted in a zone of apparent shearing measuring more than 400m in width and traceable along strike for a distance of at least 1.8km. The main pegmatite, which has been extensively trenched, dips either steeply towards the southeast or near vertical and strikes N700E.The petrographic studies show that the Mbeta pegmatites are dominated by quartz, albite-plagioclase, muscovite and minor rutile, topaz, lepidolite and garnets. Modal mineralogy studies indicate the main framework grains are subhedral crystals of quartz and albite-plagioclase comprising ~70-90%, muscovite mica constitutes 5-20% while lepidolite forms 1-5% of the samples and accessory minerals such as rutile and topaz constituted less than 4%. The apparent lack of K-Feldspar as indicated by modal and quantitative XRD analyses is confirmed by the very low K2O contents (<1%, corrected for the presence of micas) in most samples. Consequently the Mbeta albite pegmatites cannot be crystallised conventional granitic melts and have been explained as formed by secondary replacement by sodic fluids or preferential release of K in a separate fluid phase at a very late stage of pegmatite consolidation. Albitisation is a common feature in rare element enriched pegmatites, with Bikita being a prime example. It is concluded that while the pegmatites of the Mbeta area only constitute a minor lepidolite resource its economic significance is still yet to be fully ascertained. The albite rich nature of the Mbeta pegmatites resemble those hosting the main Li ore deposits at Bikita and thus suggest some similarities
A research report submitted to the Department of Geosciences, Faculty of Science, University of the Witwatersrand, in partial fulfilment of the requirements for the degree MSc: Economic Geology, Department of Geosciences, 2021