Field relationships and petro-chemical investigation of mafic sills and dykes in the vicinity of the Uitkoms complex, Mpumalanga South Africa
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Numerous mafic sills and dykes intruded into the Lower Transvaal sediments and the Archaean Basement in the vicinity of the Uitkomst Complex, which is assumed to be a satellite intrusion of the Bushveld Complex. Investigations on mafic sill intrusions near the Eastern Bushveld Complex described sills of pre-, syn- and post-Bushveld age and assigned the syn-Bushveld sills to the corresponding marginal rocks of the Bushveld Complex. Purpose of the presented combined diploma thesis was to map the mafic sills and dykes in the vicinity of the Uitkomst Complex as basis for a petrographical and geochemical characterization. The geological mapping as well as the petrographic description distinguishes three groups of mafic intrusive rocks: microgabbro sills, gabbronorite sills and gabbronoritic basement dykes. The group of elongated microgabbro sills shows a widespread spatial as well as stratigraphic distribution within the study area, whereas the gabbronorite sills form huge sill bodies within a main stratigraphic position between the Upper Timeball Hill Shale and the Klapperkop Quartzite. The gabbronoritic basement dykes intruded into the Archaean granite gneiss and represent the oldest of the investigated mafic rocks. Evaluation of the obtained geochemical data from about 160 samples verifies the classification into the three main groups. Based on incompatible element contents and element ratios different magma derivations are interpreted for the gabbronoritic sills, basement dykes and microgabbro sills. The basement dykes and gabbronorites sills are derived from a primitive partial melt, whereas the microgabbro sills show an evolved magma composition. Additional contamination with crustal material also changed the magma composition of each group as reflected by changing Ti/Zr ratios within the groups. The gabbronorite sills and basement dykes are probably contaminated with SiO2-rich material from quartzites and granite gneiss (highest SiO2 and Zr values), whereas the microgabbro sills show possible contaminations with dolomitic material (highest variation in CaO contents).The comparison to Bushveld related marginal rocks shows a similar composition of the gabbronorite sills and basement dykes to the B1 quenched textured micropyroxenites, whereas the microgabbro sills indicate little correlation with the composition of one of the Bushveld marginal rocks (B1 to B3). Furthermore the microgabbro sills are nearly conformable in composition of selected trace elements with the Basal Gabbro chilled margins at Uitkomst and Slaaihoek, which is supported by a possible palaeomagnetic age relationship in some extent. The geochemical fingerprints of the investigated sills and dykes compared to the Uitkomst Complex have identified no direct correlation between them and show only low contents of exploration relevant elements for microgabbro sills, gabbronorite sills as well as basement dykes.
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