Geology of the ultrabasic to basic Uitkomst complex, eastern Transvaal, South Africa: an overview |
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| Authors: | CDK Gauert SA De Waal T Wallmach |
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| Abstract: | The Uitkomst complex in eastern Transvaal, South Africa, is a mineralized, layered ultrabasic to basic intrusion of Bushveld complex age (2.05–2.06 Ga) that intruded into the sedimentary rocks of the Lower Transvaal Supergroup. The complex is situated 20 km north of Badplaas. It is elongated in a northwesterly direction and is exposed over a total distance of 9 km. The intrusion is interpreted to have an anvil-shaped cross-section with a true thickness of approximately 800 m and is enveloped by metamorphosed and, in places, brecciated country rocks. Post-Bushveld diabase intrusions caused considerable vertical dilation of teh complex.The complex consists of six lithological units (from bottom to top): Basal Gabbro, Lower Harzburgite, Chromitiferous Harzburgite, Main Harzburgite, Pyroxenite and Gabbronorite. The Basal Gabbro Unit, developed at the base of the intrusion and showing a narrow chilled margin of 0.2 to 1.5 m against the floor rocks, has an average thickness of 6 m and grades upwards into the sulphide-rich and xenolith-bearing sequence of the Lower Harzburgite Unit. The latter unit averages 50 m in thickness and is gradationally overlain by the chromite-rich harzburgite of the Chromitiferous Harzburgite Unit (average thickness 60 m). Following on from the Chromitiferous Harzburgite Unit is the 330 m thick Main Harzburgite Unit. The Pyroxenite and Gabbronorite Units (total combined thickness of 310 m) form the uppermost formations of the intrusion. The three lower lithological units, Basal Gabbro to Chromitiferous Harzburgite, are highly altered by late magmatic, hydrothermal processes causing widespread serpentinization, steatitization, saussuritization and uralitization.Field relations, petrography and mineral and whole rock chemistry suggest the following sequence of events, The original emplacement of magma took place from northwest to southeast. The intrusion was bounded between two major fracture zones that gave rise to an elongated body, which acted as a conduit for later magma heaves. The first magma pulses, forming the chilled margin of the intrusion, show chemical affinities to a micropyroxenite described from the Bushveld complex. The Lower Harzburgite and Chromitiferous Harzburgite Units, judged from the abundance of xenoliths, originated by crystal settling from a contaminated basic magma. The Main Harzburgite crystallized from a magma of constant, probably also basic, composition, which flowed through the conduit after formation of the lower three lithological units. At a late stage of emplacement, after replenishment in the conduit came to a standstill, closed system conditions developed in the upper part of the complex, resulting in a magma fractionation trend of increasing incompatible elements contents towards the top of the intrusion.The mineralization in the lower three rock units and at the base was most probably caused by a segregating sulphide liquid forced to precipitate by the oxidative degassing of dolomite. Sulphur isotope ratios indicate various degrees of contamination of the magma by the enveloping sedimentary rocks, which provided the necessary amounts of S to reach S saturation. |
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