GEOLOGY OF THE ULTRABASIC TO BASIC UITKOMST COMPLEX, EASTERN TRANSVAAL, SOUTH-AFRICA - AN OVERVIEW

The Uitkomst complex in eastern Transvaal, South Africa, is a minerali zed, layered ultrabasic to basic intrusion of Bushveld complex age (2. 05-2.06 Ga) that intruded into the sedimentary rocks of the Lower Tran svaal Supergoup. The complex is situated 20 km north of Badplaas. It i s elongated in a northwesterly direction and is exposed over a total d istance of 9 km. The intrusion is interpreted to have an anvil-shaped cross-section with a true thickness of approximately 800 m and is enve loped by metamorphosed and, in places, brecciated country rocks. Post- Bushveld diabase intrusions caused considerable vertical dilation of t he complex. The complex consists of six lithological units (from botto m 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 thickne ss of 6 m and grades upwards into the sulphide-rich and xenolith-beari ng sequence of the Lower Harzburgite Unit. The latter unit averages 50 m in thickness and is gradationally overlain by the chromite-rich har zburgite 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 (t otal combined thickness of 310 m) form the uppermost formations of the intrusion. The three lower lithological units, Basal Gabbro to Chromi tiferous Harzburgite, are highly altered by late magmatic, hydrotherma l processes causing widespread serpentinization, steatitization, sauss uritization and uralitization. Field relations, petrography and minera l and whole rock chemistry suggest the following sequence of events. T he original emplacement of magma took place from northwest to southeas t. The intrusion was bounded between two major fracture zones that gav e rise to an elongated body, which acted as a conduit for later magma heaves. The first magma pulses, forming the chilled margin of the intr usion, show chemical affinities to a micropyroxenite described from th e Bushveld complex. The Lower Harzburgite and Chromitiferous Harzburgi te Units, judged from the abundance of xenoliths, originated by crysta l settling from a contaminated basic magma. The Main Harzburgite cryst allized from a magma of constant, probably also basic, composition, wh ich flowed through the conduit after formation of the lower three lith ological 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 t rend of increasing incompatible elements contents towards the top of t he intrusion. The mineralization in the lower three rock units and at the base was most probably caused by a segregating sulphide liquid for ced to precipitate by the oxidative degassing of dolomite. Sulphur iso tope 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.