The Sulphides of the Uitkomst Complex, Badplaas, South Africa

Abstract

The Uitkomst Complex is situated on the farms Uitkomst 541JT and Slaaihoek 540JT, twenty kilometres north of Badplaas, in the Mpumalanga Province, South Africa. It is a mineralized, layered, basic to ultrabasic intrusion of Bushveld age (2,05 - 2,06 Ga). The Complex is intruded into sedimentary rocks of the lower Transvaal Supergroup. It is elongated in a north-westerly direction and is exposed over a total distance of 9km. The intrusion is interpreted by earlier workers to have an anvil-shaped cross section with a true thickness of approximately 800m. It is enveloped by metamorphosed, and in places brecciated, country rocks. Post-Bushveld diabase intrusions caused considerable vertical dilation of the Complex, which consists of six lithological units (from bottom to top) : Basal Gabbro (Bgab), Lower Harzburgite (Lhzbg), Chromitiferous Harzburgite (PCR), Main Harzburgite (Mhzbg), Pyroxenite (PXT) and Gabbronorite (GN). The lower three units carry potentially economic quantities of base metal sulphides with some PGE enrichment. The sulphide minerals occur as' disseminated, net-textured, and massive segregations. The primary sulphides are pyrrhotite (po), pentlandite (pn), and chalcopyrite (cp), with cubanite (cb), mackinawite (me), pyrite (py), millerite (ml), and cobaltite (cob) occurring with these at depth in the Complex. The assemblages are comprised of po+ pn + cp; po+ pn + cp + cb (Mhzbg); po + pn + cp + me (Mhzbg and PCR); po+ pn + cp + py (Lhzbg, PCR and Bgab); py + cp +ml+ po (Lhzbg and PCR) and po+ pn + cp + cob (Lhzbg and PCR). Chemical analyses of the sulphide minerals show a definite increase in Ni and decrease in Fe in pentlandite and pyrrhotite towards the base of intrusion. Image analysis indicates that the average grain size of the disseminated sulphide blebs increases towards the base of the intrusion where approximately 80 per cent of the grains are larger r-- qL>v\)- l'J'll,)c 7 than 450 micron. In the Mhzbg, PCR and Lhzbg zones the flame pentlandite forms between 1 and 5 percent of the total pentlandite which is indicative of the lower limit of the nickel losses to be expected in beneficiation. Granular pentlandite has an average grain size of greater than 11 micron and flame pentlandite measures 3 to 10 micron in section.