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.
