The Platreef is a platinum group element (PGE) and base metal enriched mafic/ultramafic layer situated along the base of the northern (Potgietersrus) limb of the Bushveld Complex. It represents an important resource of PGE which is only in its early stages of exploitation. The present study contains a detailed petrographic and geochemical investigation of a borehole core drilled on the farm Townlands. At this locality, the Platreef rests on metasedimentary rocks of the Silverton Formation of the Transvaal Supergroup, and is comprised of three medium grained units of gabbronorite/feldspathic pyroxenite that are separated by hornfels interlayers. I refer to the three platiniferous layers as the Lower, Middle and Upper Platreef. The Middle Platreef is the main mineralized layer, with total PGE contents up to 4 ppm. The Lower and Upper Platreefs are less well mineralized (up to 1.5 ppm). Trace element and S-isotope data show compositional breaks between the different platiniferous layers suggesting that they represent distinct sill-like intrusions. The study also reveals a reversed differentiation trend of more primitive rocks towards the top of the succession. For example, pyroxene shows an increase in Cr2 O3 with height couples with a decrease in TiO2 . Olivine from the Upper Platreef has Fo contents between 80-83 (averaging Fo81) and those from the Middle Platreef have Fo from 78-83 (averaging Fo79 ). The Upper and Lower Platreefs have <FONT FACE="SYMBOL">d</FONT> 34S values averaging 80/00 while the Middle Platreef has <FONT FACE="SYMBOL">d</FONT> 34S values averaging 4 0 /00. All three Platreef layers have elevated <FONT FACE="SYMBOL">d</FONT> 34S values, indicating addition of 34S-enriched crustal sulphur. The model of contamination is supported by elevated K, Ca, Zr and Y contents in the Platreef relative to Critical Zone rocks from elsewhere in the Bushveld Complex, and by high Zr/Y ratios. Well defined correlations between concentrations of the individual PGE, and between the PGE and S suggest that the concentration of the PGE was controlled by segregating sulphide melt. Alteration of the rocks, possibly due to infiltration by fluids derived from the floor rocks, caused localized redistribution of Cu and, to a lesser degree, the PGE. A model is proposed whereby the Platreef magma assimilated calcsilicate and hornfels from the country rocks. The hornfels and calcsilicate of the Silverton Formation that forms the floor rocks to the Platreef on the farm Townlands constitute a possible source of the crystal sulphur. Release of S from the floor rocks caused S-supersaturation in the magma, followed by segregation of an immiscible sulphide melt. The sulphide melt scavenged the PGE from the silicate magma. The sulphides and the xenoliths were entrained by successive, metal-undepleted magma flows, causing high metal tenors in the sulphides and undepleted Ni contents in associated olivine / Dissertation (MSc)--University of Pretoria, 2006. / Geology / unrestricted

M.Sc. / Please refer to full text to view abstract

The UG3 at Modikwa Platinum Mine occurs as a platiniferous, planar chromitite seam. It is stratigraphically located in the Upper Critical Zone of the Eastern Bushveld Complex. Field work study comprise of underground mapping, sampling, surface mapping, borehole core logging, microprobing and microscopic investigations carried out at the Rhodes University. The UG3 at the Modikwa Platinum Mine is about 22cm thick chromitite seam underlain by a white fine grained anorthosite and overlain by a brown medium grained feldspathic pyroxenite. It is an incomplete cyclic unit consisting of chromite and feldspathic pyroxenite. The UG3 reef at the Modikwa Platinum Mine lease area represents a Platinum Group Metal resource of 300 million tons of ore at an in situ grade of 2.5g/t. Under the current market conditions the UG3 reef remains unprofitable to mine in an underground operation due to the operational cost involved. However, it has been illustrated that the UG3 chromitite seam can increase profit margins in an open pit operation provided it is mined together with the economic UG2 chromitite seam. The extraction of the UG3 as ore in the four Modikwa UG2 open pits would result in a combined operating cash profit of R330 million. The UG3 chromitite seam is platiniferous. The platinum-group minerals (PGM) range in size from less than 10μm to about 70μm. The PGMs are associated with sulphides and are both located in the interstitial silicates and are concentrated in the chromitite seam. The PGMs show a strong preference to contact boundaries of the silicate grains, the chromite grains and the sulphide phases. In some instances, they are enclosed within the chromite grains in association with sulphides. The general sulphide assemblage comprises pentlandite and chalcopyrite whereas, the PGMs assemblage comprises cooperite, ferroplatinum, laurite, FeRhS and PtRhS.