Normal Merensky Reef on Northam Platinum Mine, Zwartklip Facies, Upper Critical Zone, Western Bushveld Complex D. Smith 1 , I. J. Basson 2 and D. L. Reid 2 1 Northam Platinum Mine, P.O. Box 441, Thabazimbi, 1380 2 Department of Geological Sciences, University of Cape Town, P. B. Rondebosch, 7701 email: dsmith@norplats.co.za, ibasson@geology.uct.ac.za, dlr@geology.uct.ac.za Introduction – The Bushveld Complex The Upper Critical Zone (UCZ) of the western lobe of the Rustenburg Layered Suite (RLS) of the Bushveld Complex (e.g. Eales et al., 1993; Eales and Cawthorn, 1996) hosts the Northam Platinum Mine (Fig. 1), exploiting subsurface down-dip extensions of the Merensky and UG2 chromitite seams (e.g. Von Gruenewaldt, 1977). The western Bushveld Complex comprises two main facies: the Rustenburg Facies to the south of the Pilansberg Complex and the Swartklip Facies to the north of the Pilansberg (Wagner, 1929). This subdivision relates to the much smaller UG2- Merensky separation and widespread olivine- bearing layers in the Swartklip Facies, which consists of two reef sub-facies, i.e. Normal and Regional Pothole Merensky Reef (Viljoen, 1994; Maier and Eales, 1997; Viljoen, 1999). The Merensky Reef at Northam is divided into Normal and “Pothole” reef sub-facies (Viljoen et al., 1986 a, b; Viring and Cowell, 1999). Normal and Regional Pothole Merensky Reef Normal Merensky Reef (NR) at Northam has a footwall of Upper Pseudoreef Cyclic Unit mottled anorthosite overlain by an orthocumulate stringer (4X) chromitite, followed by reconstitution pegmatoids and mg. to cg. pyroxenite and harzburgite, the latter overlain by the Merensky Cyclic Unit (MCU), consisting of a basal refractory chromitite stringer (Merensky chromitite) overlain by a mg. poikilitic orthopyroxenite. NR sub-facies occurs where the base of the MCU does not transgress the 4X chromitite although recon- stitution, petrographic character and PGE mineralization of the inter-chromitite pegmatoid vary with reef thinning. Transgression of the 4X chromitite and underlying stratigraphic markers by the MCU produces the Regional Pothole sub-facies. Three main pothole reef types, separated by transition zones, are termed the NP2, P2 and FWP2 reefs (Viring and Cowell, 1999; Reid and Basson, in press). In addition, potholing 1) affects the extent of reconstitution and PGE/sulphide distributions in inter-chromitite pegmatoid and the reef footwall (e.g. de Klerk, 1982); 2) causes zoning, with Pt-Pd tellurides at pothole edges and Pt-Fe alloys at pothole centers (Kinloch and Peyerl, 1990) and 3) show more reduced minerali- zation/reconstitution conditions, suggesting the localization of C- and S-rich reducing fluids during footwall erosion and MCU deposition (e.g. Buntin et al., 1985; Campbell, 1986). Three broad subdivisions of the Normal Merensky Reef in the proximity of the Regional Pothole sub-facies/4X transgression are defined by the Merensky-4X separation at Northam. A fourth subdivision, where Merensky merges with the 4X chromitite, is included (Figs. 1-1 to 1-4): 1) >300 cm 4X-Merensky separation: Basal pegmatoidal dunite, grading upwards into a mg. harzburgite/orthopyroxenite, succeeded by upper pegmatoidal orthopyroxenite. Sulphide phases comprise intercumulous pentlandite- pyrrhotite-chalcopyrite as coarse clusters in the upper and lower pegmatoids and as sparse, finely disseminated intergrowths in the intervening poorly mineralised mg. harzburgite/orthopyroxenite. Chalcopyrite decreases upwards, pentlandite increases downwards; pyrrhotite remains constant. Sulphide and concomitant PGE content display two distinct spikes, one centred 10-20 cm above the 4X chromitite, and the other coincident with or immediately below the Merensky chromitite. 2) 160-300 cm 4X-Merensky separation: Basal pegmatoidal harzburgite, grading upwards into a pegmatoidal pyroxenite. Sulphide phases comprise intercumulus pentlandite-pyrrhotite- chalcopyrite, as coarse clusters in a zone centred 10 cm above the 4X chromitite and coincident with the Merensky chromitite, and as smaller disseminated clusters in the pegmatoid. PGE content mimics sulphide distribution, with peaks in a zone centred 10cm above the 4X chromitite, and associated with Merensky chromitite. The two PGE peaks, evident in the >300 cm reef, merge in the inter-peak area. 3) <160 cm 4X-Merensky separation: Heterogeneous pegmatoidal pyroxenite ± minor olivine. Intercumulous pentlandite-pyrrhotite- chalcopyrite occurs as randomly distributed irregul- ar clusters to fine disseminations. Chromitite- related PGE and sulphide peaks become progress-