Reef disturbances of Critical Zone rocks of the eastern Bushveld Complex in the vicinity of the Steelpoort fault, South Africa – petrogenetic implications C. Gauert, E. Kotzé, J.J. Beukes Department of Geology, University of the Free State, Nelson-Mandela-Drive, Bloemfontein, R.S.A. R.J. Giebel Department of Geology, University of the Free State, Nelson-Mandela-Drive, Bloemfontein, R.S.A; Institut für Geologische Wissenschaften, Von-Seckendorff-Platz, Martin-Luther-Universität Halle-Wittenberg, Germany Abstract. Critical Zone rocks from the MG and UG2 intervals and the Merensky cyclic unit of various sections of the eastern Bushveld complex were investigated in terms of mineralogy and geochemistry. Variations in thickness and texture of sections south and north of the Steelpoort fault zone were compared as well as the density of pegmatoidal pipe development. Approaching the Steelpoort fault from a southerly direction, the density of pegmatoids occurrence increases and the magmatic layering shows more disturbances. In the northern part of the Steelpoort fault zone, pegmatoid density is high but decreases distally to the zone. It is suggested that the post-intrusive fault formed along the position of a magma conduit. Late-stage iron-rich liquids exploited it, intruding the cumulate pile within and in proximity to the fault zone. Increased disturbance of the cumulus pile could have occurred by magma - floor rock interaction due to turbulent flow in the fault’s/conduit’s vicinity, together with changes in magma pressure, addition of volatiles by floor rock assimilation and degassing. The economic effect caused by the increased volatile situation is, that PGE grade in the vicinity of the fault zone decreases (Two Rivers Platinum Mine) or becomes very erratic (e.g. Rhodium reefs mine). Cumulate thickness variations, as noted in the vertical distance of UG-2 and Merensky reef in the northern sector of the eastern Bushveld with increasing distance from the fault zone, are caused by basement morphology induced compartmentalization. Cumulate facies changes in thickness and composition of UG-2 and MR units within lateral distances of 100s of meters at Two Rivers Platinum mine probably reflect changes in liquid line of descent, intensity of magma convection, pulses of replenishment, contamination and volatile content of the magma. Keywords. Bushveld Complex, Critical Zone, Steelpoort fault, reef disturbances, PGEs 1 Introduction Increased mining activities in the past ten years in the eastern Bushveld complex of north-eastern South Africa lately motivated mineralogical and geochemical studies of Critical Zone (CZ) rocks from the Middle group (MG) and Upper group (UG-2) intervals and the Merensky cyclic unit of various intersections. Variations in thickness and texture of sections south and north of the Steelpoort fault were compared as well as the density of pegmatoidal pipe development. MG chromitite layers in the eastern Bushveld at Thorncliffe, Tweefontein (Fig.1) were compared to the western Bushveld complex at Kroondal, revealing an indicated PGE resource as by-product to chrome in those layers. The transition from lower to upper Critical Zone is marked by first occurrence of cumulus plagioclase, probably indicating replenishment by lighter more Al- and Fe-rich magma, resulting in higher PGE contents of the LG-6 and -7 as well as MG-2 and -3 chromitites. Figure 1. Geological map of the Eastern Bushveld, and prospect or mine drillcore sites of investigation Studies were extended to the Merensky reef (MR)and UG-2 chromitite layers because of higher PGE contents and accessibility along strike and downdip by company drill cores. This allows for petrological studies of mineralogy in undisturbed and disturbed sections close to the fault. This paper will discuss the effect of late stage fluids which exsolved during magma emplacement along feeder zones on the igneous layering and the PGE geochemistry during decompression and advanced