ORE-FORMING PROCESSES IN THE UPPER PART OF THE BUSHVELD COMPLEX, SOUTH-AFRICA

This paper describes several types of mineralization from the upper pa rt of the Bushveld complex as well as the processes leading to their f ormation. Vanadium and Ti-bearing magnetite layers, which occur throug hout the entire Upper Zone of the intrusion, have formed in response t o magma mixing events, which resulted either from a breakdown of dense ly stratified Liquid layers or an influx of a small volume of new magm a. Magma mixing,together with a depletion of the ferrous Fe content of the crystallizing magma during the formation of the thicker magnetite layers, also resulted in the formation of immiscible sulphides, which became concentrated within, or in close proximity to, these layers. A lthough no stratigraphical horizon continuously enriched in Pt-group e lements is known to occur within the Fe-rich, late differentiates of t he Bushveld complex, local concentrations of Pt and Pd, related to hyd rothermal processes, have been identified in that part of the intrusio n. Immiscible low silica and P2O5-rich liquids, which separated at lea st twice during crystallization of the topmost 1000 m of the Upper Zon e, led to the formation of massive and net-textured apatite-oxide ore. Granular ilmenite has recently been recognized as an important consti tuent of this mineralization, therefore, it may be regarded as a major , combined rock-phosphate and Ti resource. The forceful injection of h ighly fractionated interstitial liquids is considered to have given ri se to vermiculite-bearing breccia pipes, of which only one has been re cognized up to now. The association of vermiculite with Ti-bearing mag netite and diallage pegmatite, however, suggests that the former may b e a much more common constituent of the many titaniferous magnetite pl ugs of the Upper and Main Zones.