Gerhard Von Gruenewaldt

Ore-forming processes in the upper part of the Bushveld complex, South Africa

Abstract This paper describes several types of mineralization from the upper part of the Bushveld complex as well as the processes leading to their formation. Vanadium and Ti-bearing magnetite layers, which occur throughout the entire Upper Zone of the intrusion, have formed in response to magma mixing events, which resulted either from a breakdown of densely stratified liquid layers or an influx of a small volume of new magma. 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 dose proximity to, these layers. Although no stratigraphical horizon continuously enriched in Pt-group elements is known to occur within the Fe-rich, late differentiates of the Bushveld complex, local concentrations of Pt and Pd, related to hydrothermal processes, have been identified in that part of the intrusion. Immiscible low silica and P2O5-rich liquids, which separated at least twice during crystallization of the topmost 1000 m of the Upper Zone, led to the formation of massive and net-textured apatite-oxide ore. Granular ilmenite has recently been recognized as an important constituent of this mineralization, therefore, it may be regarded as a major, combined rock-phosphate and Ti resource. The forceful injection of highly fractionated interstitial liquids is considered to have given rise to vermiculite-bearing breccia pipes, of which only one has been recognized up to now. The association of vermiculite with Ti-bearing magnetite and diallage pegmatite, however, suggests that the former may be a much more common constituent of the many titaniferous magnetite plugs of the Upper and Main Zones.

Coexisting Ca-poor pyroxenes in the Main Zone of the Bushveld Complex

Electron microprobe analyses of Ca-poor pyroxenes in gabbroic rocks of the Main Zone of the Bushveld Complex reveal that inverted pigeonites have lower Mg/Fe ratios than coexisting hypersthenes. Textural relationships, however, indicate that the two Ca-poor pyroxenes did not crystallize simultaneously from the magma. Early pigeonite reacted with the magma to form hypersthene and the difference in the Mg/Fe ratio of these two pyroxenes reflects the difference of this ratio between early pigeonite and the magma at the time of reaction. Some of the grains of early pigeonite, now inverted to hypersthene, evidently escaped this reaction with the magma. Bulk compositions of pyroxenes intermediate between that of pigeonite and hypersthene are postulated on the grounds of varying amounts of exsolved augite in the hypersthene which has originated from pigeonite by reaction with magma.