The geologic setting of the platiniferous UG-2 chromitite layer on the farm Maandagshoek, eastern Bushveld Complex

Abstract

The platiniferous UG-2 chromitite layer forms part of the upper chromitite group of the critical zone. On the farm Maandagshoek 254KT in the eastern Bushveld Complex the upper chromitite group is made up of four cyclic units, namely, UG-1, UG-2, UG-3, and UG-3A, each of which consists of a basal chromitite layer with or without olivine, overlain successively by melanorite and norite, and terminated with anorthosite. The UG-2 chromitite layer and its associated basal pegmatoid is found close to the base of the UG-2 cyclic unit. The cyclic units are separated by sharp contacts and have a remarkable persistence along strike and downdip.Whole-rock Cu, total Ni, Pt, and Pd show an upward enrichment from the base of the UG-2 unit, reaching a maximum in the main chromitite layer; they are depleted in the hanging-wall sequence of melanorite. Bulk analyses of the UG-2 chromitite and its enclosing lithologies for Pt, Pd, Ru, Rh, Ir, Au, S, sulfide Ni, Cu, and Co show that the platinum-group elements and base metals are concentrated (1) close to the top and base of the main chromitite layer, (2) in the leader layer, and (3) in minor chromitite lenses and layers both in the footwall pegmatoid and the hanging-wall melanorite. The Pt/(Pt + Pd) and Cu/Ni ratios are low in the footwall pegmatoid and base of the main chromitite layer. Values of these ratios, however, increase upward, with Pt/(Pt + Pd) decreasing at the upper contact of the UG-2 layer.The calculated concentration of platinum-group metals in the sulfide phase in the UG-2 chromitite layer ranges from 3,000 to just under 12,000 ppm. The above determination is primarily controlled by the concentration of S in the layer, with the average precious metal distribution remaining relatively constant. The calculated D (super sulfide liquid) (sub silicate liquid) with the parental liquid values in parentheses are Pt: 52,575 (40 ppb), Pd: 132,940 (16 ppb), Rh: 22,080 (8 ppb), Ir: 730,000 (0.4 ppb), and Au: 2,190 (21 ppb).Evidence of an influx of magma into the eastern lobe of the Bushveld Complex at a level between the upper and middle chromitite groups is indicated by discontinuous and disturbed layers in borehole drilling on Maandagshoek. It is suggested that this magma batch was slightly more enriched in the platinum-group elements in order to produce the increased value of these elements in the UG-2 chromitite layer. It is proposed that preexisting sulfides in the UG-2 chromitite layer collected platinum-group elements from upward-migrating, highly fractionated intercumulus fluids under highly reducing conditions during the early stages of formation of the layer. The relatively large volume of sulfide initially present in the layer was later reduced by volatiles enriched in O 2 passing through the layer, resorbing the sulfides and blowing off SO 2 . The partial resorption of the sulfides accounts for the presence of minor Pt-Fe alloys and the abnormally high partition coefficients of the platinum-group elements in the sulfide liquid of the UG-2 chromitite layer.