- © 2006 Geological Society of South Africa
The main approach to PGE target selection west of the Pilanesberg was to use the geological models for the setting of PGE mineralisation (together with their associated geophysical and geochemical signatures) in the southwestern lobe of the Bushveld Complex where abundant information is available. A small gravity anomaly to the west of the Pilanesberg could represent stratigraphic thickening of the Bushveld stratigraphy or a local magma feeder zone. If the Merensky Reef is present in this area then it could be expected to be of the narrow, pegmatoidal, potholed (including contact type) and high-grade variety, as in the southwestern and northwestern lobes of the Bushveld Complex. High precision, airborne magnetic and radiometric surveys over the area, carried out by the Council for Geoscience, demarcate Upper Zone stratigraphy and a range of dykes, faults and probable scattered bodies of iron-rich ultramafic pegmatites (IRUPS). Of particular significance are a set of northeasterly trending dykes and/or faults of the Vlakfontein swarm, which could host Ni (PGE) sulphide bodies of the type already known from the area. Anomalously low radiometric signatures were used to demarcate areas of unequivocal mafic/ultramafic rock outcrops or sub-outcrops. Higher radiometric signatures reflect Transvaal floor rocks while extremely high signatures occur over the Pilanesberg alkalic rocks.
In terms of soil geochemistry, chromium is a good indicator of the Lower and Lower Critical zones of the Bushveld Complex. The transition from a high to a low chromium signature in the Rustenburg area is broadly coincident with the base of the Upper Critical Zone, which contains the important UG2 and Merensky Reef platiniferous layers. The same high to low chrome transition is seen to the west of the Pilanesberg and is considered to be an important PGE target indicator. Anomalously high nickel values occur over the Lower Zone rocks to the west of the Pilanesberg, although in the Rustenburg sector the most pronounced nickel geochemical anomalies are shown to be related to the impact of mining including tailings dams, smelters etc. The expansive nickel anomaly west of the Pilanesberg probably reflects the greater abundance of altered (including birbiritized) olivine-bearing rocks (harzburgites) and the presence of nickeliferous pipes. The transition area from a high-nickel (>220ppm) to a low-nickel signature, as in the case of chromium, defines a good PGE target area. Vanadium and titanium as expected show an anomalous build-up in the Upper Zone rocks of the Bushveld Complex. V also shows anomalous values over the upper chromitite zone and over IRUPS. The zone east of the vanadium anomalies to the west of the Pilanesberg therefore emerges as one with PGE potential.
By integrating the interpretations of the above data sets, six main PGE target areas emerge to the west of the Pilanesberg. The most important targets include a well-defined zone of probable Critical Zone rocks as well as east-northeasterly trending linear targets along the Vlakfontein and related fault/dyke sets. Possible fault-, basal- and offset-style mineralisations are identified in the northwestern part of the area.