Summary:
The platiniferous horizons of economic interest at Bokoni Platinum Mine are the Merensky Reef and the UG2 which are part of the Critical Zone of the Rustenburg Layered Suite (“RLS”). In the Eastern Limb of the Bushveld Complex, the Critical Zone is developed over a strike length of approximately 150 km but separated by regional faulted systems. The Merensky Reef and UG2 outcrop over about 130 km, but also occur in down-faulted blocks and erosional outliers. The Merensky in the Eastern Limb comprises types that are a variation of the equivalent reef developed within the Western Limb. In common however, is that in both the Eastern and Western Limbs, economic mineralisation is hosted within a pyroxenite unit and often between relatively narrow chromitite stringers. The chromitite stringers form useful mining contacts that visually define the position of the orebody.
The general stratigraphy of the Critical Zone in the Bokoni project area, from bottom to top begins with the UG1 immediately above the footwall and ends with the Bastard reef at the top. Within the Bokoni project area, both the Merensky and UG2 horizons sub-crop and in some instances outcrop in the area along a northwest-southeast trending strike length in the mountain range to the north of the project area. The Bokoni orebodies dip from north-east to south-west at approximately 25º in the north-western areas (Zeekoegat Farm), and gradually decreases to approximately 18º in the south-eastern area (Brakfontein Farm). The general structural geology of Bokoni is characterised by north-northeast and west-east trending dykes and faults with associated conjugated joint sets and these features may result in the disruption of normal Merensky and UG2 Reef occurrence. Dominant structures include potholes, bifurcation of the UG2; dolerite dykes, faults shears and joints as well as iron rich ultramafic pegmatites.
Merensky Reef
The Merensky Reef at Bokoni mine is stratigraphically positioned approximately 350 m above the UG2, in the upper portion of the Merensky Pyroxenite. It is defined as the economical part of the Merensky Pyroxenite. The Merensky unit, whose thickness ranges from 50cm to 200 cm, typically comprises a feldspathic or poikilitic pyroxenite immediately below the gradational top contact with the overlying norite.
Four Merensky reef types have been identified in the Bokoni drillhole database based on the number of chromitite stringers within the reef. The Merensky types at Bokoni, in order of frequency, are:
• Two chromitite stringers
• Single chromitite stringer at the top
• No chromitite stringer
• Single chromitite at the bottom.
Although the upper chromitite stringer of the Merensky Reef is usually associated with the highest PGE grades, mineralisation is not always at its highest in the chromitite stringer samples and higher grades can typically occur between them. The top chromitite stringer is narrow and can be difficult to define at times, occurring as particles of chromite rather than a welldefined layer. The bottom stringer is often thick and clearly visible when present.
The Merensky footwall has a sharp contact, usually marked by the lower chromitite stringer. While the top contact tends to be planar, the basal contact is undulating as a result of thermo-chemical erosion of the more mafic Merensky lithologies. The footwall contact is also often associated with a thin anorthosite layer.
UG2 Reef
The UG2 chromitite layer occurs as a tabular massive chromitite layer in the upper critical zone approximately 350 m below the Merensky Reef at Bokoni Platinum Mine. The UG2 at Bokoni Platinum Mine occurs as a single layer of chromitite, with some internal pyroxenite lenses, and has a thickness of approximately 65 cm. Thicker reef areas are known to occur (up to approximately 1.6 m thick) where the chromitite is diluted by a higher than normal proportion of irregular layers and lenses of pyroxenite. PGE mineralisation typically peak at the top contact and bottom contacts, with the mineralisation at the bottom contact being higher. However, this pattern is not consistently observed.
The hangingwall to the UG2 is made up of feldspathic pyroxenite which is generally barren, except for sporadic higher-grade samples typically associated with the Leader chromitite stringers. Chromitite stringers, referred to as leaders at Bokoni Platinum Mine, occur in the hangingwall.
The UG2 is underlain by a pegmatoidal feldspathic pyroxenite layer of approximately 0.75 m in width which is commonly host to disseminated chromite and some base metal sulphide occurrences within close proximity to the UG2.The UG2 elevation isopachs at Bokoni indicate a relatively undisturbed tabular and gently dipping layer with widths generally increasing to the northwest from an average of 67 cm on Umkoanesstad Farm to about 74 cm on Zeekoegat Farm.