Geology
Regional geology
The Rustenburg Layered Suite (RLS) in the eastern limb attains a maximum thickness of 8 600m and covers an area of approximately 66 000 km2. The major units of the RLS stratigraphy are:
- A variably thick marginal zone containing dark coloured and dense ultramafic rocks;
- An approximately 1 850m thick lower zone composed of cyclical ultramafic units of harzburgite, pyroxenite and feldspathic pyroxenite making up 21% of the RLS;
- An approximately 1 250m thick critical zone composed of cyclic units of norite, anorthosite, pyroxenite and harzburgite making up 15% of the RLS;
- An approximately 3 500m thick main zone composed of norite, gabbronorite, gabbro and anorthosite comprising 41% of the RLS; and
- An approximately 2 000m thick upper zone composed of magnetite layers, magnetite gabbro, anorthosite and ferro-diorite making up 23% of the RLS.
The upper critical zone is characterized by persistent differentiated cyclicity, culminating with the Merensky reef. The chromitite layers within the upper critical zone are designated the UG1 and UG2, although a UG3 layer is present in the eastern limb. The chromitites, with a Cr:Fe ratio of 1.3:1, commonly occur at the base of a cyclic layer. Each cycle starts with a basal chromitite and grades upwards into leader chromitite layers, into an iron-rich pyroxenite, norite and, on occasion, into an anorthosite.
The UG2 reef
The UG2 reef is generally underlain by a pegmatoidal feldspathic pyroxenite, with a diffuse basal contact and frequently a thin (2cm) chromite stringer below the base of the main UG2 chromitite. The absence of this stringer would generally indicate the presence of potholing.
The UG2 reef varies between 60 and 80cm in thickness and often displays a mottled appearance due to the presence of large bronzite crystals within the chromitite. The main chromitite is overlain by a relatively thick (up to 6m) porphyritic pyroxenite layer, which contains up to three leader chromitite layers, which are known to bifurcate and coalesce on a local. PGEs are concentrated at the upper and lower contacts of the main chromitite, with lesser concentrations in the leader layers. The highest PGE concentration is generally recorded at the base of the UG2 Reef.
The footwall layers
Overlying the upper members of the UG2 reef is a series of anorthosite and anorthosite-norite layers which are collectively referred to as the footwall layers. The detailed descriptions and recognition of these units is a prerequisite for understanding the impact and magnitude of the potholing in the overlying Merensky reef.
The Merensky reef
In the eastern limb, the Merensky reef exhibits variations in lithology and grade and is the most regular and complete cyclic unit within the critical zone. The terms Merensky pegmatoid, Merensky pyroxenite and Merensky chromitite have been used to describe the various layers comprising the Merensky reef. The Merensky reef is located between 60 to 100m below the top of the critical zone and grades upward through the cycle into norite, a 'spotted' anorthosite and, finally, into a 'mottled' anorthosite (at the top of the cycle).
The Merensky chromitite always registers peak values of the PGEs. The entire thickness of the Merensky pyroxenite contains approximately the same total content of PGEs all around the Bushveld Complex and the variation in grade is (crudely) inversely proportionate to the thickness of the pyroxenite. The Merensky pyroxenite does not, however, constitute a mining horizon in itself as it can be many metres in thickness that contains the Merensky reef.
Structural geology
In the central part of the eastern limb, the mafic rocks generally dip at 10-15° towards the west. Dips increase to 20° at Lebowa Platinum Mine and reach as much as 65° at Lonmin’s Messina Mine. This steepening of dip is interpreted as part of the complex (and unresolved) structural architecture of the floor rocks of the Bushveld Complex. Structural disturbances are most pronounced around the Steelpoort fault and in the vicinity of the Wonderkop fault and Grootboom Anticline. The Steelpoort fault has been considered a structural element that was active at the time that the RLS was emplaced. Stratigraphic variations are quite pronounced across this structure, exemplified by the fact that the Merensky reef to UG2 reef separation varies from 150m to 300m across this fault zone. Other subsidiary sets of faults and fracture zones strike north to south, east-north-east to west-southwest and west-northwest to east-southeast. Dolerite dykes of post-Karoo age have been emplaced along east-north-east to west-south-west and west-north-west to east-south-east striking extensional fracture sets.
