Summary:
The Mototolo – Der Brochen Complex, is an important part of Anglo American’s PGMs portfolio, located in the eastern limb of the Bushveld Complex, South Africa. It lies within mountainous topography southwest of Burgersfort and west of Lydenburg with elevations varying between 975m and 2,000m above mean sea level.
Mototolo is located in the Eastern Limb of the Bushveld Complex, where the Merensky and UG2 Reefs outcrops strike north to south over approximately 13km and dip at an average of 10° to the west. The UG2 Reef is characterised by a single thick chromitite layer known as the main band, followed by an overlying poikilitic feldspathic pyroxenite and a series of chromitite layers. These narrow chromitite layers that occur in the hangingwall of the UG2 main band are collectively termed the triplets. The three chromitite stringers vary in thickness from 2cm to 5cm (triplet 1), 10cm to 25cm (triplet 2), and approximately 5mm (triplet 3). The immediate footwall of the UG2 is usually a pegmatoidal feldspathic pyroxenite, which varies in thickness from a few centimetres to over 1m. Localised internal pyroxenite or anorthosite-rich layers can occur within the UG2 main chromitite band, creating areas of ‘split-reef’ facies. The vertical separation between the Merensky Reef and UG2 Reef horizons is approximately 170m.
The Merensky and UG2 Reefs are vertically separated by approximately 175m and strike almost north-south over an outcrop length of 11 to 12km, dipping between 10° and 12° in a westerly direction. Continuity to a depth in excess of 2,000m has been established on the western limit of the lease area.
The UG2 Reef is characterised by a single chromitite layer known as the Main Band (averaging 1.18m in thickness) overlain by a poikilitic feldspathic pyroxenite (approximately 1.07m) and a series of narrow chromitite layers. These chromitite layers occurring in the hangingwall of the UG2 have been collectively classified as the ‘Triplets’ with an average thickness of 0.80m. The first triplet, UT1, is normally 2 to 5cm thick followed by UT2, a diffuse chromitite layer that is approximately 10 to 25cm thick with a characteristic pegmatoidal, pyroxenite hangingwall (UTPEG). The UTPEG is overlain by a thin chromitite layer (UT3) approximately 5mm in thickness.
The Merensky Reef is a 2 to 5m thick feldspathic pyroxenite with the base of this unit marked by a well-defined 1 to 2cm chromitite layer (MRB). A second less consistent chromitite layer (MRT) occurs near the top of the Merensky pyroxenite. Platinum Group Element (PGE) mineralisation is concentrated in the vicinity of these chromitite layers. The Merensky Reef pyroxenite is overlain by a gradational succession of melanorite, norite, leuconorite and anorthosite.
The north/south trending St George fault traverses through the mine and represents a natural boundary that divides the mine into a ‘shallow’ eastern, and an upthrown ‘deep’ (offset of 30-60m from north to south) western portion. On average, an 80-300m wide corridor in the proximity of the fault including a highly fractured damage zone (interpreted from 3D seismic surveys), has been assigned a 100% geological loss.
The Helena pothole is located immediately south of Borwa shaft and represents an area of severe slumping and destructive potholing. Dykes are predominantly dolerite/diabase in composition and of Karoo age with little variation in strike directions and steep dips, with the Caracle dyke swarm traversing the Der Brochen south area. Whilst the main structural trend is north north-east/south south-west, most small-scale faulting in the mining operations is trending north-west/south-east. A regional depression is currently interpreted west of the St George fault on Richmond farm.