Summary:
Amplats’ Mineral Resources of Platinum Group Metals (PGM) occur exclusively within southern Africa and are hosted by two distinct layered intrusions: the Bushveld Complex in South Africa and the Great Dyke in Zimbabwe.
The mines Tumela and Dishaba are forms part of the Amandelbult section and fully developed situated on the north-western limb of the Bushveld Complex.The mining occurs on both the Merensky Reef and the UG2 Reef horizons.
Bushveld Complex
Formed over two billion years ago as a result of multiple injections of magma into the earth’s crust many kilometres below the surface, the Bushveld Complex is geologically unique due to its size, uniformity of its layering and extent of known mineral content. This saucer-shaped intrusion is over 350km wide, 250km long and up to 12km thick. Over time, the rim of the intrusion has been exposed by erosion, revealing three major separate segments known as the Western, Eastern and Northern Limbs. The Western Limb is split into two lobes (north-western and south-western) by the Pilanesberg Complex, a remnant of an alkaline volcanic plug that intruded into the Bushveld Complex about 1.2 billion years ago. The north/east trending Steelpoort fault divides the Eastern Limb into two lobes: north-eastern and south-eastern. The exposed segments exhibit layering of pyroxenites, norites, gabbros, anorthosites and chromitites across the entire extent of the complex.
The Bushveld Complex comprises three main suites, namely the Rooiberg Group, Lebowa Granite Suite and Rustenburg Layered Suite. The Rustenburg Layered Suite comprises four major subdivisions: the upper zone, main zone, critical zone and lower zone. Economic concentrations of PGMs occur mainly in three distinct units within the critical zone: Merensky Reef; Upper Group 2 (UG2) chromitite; and Platreef. The Merensky Reef and UG2 Reef occur along the Eastern (EL) and Western (WL) Limbs of the complex, while the Platreef is restricted to the eastern edge of the Northern Limb (NL). The UG2 and Merensky Reefs are extracted at Amandelbult, Mototolo, Twickenham and Modikwa while the Platreef is extracted at Mogalakwena.
The Merensky and UG2 Reefs are narrow tabular orebodies that extend laterally over hundreds of square kilometres, resulting in extensive mineralisation.
The Merensky Reef
The Merensky Reef has been the principal source of PGMs since it was first mined in 1925. The reef contains economically important PGMs and base metal sulphide mineralisation.
It is extensively developed in both the EL and WL of the Bushveld Complex. The reef typically consists of a pegmatoidal feldspathic pyroxenite layer, bound at the top and bottom by thin chromitite layers (stringers) that range from 5mm to 20mm in thickness. Mineralisation of the reef generally occurs within the pegmatoidal feldspathic pyroxenite and, to a limited extent, in the hanging wall and footwall, with the highest PGM concentration peaking in the chromitite stringers.
In the WL, the Merensky Reef dips in an arc ranging from 18º to 27º south-easterly direction in the Amandelbult area and at 9º to 12º northerly direction in the Rustenburg area. It has a variable reef thickness ranging from 1cm (Contact Reef facies) to >2m, over large areas. At Amandelbult complex in particular, the reef comprises up to five different facies. Each facies type exhibits unique geomorphological, geochemical and mineralisation characteristics and plays a fundamental role in geozone delineations and Resource and Reserves widths. In the EL, the Merensky Reef is slightly thicker and dips at 8º to 18º in a south-westerly direction.
The UG2 Reef
With the depletion of shallow Merensky Mineral Resources, the UG2 Reef has grown steadily in importance to the point where it now accounts for most of the PGM-bearing ore processed in South Africa. The UG2 Reef, which is consistently developed throughout the EL and WL, is rich in chromitite, with lower gold and base metal but slightly higher rhodium grades compared to the Merensky Reef.
The UG2 Reef occurs vertically below the Merensky Reef and the separation distance varies between 12m and 150m in the WL and averages at 350m to 400m in the EL. The UG2 Reef normally comprises a 0.6m to 1.0m main chromitite layer overlain by three to five chromitite layers (UG2 leaders) varying in thickness from 5cm to 30cm, separated by feldspathic pyroxenite. The immediate footwall of the UG2 is usually a pegmatoidal feldspathic pyroxenite, which varies in thickness from a few centimetres to over 1m. The separation distances between these UG2 leader chromitite layers and the UG2 main layer has important implications for geotechnical considerations for mining across the Bushveld Complex.
The UG2 Reef, which is consistently developed throughout the EL and WL, is rich in chromitite, but with lower gold and base metal content compared to Merensky Reef. In the WL, the UG2 Reef occurs between 12m and 150m vertically below the Merensky Reef and dips at 18º to 27º in a south-easterly direction (Amandelbult area). In the EL, the reef occurs between 120m and 400m vertically below the Merensky Reef and dips at 8º to 18º in a south- westerly direction.
In the WL, the UG2 normally comprises a 0.6m to 1.0m main chromitite band overlain by three chromitite bands (UG2 leaders) varying in thickness from 5cm to 30cm, separated by feldspathic pyroxenite. The immediate footwall of the UG2 is usually a pegmatoidal feldspathic pyroxenite, which varies in thickness from a few centimetres to over 1m. The separation distances between these UG2 leader chromitite bands and the UG2 main band has important implications for geotechnical considerations for mining.
In the EL, the UG2 normally comprises a main chromitite band varying in thickness from 0.3m to 1.0m, overlain by three or up to four chromitite stringers varying in thickness from 2mm to 1cm. The immediate footwall of the UG2 is usually a pegmatoidal feldspathic pyroxenite, which varies in thickness from a few centimetres to over 1m. The separation distances between these UG2 hanging wall stringers has important implications for geotechnical considerations for mining.
In the EL south of the Steelpoort fault, UG2 chromitite is an amalgamation of UG2 main chromitite with the leader chromitite unit, the two usually indistinguishable but may be separated based on their Pt/Pd ratios varying in thickness from 0.60m to 2.25m (averaging 1.20m). This is overlain by a poikilitic feldspathic pyroxenite parting varying in thickness from millimetres to 15m and averaging 1m. This parting is overlain by three chromitite bands (up to 20cm), separated by poikilitic feldspathic pyroxenite and pegmatoidal pyroxenites commonly referred to as the ‘triplets’ with an average thickness of 80cm.