Implats and its associated companies exploit platiniferous horizons within the two largest known deposits of platinum group minerals in the world, namely the Bushveld Complex in South Africa and the Great Dyke in Zimbabwe.
Both these deposits are unique in terms of size and continuity. Mining mostly takes place as underground operations focusing on relatively narrow mineralised horizons with specific mining methods adapted to suit the local geology and morphology of the mineralised horizon.
The Bushveld Complex is an extremely large, 2-billion-year-old saucer-shaped layered igneous intrusion occurring in the northern part of the country within the boundaries of South Africa. The complex comprises a diversity of igneous rocks ranging in composition from ultramafic to felsic. It is generally understood that the Bushveld Complex was formed by the repeated injection of magma into an enormous chamber. Due to the huge volumes of magma involved, cooling and subsequent mineral crystallisation out of the magma was a slow process. Different minerals were formed as the magma cooled. These minerals accumulated into sub-horizontal layers, building from the base of the chamber. These processes were repeated by the intermittent replenishment and addition of existing and new magma as the case may be, thus producing a repetition of the mineral layering.
Some individual layers or groups of layers can be traced for hundreds of kilometres. This layered sequence, the Rustenburg Layered Suite, comprises five principal zones, the Marginal, Lower, Critical, Main and Upper Zones. The Bushveld Complex, dipping in general to the centre of the complex, is, horizontally, roughly clover-leaf shaped, consisting of four compartments or limbs, the western, eastern, northern and southern limbs.
The Bushveld Complex is unique both in its size, covering an extent of some 66 000km2, and in the economic importance of its minerals. Contained within the well-layered ultramafic to mafic succession are two horizons in the Critical Zone which host economically exploitable quantities of PGMs, namely the Merensky Reef and the underlying UG2 Reef. These two economic horizons can be traced for hundreds of kilometres around the complex and are the focus of Implats’ operations from which the PGMs – platinum, palladium, rhodium, ruthenium and iridium – are recovered, together with quantities of gold, nickel, copper and numerous other metals and compounds.
The Merensky Reef is generally composed of an upper feldspathic pyroxenite, overlying a thin basal chromitite stringer, followed by an anorthosite to norite footwall and with mineralisation decreasing from the basal chromitite stringer into the hanging wall and footwall. The UG2 Reef is defined as a main chromitite layer, with most of the mineralisation contained within this unit, followed by a poorly mineralised pegmatoidal pyroxenite footwall. Below the UG2 Reef are numerous other chromitite layers that are mined by others for chromium, as their PGM content is too low.
Implats’ operations on the Bushveld Complex comprise Impala Platinum, located north of Rustenburg in North West Province, and Marula, situated north-west of Burgersfort in the province of Limpopo. The Two Rivers mine, a joint venture between Implats and African Rainbow Minerals Limited (ARM), is located south-west of Burgersfort in the province of Mpumalanga. The Leeuwkop Project and contiguous prospecting areas of Afplats are situated west of Brits, also in North West Province.
The Great Dyke is a highly elongated, slightly sinuous, 550km long, layered igneous intrusion, with a maximum width of 11km, occurring in the centre of Zimbabwe. The Great Dyke, which bisects the country in a north-north-east trending direction, is a 2.5-billion-year-old, layered igneous complex similar to the Bushveld Complex. It comprises igneous rocks ranging in composition from ultramafic to mafic.
The Dyke is divided vertically into an ultramafic sequence, dominated from the base upwards by cyclic repetitions of dunite, harzburgite and bronzitite, and an upper mafic sequence consisting mainly of gabbro and gabbronorite. It is V- to Y-shaped in section, with the layering dipping from the sides of the Dyke towards the axis of the intrusion near the margins and flattening out near the centre to form a flat-lying floor.
Much of the mafic sequence has been removed by erosion. Contained within the ultramafic sequence is the P1 pyroxenite, directly below the mafic-ultramafic contact. The P1 pyroxenite in turn hosts economically exploitable quantities of PGMs in the Main Sulphide Zone (MSZ), which is generally 10 to 50m from the top of the ultramafic sequence.
The Great Dyke developed as a series of initially discrete magma chambers or compartments, which joined up as the chambers filled. The chambers coalesced below the MSZ and before erosion, the MSZ would have been continuous along the length of the Dyke.
In its present plane of erosion, the Great Dyke is exposed as a series of narrow contiguous layered complexes or chambers, namely a northern chamber consisting of the Musengezi, Darwendale and Sebakwe sub-chambers; and a southern chamber consisting of the Selukwe and Wedza sub-chambers. The Darwendale and Sebakwe sub-chambers collectively are known as the Hartley Complex.
The MSZ is a lithologically continuous layer, typically between 2 and 3m thick, that forms an elongated basin. It generally contains iron-nickel-copper sulphides, while elevated PGM concentrations occur towards its base. Peak values for the PGMs and base metals are commonly offset, while the ratio between platinum and palladium also varies vertically. In contrast to the Bushveld Complex, it is often difficult to identify mineralisation visually in the MSZ. Below the MSZ are several chromitite layers that are mined by others for chromium, as their PGM content is too low.
Implats' operations on the Great Dyke comprise Zimplats’ Ngezi mine, located south-west of Harare in Zimbabwe, and the Mimosa mine, a joint venture between Implats and Aquarius Platinum Limited (Aquarius), which is situated east of Bulawayo.
