Geology

The mining operations of 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. Mining mostly takes place as underground operations, focusing on relatively narrow mineralised channels with the specific methods adapted to suit the local geology and morphology of the mineralised horizon.

The Bushveld Complex

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 an array of diverse 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, in order of economic importance.

The Bushveld Complex is unique both in its size, covering an aerial 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 a feldspathic pyroxenite, overlying a thin basal chromitite stringer, followed by an anorthosite to norite footwall and with mineralisation decresasing from the basal chromitite stringer into the hangingwall 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 pryoxenite footwall. Below the UG2 Reef are numerous other chromitite layers that are mined for chromium, as their PGM content is too low.

Implats’ operations on the Bushveld Complex comprise Impala Rustenburg, 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 (ARM), is located south–west of Burgersfort in the province of Mpumalanga. The Leeukop project and contiguous prospecting areas of Afplats are situated west of Brits, also in the North West Province.

The Great Dyke

The Great Dyke is a 2.5 billion year old highly elongated, layered igneous intrusion occurring in the centre of Zimbabwe. It bisects the country in a north–north–east trending direction and comprises an array of igneous rocks ranging in composition from ultramafic to mafic. The Great Dyke is a layered complex similar to that of the Bushveld Complex. The Dyke is divided vertically into three major successions, a lower mafic sequence consisting mainly of steeply–dipping, fine–grained rocks of variable composition, including pyroxenites and norites, an overlying 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 gabbro–norite. 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, which in turn hosts economically exploitable quantities of PGMs in the Main Sulphide Zone (MSZ), which is generally 10m to 50m from the top of the ultramafic sequence. Disseminated sulphides with anomalous base metals, but a low PGM content are also present locally at the mafic–ultramafic contact. The 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 longitudinally subdivided into 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 are known as the Hartley Complex. The Dyke is highly elongated, slightly sinuous, 550km long, with a maximum width of 11km.

The MSZ is a lithologically continuous layer that is typically between 2m and 3m thick that forms an elongated basin. It general 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. Below the MSZ are several chromitite layers that are mined for chromium, as their PGM content is too low.