The 2 billion year old Bushveld Complex is the largest layered igneous complex in the world, and is the repository for around 85% of known global PGM resources. Extending over an area of some 67 000km² within the north-eastern portion of the Republic of South Africa, it contains the intrusive, mafic-ultramafic Rustenburg Layered Suite (RLS), which outcrops as three main acicular limbs and ranges in thickness from 7km to 12km.
The magmatic layering in the RLS is laterally persistent and can be correlated throughout most of the complex. Layering is generally shallow dipping towards the centre of the complex. The RLS stratigraphy is subdivided into five zones, which are, from lowest to highest; the marginal zone, the lower zone, the critical zone (which is further subdivided into a lower and upper unit), the main zone and the upper zone.
PGM and associated base metal mineralisation is hosted in or adjacent to chromitite seams located within the critical zone of the RLS. There are two significant ore bodies from which 75% of world PGM production is derived, these being the UG2 and Merensky reefs. The vertical separation between the UG2 and Merensky reefs is variable across the Bushveld Complex, ranging from 20m to 140m on the western limb, to between 170m and 400m on the eastern limb.
Historically, PGM production was concentrated on the western limb, but in recent years the eastern limb has been the focus of new mine development. The two wholly-owned Northam properties, Zondereinde mine and the Booysendal project, contain resources of both the UG2 and Merensky reefs.