The Merensky Reef, is a layer of igneous rock in the Bushveld Igneous Complex (BIC) in the Transvaal which together with an underlying layer, the Upper Group 2 Reef (UG2), contains most of the world's known reserves of platinum group metals (PGMs) or platinum group elements (PGEs) - platinum, palladium, rhodium, ruthenium, iridium and osmium.

The UG2 Reef, the composition of which is relatively consistent throughout the BIC, is rich in chromite, but lacks the Merensky's gold, copper and nickel by-products, though its PGM reserves may be almost twice those of the Merensky Reef.

Chromitite layers occur commonly in large mafic layered intrusions. A current theory is that chromitites form as a result of introduction and mixing of chemically primitive magma with a more evolved magma, which leads to supersaturation of chromite in the mixture, which in turn leads to the formation of a nearly monomineralic layer on the magma chamber floor.

The initial recovery of platinum in South Africa took place on several of the large East Rand gold mines and the first separate platinum mine was a short lived venture near Naboomspruit that worked very patchy quartz reefs. The discovery of the Bushveld Igneous Complex deposits was made in 1924 by a Lydenburg district farmer, A F Lombaard. This was an alluvial deposit but its importance was recognised by Hans Merensky whose prospecting work discovered the primary source in the Bushveld Igneous Complex and traced it for several hundred kilometres by 1930. Extensive mining of the Reef did not take place until an upsurge in the demand for platinum group metals used in exhaust pollution control in the 1950s, made exploitation economically feasible. Extraction of metals from the UG2 chromitite could only take place in the 1970s with major advances in metallurgy.