Merensky Reef, Republic of South Africa

The Merensky Reef lies close to the top of the Critical Zone and comprises a 0.3-1.5 m thick, coarse-grained pegmatoidal feldspathic pyroxenite that carries an upper and lower chromite layer. Base metal sulphides and platinum group minerals are associated with these layers.

Major Minerals:

Chromite, hexagonal and monoclinic pyrrhotite, pentlandite, chalcopyrite, pyrite, troilite

Minor Minerals:

Trace Minerals:

Include braggite, cooperate, laurite, sperrylite, platinum-iron alloys, electrum, Ni-mackinawite, bravoite, valleriite, millerite, niccolite, Ni-skutterudite, bornite, galena, sphalerite, cassiterite, molybdenite, graphite, violarite.


Chromite is euhedral to subhedral and carries small base metal sulphide, platinum group mineral, ilmenite and exsolved rutile inclusions. Primary magnetite and chrome-rich titanomagnetite are associated with chromite; secondary magnetite forms rims around sulphides. The sulphides are interstitial to the silicates and are redistributed along the cleavage of secondary and altered silicates. The sulphides are zoned with pyrrhotite cores surrounded by granular pentlandite rims themselves enclosed within chalcopyrite. Locally troilite is present rather than pyrrhotite, and pyrrhotite carries flame-like exsolution bodies of pentlandite. Chalcopyrite forms composite grains with cubanite, with broad exsolution laths of cubanite within the host chalcopyrite. Chalcopyrite is associated with minor bornite, chalcocite, valleriite and Ni-mackinawite. Platinum group minerals are associated with base metal sulphides or form small inclusions in silicates. Pentlandite is altered to Ni-mackinawite, and to violarite and bravoite in oxidized ores


Cousins, 1969; Schwellnus et al., 1976; Brynard et al., 1976; Vermaak and Hendriks, 1976; de Villiers et al., 1978; Viljoen et al., 1986