Bushveld Igneous Complex

  The Bushveld Igneous Complex (or BIC) is a large igneous intrusion within the Earth's crust which has been tilted and eroded and now outcrops around what appears to be the edge of a great geological basin. Located in South Africa, the BIC contains some of the richest ore deposits on Earth. The reserves of platinum group metals (PGMs), platinum, palladium, osmium, iridium, rhodium, and ruthenium are the world's largest, and there are vast quantities of iron, tin, chromium, titanium and vanadium. Gabbro or norite is also quarried from parts of the Complex and rendered into dimension stone.

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Origin

The Bushveld Igneous Complex is divided into an eastern and western lobe, with a further northern extension. All three sections of the system were formed around the same time - about 2 billion years ago - and are remarkably similar. Vast quantities of molten rock from the earth's mantle were brought to surface through long vertical cracks in the earth's crust - huge arcuate differentiated lopolithic intrusions - creating the geological intrusion known as the Bushveld Igneous Complex. The effects of these injections of molten rock over time, combined with the crystallisation of different minerals at different temperatures, resulted in the formation of a structure rather like a layered cake consisting of distinct rock strata, including three PGM-bearing layers, referred to as reefs.

The Complex includes layered mafic intrusions (the Rustenburg Layered Suite) and a felsic phase. It has its geographic centre located north of Pretoria in South Africa at about 25° S and 29° E. It covers over 66,000 km², an area the size of Ireland. The complex varies in thickness, sometimes reaching 9,000 meters thick. Lithologies vary from largely ultramafic peridotite, chromitite, harzburgite, and bronzitite in the lower sections to mafic norite, anorthosite, and gabbro toward the top, and the mafic Rustenburg Layered Suite is followed by a felsic phase (the Lebowa Granite Suite).

The orebodies within the complex include the UG2 reef containing up to 43.5% chromite, and the platinum-bearing horizons Merensky Reef and Plat Reef. The Merensky Reef varies from 30 to 90 cm in thickness. It is a norite with extensive chromitite and sulfide layers or zones containing the ore. The Reef contains an average of 10 ppm platinum group metals in pyrrhotite, pentlandite, and pyrite as well as in rare platinum group minerals and alloys. The Merensky and UG-2 reefs contain approximately 90% of the world's known PGE reserves. About 80% of the platinum and 20% of the palladium mined each year are produced from these horizons.

See also

• Platinum group metals
• Igneous differentiation
• Cumulate rocks
• Ultramafic to mafic layered intrusions
• Merensky Reef
• Hans Merensky
• Stillwater igneous complex

References

• Viljoen, M.J. & Schürmann, L.W. (1998), 'Platinum-group metals' in Council for Geoscience Handbook 16, Mineral Resources of South Africa, edited by M.G.C. Wilson and C.R. Anhaeusser. Pretoria: Council for Geoscience, ISBN 1-875061-52-5
• Guilbert, John M., and Park, Charles F., Jr. (1986) The Geology of Ore Deposits, Freeman, ISBN 0-7167-1456-6