The Merensky Reef of the Bushveld Igneous Complex, a regular layer of pyroxenite, mineralized by the sulfides of nickel, iron and copper, together with thin bands or concentrations of chromite, contains by far the greatest known ore-reserve of platinoids in the world.
The Reef occupies a specific position, within a regularly layered sequence of anorthositic, gabbroidal and pyroxenitic rocks. The Mafic Zone of the Bushveld, in which it is found, can be divided into readily recognizable sub-zones, with definite markers, and has an almost identical succession in both East and West Belts.
The reef contains the platinoid metals, principally as sulfides, but also as ferroplatinum, together with arsenides, tellurides, bismuthides, antimonides and possibly stannides and selenides.
The reef has been followed over a total distance of 155 miles and to vertical depths of greater than 6,000 feet. All evidence suggests that the Merensky Reef is the basal portion of a relatively thin unit-heave of magma with a clear pattern of magmatic differentiation. The heavy minerals appear to be primarily precipitated by gravity from solution in the magma. The Merensky unit appears to be one of many heaves that built up the Mafic Zone.
The amazing regularity of the reef and its associated rocks is in places disturbed by depressions and domes, mostly small but a few are large. Alteration pegmatites associated with these structures and evidence indicating the rocks were solid when they were formed, suggest a probable mode of genesis.
Figures & Tables
This monograph on Magmatic Ore Deposits has resulted from a Symposium held at Stanford University on November 12 and 13, 1966. All except three of the papers that were presented are published in this volume as well as some of the discussion and the summation of the symposium. Unfortunately much of the discussion cannot be included because the volume is already so large. The best introduction to this volume is, perhaps, the introduction as it was presented at the symposium:
This symposium was conceived in 1962 when the Program Policy Committee recommended that the . Society of Economic Geologists should sponsor a symposium on magmatic ore deposits. The Committee under the chairmanship of John K. Gustafson believed this to be an effective method of advancing geologic thought. It is fitting that the symposium should finally be held during Gustafson’s presidential year. The proposal of the Program Policy Committee was approved by Council at its meeting in November, 1962. A special committee consisting of G. Kullerud, J. A. Noble, C. H. Smith, T. P. Thayer, with H. D. B. Wilson as chairman, was appointed by the President, Olaf N. Rove, in February 1963 to make arrangements for the symposium. E. N. Cameron, Secretary of the Society, was ex officio member of the special committee and remained as an active member when he resigned the secretaryship. C. H. Park, Jr. joined the committee shortly after its formation.
The Program Policy Committee was prompted to recommend the symposium by the realization that the underlying theory of the formation of magmatic ore deposits was formulated many decades ago., In the intervening years, much new data have been acquired from systematic research. It seemed to the Program Policy Committee that it was time for those with an abiding interest in the magmatic deposits to meet to assess this new data and to point out the unresolved problems.
The symposium was entitled “Symposium on Magmatic Ore Deposits.” The special committee accepted the terminology in the “Glossary of Geology and Related Sciences,” Edition 2, page 175.
Magmatic Deposits Certain kinds of mineral deposits form integral parts of igneous rock masses and permit the inference that they have originated, in their present form, by processes of differentiation and cooling in molten magmas. (Lindgren p. 863, 1929).
The symposium committee has added the term “ore” to attempt to keep the discussions centered on ore, or near ore material, or with