The Geology of the Bushveld Igneous Complex, the Largest Repository of Magmatic Ore Deposits in the World
Compared with other layered intrusions, the Bushveld Complex occupies an area about 26,000 square miles in extent.
The Complex consists essentially of five adjoining, basin-shaped masses of igneous-rocks. The outline of the whole mass is accordingly lobulate. The Bushveld episode was evidently initiated by the outpouring of andesitic lava on three occasions, and of agglomerate and tuff on one occasion, during the deposition of the Transvaal System.
Sills of diabase in the Transvaal System represent the first intrusive phase of the Complex.
The Epicrustal rocks, so-called because they formed close to and at surface, include felsite, granophyre, microgranite and leptite. Their combined thickness in places of more than 15,000 feet indicates an extraordinary thick roof for the mafic and ultramafic rocks, which constitute the Layered Sequence.
The total thickness of the exposed Layered Sequence in the Eastern Transvaal is now placed at 25,000 feet. The mineral and chemical variation upward in the succession is towards enrichment in iron rather than in alkalis, although felspathization of the roof rocks is also recognized. The textural relationships of the minerals in the gabbroic rocks suggest that considerable change took place after cumulation, but still during consolidation. Rhythmic and phase layering is very pronounced; cryptic layering is indicated to some extent by the ferromagnesian constituents, but not much by the feldspars.
Whereas the Main Zone of the Layered Sequence was always considered to be rather uniform in composition, being largely gabbro and norite, it can now be subdivided according to certain marker horizons.
A wide range in composition of ultramafic pegmatoids is indicated. They are transgressive to the layering.
The transgressive character of the Layered Sequence to the encircling sedimentary rocks is emphasized and is considered to be due to funnel intrusion, perhaps at more than one center.
The correspondence in the layering in areas some 200 miles apart presents a major problem. However, the effect of turbidity currents on a crystal mush accumulated at the margin of a gently dipping basin should receive serious consideration. To explain the very large volume of the Bushveld granite one has to think in terms of anatexis of the Epicrustal and the sedimentary rocks.
The magmatic ore deposits comprise: chromite in the lower layered sequence; platinum in the Merensky Reef, which is a layered pegmatoid, and in hortonolite dunite pipes; vanadiferous magnetite in seams in the upper layered sequence and in plugs; copper and nickel in the Merensky Reef and in bronzitite pipes; tin and fluorspar in the granite and the roof rocks; telemagmatic ores of lead, zinc, fluorspar and gold in the encircling sedimentary rocks.
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