Occurrence and Characteristics of Chromite Deposits—Eastern Bushveld Complex
The chromite deposits of the eastern Bushveld Complex occur in the Critical Zone, a layered pile consisting mainly of pyroxenite, norite, and anorthosite, with minor gabbro, dunite, and harzburgite. With probable reserves in excess of one billion tons, the deposits are among the major mineral resources of the world. The Critical Zone is more than 75 miles long and is divisible into several sectors. The central and southern sectors are best known. Field studies indicate that they differ in rock sequences and interlayered chromite deposits.
An 11,000 foot sequence of rocks in the central sector, extending from the Basal Zone upward into the Main Norite Zone, has been sampled in detail, and its evolution is being studied in terms of field data, mineralogy, chemical composition, and textures. The sequence is divisible into successive units on the basis of silicate mineralogy. The Critical Zone is also divisible into chromitic and non-chromitic intervals, according to the presence or absence of cumulate chromite. Within the chromitic intervals occur the chromitites, in which chromite is the sole cumulate. Most chromitic intervals consist of chromitic pyroxenite or anorthosite, or both, but two consist mainly of chromitic dunite and harzburgite.
Broad mineralogical and compositional trends—upward change from predominant pyroxenite to predominant norite and anorthosite, upward increase in total Fe, Fe/Mg, and Fe/Cr of chromite, in Fe/Mg of orthopyroxene, and in Ab/An of plagioclase—suggest that the Critical Zone is grossly the result of progressive magmatic differentiation. Departures from these trends, however, together with major disconformities in the sequence, indicate a complex history. This is reflected in the distribution and characteristics of chromitites.
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