Primary Oxide Minerals in the Layered Series of the Muskox Intrusion
The layered series of the Muskox intrusion consists of 42 layers of 18 different rock types and has a total thickness of about 6,500 feet. The series ranges with much repetition from dunite at the base, through peridotite and various pyroxenites and gabbros, to a discontinuous capping of granophyre. The layers are accumulations of mineral grains fractionally crystallized and settled from basaltic magma, and the sequence of layers reflects the order of crystallization of the minerals periodically interrupted by the introduction of fresh magma. The primary oxide minerals are chromite, magnetite and ilmenite.
Chromite occurs as tiny octahedral crystals. It is disseminated in amounts of 1–3 percent (by volume) throughout layers of dunite and peridotite having a total thickness of about 4,000 feet, and it is concentrated in two thin layers in peridotite just below orthopyroxenite layers. Textural, modal and chemical data show that the chromite crystallized simultaneously with olivine, and the thin concentrated layers apparently formed as a result of gravity sorting accompanying a change in crystallization from relatively coarse olivine to finer-grained orthopyroxene. Chromite ceased to precipitate when pyroxene began to form.
The magnetite and ilmenite are settled minerals in granophyre-bearing gabbro near the top of the intrusion. Magnetite was first to crystallize and co-precipitated with plagioclase and pyroxene in amounts of 3–5 percent. With the appearance of ilmenite, the two oxide minerals generally total 8–10 percent. The settled grains of magnetite have equant forms; those of ilmenite, a distinctive platy habit. The magnetite commonly shows lamellae of ilmenite probably formed by "exsolution" in response to deuteric or later oxidation. The parent liquid of the magnetite and ilmenite apparently was fairly rich in FeO and TiO2 but did not have an exceptionally large content of Fe2O3.
The crystallization relations of the Muskox oxide minerals obey closely certain theoretical rules of fractional crystallization and are generally consistent, both qualitatively and quantitatively, with available phase equilibria data. They may therefore be applicable in other intrusions, in which case they place some fairly stringent limitations on processes of fractional crystallization as possible means of producing ore-grade deposits of chromite, magnetite and ilmenite.
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