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Book Chapter

Primary Oxide Minerals in the Layered Series of the Muskox Intrusion

By
T. N. Irvine
T. N. Irvine
Geological Survey of Canada, Ottawa, Ontario,
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C. H. Smith
C. H. Smith
Geological Survey of Canada, Ottawa, Ontario,
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Published:
January 01, 1969

Abstract

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.

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Contents

Society of Exploration Geophysicists Geophysical Monograph Series

Magmatic Ore Deposits

H. D. B. Wilson
H. D. B. Wilson
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Society of Economic Geologists
Volume
4
ISBN electronic:
9781934969991
Publication date:
January 01, 1969

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