The texture, mineralogy and composition of chromite in the upper chromitite of the Muskox intrusion, in the Northwest Territories, have been studied in two 0.5-meter sections of drill core. The principal rock-type is an orthopyroxenite that contains cumulus olivine, orthopyroxene and chromite, and the intercumulus minerals clinopyroxene and plagioclase. The minor minerals ilmenite and biotite are found, together with a number of accessory minerals, in pockets that are interpreted as sites of late intercumulus melt. The chromitite seam is up to 10 cm thick and contains chromite with a narrow range in composition: 0.64 < Cr/(Cr+Al) < 0.74, 0.62 < Fe (super 2+) /(Fe (super 2+) +Mg) < 0.69, and 0.18 < Fe (super 3+) /(Fe (super 3+) +Al+ Cr) < 0.26. The average composition of chromite in the chromitite, and the olivine and orthopyroxene in the orthopyroxenite, were used to calculate a temperature of 1146 degrees C and log f(O 2 ) = -9.1. The disseminated chromite in the orthopyroxenite shows a much greater range in composition, and increases in Fe (super 2+) /(Fe (super 2+) +Mg), Fe (super 3+) /(Fe (super 3+) +Al+Cr), Ti and Ni with stratigraphic height above the massive chromitite. The chromite in the Muskox chromitite is significantly higher in Fe (super 3+) , Ti and Fe (super 2+) /(Fe (super 2+) +Mg) than chromite in the Bushveld, Stillwater and Great Dyke chromitites; furthermore, the Muskox chromitites formed much higher in the stratigraphic section of the layered series than in these other intrusions. The Muskox chromitites are considered to have formed late in the magmatic history of the intrusion as a result of mixing of a fractionated magma with a more primitive magma and a component due to wall-rock assimilation.

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