Abstract

The opaque minerals in ten lava flows of the Keweenawan series of Michigan were studied microscopically by the writer. The basaltic lavas, which range in thickness from 100 to 1,400 feet, contain ilmenite, magnetite, hematite, intergrowths of magnetite-ilmenite and ilmenite-hematite, copper sulfides, native copper, and pyrite. Variations of opaque iron minerals with thickness of flow are slight, but native copper predominates in the thinner flows and copper sulfides in the thickest. Pyrite occurs only in the thickest flow. Two groups of opaque minerals and their alteration products are distinguished. The earlier group includes magnetite, ilmenite, magnetite-ilmenite and ilmenite-hematite intergrowths, copper sulfides, and native copper. The later group includes hematite, native copper, chalcocite, pyrite, chlorite, and sphene. The second-generation minerals formed by the action of volatiles escaping from the lavas. The ratio of Ti to total Fe correlates with the degree of differentiation of the lavas, but the ratio of ferric to ferrous iron does not. The latter ratio was controlled primarily by the action of escaping volatiles.

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