Henry Rowland Cornwall
1951
<p><span>The opaque minerals </span>in<span> ten </span>lava<span> flows of the Keweenawan </span>series<span> of Michigan were studied microscopically by the writer. The basaltic </span>lavas<span>, which range </span>in<span> thickness from 100 to 1,400 feet, contain </span>ilmenite<span>, </span>magnetite<span>, </span>hematite<span>, intergrowths of </span>magnetite<span>-</span>ilmenite<span> and </span>ilmenite<span>-</span>hematite<span>, </span>copper<span> sulfides, native </span>copper<span>, and pyrite. Variations of opaque iron minerals with thickness of flow are slight, but native </span>copper<span> predominates </span>in<span> the thinner flows and </span>copper<span> sulfides </span>in<span> the thickest. Pyrite occurs only </span>in<span> the thickest flow. Two groups of opaque minerals and their alteration products are distinguished. The earlier group includes </span>magnetite<span>, </span>ilmenite<span>, </span>magnetite<span>-</span>ilmenite<span> and </span>ilmenite<span>-</span>hematite<span> intergrowths, </span>copper<span> sulfides, and native </span>copper<span>. The later group includes </span>hematite<span>, native </span>copper<span>, chalcocite, pyrite, chlorite, and sphene. The second-generation minerals formed by the action of volatiles escaping from the </span>lavas<span>. The ratio of Ti to total Fe correlates with the degree of differentiation of the </span>lavas<span>, but the ratio of ferric to ferrous iron does not. The latter ratio was controlled primarily by the action of escaping volatiles. </span></p>
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10.2113/gsecongeo.46.1.51
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Society of Economic Geologists
Ilmenite, magnetite, hematite, and copper in lavas of the Keweenawan series
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