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July-August 2010

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The Pewabic Copper-bearing Amygdaloidal Lode, Houghton County, Michigan Lake Superior Native Copper District

Sixteenth in a series of articles on the mines and minerals of Michigan's Copper Country

The Lake Superior native copper district, also known as the Michigan Copper Country, is located on the sparsely populated, isolated, and scenic Keweenaw Peninsula that juts out into Lake Superior near the western end of Upper Michigan. The Keweenaw winters can be long and cold with lake-effect snow accumulations of 200–300 inches not uncommon. The summer season can be a mixture of cold-mild-hot weather with a short growing season. From midspring to midsummer the Keweenaw “bug season” is in full swing and is an entomologist's paradise with swarms of mosquitoes, blackflies, deerflies, and horseflies filling the sky.

A narrow, erosion-resistant ridge of tilted basalt flows forms a prominent spine that traverses Ontonagon, Houghton, and Keweenaw counties. This series of basalt flows with interbedded conglomerate beds is collectively known as the Portage Lake Volcanics (PLV) and is host to the native copper deposits. The now-dormant mines, which comprised the district, are strung out along the northeast strike of the PLV for approximately 90 miles. During the 132 years of active mining, from 1844 through 1976, the district produced 10,525,922,136 pounds of copper from fissure veins and amygdaloidal and conglomerate lodes (Rosemeyer 2007). This production total does not include an additional 4,088,269,603 pounds of copper recovered from the sediment-hosted stratiform sulfide (chalcocite) deposit at the White Pine mine, Ontonagon County.

Tom Rosemeyer, a native of Upper Michigan, is a graduate of Michigan Technological University. He now spends summers in the San Juan Mountains and the Copper Country researching and writing articles on the mineralogy of the districts.

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