By Richard I. Gibson
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|Keweenaw copper district|
Coarse sediments, deposited relatively close to the mountains, included pebbles and cobbles that solidified into a rock called conglomerate, nicely exposed in the area around Copper Harbor, on Michigan’s Keweenaw Peninsula. In some places, the cement holding those pebbles together has been replaced by pure native copper that was brought to the surface by the volcanism associated with the mid-continent rifting about 1.1 billion years ago.
On average, only about 1½% of the rock is copper, but it still adds up to one of the greatest copper deposits on earth. And unlike most of the world’s copper mines today, which are copper sulfides, the Keweenaw copper is pure – already in metallic form, called native copper. Relatively easy to mine, and because of the lack of sulfur, environmental problems are much less serious than in copper sulfide mines where sulfur generates sulfuric acid and pollution that can be challenging to deal with.
Photo by Jonathan Zander under GNU free documentation license.
Map from Michigan Tech
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