January 27. Keweenaw Copper

By Richard I. Gibson

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Keweenaw copper district

The mid continent rift system ruptured the crust of Proterozoic North America, allowing magmas to flow into parts of the rift. But don’t think of it as a simple low-lying trough: the flanks of the trough were probably fairly high mountains, and basins bordered the trough and mountains. Lots of sediment came into those basins.

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 In some places, the cement holding those pebbles together has been replaced by pure native copper that was brought to the surface by the volcanism associated with the mid-continent rifting about 1.1 billion years ago.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.

Native copper

The first copper rush to northern Michigan was in 1843-1846, and by about 1870, 95% of America’s copper came from this area. But after decades of mining, the area was surpassed by western mines in Butte, Montana, and in Utah and Arizona in the late 19th century. The copper industry in Michigan was pretty much dead by the 1970s, but relatively high copper prices mean that there are several projects in the works today to rejuvenate copper mining there.

Photo by Jonathan Zander under GNU free documentation license.


Map from Michigan Tech

Useful links for further reading:
http://www.nps.gov/history/history/online_books/geology/publications/pp/754-b/sec1.htm
http://pubs.usgs.gov/of/1999/of99-149/
http://www.geo.mtu.edu/~raman/SilverI/BlackLavas/Stratigraphy/Stratigraphy.html
http://www.geo.mtu.edu/~raman/SilverI/BlackLavas/Copper_Mining.html