native copper - untreated native copper from Michigan's upper peninsula - display specimen
Native copper has been mined on Michigan's Keweenaw Peninsula for at least 7,000 years, when Native Americans started mining copper there. Objects made of Keweenaw copper have been found in archaeological sites across the continent.
The first commercially successful copper mine, the Cliff Mine, opened near Eagle River in 1845 and by 1870 had returned $2,519,000 to its investors. In 1852, the mine installed a 45 ton steam engine to power a stamp mill and to pump water out of the mine. Miners had to climb almost 1,000 feet of ladders to get to the working faces of the mine, where they drilled blasting holes using hammers and drill bits.
The copper deposits of the Keweenaw Peninsula are unusual. This is the only copper mining district in the world where the ore is primarily native copper. The ore is hosted by amydaloidal basalts and conglomerates of the Portage Lake Volcanics. Mineralization occurred between 1,060 and 1,047 million years ago, with the dates from Rb-Sr radiometric dating of amygdule fillings. These native pieces filled crevasses in the rocks and have been bent during mining. The oxidized surface has been left natural.
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