One hammer drill leads to another, and we find ourselves deep underground in an old copper mine in Northern Michigan. Herewith we have a condensed version of Wikipedia's story about Copper Mining in Michigan:
The western portion of the Upper Peninsula of Michigan is highly unusual among copper-mining districts, because copper is predominantly found in the form of pure copper metal (native copper) rather than the copper oxides or copper sulfides that form the copper ore at almost every other copper-mining district. The copper deposits occur in rocks associated with the Keweenawan Rift.
The miners sometimes found masses of native copper up to hundreds of tons (fissure deposits). To extract a single mass of copper, miners could spend months chiseling it into pieces small enough to hoist out of the mine. Although they were pure copper, removing the masses took a great deal of effort, and was sometimes not even profitable.
In the 1850s, mining began on stratiform native copper deposits in conglomerates and in basalt lava flows (locally called amygdaloids). Although these deposits tended to be lower-grade than the fissure deposits, they were much larger, and could be mined much more efficiently, with the ore blasted out, hoisted to the surface, and sent to stamp mills located at a different site. Amygdaloid and conglomerate mining turned out to be much more productive and profitable than fissure mining.
A number of copper mines also contained a notable amount of silver, both in native form and naturally alloyed with the copper. Halfbreed is the term for an ore sample that contains the pure copper and pure silver in the same piece of rock; it is only found in the native copper deposits of the Upper Peninsula of Michigan.
Copper mining in the Upper Peninsula boomed, and from 1845 until 1887 (when it was exceeded by Butte, Montana) the Michigan Copper Country was the nation's leading producer of copper. In most years from 1850 through 1881, Michigan produced more than three-quarters of the nation's copper, and in 1869 produced more than 95% of the country's copper.
Annual production peaked in 1916 at 266 million pounds (121,000 metric tons) of copper. By 1968 Michigan's native copper industry was essentially dead, after producing 11 billion pounds (5.0 million metric tons) of copper.
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