Copper mining in Michigan

While it originated thousands of years earlier, copper mining in Michigan became an important industry in the 19th and early 20th centuries. Its rise marked the start of copper mining as a major industry in the United States.

Within the state of Michigan, copper is found almost exclusively in the western portion of the Upper Peninsula, in an area known as the Copper Country. The Copper Country 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 of Precambrian age, in a thick sequence of northwest-dipping sandstones, conglomerates, ash beds, and flood basalts associated with the Keweenawan Rift.

The native copper deposits originate in fissures, steeply dipping veins or in the amygdaloid top portion of the Portage Lake Lava Series lava tops and conglomerate beds. This series of lava "is at least 15,000 feet thick in the Michigan copper district" and consists of "several hundred flood basalt flows." The district rocks are Precambrian in age and belong to the Keweenawan Series. The first six years of mining exploited the fissure deposits, then gave way to the amygdaloidal deposits.

Although native copper was the dominant ore mineral, chalcocite (copper sulfide) was sometimes present, and, especially in the Mohawk mine, copper arsenide minerals such as mohawkite and domeykite. Gangue minerals included calcite, quartz, epidote, chlorite, and various zeolites. 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.

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