Copper

specimen number:
2262326

location:
Houghton Co., Michigan, USA

description:
Crudely crystallized grouping of native copper. Some matrix attached.

The Keweenaw Peninsula mining district, located in the Upper Peninsula of Michigan, was an immense and uniquely prolific producer of native copper. Geologically, the peninsula forms the rib of the Midcontinent Rift System, where a thick Mesoproterozoic sequence of basaltic lava flows (the Portage Lake Volcanics) is interbedded with layers of volcanic conglomerate. Hydrothermal fluids circulating through this thick sequence leached copper from the deep basalts and deposited it as pure, uncombined native metal within the porous amygdaloidal flow-tops and the matrix of the conglomerate beds, forming a massive mineralized belt alongside lesser amounts of native silver. Historically, indigenous peoples mined the shallow outcropping metal for thousands of years before the modern industrial boom was triggered by Douglass Houghton's geological report in 1841. Industrial exploitation began in 1844 with the Cliff mine, rapidly evolving from shallow surface quarrying into some of the deepest underground vertical and inclined shaft mines in the world, including the legendary operations of the Calumet and Hecla Mining Company and the Quincy Mining Company. These highly capitalized corporate operations produced millions of tons of high-grade copper to support the expansion of the American electrical grid through the late nineteenth and early twentieth centuries. Production peaked during World War I, after which deep-level pumping costs, labor disputes, and severe competition from cheaper Western open-pit copper mines caused a steady contraction, culminating in the closure of the major Calumet and Hecla shafts in 1968 and the final cessation of the last localized corporate underground operations at the White Pine mine extension on the edge of the district in 1995.

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