Copper (CU-0022)

specimen number:
9273051

location:
Keweenaw Peninsula, Michigan, USA

description:
An aesthetic branching crystallized grouping of native copper. In great condition.

The Keweenaw Peninsula of Upper Michigan hosted the first major mining boom in the United States, targeted at the world's largest concentration of native copper. Geologically, this unique deposit formed roughly 1.1 billion years ago within the Midcontinent Rift System, where massive flows of basalt lava alternated with layers of conglomerate sandstone and shale. Hot, metal-bearing hydrothermal fluids rose through the fractured rock, depositing pure metallic native copper within the vesicular tops of the basalt flows (amygdaloids) and filling spaces between the pebbles of the sedimentary layers. Indigenous peoples mined the surface deposits for thousands of years, but commercial Euro-American mining officially launched in 1843 following a federal land treaty and a seminal geological report by Douglass Houghton. This sparked an industrial rush led by iconic operations like the Cliff, Central, Quincy, and the massive Calumet and Hecla mines, which required some of the deepest vertical shafts on earth to follow the rich ore bodies underground. Production peaked during the late nineteenth and early twentieth centuries, playing a critical role in wiring the expanding American electrical grid, before high costs and depleted shallow reserves led to a gradual decline. Large-scale corporate mining on the peninsula effectively concluded in 1968 with the closure of the Calumet and Hecla properties, though intermittent exploration and minor reclamation efforts continued until the White Pine mine shut down in nearby Ontonagon County in 1995.

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