Copper

specimen number:
1052850

location:
Keweenaw Peninsula, Michigan, USA

description:

Crystallized grouping of native copper with little attached breccia matrix.

The Keweenaw Peninsula, projecting into Lake Superior in Upper Michigan, contains one of the largest concentrations of native copper in the world, deposited within the Midcontinent Rift System approximately 1.1 billion years ago. During this rifting event, thick sequences of basaltic lava flows and conglomerate sedimentary rocks accumulated, and subsequent hot, mineral-rich brines migrated upward, depositing elemental copper inside volcanic gas vesicles and gravel pore spaces. Indigenous peoples began surface mining this metal thousands of years ago, using cold-working techniques to produce tools and trade goods. In 1843, following the Treaty of La Pointe and early geological surveys, a major copper boom commenced, establishing the first large-scale mining industry in the United States. For nearly a century, deep underground mines like the Quincy and Calumet and Hecla extracted millions of tons of pure native copper to support national industrialization, before declining reserves and high production costs led to the closure of the major mines in the mid-twentieth century.

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