Calcite with Quartz

specimen number:
1052786

location:
Keweenaw Peninsula, Michigan, USA

description:
A unique specimen for this area, a gemmy scalenohedral crystal of colorless calcite just over 4.0 cm in height is set on matrix with many large glassy gemmy crystals of quartz. This is in very good condition with just minimal damage to some of the quartz crystals. It is unusual to see quartz crystals in this size from this area. The combination is especially unusual. Minor native copper is present within the matrix.

The Keweenaw Peninsula, situated in the Upper Peninsula of Michigan, is globally renowned for hosting the largest concentration of native copper on Earth. Geologically, the deposit is situated within the Mesoproterozoic Midcontinent Rift System, specifically within the Portage Lake Volcanics formation, which consists of hundreds of thick, basaltic lava flows interbedded with sedimentary conglomerates. Hydrothermal solutions circulating through the deep rift basin carried dissolved copper upward, depositing it as pure native metal within the porous amygdaloidal tops of the basalt flows, the matrices of conglomerates, and cross-cutting veins. Native Americans mined the shallow copper deposits for thousands of years, but commercial mining began following a geological report by Douglass Houghton in 1841. The Michigan copper boom triggered systemic industrial operations that spanned from 1845 through the mid-twentieth century, dominated by major operations like the Cliff, Minnesota, and Calumet and Hecla mines. Industrial extraction peaked during World War I, after which declining ore grades and deep operating costs caused a steady decline; the traditional native copper mines closed by the late 1960s, and the nearby White Pine mine permanently ceased operations in 1995, marking the end of large-scale copper mining in the region.

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