Copper

specimen number:
8311562

location:
Central Mine, Keweenaw Co., Michigan, USA

description:

Attractive bright crystallized cluster of native copper without matrix. In great condition.

The Central mine, situated in Keweenaw County on the northern portion of the Keweenaw Peninsula, Michigan, United States, was one of the most celebrated and highly profitable fissure-vein underground native copper operations in the Lake Superior district. Geologically, this deposit is hosted within the Mesoproterozoic-age Portage Lake Volcanics of the Midcontinent Rift System, where mineralizing hydrothermal fluids filled a major, steeply dipping, north-trending fault fissure that cut directly across the regional basaltic lava flows and conglomerate layers. Unlike the stratiform amygdaloid lodes common to the region, the Central fissure vein was exceptionally rich in massive native copper blocks, often weighing many tons each, alongside native silver, calcite, prehnite, and quartz. Historically, the Central Mining Company was organized in 1854 after investigators discovered a prehistoric native mining pit containing a mass of copper, and active underground shaft sinking began immediately in 1855. The operation proved to be a spectacular success, paying its first dividend in 1862 and operating continuously for over forty years, which allowed a thriving, self-contained company town to develop around the shafts. As miners pushed deeper, the rich copper mineralized zone began to pinch out against underlying rock strata, and combined with dropping copper prices, the company permanently ceased all commercial underground operations in July 1898.

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