Copper

specimen number:
2062242

location:
Mass Mine, Ontonagon Co., Michigan, USA

description:
A sharp large crystal of native copper over 3.0 cm in size on a breccia matrix. This is in good condition.

The Mass mine, situated near the town of Mass City in Ontonagon County, Michigan, was a major underground native copper operation located on the southern bedrock package of the historic Keweenaw Peninsula copper range. Geologically, the deposit consists of strata-bound native copper mineralization localized within the volcanic flow tops of the Mesoproterozoic Portage Lake Volcanics, specifically targeting the highly amygdaloidal horizons of the Evergreen, Butler, Mass, and Knowlton basalt flows. Late-stage, low-temperature hydrothermal fluids migrating through these porous, gas-bubble-filled volcanic units precipitated metallic native copper alongside secondary minerals such as epidote, quartz, calcite, and prehnite, occasionally forming large, isolated sheets of copper known locally as mass copper. Historically, the property was initially explored in 1848 after prospectors discovered ancient indigenous mining pits, leading to the organization of the Mass Mining Company and sporadic production through the mid-nineteenth century. In 1899, a massive corporate consolidation merged the original Mass property with several adjacent historic tracts, including the Ridge, Ogima, and Merrimac mines, to form the Mass Consolidated Mining Company, which constructed extensive modern surface plants and developed four deep shafts. This consolidated operation ran continuously and produced millions of pounds of copper until changing economic conditions following World War I forced the permanent cessation of all mining and milling activities in 1919.

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