Prehnite with Copper

specimen number:
2062308

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

description:
Green botryoidal lustrous crystallized prehnite on matrix with minor copper crystals. In good condition. 1997.

The Cliff mine, located near Eagle River in Keweenaw County, Michigan, was the first highly successful underground copper mine in the Keweenaw Peninsula native copper district. Geologically, the deposit was situated along an extensive fissure vein that cut perpendicularly through layered Neoproterozoic basalt flows of the Portage Lake Volcanics, containing massive, unoxidized masses of native copper alongside localized concentrations of native silver in a calcite and prehnite gangue. Discovered in 1844 by the Pittsburgh and Boston Mining Company, formal extraction operations commenced in 1845 and continued with massive profitability through the mid-nineteenth century, proving that native copper deposits could be commercially mined at great depths. Production began to decline significantly after 1870 as the richest portions of the fissure were depleted, and formal corporate mining operations permanently ceased around 1887, although secondary processing of the waste rock dumps and sporadic leasing work continued into the early twentieth century.

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