Copper with Analcime

specimen number:
2062263

location:
Seneca #3 Mine, Keweenaw Co., Michigan, USA

description:
Nicely crystallized branching native copper specimen with minor analcime. Minor cuprite alteration. Collected in 1998.

The Seneca Number 3 mine, located near Mohawk in Keweenaw County, Michigan, was a native copper underground operation targeting the steeply dipping Kearsarge amygdaloid lode. Geologically, the deposit consists of elemental native copper hosted within a thick sequence of Precambrian amygdaloidal basalts and interbedded conglomerates of the Portage Lake Volcanics. High-temperature metamorphic fluids altered the basalts, filling the gas cavities with native copper accompanied by secondary minerals including quartz, calcite, epidote, pumpellyite, and the rare blue silicate kinoite. Sunk as a secondary production and ventilation shaft to exploit the northern extension of the lode, the Seneca Number 3 shaft was actively utilized through the middle of the twentieth century under the management of the Calumet and Hecla Consolidated Copper Company. Commercial extraction from the interconnected Seneca system fluctuated due to economic cycles and varying mineral grades before all underground operations ceased permanently in 1965.

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