Domeykite

specimen number:
2062247

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

description:
A cut thick slab through massive quartz showing rich metallic domeykite. One side is slightly coated with epoxy to bring out the color.

The Ahmeek mine, located near Mohawk in Keweenaw County, Michigan, is a historic underground copper operation that consisted of four primary shafts working the copper-bearing Kearsarge amygdaloidal lode. Geologically, the deposit is situated within the Keweenawan Portage Lake Volcanics, a thick sequence of Neoproterozoic basaltic lava flows where ascending hydrothermal fluids deposited native copper within the porous, gas-bubble cavities and associated flow-top breccias of the basalt. Historically, the Ahmeek Mining Company was organized in 1880, but large-scale industrial shaft sinking and regular commercial production did not begin in earnest until 1903 following successful exploration of the Kearsarge lode. The mine became one of the most prolific producers in the district, continuing operations after its acquisition by the Calumet and Hecla Mining Company in 1923, and surviving the Great Depression to supply critical metals during World War II. Industrial underground stoping and ore extraction operated continuously until 1966, when escalating operational costs and the approaching exhaustion of accessible high-grade reserves led to the permanent closure and flooding of the underground workings.

Mohawkite is a rare, complex mineral mixture consisting primarily of copper arsenides, specifically algodonite, domeykite, and arsenian copper, often intergrown with nickel and cobalt minerals. Geologically, this unique material is found within hydrothermal vein deposits that crosscut the native copper-bearing basalt flows of the Keweenaw Peninsula, where localized arsenide-rich fluids precipitated distinct metallic masses rather than pure native copper. Historically, the material was first discovered in the nearby Mohawk mine around 1900 and named for the locality, but excellent specimens were also recovered from the fissure veins at the Ahmeek mine. When freshly mined, mohawkite displays a bright, brassy-yellow metallic luster, but it rapidly tarnishes to a dark gray or rich iridescent bronze upon exposure to air. Because it lacks the structural stability required for industrial copper smelting and was found in highly localized pockets rather than massive strata-bound orebodies, mohawkite was never mined as a primary commercial ore.

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