Wurtzite with Enargite

specimen number:
2282070

location:
West Colusa Mine, Butte, Montana, USA

description:
Metallic crystals of wurtzite to 0.4 cm in size on matrix with lustrous crystals of enargite. Very slight micro-damage present, still a choice thumbnail for the species and locality.

The West Colusa mine, situated within the famed Butte district of Silver Bow County, Montana, is a historic underground operation located on the eastern side of the Anaconda hill complex. Geologically, the deposit is hosted entirely within the late Cretaceous Boulder Batholith, specifically the Butte Quartz Monzonite, which is cross-cut by a complex network of quartz-porphyry dykes. The ore body consists of massive hydrothermal vein systems and structural horsetail shear zones where intense, ascending magmatic fluids caused widespread sericitic and argillic alteration, depositing rich concentrations of primary copper-iron sulfides like enargite, chalcocite, bornite, and chalcopyrite, alongside substantial byproduct zinc and manganese minerals. Underground mining at the West Colusa shaft began in the early 1880s under the Colusa Land and Mining Company before becoming a major production asset of the Boston and Montana Consolidated Copper and Silver Mining Company, and later the Anaconda Copper Mining Company. The shaft reached deep vertical levels to extract high-grade copper ores continuously through the peak industrial periods of the early to mid-twentieth century until underground operations gradually wound down, shifting toward open-pit extraction at the nearby Berkeley Pit by the 1950s and 1960s.

Wurtzite is a zinc iron sulfide mineral that represents the high-temperature, hexagonal polymorph of zinc sulfide, sharing the same chemical composition as its isometric counterpart, sphalerite. Geologically, it forms in low-temperature to moderate-temperature hydrothermal vein environments, often depositing during late-stage mineralization cycles or under specific geochemical conditions where rapid precipitation or acidic solutions favor the hexagonal structural arrangement over the cubic one. The mineral typically manifests as deep brownish-black to jet-black, opaque crystals that exhibit a distinct resinous to submetallic luster, commonly forming sharp hemimorphic pyramids, thin tabular plates, or fibrous, radiating column-like aggregates known as schalenblende when intergrown with sphalerite and galena. Because it is structurally less stable than sphalerite at ambient surface conditions, its presence in unweathered zones provides crucial thermodynamic insights into the temperature fluctuations, sulfur fugacity, and specific chemical kinetic factors governing the depositional history of polymetallic sulfide ore bod

Questions about this specimen?
Send an email or call us: 314-341-1811.