Arsenopyrite cluster, about 3 mm across. Flambeau Mine, Rusk County, WI. (Photograph by Dan Behnke.)

Formula: FeAsS Monoclinic


Arsenopyrite is formed in hydrothermal deposits, especially those formed at medium to high temperatures. It is often associated with other sulfides such as pyrite and chalcopyrite.

CLARK COUNTY: Reported from an unspecified locality in the county as particles in a quartz veins that also yielded some silver and gold values when assayed (Irving, 1882).

FLORENCE COUNTY: Arsenopyrite occurs as an accessory in numerous pegmatite dikes cutting the Hoskins Lake Granite (Sims et al., 1992).

FOREST COUNTY: Found as a rare component of the massive sulfide zone in the Crandon Deposit. It may occur as tiny euhedral crystals several mm. in diameter with chalcopyrite, pyrite and sphalerite. Higher gold values tend to be found in zones where the arsenopyrite is more common (May and Schmidt 1982; Lambe and Rowe 1989).

GREEN LAKE COUNTY: Small tabular to wedge-shaped crystals of arsenopyrite occur as an accessory mineral in a diabase dike approximately 2 kilometers SSW of Marquette (Hobbs, 1895).

MARATHON COUNTY: Falster (1987) notes that arsenopyrite occurs rarely as small crystals in the vugs of pegmatite bodies in the Wausau pluton.

RUSK COUNTY: Arsenopyrite was found as euhedral crystals, at times several centimeters across in the Flambeau Copper Mine, near Ladysmith. Some samples labeled as arsenopyrite, however, have later been shown upon analysis to be marcasite with little or no arsenic. Energy-dispersive spectroscopy (EDS) does verify some arsenopyrite in pockets with chalcocite crystals associated with bornite, quartz, and pyrite. (Tom Buchholz, 2002, 2016, pers. com.)

WAUPACA COUNTY: Arsenopyrite was reported as small grains in road cuts in pegmatite cutting granite along County G near Keller Lake, NE of Big Falls. Associated minerals include euxenite-(Y), biotite, fluorite, K feldspar, bastnaesite group minerals, allanite and various sulfides. (Tom Buchholz, 2002, pers. com.)