Formula: Cu2S Monoclinic
Chalcocite is formed either as a primary vein mineral or, more typically, in supergene deposits where it forms as a result of the redeposition of copper from the breakdown of minerals such as chalcopyrite or bornite. Chalcocite is distinguished with difficulty from digenite (Cu9S5) and djurleite (Cu31S16), both of which may form by the leaching of chalcocite. Chalcocite is thus used here to covering all of these species. The chalcocite at any of these localities may, in fact, be any of these three minerals or an intergrowth. Minerals commonly associated with chalcocite include malachite, azurite, chalcopyrite, bornite and covellite.
CRAWFORD COUNTY: Chalcocite is one of the components of the ore at the Plum Creek copper mine in the SE SW Sec. 26 T.8N R.5W northwest of Wauzeka and in adjacent areas along Otter Creek (Heyl and West, 1983).
DOUGLAS COUNTY: Veins of quartz containing chalcocite and galena were reported cutting Keweenawan basalt just south of the Douglas Fault in SE sec. 29 T.48N. R.12W. (Grant, 1901).
—Chalcocite occurs with chalcopyrite and malachite at the Amnicon Copper Prospect, NE NW sec. 11 T.47N. R.13W. (Dutton, 1972).
FOREST COUNTY: Thin coatings and veins of sooty chalcocite are found as a secondary supergene mineral coating chalcopyrite at the Crandon deposit near Little Sand Lake SE Sec. 25 T.35N R.12E (May and Schmidt, 1982; Lambe and Rowe, 1989).
IOWA COUNTY: Chalcocite is found at the old Eberle Mine, 5 miles N of Cobb at the NW 1/4 NW 1/4 Sec. 2 T.6N R.1E. Here it is associated with malachite, azurite, chrysocolla, aurichalcite, sphalerite and smithsonite (WGNHS files, Heyl et al., 1959).
—Chalcocite is found on the Steppler Farm, 4 miles SW of Highland, in the SE 1/4 NE 1/4 Sec. 7 T.6N R.1E. Here it is associated with malachite, azurite, chysocolla, aurichalcite, sphalerite and smithsonite (WGNHS files; Heyl et al., 1959).
—Chalcocite was locally mined near Mineral Point in Sec. 4 and 5 T.4N R.3E; Sec 32 T.5N R.3W. It was found with azurite, malachite and chalcopyrite (Heyl et al., 1959).
LAFAYETTE COUNTY: Chalcocite was mined in the SE NW Sec.36 T.2N R.3E, about 4 miles NW of Gratiot. Associated minerals are copper, bornite and chalcopyrite. (Heyl et. al., 1959).
—Chalcocite is reported as a secondary component of the lead-zinc ores in the Belmont and Calamine quadrangles (Heyl et. al., 1959).
POLK COUNTY: Chalcocite is present in masses and veins cutting sandstones of the Cambrian Mazomanee Formation, found in an old copper prospect near the Silver Brook site (NE 1/4 NW 1/4 SW 1/4 Sec. 1 T.33N R.19W) in Interstate Park, south of St. Croix Falls. Malachite is associated (Cordua et. al., 1979).
—Chalcocite occurs in quartz near cutting basalt at the Dresser trap Rock quarry, with cuprite, bornite, malachite (Cordua, 1989b).
—Chalcocite is reported at numerous localities in T.37N R.16W. It is found in vesicles and the ground mass of Keweenawan volcanic rocks such as at SE SE NW Sec. 18; NE NE NE Sec. 28 and NW NW Sec. 35. It is associated with cuprite and native copper (Hansell, 1928).
RICHLAND COUNTY: Chalcocite was found in float with chalcopyrite and malachite in Prairie du Chien group rocks in a stream wash in NE 1/4 Sec. 25 T.10N R.1E, about 5 miles west of Richland Center (Heyl et al., 1959).
RUSK COUNTY: Chalcocite was abundant at the Flambeau Mine, sec. 9 T.34N. R.6W., south of Ladysmith, where it occurred in a wide variety of habits, some of world-class specimen quality. It was found generally as sooty to coarsely crystalline granular masses in the supergene zone, where it formed one of the mine’s primary ore materials. It was associated with chalcopyrite, malachite, azurite, bornite and pyrite. Crystallized chalcocite was frequently encountered in cavities. Occasionally pockets several cubic meters in size would form at the intersections of fractures, along water courses active during supergene enrichment. The walls of these cavities were generally lined with curving or botryoidal growths, tending to be locally skeletal or stalactitic in nature. A later generation of euhedral xls., some individuals several centimeters long were perched on these. Most of the crystals were cyclically twinned on (110), giving pseudohexagonal symmetry. More rarely, X-shaped twins on (112) or (032) were found. Clusters often showed these crystals in parallel growth. Many specimens had a thin coating of bornite which imparted an attractive iridescence in shades of purple, brassy, bronze, green or blue. A number of crystals were partially or completely replaced by chalcopyrite and bornite, probably as a result of water table fluctuation. Some have parting that is likely the result of polysynthetic twinning. A number of fine specimens were preserved due largely to the efforts of a contract collector, Mr. Casey Jones. Jones assigned names to the larger vugs such as the Drill Pocket, Lucky Friday Pocket, Sunset Pocket and Quick Pocket. Excellent specimens have made their way into museum and private collections. A number of the best specimens are illustrated in LaBerge, 1997. (May, 1973; Cordua, 1994- 1997 pers. obs.; LaBerge, 1997; Jones and LaBerge, 1997).
SAUK COUNTY: Chalcocite was found with chalcopyrite, malachite in a deposit on upper east side of Hagerman Hill, Sec. 34 T.13N R.3E, 2 km S. of Lavalle (Heyl and West, 1982).
WOOD COUNTY: Chalcocite occurs as small xls. in vugs with chalcopyrite in the Frederick Schill quarry, north of Vesper (SW NW sec. 5 T.23N. R.4E.). (Buchholz, 1999).