Formula: Cu5FeS4 Orthorhombic
Description: Bornite forms as a primary hydrothermal precipitate or a later supergene replacement mineral. It occurs with other copper minerals such as covellite, malachite and azurite. It is marked by its intense purple to blue iridescence on weathered surfaces. Other minerals, especially sulfides and oxides, are also iridescent and are mistaken for bornite. This may well be the case for several reported Wisconsin occurrences. Bornite’s peculiar brownish-purple color on a fresh surface is diagnostic, and should be used in identification instead of the iridescence.
ASHLAND COUNTY: Bornite has been tentatively identified .along shear zones in anorthositic gabbro in a quarry in NW NW Sec. 4 T.44N R.3W west of Mellen (WSC, 1994 notes).
BAYFIELD COUNTY: Small grains embedded in pink agates are found in amygdules in the S 1/2 Sect. 5 T.44N R.6W. (south of Grand View) with epidote, chlorite, calcite, pumpellyite, and K. spar-in Keweenawan basalts (WSC, 1990 Field notes).
DOUGLAS COUNTY: Small masses were found with barite and siderite in the quarry on Mabel Nelson quarry, south of South Range. (WSC, 1990 Field notes).
FOREST COUNTY: Bornite is found as thin coatings with chalcocite and covellite formed by supergene processes on massive sulfide ore at the Crandon deposit (May and Schmidt, 1982; Lambe and Rowe, 1989).
IOWA COUNTY: Reportedly found in the copper mines near Mineral Point (Strong, 1877).
LAFAYETTE COUNTY: Found with chalcopyrite and chalcocite at the copper prospect in the SE 1/4 NW 1/4 Sec. 36 T.2N. R.3E., 4 miles northwest of Gratiot (Heyl et al., 1959).
POLK COUNTY: Massive bornite is found in quartz veins and along joint surfaces at the Dresser trap rock quarry (sec. 5 and 6 T.33N. R.18W.) with chalcocite, cuprite and malachite (Cordua, 1989b).
RUSK COUNTY: Bornite is a major component of the supergene enrichment zone at the Flambeau deposit, south of Ladysmith in T.34N R.6W. It is found as masses and veinlets where it replaces chalcopyrite and is in turn replaced by chalcocite (May, 1977). During mining it was found in pure masses of considerable size intergrown with chalcopyrite and chalcocite. The bornite also coats large chalcocite crystals giving them a distinctive purple, blue, yellow or green patina.
TAYLOR COUNTY: Bornite occurs at the Bend massive sulfide deposit NW, sec. 2 T.32N. R.2W. northeast of Gilman. It is associated with pyrite, chalcopyrite and gold tellurides. (DeMatties and Rowell, 1991).
WINNEBAGO COUNTY: Bornite is reportedly found in cavities in dolostone with pyrite, marcasite, sphalerite, galena, covellite and cuprite in the Lutz Quarry, in the west part of Oshkosh along Rte. 44, T. 18N R.16W (USGS, 1976).