Formula: Cu2(CO3)(OH2) Monoclinic
Description:
Malachite is a common alteration product of other copper-bearing minerals. Its bright green color makes it not only easily recognized, but also highly sought after by mineral collectors. It occurs most notable in the oxidized and supergene zones of copper deposits, especially where CO2 is available. As such, it occurs in the state in the small copper deposits associated with the Upper Mississippi Valley zinc-lead district, the volcanogenic massive sulfide deposits of the Wisconsin magmatic belt, and the native copper deposits of the Keweenawan. It may be in sufficient quantities to use as ore. It often occurs with azurite, cuprite, chalcocite, copper and chalcopyrite. One caution: there is a common tendency to call any bright green mineral “malachite”. The author has seen rare minerals as atacamite, brochantite, pseudomalachite, chrysocolla and cornwallite, as well as more common chlorite and pumpellyite, mislabeled as malachite by the careless. A good test for malachite is its effervescence in HCl.
CRAWFORD COUNTY: Malachite occurs with azurite at the Copper Creek Mine, about 1 km west of Mt. Sterling (W 1/2 sec. 26 T.10N. R.5E.). It is found with azurite in Ordovician carbonate rocks (Heyl et al., 1959).
— Malachite occurs at the Plum Creek Copper Mine, 8 km. NW of Wauzeka along Otter Creek (SE SW sec. 26 T.8N. R.5W.) It occurs in Ordovician carbonate rocks associated with azurite, chalcocite and chalcopyrite (Heyl, et al., 1959).
— Reported occurring with chalcopyrite and cuprite near Soldier’s Grove (Hobbs, 1905).
DOUGLAS COUNTY: Occurs at virtually all of the mines and prospects described under native copper. These formed in Keweenawan volcanic rocks, with the malachite forming as a secondary alteration product on primary copper and related minerals. Some specific occurrences are:
— At the Starkweather Mine, NW NW sec. 2 T.47N. R.13W. (Dutton, 1972).
— At the Amnicon copper prospect, NW sec. 11 T.47N. R.13W. (Dutton, 1972).
— Found in amygdules and along fractures in a small pit in Pattison State Park in the center of sec. 15 T47N. R.14W, west of Copper Creek (Grant, 1901).
— Malachite occurs with calcite and laumontite in veins cutting altered Keweenawan basalt along the Middle River (Irving, 1883).
— Malachite is found in a small pit in amygduloidal Keweenawan basalt exposed at the falss of an unnamed stream in the NE 1/4 sec. 28 T.48N. R.10W. (Irving, 1883).
FOREST COUNTY: Malachite occurs as an alteration of chalcopyrite in quartz veins cutting small pegmatites along State Hwy. 55 south of Alvin (Tom Buchholz, 2003, personal communication).
GRANT COUNTY: Malachite was reported occurring with cuprite and chalcopyrite near Boscobel (Hobbs, 1905).
GREEN LAKE COUNTY: Malachite occurs in tiny sprays and coatings in vuggy dolostone at the Morris Pit in Green Lake County.
IOWA COUNTY: Malachite was found at numerous small copper deposits in the county, which formed as part of the Upper Mississippi Valley zinc-lead district. The copper deposits were particularly common in the Mineral Point area. Raasch(1924) describes the material as earthy masses and coatings. Earlier, however, Hobbs (1895) describes the malachite as occurring in “beautiful radial and sheath-like forms and as minute druses 0.3 mm or less in their longest dimension”. Associated minerals are azurite, tenorite, chalcocite, chalcopyrite and iron oxides. Some localities are:
— The Beach Mine, N 1/2 sec. 4 T.4N. R.3E. (Heyl et. al., 1959).
— The Wasley Mine, N 1/2 SE sec. 5 T4N. R.3E. (Heyl et. al. 1959).
— The Amsley Mine N 1/2 SE sec. 32 T.5N. R.3W. (Heyl et. al., 1959).
— Malachite occurs at the Eberle Mine, 5 miles north of Cobb (NW NW sec. 2 T.6N. R.1E. where it occurs with azurite, chalcocite, cuprite, chalcopyrite, aurichalcite, sphalerite and smithsonite (WGNHS files).
— Malachite is found at the Steppler Farm, 4 miles SW of highland at Centerville in the SE NE sec. 7 T.6N. R.1E. Here it occurs with azurite, chalcocite, cuprite, chalcopyrite, aurichalcite, sphalerite and smithsonite (WGNHS files).
— Malachite occurs with zinc and lead minerals in the SW sec. 22 near Dodgeville (Strong, 1877).
LAFAYETTE COUNTY: Malachite occurs sporadically throughout the mines of the Upper Mississippi Valley zinc-lead district. It is commonly as thin crusts associated with other copper minerals.
— Malachite occurs in the SE NW sec. 36 T. 2N. R.3E. near Gratiot. Here it is found with azurite, cuprite, native copper, tenorite, pyrolusite, “wad”, bornite, chalcopyrite and chalcocite (Heyl et al., 1959).
— Crusts and films of malachite occur on dolostone with goethite at a copper prospect south of Ames Branch, NE SE sec. 22 T.2N. R.3E. (Klemic and West, 1964).
MARATHON COUNTY: Malachite occurs with chalcopyrite, pyrite and quartz in chlorite schist at the shaft of the old “gold mine” on the east side of the Eau Claire River near the town of Eason, SW of the center of sec. 26 T.29N. R.9E. (LaBerge and Myers, 1983).
— Malachite is noted with chalcopyrite and pyrite in quartz in sheared gneisses and greenstones in the NW SW SW sec. 9 T.26N. R.7E. (LaBerge and Myers, 1983).
POLK COUNTY: Malachite occurs in the Dresser Trap Rock Quarry, sec. 5 and 6, T.33N. R.18W. It is found as thin crusts along joint surfaces, coatings on native copper, or a globular and fibers in small vugs in quartz veins associated with chalcocite and cuprite (Cordua, 1989b).
RICHLAND COUNTY: With chalcopyrite and chalcocite in Prairie du Chien Group dolostones found as float blocks in a wash in NE sec. 25 T.10N. R.1E., about 5 km. west of Richland Center (Heyl and West, 1982).
RUSK COUNTY: Malachite occurs at the Flambeau Copper Mine, south of Ladysmith (Sec. 9 T.34N. R.6W.) in the supergene and oxidized zones. It occurs as coatings and masses. The masses may be vuggy with botryoidal, fibrous or drusy malachite associated with azurite and other rare copper minerals. Interesting coiled, ringed or helical growths may occur (May, 1977, Rosemeyer, 1995, 1997).
— Found in quartz veins cutting foliated greenstone in the SE sec. 35 T.36N. R.8W. (Dutton and Bradley, 1970).
SAUK COUNTY: Malachite is associated with chalcopyrite and hematite in brecciated Cambrian sandstone on the upper east side of Hagerman Hill in SE sec. 34 T.13N. R.3E., south of LaValle (Heyl and West, 1982).