Formula: Fe2+Fe3+2O4 Isometric
Magnetite is a common mineral found at many localities in Wisconsin. Only localities that where the mineral is present in great abundance or as unusual material are listed here. Magnetite occurs as an accessory in most igneous, metamorphic and sedimentary rocks, where it is found as small equant, euhedral to rounded grains. It is an abundant heavy mineral in sediments and is usually the major constituent of certain metamorphosed iron formations, where it forms an important ore mineral. It is exceptionally common on Wisconsin’s iron ranges, such as at Black River Falls, the Gogebic Range in Iron County and in Florence County. Magnetite may also form in high-temperature hydrothermal veins in association with ore minerals such as pyrrhotite, sphalerite and galena.
ASHLAND COUNTY: Abundant constituent of the iron formation in the Gogebic Range from Mineral Lake to Hurley where it is found as small crystals and masses intergrown with siderite, chamosite, stilpomelane, ferrodolomite, chlorite, calcite and quartz. Magnetite is also in actinolite schists associated with the iron formation where it forms small crystals, masses and clusters. (Van Hise and Irving, 1892; Huber, 1959; USGS, 1976) Outcrops occur along Ballou Creek (NW NE Sec. 11 T.44N. R.2W). where magnetite occurs with quartz, grunerite, siderite and minnesotaite. Also in outcrop on the west side of Hwy. 13 near Mellen (SE NW Sec. 13 T.44N. R.3W.) where it occurs with grunerite, tremolite, orthopyroxene, clinopyroxene and talc. Also in Penokee Gap (NW NW Sec. 14 T.44N. R.3W.) on the west side of Bad River where it occurs with quartz, actinolite, orthopyroxene and clinopyroxene. (Laybourn, 1979).
— Magnetite is a major component of the ore at the Berkshire Mine, (SW 1/4 SW 1/4 SE 1/4 Sec. 9 T.44N. R.2W) near Mellen. Associated minerals are quartz, talc, dolomite, grunerite and almandine (Mudrey, 1979, Laybourn, 1979).
— Small octahedra of magnetite are found in slatey layers of the Palms Formation west of the Tyler Forks River. Mudrey (1979) suggests this is due to contact metamorphic effects from the Mellen gabbro.
— With specular hematite and chert at Michigan Mine, 20 km. NE of Butternut, N.42N. R.1E. (USGS, 1976).
— At Hanna Mining Co. Test Pit in SW SE Sec. 18 T.44N. R.4W. with clinopyroxene, orthopyroxene and abundant grunerite (Laybourne, 1979).
— Near Mineral Lake Lookout Tower (NE NE Sec. 24 T.44N. R.4W.) with grunerite, actinolite, orthopyroxene, clinopyroxene and tremolite (Laybourne, 1979).
— In SE NE Sec. 14 T.44N. R.4W. 20 km. west of Mellen, in hornfels with fayalite, pigeonite, grunerite and quartz (Laybourn, 1979).
— Magnetite and ilmenite make up nearly 60% of the rock in drill core through gabbroic rocks near Clam Lake in T.43N. R.4W. (Dugan and Ervin, 1977).
CLARK COUNTY: Magnetite occurs with quartz in “thin discontinuous” iron formation found in scattered outcrops about 5 km. south of Neillsville (Dutton and Bradley, 1970).
FLORENCE COUNTY: Massive magnetite associated with stilpomelane is found with sideritic carbonate and pyritic chert on the dump from the Dunkel exploration shaft and adit in sec. 25 T.40N. R.17E. (Dutton, 1971)
— Magnetite crystals, partly to completely replaced by hematite (“martite”), occurs in quartzite in the Michigamme Slate in outcrop and small pits in Sec. 31 and 32 T.40N. R.18E. near Commonwealth. The magnetite is associated with iron silicates, garnet, chlorite and tourmaline. (Dutton, 1971).
— Small magnetite crystals in a grunerite-quartz iron formation within the Michigamme slate is found at SW 1/4 NE 1/4 Sec. 28 T.39N. R.18E. (Dutton, 1971).
— Magnetite-grunerite-stilpnomelane-garnet assemblages outcrop in the W 1/2 Sec. 26 and in Sec. 35 and 36 T.40N. R.17E. (Dutton, 1971).
— Euhedral xls. about 1 mm across are abundant in the Riverton iron formation in Sec. 26, 27, 34 and 35 T.40N. R.17E. (Dutton, 1971).
IRON COUNTY: Magnetite is abundant in the iron formation in the Gogebic Range from Upson to Hurley where it is found as small crystals and masses intergrown with siderite, ferrodolomite, chlorite, calcite and quartz. It is also a component of the actinolite schists with the iron formation, where is forms small crystals, masses and clusters intergrown with actinolite and other iron silicates. (Van Hise and Irving, 1892) (Huber, 1959).
— Found as small octahedral xls. in chloritic slate in sec. 1 T.45N. R.1E. (Van Hise and Irving, 1892).
— Magnetite-rich iron formation occurs in the Potato River section (SE SE Sec. 19 T.45N. R.1E.) associated with minnesotaite and stilpnomelane (Laybourne, 1979).
— Found as crystals in ore vugs at the Montreal Mine, Montreal. (Laybourne, 1979).
– Fine-grained laminated rocks with magnetite, siderite, chlorite, minnesotaite, stilpnomelane, specular hematite and quartz are found along the gorge of Tyler Fork (SE 1/4 NE 1/4 Sec. 33 T.45N. R.1W.) (Van Hise and Irving, 1892, Laybourn, 1979).
— Magnetite occurs with hematite and siderite in the banded iron formation in the Pine Lake area, T.44N. R.3E. (U.S.G.S., 1976).
— Magnetite and grunerite occur at the Ford and Lucas Whitside exploration near Butternut in sec. 2 and 4 T.41N. R.1E. (Dutton and Bradley, 1970).
JACKSON COUNTY Magnetite was the major ore mineral at the Jackson County Iron Mine near Black River Falls (Sec. 15 T.21N. R.3W). It is found as small octahedra and masses associated with quartz, cummingtonite, grunerite, ferroactinolite, biotite, hastingsite, hedenburgite and garnet. (Jones, 1978). It also occurs as euhedral octahedra up to 1 cm. in diameter in talc schist.
— Magnetite partly converted to hematite (“martite”) occurs in iron formation on several mounds near Black River Falls. Some examples are in NE sec. 12 and SW SE sec. 11 T.21N. R.4W. (Klemic and Ohlson, 1973).
MARATHON COUNTY: Magnetite may form up to 10% of some quartz syenite bodies in outcrops and roadcuts near the Employer’s Mutual Insurance Co. offices (NW SE sec. 27 T.29N. R.7E. , accounting for a magnetic anomaly there. (LaBerge and Myers, 1983).
— Coarse magnetite octahedra occur locally in the nepheline syenite of the core zone of the Stettin Pluton, with K feldspar, fluorite, zircon, apatite, and a variety of amphiboles and pyroxenes. (LaBerge and Myers, 1983).
MARINETTE COUNTY: Magnetite forms up to 10% of the iron formation at the Duval massive sulfide deposit Sec. 2 and 3 T.35N. R.18E. and Sec. 28 T.36N. R18E. Here it is associated with grunerite, pyrrhotite and stilpnomelane. (Hollister and Cummings, 1982).
— Described as large crystals with serpentine and carbonates in Sec. 21 and 22 T.37N. R.21E. , near Pembine (Sims et. al. 1984).
ONEIDA COUNTY: Magnetite is a minor component of the Pelican River massive sulfide deposit east of Rhinelander in Sec. 29 T.36N. R.9E., here it formed during metamorphism of the body. (Mudrey, 1979).
POLK COUNTY: Magnetite is a widespread but minor component of the Keweenawan volcanic rocks exposed throughout the county.
— Magnetite is found in masses with gabbroic lenses in the thick lava flow exposed in the north quarry of the Dresser Trap Rock Quarry NW 1/4 Sec. 5 and NE 1/4 Sec. 6 T.33N. R.18W. Some lodestone present affects instruments lowered down drill holes in the quarry, causing them to stick to the sides of the holes. (Cordua, 1989b).
SAWYER COUNTY: Titaniferous magnetite is abundant in gabbro and troctolite in the Round Lake Intrusive east of Hayward (T.41N. R.8W.). The pluton is only poorly exposed. The magnetite occurs with olivine, plagioclase, augite and apatite (Stuhr and Cameron, 1976).
WAUPACA COUNTY: Magnetite was reported as small crystals in road cuts in pegmatite cutting granite along County G near Keller Lake., NE of Big Falls. (Tom Buchholz, 2002, personal communication)
WOOD COUNTY: Magnetite octahedra occur in talc schist and chlorite schist at the Trowbridge Mine, North of Milladore (SW NW Sec. 11 T.25N. R.5E.) (U.S. Bureau of Mines, 1963, Crowns, 1976).
— Magnetite octahedra occur in talc, chlorite, and tremolite schists at the Something Mine (old Czaikowski property) N. of Milladore in the NE SW Sec. 15 T.25N. R.5E. (U.S. Bureau of Mines, 1963, Crowns, 1976).
— Tiny euhedral to rounded octahedra of magnetite occur in calcite veins and serpentine country rock in excavations on Tower Hill, north of Rudolph. (Buchholz, 1994, personal communication).