WGNHS
Wisconsin Geological and Natural History Survey

Siderite

Siderite as yellow crystals on dolomite. From Flambeau Mine in Rusk County, Wisconsin. Siderite crystals are about 0.3 mm across. (Specimen and photo by Dan Behnke.)

Formula: FeCO3 Hexagonal

Description:

Siderite forms as a sedimentary precipitate, in hydrothermal veins, in metamorphic rocks formed from such protoliths, and, rarely, in pegmatites. In sedimentary environments it forms in extensive beds of banded iron formations and as more localized deposits of bog-iron ore. It readily weathers to iron oxides.

ASHLAND COUNTY: Siderite is a common component of the Ironwood Banded iron formation, exposed throughout the Gogebic Range. It is associated with calcite, dolomite, ankerite, magnetite, hematite, goethite, grunerite, chamosite, stilpnomelane, minnesotaite and quartz ( Van Hise and Irving, 1892; Laybourne, 1979; Schmidt, 1980; LaBerge, 1984). The siderite is fine-grained, forming matrix material and some recrystallized rhombs up to 4 mm. in diameter.
—Van Hise and Irving (1892) cite numerous outcrops of the Ironwood Formation in T.44N. R.2W including SW sec. 1, SW sec. 10 and NW sec. 14 (Penokee Gap). It also outcrops in T.44N. R.3W. at NE sec.15 and SW sec. 17.
— Laybourne (1979) cites an outcrop at Ballou Creek, NW NE Sec. 11 T.44N. R.2W. where siderite is associated with grunerite, minnesotaite, quartz, magnetite and hematite.
Siderite-bearing Ironwood Iron Formation occurs at the Berkshire Mine on Mt. Whittlesey (SW SW SE sec. 9 T.44N. R.2W.) (Mudrey, 1979).

DODGE COUNTY: Siderite is a sparse component of the oolites and matrix of the Neda iron formation, as mined in the east 1/2 NW sec. 1 T.11N. R.16E. (Hawley and Beavan, 1934).

DOUGLAS COUNTY: A vein bearing siderite, with barite and bornite, was found at the Mabel Nelson quarry, south of South Range.

FLORENCE COUNTY: Siderite occurs with quartz in banded iron formation at several spots in the T.40N. R.18E., including the Florence Mine (sec. 20 and 21; the Ernst Mine SW SW sec. 27 and the Welch Exploration pits (NW sec. 34) (Dutton, 1971).

IOWA COUNTY: Siderite is reported as small lenticular crystals perched on cerussite from the mines near Mineral Point. (Hobbs, 1895).

IRON COUNTY: Siderite is a common component of the Ironwood Banded iron formation, exposed throughout the Gogebic Range. It is associated with calcite, dolomite, ankerite, magnetite, hematite, goethite, grunerite, chamosite, stilpnomelane, minnesotaite and quartz ( Van Hise and Irving, 1892; Laybourne, 1979; Schmidt, 1980; LaBerge, 1984). The siderite is fine-grained, forming matrix material and some recrystallized rhombs up to 4 mm. in diameter.
— Van Hise and Irving (1892) cite numerous mines working the Ironwood Formation in Iron County. These include:

  • Tyler Fork Mine SE NE sec. 33 T.45N. R.1W.
  • Iron Belt Mine NE NE sec. 11 T.45W. R.1E.
  • Atlantic Mine NW NW sec. 12 T.45N. R.1E.
  • Germania Mine SW SW sec. 24 T.46N. R.2E.
  • Superior Mine SW NW sec. 26 T.46N. R.2E.
  • Kakagon Mine SE NW sec. 26 T.46N. R.2E.
  • Bessemer Mine SW NW sec. 26 T.46N. R.2E.
  • Nimikan Mine NW NE sec. 26 T.46N. R.2E.
  • Trinkle Mine SW NE sec. 27 T.46N. R.2E.
  • Odanah Mine NW SE sec. 27 T.46N. R.2E.
  • Pence Mine, at Hoyt, NE SE sec. 32 T.46N. R.2E.
  • Father Hennepin Mine, Hoyt, NE SE sec. 32 T.46N. R.2E.
  • Cary Mine NW SE sec. 26 T.46N. R.2E.
  • Montreal Mine SW NW sec. 33 T.46N. R.2E.

— Laybourne (1979) studied outcrops of the Ironwood formation on Tyler Fork and noted that the siderite occurred there with quartz, stilpnomelane, magnetite, chlorite and minnesotaite.
— The Montreal Mine ore body has also been the subject of later studies. The siderite iron ore there occurred with a host of minerals, many well-crystallized, including manganite, rhodochrosite, pyrolusite, and manganoan calcite (Dickey, 1938; LaBerge,1984).
— Siderite occurs with magnetite and hematite in drill core and small pits in the Pine Lake area in sec. 23, 26, 27, and 28 T.44N. R.3E. (Dutton and Bradley, 1970, USGS, 1976.).

MARATHON COUNTY: Siderite is abundant as large crystals and masses up to 20 cm. across in cavities in the Wausau Pluton, such as is exposed in the “rotten granite” quarries south of Rib Mountain. The siderite is black to brown to yellow and often forms crystals in parallel growth. The siderite is generally partly to completely replaced by iron oxides. (Falster, 1985, 1986, 1987; Falster and others, 2000). Such siderite can be found in the Koss Pit (SW sec. 2 T27N R.5E), the Red Rock Granite North Pit (along Spring Brook Road), the Ladick Quarry (sec. 19 and 20 T. 27N. R.6E.) and the Wimmer #3 pit (NW sec. 19 T.28N R7E) (Buchholz and Simons, 2002).

RUSK COUNTY: Siderite is a heavy mineral in the Barron Quartzite ,as exposed in Rusk and adjacent Sawyer Counties (Ernst and others, 1980).

SAUK COUNTY: Siderite is a major component of the Freedom Iron Formation once mined in the Baraboo region. Leith (1904) cites the following mines, all in T.11N. R.5E. : Judevine Property SW sec. 17, Patterson Mine NW sec. 20, LaRue Mine, SW sec. 16, Illinois Mine, E 1/2 sec. 16, Wisconsin Mine, SE sec. 10 and the Sauk Mine, NE sec. 10. None of these working are accessible at present.

WAUPACA COUNTY: Siderite, pseudomorphically replaced by goethite, was reported from 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, personal communication).

WOOD COUNTY: Siderite occurs as oxidized masses and drusy xls. along vugs in granite at the Haske Quarry, Cary Bluffs, Sec. 25 T.24N. R.2E. (Buchholz, 1997, personal communication).
— Siderite occurs as small crusts and masses in vuggy veins at the Cary Rock Road Quarry, NE sec. 4, T23N R2E. (Buchholz, 1997).
— Siderite occurs as small crusts and masses in vuggy veins at the John Cepress Quarry, sec. 11, T23N R2E. (Tom Buchholz, personal communication).
— Siderite occurs as small iridescent drusy crusts and masses in vuggy veins at the Haske Quarry, sec. 25, T24N R2E. (Tom Buchholz and others, 1998).