DOLOMITE CaMg(CO3)2 Hexagonal

Description: Dolomite is an abundant mineral in Wisconsin. It is a major rock-forming mineral in the sedimentary dolostones within the Prairie du Chien group (Oneota and Shakopee formations), Sinnepee Group (Platteville, Decorah and Galena Formations) and the Niagaran rocks. These dominate the outcrops in a band reaching from the lower St. Croix River to the west, south along the Mississippi to the Illinois border, west across southern quarter of the state, and continuing north along the entire Lake Michigan shoreline. The dolomite is believed to be a secondary replacement of original limestone while the sediment was undergoing lithification. Dolomite also forms as later drusy crystals lining vugs in dolostones. Dolomite may form as a hydrothermal replacement mineral and, as such, is especially abundant as a gangue mineral in the ore deposits of the Upper Mississippi Valley zinc-lead district in southwestern Wisconsin. Ferroan dolomite appears in several of the state's Precambrian iron formations. Finally, dolomite locally occurs in the state in marbles. Only some of the most typical or important localities for dolomite are mentioned below.

Bluffs near LaCrosse

Dolomite forms the main mineral in the bluff-forming rocks along the Mississippi River, such as Granddad Bluff near LaCrosse.

close-up of pink dolomite

Pink dolomite with white quartz and green chlorite. Field of view is about 5 cm. Quarry in Tyler slate, southwest of Hurley, WI. W. Cordua photo.

ASHLAND COUNTY: Dolomite is a common rock-forming mineral in the Bad River dolostone found throughout the Penokee Range area. It is associated with calcite, tremolite, chlorite, quartz, sericite, pyrite and magnetite. Some localities mapped by Van Hise and Irving (1892) are: NW NW Sec. 22 T.44N R.5W; along the Marengo River in the SE sec. 15 T.44N R.5W, and at Penokee Gap in the SE NW sec. 14 T.44N R.3W.

A ferroan variety of dolomite is a rock-forming mineral in the Ironwood Iron Formation through the Penokee iron range. Here it forms fine-grained to sparry masses intergrown with bands of siderite. Other associated minerals are magnetite, hematite, grunerite, chlorite and quartz. It may be found at numerous spots within Ashland County and adjacent Iron County. Some specific localities are at Penokee Gap in the NW Sec. 14 T.44N R.3W (Van Hise and Irving, 1892) and at the Berkshire Mine in Mt. Whittlesey in the SW SE Sec. 9 T.44N R.2W (Laybourn, 1979).

BAYFIELD COUNTY: Dolomite is a rock-forming mineral forming sparry masses in the marble at the Grandview Quarry, NW NW NW Sec. 22 T.44N R.5W. Here it is associated with tremolite, calcite and quartz (Mudrey, 1979).

BUFFALO COUNTY: Drusy crystals of rhombohedral dolomite are common in the dolostones of the Prairie du Chien group throughout the county. These are gray to buff to rarely a very pale pink. Some quarries where these crystals are found are in the NW NW Sec. 24 T.22N R.12W near Cream, SE SE Sec. 3 T.20N R.12W east of Cochrane and SE NW NE Sec. 14 T.20 N R.12W, also east of Cochrane (W.S. Cordua, field notes).

DODGE COUNTY: Dolomite is an accessory mineral in the oolitic iron ore of the Neda formation, as exposed in the old mines in T.11N R.16E, east of Neda (Hawley and Beavan, 1934).
- Dolomite occurs as an important rock forming mineral in the Silurian dolostones exposed in a quarry 3 km. north of Knowles (Soderman and Carozzi, 1963).

DOOR COUNTY: Dolomite is an important rock-forming mineral in the dolostones throughout the county. It may be found, among many other places, at a quarry near Nasbro in the center of Sec. 18 T.13N R.17E; at a quarry near Chilton in Sec. 25 T.18N R.19E, and at a quarry 2 miles north of Sturgeon Bay (Soderman and Carozzi, 1963).
- Dolomite occurs as rhombohedral drusy crystals with fluorite at a quarry in Sturgeon Bay. (Carl Cochrane, 1999, pers. comm.).

FOND DU LAC COUNTY: Dolomite occurs in dolostone and as drusy crystals at the Ripon Lime and Materials Company Quarry, NW sec. 20 T.16N R.14E., just west of Ripon.

GRANT COUNTY: Dolomite is abundant throughout the Upper Mississippi Valley zinc-lead district in the county both as a component of the host rock dolostones and as a later hydrothermal gangue mineral. In the ore deposits the original carbonates are often replaced by a poorly cemented "sandy" aggregate of euhedral dolomite rhombohedrons. Later veins contain pink to gray crusts of dolomite crystals in association with galena, sphalerite, marcasite, pyrite and calcite. Most localities listed for the county under these minerals contain dolomite as well.
- Dolomite occurs at many localities in the Platteville Quadrangle (Agnew, 1963) such as the Trego Mine in the SE NE Sec. 10 and S 1/2 NW Sec. 11 T.3N R.1W (Heyl et. al., 1959).
- Pink dolomite is associated with galena, sphalerite, and malachite at the Bridgeport Quarry, SW Sec. 20 T.6N R.5W on the north side of U.S. Hwy 18 (Heyl and West, 1982).

IOWA COUNTY: Dolomite is abundant throughout the Upper Mississippi Valley zinc-lead district in the county both as a component of the host rock dolostones and as a later hydrothermal gangue mineral. In the ore deposits the original carbonates are often replaced by a poorly cemented "sandy" aggregate of euhedral dolomite rhombohedrons. Later veins contain pink to gray crusts of dolomite crystals in association with galena, sphalerite, marcasite, pyrite and calcite. Most localities listed for the county under these minerals contain dolomite as well.

JEFFERSON COUNTY: Drusy dolomite xls. occur with pyrite, marcasite, calcite and sphalerite in the Galena Formation in the Hausz Brothers Quarry, Sec. 11 T.5N R.14E near the intersection of U.S. Hwy. 12 and county M, south of Fort Atkinson (U.W. Parkside, 1980).

LAFAYETTE COUNTY: Dolomite is abundant throughout the Upper Mississippi Valley zinc-lead district in the county both as a component of the host rock dolostones and as a later hydrothermal gangue mineral. In the ore deposits the original carbonates are often replaced by a poorly cemented "sandy" aggregate of euhedral dolomite rhombohedrons. Later veins contain pink, pale or gray crusts of dolomite crystals associated with galena, sphalerite, marcasite, pyrite and calcite. Most localities listed for the county under these minerals contain dolomite as well. Dolomite is noted in particular as occurring in deposits in the Cuba City-New Diggings and Shullsburg district (Mullens, 1964) and the Belmont and Calamine Quadrangles. Hosterman, Heyl and Jolly (1964) describe it and associated minerals at the Thompson-Temperly Mine on the SW edge of New Diggings.

MANITOWOC COUNTY: Dolomite occurs as drusy crystals with calcite, marcasite, pyrite, fluorite, and strontianite in vugs in dolostone at the quarry on Highway 151 on the north edge of Valders. (Carl Cochrane, 1999, pers. com.).

MONROE COUNTY: Calcite occurs as crystals with dolomite in the Schraufnagel Quarry, near Oakdale.

OUTAGAMIE COUNTY: Dolomite occurs as drusy crystals in vugs in the Oneota dolostone in the New London Quarry, approximately 2 miles south of New London on County D (U.W. Oshkosh, 1989).

PIERCE COUNTY: Dolomite occurs as drusy crystals in vugs in the Prairie du Chien rocks at numerous localities in the county where it is associated with calcite and iron oxide pseudomorphs after pyrite and marcasite. Specimens can be found, for example, at the quarry east of Maiden Rock on the north side of County S in the NW Sec. 14 T.24N R.16W (Cordua, 1989) and at the quarry north of Hager City along County VV in Sec. 35 T.25N R.18W. (W.S. Cordua, field notes).

Drusy dolomite

Drusy dolomite crystals with a few goethite pseudomorphs after marcasite.

SAUK COUNTY: Ferroan dolomite is a major constituent of the Freedom iron formation in the Baraboo Iron Range, such as at the Illinois Mine, 3 miles SW of Freedom (Leith, 1904).

WINNEBAGO COUNTY: Dolomite crystals occur with marcasite, pyrite, calcite and rare sphalerite in the Platteville Formation at the Grundy Quarry in the NE NW Sec. 33 T.19N R.16E (U.W.-Oshkosh, 1989).

WOOD COUNTY: Ferrodolomite occurs at the Trowbridge talc mine north of Milladore in the SW NW Sec. 11 T.25N R.5E where it is associated with talc, chlorite and serpentine (Aldrich, 1928).

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