Formula: ZnCO3 Hexagonal
Description: Smithsonite is a secondary mineral formed primarily by the weathering and alteration of sphalerite in the presence of CO2. It is typical of the oxidized zone of hydrothermal zinc deposits. The mineral is widespread in the Upper Mississippi Valley zinc-lead district found in the southwestern part of the state. At one time smithsonite was a major zinc ore mineral in the state. Eventually mining worked through these surficial deposits and zinc production shifted to the mostly unaltered sphalerite found at depth. Smithsonite, however, was encountered in blankets and veins reaching well below the present groundwater table (Heyl et al., 1959). Minerals frequently associated with smithsonite galena (often encrusted with cerussite), relict sphalerite, hemimorphite, hydrozincite, and iron oxide.
The smithsonite of the Upper Mississippi Valley zinc-lead district occurs in a variety of habits. These include solid white masses known as “dry bone ore,” massive rather cellular masses and coatings on rock known as “sheet ore,” and yellowish plates and fragments known as “skull ore.” The material is often ocherous due to the intergrowth with iron oxides from the deterioration of the abundant pyrite and marcasite in the deposits. Small colorless to white rhombohedral crystals, stalactitic, fibrous coatings and pearly reniform green to yellow coatings in small vugs are also known (Heyl et al., 1959). Irving (1883) notes smithsonite as pseudomorphous after sphalerite, galena, and calcite. The pseudomorphs after calcite could be after either rhombohedral or scalenohedral forms, and could be partially hollow where the remaining calcite was leached away.
GRANT COUNTY: Smithsonite is common in the county’s many old zinc-lead mines. Major subdistricts within this county are the Platteville subdistrict and the Potosi subdistrict. Many of the mines listed in the county under sphalerite are also localities for smithsonite.
—Smithsonite was mined with sphalerite in a group of small mines and pits of the Grab diggings in SW Sec. 19 and NW NW Sec. 30 T.4N. R.2W. along both sides of McPherson Branch of the Platte River, west of Ellenboro (West and Blacet, 1971). It was also mined south of Ellenboro in NE NE Sec. 16 T.3N. R.2W. (Whitlow and West, 1966b).
—Smithsonite was mined at several spots in T4N R4W, with sphalerite, galena, pyrite and marcasite such as at Days Diggings (SE Sec. 15), East Grant River Diggings (SW SE Sec. 22) and Bass Diggings (NW Sec. 24) (West and Heyl, 1985).
IOWA COUNTY: Smithsonite is abundant in this county at many old zinc-lead mines. The localities listed under sphalerite in this county are also localities for smithsonite. Some of the more important or unusual localities are cited below.
—Smithsonite was found at the Eberle Mine, (NW NW Sec. 2 T6N R1E), SE of Highland. Smithsonite occurs here as colorless to white crystals in vugs and as thick fibrous coatings showing well developed terminations. It is here associated with a complex suite of secondary minerals including pyromorphite, aurichalcite, cuprite, azurite and malachite (Heyl et al., 1959).
—Smithsonite occurs at the Steppler Farm, 4 miles SW of Highland (SE NE Sec. 7 T.6N. R.1E.) where it occurs with a complex suite of secondary minerals including aurichalcite, chrysocolla, malachite and azurite (Heyl et al., 1959).
—Smithsonite was widely mined throughout the Mineral Point region. Some of the larger mines producing smithsonite, according to Heyl et al. (1959), were the Liverpool Mine (NE Sec. 1 T.4N. R.2E.), the Ben Hur Mine (SW NW Sec. 5 T.4N. R.3E.) and the Argall Mine (NE NE Sec. 28 T.5N. R.2E.). Large chunks of cavernous smithsonite with molds of calcite crystals are on display at the Geology Museum at UW–Madison and are labeled as collected from the Hutchinson & Co. Mine near Mineral Point.
—A number of mines near Linden produced smithsonite such as the Optimo #2 Mine, E 1/2 NW Sec. 17 T5N R2E (Heyl et al., 1959).
—Smithsonite was mined widely near Dodgeville where deposits included (Heyl et al., 1959): the Williams Mine (SW SW Sec. 25 T6N R3E), the Oxman Mine (N 1/2 SE Sec. 26 T.6N. R.3E.) and the Demby-Weist Mines, (NW Sec. 28 to the SW Sec. 21 T.7N. R.4E.).
—Smithsonite occurs at the John Vivian Mine (SE SW Sec. 33 T.5N. R.2E.) near Lost Grove (Heyl et al., 1959).
—Smithsonite is a placer mineral with galena at NW NW Sec. 8 T.5N. R.2E. near Linden (Heyl et al., 1959).
LAFAYETTE COUNTY: Smithsonite was mined at many localities in the county. Many of the localities listed under sphalerite are also smithsonite localities. Some of the more important or unusual deposits of smithsonite are listed below.
—Smithsonite is abundant throughout the Hazel Green Shullsburg subdistrict, where it was mined at a number of localities (Heyl, et al., 1959). One of the larger producer in the district was the Little Giant Mine east of Shullsburg (E 1/2 SW Sec. 4 T.1N. R.2E.) . Another major mine was the Sally Waters or Monarch Mine (NE NE Sec. 23 T.1N. R.1E.). Near Leadmine, smithsonite was common at the Empress open cut (SW NW Sec. 13 T.1N. R.1W.), the Robson Mine (SW SE Sec. 14 T.1N. R.1E.), the New and Old Ida Blende Mines (NW Sec. 15 T.1N. R.1E.), the Leadmine Mine (SE NE Sec. 15 T.1N. R.1E.) and the Jack of Diamonds Mine (NW SE Sec. 22 T.1N. R.1E..) Smithsonite is found at the Little Dad Mine (SE NW Sec. 32 T.1N. R.1E.) near Shullsburg. It was a major ore mineral at the Hofer Mine (SW NE Sec. 4 T.1N. R.2E.), the Little Bennie Mine (SW SW Sec. 4 T.1N. R.2E.), and the Nip and Tuck Mine (SW NW Sec. 7 T.1N. R.2E.). At the Rodham Mine (N 1/2 Sec. 25 T.2N. R.2E.) a large cave filled with smithsonite, clay and iron oxides was found above the main sulfide ore body. Smithsonite also occurs at the Lucky Hit Mine (SW SE Sec. 33 T.2N. R.2E.).
—Smithsonite is widespread in the Meeker’s Grove subdistrict, (T.2N. R.1E.) where it has been mined in numerous places. Strong (1877) notes smithsonite in abundance at the Drybone diggings in the (SW Sec. 28). This is likely the same as the Roosevelt Mine noted in the NE SW of this same section by Heyl, et al.(1959). A substantial tonnage of “dry bone ore” was noted at the Kesting and Hines diggings (SE Sec. 21) near Calamine (Klemic and West, 1964). Heyl et al. (1959) note smithsonite as a common ore forming locally pearly coating on calcite at the Liberty Mine (NE Sec. 16). It also occurs at the Trego-Anthony Mine (N 1/2 SE Sec. 21), the Pittsburg Mine (W 1/2 SE Sec. 28), and the James Mine (NW Sec. 28 ).
—Smithsonite was common in the ores at Etna Hill, 3 miles east of Benton (Wheeler, 1908).
WINNEBAGO COUNTY: Smithsonite is found with sphalerite, galena, cerussite, hemimorphite, cuprite, calcite and dolomite at the Lutz Quarry in the west part of Oshkosh along Hwy 44 in T.18N. R.16W. (USGS, 1976).