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Iowa Co., Wisconsin, USA

Iowa County is underlain by Cambrian and Ordovician sedimentary rocks. It is in Wisconsin’s Driftless Area, hence glacial sediments are sparse and the landscape is dominated by bedrock, locally very well exposed. Erosion has developed a low topography punctuated by mounds or bluffs, generally held up by carbonate rocks and deep ravines or coulees draining to the Mississippi River and its tributaries.

The County is part of the Upper Mississippi Valley Zinc-Lead District. This is the classic Mississippi Valley Type deposit. The first workings by Europeans settlers date back to 1685, with the deposits known to Native Americans back into antiquity. The last mines closed in 1978,, thus mining in the area spans nearly 300 years, with peak production between 1917-1952. An estimated 1.2 million tons of zinc and 100,000 tons of lead were produced. There were thousands of small to large mines, prospects and “diggings” in the County. Wisconsin’s state mascot, the badger, refers to the scruffy appearance of lead miners emerging from their small mines. Many of these mines have now been covered or reclaimed.

The mineralization was largely confined to Middle Ordovician limestone and dolostone of the Galena, Decorah and Platteville Formations. The deposits are thought to be the result of movement of low temperature (90 to 150 degrees C) connate brines out of adjacent basins due to stress imposed further east due to the rise of the Appalachians. Local ore controls were fracture zones, solution collapse structures (“pitches and flats”) and subtle folds.

The mineralogy was relatively simple, but coarsely crystallized specimens were common. The main ore minerals were galena (Wisconsin’s official state mineral), sphalerite, smithsonite and, to a lesser extent, barite and copper sulfides. Notable accessories are calcite, dolomite, marcasite and pyrite. The abundant marcasite is notoriously prone to deterioration, destroying many fine samples. This makes large stable samples relatively scarce and probably is the reason why specimens from this District are less well represented in collections. The best known materials, from a collector point of view, is calcite which forms fine scepter crystals with rhombohedrons gowning on steep scalenohedral bases.

Mineral Point is a town in the heart of the district and its restored buildings, especially at the Pendarvis site, give a good picture of life in the heyday of mining in the area. This is also a place to buy Cornish pasties, another reminder of those who came to mine the deposits. Governor Dodge State Park north of Dodgeville is a great place to enjoy the beautiful southern Wisconsin scenery. Near the park the Museum of Minerals and Crystals has a good collection of specimens on display. Tower Hill State Park along the Wisconsin River preserves workings where the abundant lead ore was smelted.

Mineral List

Mineral list contains entries from the region specified including sub-localities
Aurichalcite
Azurite
Baryte
Calcite
Cerussite
Chalcocite
Chalcopyrite
'Chert'
Chrysocolla
Copper
Covellite
Cuprite
var: Chalcotrichite
Dolomite
'Flint'
Fluorite
Galena
Glauconite
Goethite
Greenockite
Gypsum
Honessite (TL)
Hydrozincite
Leadhillite
'Limonite'
Malachite
Marcasite
Melanterite
Millerite
Minium
Muscovite
var: Sericite

'Psilomelane'
Pyrite
Pyromorphite
Quartz
Smithsonite
Sphalerite
Tenorite
Violarite


40 entries listed. 33 valid minerals. 1 type locality (valid mineral).

Localities in this Region

USA
USA

The above list contains all mineral locality references listed on mindat.org. This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in mindat.org without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.

References

Heyl, A., A. F. Agnew, E. J. Lyons & C.H. Behre Jr. (1959), The geology of Upper Mississippi Valley Zinc-Lead District: U.S. Geological Survey Professional Paper 309, 310 p.

Heyl, A., W. Broughton & W. West (1970 – revised 1978) Geology of the Upper Mississippi Valley base-metal district: Wisconsin Geological and Natural History Survey: Information Circular 16, 45 p.

Mudrey, M. G., editor (1978) Upper Mississippi Valley Base-Metal District – companion volume to Information Circular 16: Wisconsin Geological and Natural History Survey: Field trip guidebook #1, 39 p.

Brannon, J., F. Podosek & R. McLimans (1992) Alleghenian age of the Upper Mississippi Valley zinc-lead deposits determined by Rb-Sr dating of sphalerite: Nature: 356 (9): 509-511.

Rowen, E.L. & M. Goldhaber (1996) Fluid inclusions and biomarkers in the Upper Mississippi Valley zinc-lead district – implications for the fluid flow and thermal history of the Illinois Basin: U. s. Geological survey Bulletin: 2094 F: F1 – F33.

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Copyright © Jolyon Ralph and Ida Chau 1993-2014. Site Map. Locality, mineral & photograph data are the copyright of the individuals who submitted them. Site hosted & developed by Jolyon Ralph. Mindat.org is an online information resource dedicated to providing free mineralogical information to all. Mindat relies on the contributions of thousands of members and supporters. Mindat does not offer minerals for sale. If you would like to add information to improve the quality of our database, then click here to register.
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