Lafayette County, Wisconsin, USAi

Regional Level Types
Lafayette County	County
Wisconsin	State
USA	Country

Lafayette 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, baryte 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. The deposits near Shullsburg yielded many specimens of this Wisconsin classic. See http://www.mindat.org/photo-271690.html for an example.