WONEWOC SANDSTONE AS A POTENTIAL CONDUIT FOR MISSISSIPPI VALLEY-TYPE MINERALIZATION IN WESTERN AND SOUTH-CENTRAL WISCONSIN

Wisconsin was one of the largest lead and zinc producers in the US from the 1830’s into the 1900’s due to a sizeable abundance of low-grade metal sulfide ores in the southwestern portion of the state. The assemblage of sulfur-based minerals, composed primarily of zinc, lead, copper, and iron, are collectively known as Mississippi Valley-type (MVT) deposits. These types of deposits are identified globally and interpreted to have been emplaced into a host carbonate rock by the introduction of briny basinal hydrothermal fluids. However, the geographic origin, regional extent and pathways of these fluids into the study area are still subject to debate.

A ubiquitous presence of sulfides from recently drilled rock cores in Wisconsin but outside of the mining district suggests that the Wonewoc Formation, a sandstone unit 200 to 500 feet (60 to 150 meters) below the ore-hosting dolostone, could have been a potential conduit for the basinal hydrothermal fluids, if the sulfides are genetically similar to MVT deposits. We have compiled a dataset of pXRF and ICP-MS elemental, XRD mineralogical, sulfur isotopic and petrographic analyses of pyrite observed in the Wonewoc from two different sites in western and south-central Wisconsin. The data were then compared to similar analyses of pyrite from the ore district in southwestern Wisconsin. The data suggests that our south-central site, about 78 miles (125 km) from our MVT-district sample site, is geochemically similar to the MVT-district samples. In contrast, our western study site, 115 miles (185 km) from our MVT-district sample site, contains both MVT and authigenic pyrite signatures. These data imply that the hydrothermal fluids responsible for the MVT deposits in southwestern Wisconsin could have extended into both south-central and western Wisconsin, but to varying extents. Constraining the regional extent of the MVT mineral emplacement will improve our understanding of this type of mineralization globally, and provide useful information to the mining industry and environmental agencies. Locally, this study will provide municipal groundwater suppliers in western and southcentral Wisconsin with a better understanding of the presence of sulfide minerals in the Wonewoc aquifer system.