WONEWOC SANDSTONE AS A POTENTIAL CONDUIT FOR MISSISSIPPI VALLEY-TYPE MINERALIZATION

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 of primarily zinc, lead, copper, and iron are collectively known as Mississippi Valley-type (MVT) deposits; these types of deposits have been identified globally and interpreted to have been emplaced into the host carbonate rock by the introduction of hydrothermal fluids. However, the origin of the hydrothermal fluids that emplaced these deposits and their pathways into the study area are still subject to debate. Preliminary geochemical data from recently drilled rock cores outside of the mining district show that the Wonewoc Formation, a sandstone unit below the ore-­hosting dolostone, contains similar types of mineral suites and therefore was a potential conduit for the MVT-emplacing fluids. Our current dataset of pXRF and IPCMS elemental analyses, XRD mineralogical analyses, and sulfur isotopic analyses show that some of the sulfides observed in the Wonewoc have a similar chemical signature to the historically mined ore. These results suggest that the Wonewoc Formation sandstone played a role in MVT mineralization in the study area. Further work will be undertaken to confirm our hypotheses regarding the origin, path of transport, and spatial extent of sulfide emplacing hydrothermal fluids. Going forward, this 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 a better understanding of the presence of sulfide minerals in the Wonewoc aquifer system to municipal groundwater suppliers.