TRANSFER ZONES IN THE UPPER MISSISSIPPI VALLEY ZN-PB DISTRICT, SW WISCONSIN, USA, AND THEIR POTENTIAL ROLE LOCALIZING MISSISSIPPI VALLEY-TYPE MINERALIZATION

The Upper Mississippi Valley Zn-Pb district (UMVD) in southwestern Wisconsin has the potential to host various critical minerals in Mississippi Valley-type deposits, including barite, Zn, Co, Ge, In, and Ga. Historically, small folds and faults in overall weakly deformed Paleozoic sedimentary rocks were thought to concentrate sulfide mineralization. However, mines and prospects across the district are not homogeneously distributed along major structures, suggesting the relationship between map-scale structures and ore bodies is more complicated.

Earth MRI (Mapping Resources Initiative) 1:24,000-scale geologic mapping in the Crow Branch mining area of the northern UMVD was initiated to clarify the relationship between folding and mineralization. The Mineral Point anticline is the dominant structure in the northern UMVD. It is a northwest-trending, gentle, asymmetric anticline with an amplitude of roughly 200 feet. The fold is composed of several doubly plunging segments, each of which is probably cored by a reactivated Precambrian fault. Historic mines in the Crow Branch area are concentrated in the overlap region between fold segments. This area is interpreted to represent the area above a transfer zone between buried fault segments. Fracturing in the transfer zone may have focused mineralizing fluids upward, leading to the development of Mississippi Valley-type sulfide deposits. New geochemical analyses of cuttings sets from historic exploration holes show high concentrations of iron and sulfur in the subsurface below the mined area, but generally show low values of critical minerals Zn, Ge, In, and Ga.

Finally, a recent airborne electromagnetic (AEM) survey flown over the UMVD may provide insight into the subsurface geometry of sulfide bodies. AEM results show a north-dipping moderately conductive zone underlying the Crow Branch mining area, as well as several other moderately conductive zones in the outlying areas. We hypothesize these conductive zones reflect areas of increased sulfide mineralization. If correct, the orientation of the dipping conductive bodies may help test models of mineralizing fluid flow paths.