Paleomagnetic mapping of the regional fluid flow event that mineralized the Upper Mississippi Valley Zn-Pb ore district, Wisconsin, U.SA

摘要

The Upper Mississippi Valley (UMV) zinc-lead district contains archetypal examples of Mississippi Valley-type (MVT) ore deposits. Specimens (332) from 25 sites in the Lower Ordovician Oneota dolomite were collected from around the northern and eastern sides of the UMV district on the Wisconsin Arch in southwestern Wisconsin. Most specimens yield one of two characteristic remanence (ChRM) components, A and B, in varying proportion at each site. Component B resides in magnetite mostly and gives a primary Early-Middle Ordovician paleopole at 15.1°N, 150.5°E (δp = 2.4°, δm = 4.7°), and component A resides mainly in pyrrhotite and gives an Early Permian remagnetization paleopole at 47.4°N, 127.3°E (δp= 1.1°, δm = 2.2°) that matches the UMV zinc-lead ore's paleopole. The percentage of A component specimens at each site is used to map the boundary of the Early Permian fluid flow that formed the UMV ore over a distance of >200 km. The map also delineates areas of intense pyrrhotite authigenesis created by sulphidic fluid flow within the UMV district, and locates potential conduits in the Oneota dolomite through which fluid flow was delivered from an adjacent basin.