Fluid Inclusion and Stable Isotope Study of Minor Upper Mississippi Valley-type Sulfide Mineralization in Iowa, Wisconsin, and Illinois.
The Upper Mississippi Valley (UMV) District is one of the largest base metal districts in the United States; it covers more than 7,800 square km in northeast Iowa, southwest Wisconsin, and northwest Illinois. The largest deposits are close to the southern margin of the district in Iowa and Illinois. It has been suggested that potentially productive extensions of the district may occur up to 100 km to the south and southwest in these states. The deposits in the UMV District are primarily located in ordovician sedimentary rocks, whereas those in the outlying districts occur in rocks ranging from Cambrian to Mississippian in age. Although considerable attention has been directed toward understanding the genesis of the lead-zinc mineralization in the main UMV district, an increasing number of investigations have centered on the origin of the minor base metal occurrences that surround the main UMV District. This report uses stable isotope (s, c, o, and h) and fluid inclusion data to (1) determine the geochemical conditions under which minor occurrences in Iowa, Illinois, and Wisconsin formed, (2) identify possible sources of ore-forming constituents (i.e., fluids, sulfur, and metals), and (3) use the above data, in conjunction with available geological, mineralogical, and paragenetic information, to evaluate whether the outlying occurrences are cogenetic with the main UMV District as has been proposed previously.