PALEOHYDROLOGY OF THE UPPER MISSISSIPPI VALLEY ZINC-LEAD DISTRICT

The regional scale of the paleohydrologic system responsible for formation of the Mississippi Valley-Type ores of the U.S. midcontinent is well established. However, characterization of the 3-dimensional flow vectors and details of district scale hydrology are lacking. Geochemical data in the Illinois Basin provide paleohydrologic constraints for the Upper Mississippi Valley (UMV) zinc-lead district of southwestern Wisconsin, northwestern Illinois, and eastern Iowa. This is possible because fluorine-rich alkalic igneous activity at the south end of the basin (at Hicks Dome) and formation of the UMV district took place concurrently within the uncertainty of radiometric dates. Fluorine concentrations (as fluorite) in Ordovician rocks show an enrichment that decreases systematically northward from a maximum at Hicks Dome to zero south of the UMV district. This trend implies that brines originating in the deep southernmost portion of the Illinois Basin acquired fluorine from Hicks Dome igneous volatiles, flowed northward and deposited fluorine by rock-water interaction to form fluorite. The resulting low fluorine brines provided the mineralizing fluid to the UMV district.

Constraints on district scale hydrology derive from Pb isotope studies. Pb isotope data from ore and trace mineralization show a district-wide zonation that closely correlates with the geology of underlying basement rocks. More radiogenic Pb isotope values in UMV galena correlate with more acidic basement rock types whereas less radiogenic isotope values overlie mafic rock types. These correlations indicate that fluids interacted with and acquired their Pb from basement rocks. New sulfur isotope data from non-ore drill holes within the district indicate that the mineralization is centered on a stratigraphic interval that is sulfidized by the uniquely heavy H₂S (d³⁴S=+14 permil) that also characterizes ore sulfides. The ore-bearing interval was likely a regional reservoir for aqueous sulfide of this isotopic composition. Our interpretation is that the UMV district represents the upflow zone of an Illinois Basin fluid flow system. Ore minerals precipitated from metal-bearing fluids by reaction with H₂S that had accumulated in the Ordovician host rocks.