GEOCHEMICAL AND ISOTOPIC DIFFERENCES BETWEEN TRACE OCCURRENCES AND ORE DEPOSITS OF MISSISSIPPI VALLEY-TYPE MINERALIZATION IN THE U.S. MID-CONTINENT

Mississippi Valley-type (MVT) mineralization occurs over diverse scales in the U.S. mid-continent, ranging from trace occurrences that are widespread throughout the Paleozoic carbonate succession, to comparatively rare, large ore bodies that contain millions of tons of metals. The factors that govern the scale of mineralization, however, are not clear, as the trace MVT occurrences have not been nearly as well characterized as the MVT ore deposits. The present study has sought to address this problem through analyses of fluid inclusions and sulfide mineral matrix composition.

Results of the study thus far have identified some systematic differences between trace MVT occurrences and ore deposits. Relative to ore deposits, sphalerite-hosted fluid inclusions in trace occurrences have lower homogenization temperatures, salinities, K and methane concentrations, and Ca/Mg ratios. Sphalerite from the trace occurrences tends to have lower matrix concentrations of Cd and Fe but similar concentrations of Pb and Ag compared sphalerite from ore deposits, with the exception of sphalerite in the Southeast Missouri district, which has anomalously high Ag concentrations. The Pb isotope composition of the trace occurrences is much less radiogenic than that of the ore deposits. These results suggest that the fluids that form trace MVT occurrences broadly resemble the sedimentary brines that form MVT ore deposits, while still having distinguishing characteristics. The fluids that formed trace MVT occurrences may not have descended as deeply and not had as much interaction with igneous basement rocks as the fluids that formed MVT ore deposits. The fluids that formed trace MVT occurrences are likely to have had similar Pb concentrations as the fluids that formed MVT ore deposits, suggesting that Pb availability was not a limiting factor governing the scale of MVT mineralization. A stronger limiting factor may instead have been the local availability of sulfide.