GEOCHEMISTRY OF FLUID INCLUSIONS IN MINOR AND TRACE OCCURRENCES OF MISSISSIPPI VALLEY-TYPE MINERALIZATION IN THE U.S. MID-CONTINENT

The U.S. mid-continent hosts some of the largest Mississippi Valley-type (MVT) Pb-Zn deposits in the world. Previous research has shown these deposits to have formed from ~100° C brines characterized by high Ca/Na ratios, Pb and methane contents, and low Ca/Mg ratios. Numerous minor and trace occurrences of MVT mineralization have been documented in the U.S. mid-continent, many of which are far from known ore bodies. Some of these occurrences have previously been characterized with respect to fluid inclusion bulk salinity and homogenization temperature. However, none has so far been characterized with respect to elemental fluid inclusion composition. This raises the question of whether the minor and trace MVT occurrences were formed from fluids that are geochemically similar to those that formed the large mid-continent MVT ore deposits, and what conditions and processes govern the scale of mineralization.

The present results are the first from an ongoing investigation of the composition of fluid inclusions in sphalerite from minor and trace MVT occurrences in the U.S. mid-continent. Samples have so far been analyzed by microthermometry, Raman spectroscopy, and LA-ICP-MS from seven different occurrences in northern Missouri, southeastern Kansas, eastern Iowa, northern Illinois, and eastern Indiana. Compared to fluid inclusions in the Ozark and Illinois-Kentucky MVT ore deposits, fluid inclusions in the minor and trace MVT occurrences have lower Ba/Na and K/Na ratios, methane contents, and homogenization temperatures but similar Ca/Na ratios. The Mg/Na ratios in fluid inclusions in minor and trace MVT occurrences resemble those in Ozark MVT ore fluid inclusions but are higher than those in Illinois-Kentucky fluid inclusions. Pb was detected in fluid inclusions of minor and trace MVT occurrences much less frequently than in sphalerite-hosted fluid inclusions of Ozark MVT ore bodies, but in both cases reach maximum apparent values of 1000’s of ppm. The results thus far suggest that minor and trace MVT occurrences in the U.S. mid-continent formed from fluids that were cooler and chemically distinct from those that formed the Ozark and Illinois-Kentucky MVT ore districts. If low methane contents indicate oxidizing conditions, then this would have limited sulfide precipitation in the minor and trace MVT occurrences.