DETERMINATION OF FLUORINE CONCENTRATIONS IN MISSISSIPPI VALLEY-TYPE ORE FLUIDS THROUGH SEM-EDS ANALYSIS OF FLUID INCLUSION DECREPITATES

The majority of Mississippi Valley-type (MVT) deposits are primarily sulfide deposits with fluorite present in minor to moderate quantities. However the degree of fluorite mineralization can vary widely among deposits, ranging from anomalously fluorite dominant deposits to fluorite negligible deposits (Leach and Taylor, 2009). It has been hypothesized that anomalously fluorite rich deposits were precipitated from brines enriched in F relative to other MVT ore fluids and typical sedimentary brines. Knowing the ore fluid F concentrations for typical and atypical MVT deposits is necessary to test this hypothesis, however for the majority of MVT deposits current data on F concentrations in MVT ore fluids are negligible or non-existent. Kenderes and Appold (2017) used SEM-EDS to analyze evaporate mounds from thermally decrepitated fluid inclusions in sphalerite from two districts, the fluorite dominant Illinois-Kentucky (IL-KY) district and the fluorite-poor Tri-State (TS) district. Kenderes and Appold (2017) determined an ore fluid F concentration between 680 to 4300 ppm for IL-KY and found the TS ore fluid F concentration to be below the ~100 ppm limit of detection. The present study has been undertaken to determine whether the relative ore fluid F concentrations found for IL-KY and TS are part of a broader trend. The goal of the study is to determine ore fluid F concentrations for five additional MVT deposits with variable fluorite abundances. These include Hansonburg, New Mexico, a rift associated fluorite dominant district similar to IL-KY; Sweetwater, Tennessee, a fluorite rich district with no known igneous association; Central Kentucky, a district with zones containing minor to abundant fluorite; Central Tennessee, a sphalerite-dominant MVT deposit with moderate fluorite abundance; and the Viburnum trend of Southeast Missouri, a district containing negligible fluorite. Results have thus far been obtained from the Hansonburg district, and suggest that it was precipitated from an ore fluid containing very high F concentrations on the order of 100’s to 1,000’s of ppm, similar to what has been reported for the IL-KY district. These results support the hypothesis that fluorite-rich MVT deposits are formed from F-rich fluids.