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Abstract

The stratiform fluorite deposits of Southern Illinois constitute an important aspect of the widespread Cretaceous mineralization which also produced the numerous veins in the Illinois-Kentucky region. Although fluorite is the dominant ore mineral, some deposits contain abundant sphalerite and generally minor galena. Calcite, pyrite, quartz and barite are important gangue minerals.

The bedded ores occur in a limited section of sedimentary carbonate rocks of Mississippian age, closely associated with faults which have very minor displacement but are always clearly related to major faults that evidently served as master conduits. Ore emplacement, clearly epigenetic, involved extensive replacement as well as the filling of spaces resulting from solution of the host strata by the ore fluid. Fluid inclusions identify the ore fluid as a concentrated hydrothermal brine basically of connate type but somewhat modified by successive additions of magmatic and meteoric waters. The occurrence of numerous alkalic igneous dikes and intrusive breccias supports the assumption of a magmatic source for most or all of the fluorine and metals, and of thermal convection of the ore fluid. Isotopic data identify the lead components as of J-type, similar to that of other Mississippi Valley deposits, and as having a close kinship to the igneous materials.

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