The potential role of magmatic gases in the genesis of Illinois-Kentucky fluorspar deposits; implications from chemical reaction path modeling

Abstract

In this paper, we present results of reaction path calculations using the chemical speciation and reaction path programs SOLVEQ and CHILLER to model possible fluorite deposition mechanisms in the Illinois- Kentucky fluorspar district. Input to the reaction path calculations were the temperatures, salinities, and concentrations of major cation and anions (Na, K, CI, SO 4 , etc.) and dissolved gases (H 2 S, CO 2 , etc.) in the hydrothermal fluids, based on published fluid inclusion data and unpublished fluid inclusion gas compositions. Fluid pH values and concentrations of fluorine and various metals were then calculated assuming saturation with observed hydrothermal minerals. We assumed saturation with fluorite, dolomite, quartz, muscovite (approximating illite), anhydrite, sphalerite, galena, and pyrite. Fluorite ore deposition mechanisms which we initially modeled include (1) simple cooling (from 150 degrees - 140 degrees C, based on data and conclusions of Richardson and Pinckney, 1984), (2) reactions of the hydrothermal fluids with limestones (based upon the observed extensive replacement of limestones by fluorite), (3) combinations of cooling and limestone reaction, (4) isothermal or near-isothermal boiling of the fluids in response to pressure drops, and (5) mixing of two fluids with different Ca and F contents. Using the assumed initial fluid composition discussed above, the modeling predicts that none of these mechanisms accurately reproduces the observed fluorite-dominant mineralization stages and limestone replacement features. We then modeled how interactions of the hydrothermal fluids at 300 degrees C with HF and CO 2 (approximating a CO (sub 2 and HF-bearing gas phase expelled from a crystallizing alkalic magma) might affect fluid chemistries and results of the above deposition mechanisms. The influence of alkalic magmatism on Illinois-Kentucky fluorspar district mineralization is probable based on the geologic relations of the district. Titration of 0.5 g HF and 0.5 g CO 2 into 1 kg of a fluorspar fluid composition produces extensive fluorite precipitation and acidic (pH 2.5 or lower), F-rich, Ca-poor fluids. Cooling of these fluids to 150 degrees C leads to further fluorite precipitation. Other possible magmatic gas compositions were modeled and yielded generally comparable...