Additional fluid inclusion data for the Illinois-Kentucky fluorspar district; evidence for the lack of a regional thermal gradient

Abstract

Fluid inclusion and paragenetic studies were undertaken on fluorite samples in bedded replacement, vein, and breccia-hosted deposits in the Illinois-Kentucky fluorspar district. Paragenetic relationships show that color bands in fluorite can be correlated from one crystal to another within a bedded replacement deposit, among bedded replacement deposits in the same subdistrict, and among subdistricts. Although the paragenetic sequence of vein deposits superficially resembles that of bedded replacement deposits, it is difficult to correlate the paragenetic sequence of minerals between these two deposit types because of the general lack of color banding in most vein deposits and because fluorite of only one color may be present in a given vein deposit.Fluid inclusion homogenization temperatures (T h ) for primary, aqueous, liquid-vapor inclusions in bedded replacement deposits range from 98.4 degrees to 150.3 degrees C, whereas those in vein deposits range from 60.2 degrees to 143.9 degrees C. T h values of primary, organic liquid-vapor inclusions in the Minerva bedded replacement deposit range from 88.5 degrees to 116.6 degrees C. Secondary inclusions in several bedded replacement and vein deposits exhibit T h values that exceed those of primary inclusions (up to 157.9 degrees C). The highest homogenization temperatures were obtained from the Rose breccia-hosted deposit at Hicks Dome (155.5 degrees -168.4 degrees C). Salinities of primary inclusions in fluorite from bedded replacement (prepurple 7 band), vein, and breccia-hosted deposits range from 17.1 to 22.7, 17.7 to 21.7, and 19.7 to 20.7 wt percent NaCl equiv, respectively. Data obtained here are in general agreement with those reported previ ously from the district.Salinity-T h relationships for the Annabel Lee, Minerva, Hill, and Denton deposits suggest that bedded replacement deposits formed from at least three fluids. A lower temperature higher salinity connate fluid (F1: <125 degrees C, 23 wt % NaCl equiv) and a higher temperature-lower salinity connate fluid (F2: approx 150 degrees C, 19 wt % NaCl equiv) mixed at the site of deposition. These fluids were followed by a lower temperature-lower salinity meteoric water-dominated fluid during the waning stages of purple 6 and 7 (P6 and P7), calcite, and barite deposition. F1 dominated during the precipitation of early fluorite, whereas F2 dominated during the late-stage (but pre-P7) fluorite mineralizing event. Fluid mixing identified during fluorite deposition in the Illinois-Kentucky district is similar to that reported previously in other mid-continent Mississippi Valley-type deposits.By combining T h measurements of fluid inclusions in this study with data obtained from other fluid inclusion studies, it is apparent that there is no evidence for a systematic regional temperature variation throughout the Illinois-Kentucky district, as has been proposed by Rowan et al. (1991) and Taylor et al. (1992). Although T h values in fluorite are highest near Hicks Dome, ranges of T h values for individual deposits can exceed 30 degrees C, with average temperatures for individual deposits that appear to show a random distribution throughout the district.