The chemical and thermal evolution of the fluids in the Cave-in-Rock fluorspar district, Illinois; mineralogy, paragenesis, and fluid inclusions

Abstract

A detailed fluid inclusion and paragenetic study was undertaken on samples from the Deardorff and Hill mines in the Cave-in-Rock fluorspar district, Illinois. Fluid inclusion homogenization temperature and salinity data were collected on all available transparent minerals in the paragenetic sequence: fluorite, sphalerite, quartz, calcite, and barite. The bulk of the fluid inclusion data, however, was from fluorite crystals.The fluorite crystals from the bedded replacement deposits are color banded. The banding sequence is observable and correlated from crystal to crystal within a mine, and either the general sequence or parts of it have been observed in crystals from throughout the district. Measurements were made on fluid inclusions from each band of several large crystals from the Hill mine which contained the entire banding sequence. Inclusions in these samples yielded homogenization temperatures between 132 degrees and 151 degrees C for most of the sequence and between 115 degrees and 120 degrees C in the outermost band. The salinity of the fluids ranged between 18 and 20 equivalent weight percent NaCl except in the outermost band where the salinity dropped to 9 to 10 equivalent weight percent NaCl.A group of purple fluorite bands (P3), which are easily recognizable in many deposits, was correlated from crystal to crystal at over 40 sample localities in the Deardorff mine. Measurements on inclusions in these bands yielded information on the thermal and salinity variations at a single time plane. These measurements denote a series of concentrically zoned hot areas within the mine which were areas of upwelling fluids. The temperature of the fluids changed as much as 10 degrees C over 50 to 200 ft. The salinity of the inclusions in these samples varied only between 19 and 21 equivalent weight percent NaCl and did not correlate with either sample location or temperature.Fluid inclusion measurements on quartz, sphalerite, calcite, and barite yield temperatures of 68.3 degrees to 165.1 degrees C for early quartz; 160.1 degrees to 226.5 degrees C for milky quartz bands; 130.4 degrees to 160.0 degrees C for sphalerite; 81.9 degrees to 137.2 degrees C for calcite, and 115.2 degrees to 157.0 degrees C for barite. Inclusions in sphalerite and quartz had salinities like those for fluorite of 18.3 to 21.0 equivalent weight percent NaCl, but the salinity of the fluid dropped during calcite and barite deposition: 7.0 to 8.9 equivalent weight percent NaCl for calcite and 1.2 to 1.3 equivalent weight percent NaCl for barite.The fluid inclusion measurements on samples from the Hill mine indicate that the fluids stayed at nearly the same temperature and salinity for most of the mineralizing period. The drop in temperature and salinity in the final stages of deposition strongly suggests an influx of cooler, probably meteoric water. The temperature measurements on samples from the Deardorff mine indicate that there were thermal variations of as much as 10 degrees C within the ore fluid. The narrow salinity range and lack of correlation between salinity and temperature or sample location seem to preclude mixing of two fluids as the deposition mechanism. Mixing of two or more fluids at depth, however, is not ruled out. A cooling mechanism for fluorite deposition requires rather large quantities of fluid to have passed through the deposit; however, cooling combined with chemical changes in the fluid is the best working hypothesis.