MICRO-IR ANALYSIS OF FLUORITE FROM THE CAVE-IN-ROCK FLUORSPAR DISTRICT, ILLINOIS: INSIGHTS INTO CRYSTAL GROWTH, COLOR, AND HOST FLUID EVOLUTION

Yellow, purple, and colorless banded fluorite crystals sampled from the Cave-In-Rock Fluorspar District offer insights into the evolving hydrothermal fluids that hosted sulfide ore deposition. Earlier studies have shown that the fluids responsible for the Cave-In-Rock deposits followed similar mineral paragenesis, crystallization temperature, salinity, and isotope systematics (Richardson & Pinckney, 1984; Richardson et al., 1988). A two-stage fluid evolution sequence was identified. Early fluids were associated with the precipitation of fluorite, sphalerite, galena, and quartz at relatively higher temperatures (~145 °C), higher salinities (~20 wt % NaCl), and heavier H isotopes (-25 ‰), whereas a later fluid was associated with calcite and barite at lower Ts (~125 °C), lower salinities (<10 wt % NaCl), and lighter isotopic compositions (~ -40 ‰). The earlier studies failed to identify chemical and/or environmental factors that correlate with the prominent color banding in fluorite.

We present detailed micro-FTIR analyses of hydrous species within a gemmy fluorite specimen (#65P45) sampled from the Hill mine, IL. Our analyses represent the first high-resolution (100 µ) IR analyses on fluorite. The Hill specimen consists of a brilliant yellow core with a thick coating of alternating purple and colorless bands. We have identified a prominent broad peak at 3400 cm^-1 and three sharp peaks at 1550, 1650, & 1750 cm^-1. The 3400 & 1650 cm^-1 peaks are common features observed in hydrothermal quartz crystals (associated with hydroxyl stretching and bending of molecular water, respectively). We show that the yellow color of fluorite correlates well with absorption characteristics of the crystal. Yellow regions have high concentrations of all four species, whereas colorless and purple bands show order-of-magnitude weaker absorption at 3400 & 1650 cm^-1 and no absorption at 1750 & 1550 cm^-1. The peak heights of the 3400 & 1650 cm^-1 peaks generally correlate with one another within bands, but do not show distinct differences across purple and colorless bands. Our spectroscopic analyses correlate with the transition associated with the second stage of fluid evolution identified in earlier studies; hydrous species concentrations increase progressively toward the rim of the crystal beginning at this transition.