STABLE ISOTOPE COMPOSITIONS OF CARBONATES IN THE HICKS DOME CRITICAL MINERAL RESOURCE, IL-KY FLUORSPAR DISTRICT, USA: IMPLICATIONS FOR GENETIC MODELS

Mississippi Valley Type deposits in the Illinois-Kentucky Fluorspar District have been mined for fluorspar ± Zn, Pb, Ag, and Cu, and are hosted in Mississippian limestones. Within the district, the Hicks Dome critical mineral resource contains enrichments in fluorite, barite, Be, Ti, Nb, REE, and Y hosted in Ordovician limestones. The district differs from other Mississippi Valley Type deposits in that it is associated with ~271 Ma lamprophyre dikes and diatremes that are inferred to be the original source of F. The Hicks Dome critical mineral resource is hypothesized to have been sourced from both basinal brines and mafic alkaline magmas and/or a carbonatite intrusion. To evaluate the source(s) of H₂O and CO₂ at Hicks Dome, the isotopic composition of O and C in calcite and dolomite (n=22) from veins and cements in fractured and brecciated lamprophyre dikes and limestones were collected from drill cores (n=7) at depths of 34 to 1000 meters depth below the surface. The measured δ¹⁸O_V-SMOW and δ¹³C_V-PDB values range between +16.5 to +24.1 ‰ and -6.8 to +0.6 ‰, respectively. The range of δ¹⁸O and δ¹³C values suggests that carbonates precipitated from a mix of both basinal brine and magmatic fluid that dissolved Ordovician carbonates. Isotopic exchange temperatures, the extent of isotopic equilibrium, and brine/magmatic fluid with total fluid/rock ratios can account for the positive shift of δ¹⁸O and δ¹³C values away from the established primary igneous carbonate field and towards the compositions of host limestones. Calcite veins in shallow (<60 m) Devonian host rocks have the lowest δ¹⁸O values, which may be due to an overprint by a late stage, low temperature, predominantly meteoric fluid. There is no clear heavy isotopic enrichment in ore-host carbonates relative to pre- or post-ore stage carbonates often found in other Mississippi Valley Type deposits. This difference could be indicative of the magmatic contribution at Hicks Dome. Moreover, these data are consistent with recent O and H isotope data on muscovite from metasomatized lamprophyres and ore breccias, which indicate formation from a magmatic fluid that exchanged with host carbonates. These data support a magmatic hydrothermal model for mineralization at Hicks Dome, and allow, but do not prove, that H₂O and CO₂ were derived from a concealed carbonatite intrusion.