STABLE ISOTOPE COMPOSITIONS OF MINERALS AT HICKS DOME, IL-KY FLUORSPAR DISTRICT, USA: IMPLICATIONS FOR REE+Y-HFSE-FLUORSPAR DEPOSIT GENESIS

The Mississippi Valley type (MVT) deposits of the Illinois-Kentucky Fluorspar District (IKFD) have been resources for fluorspar ± Zn, Pb, Ag, and Cu. Contrastingly, the Hicks Dome deposit in IKFD is a potential resource for different suite of elements and minerals, several of which are currently considered critical: fluorite, barite, Be, Ti, Nb, and REE. The contrast extends also to the host rocks: Ordovician limestones for Hicks Dome rather than Mississippian limestones for the MVT type deposits in the IKFD. Hicks Dome is also associated with ~271 Ma lamprophyre dikes, diatremes, and hypothesized carbonatite, that have been argued to be the source of the critical minerals. To evaluate the source(s) of fluids and genesis of critical minerals at Hicks Dome, the isotopic composition of O and H in phyllosilicates (n=16) from lamprophyre dikes, breccias, and fault gouge and O and C in carbonates (n=35) 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 δ²H_V-SMOW values of phyllosilicates range from +5.5 to +16.1‰ and -66 to -33‰, respectively. The measured δ¹⁸O_V-SMOW and δ¹³C_V-PDB values of carbonates range from +14.0 to +25.0‰ and -6.8 to +2.6‰, respectively. Muscovite from metasomatized dikes and breccias are enriched in ¹⁸O compared to phlogopite from unaltered lamprophyre dikes. Calculated isotopic compositions of the fluids from which the phyllosilicates precipitated indicate that phlogopite retained a magmatic composition while muscovite likely formed from magmatic fluids that exchanged with limestone host rocks. The δ¹⁸O and δ¹³C values of carbonates lie between the two reservoirs of primary magmatic carbonate and limestone. Fluid-rock isotope exchange modelling suggests that if carbon was initially magmatic, the carbonate could have been isotopically enriched due to interaction with a fluid at low temperatures and high fluid to rock ratios. These data suggest that mineral formation at Hicks Dome occurred following magmatism, exchange with host carbonate rock, and alteration with multiple hydrothermal fluids of brine and meteoric water composition. The data support a magmatic hydrothermal model for mineralization, and allow, but do not prove, that H₂O and CO₂ were derived from a concealed carbonatite intrusion.