LITHOGEOCHEMISTRY OF THE HICKS DOME CRITICAL MINERAL RESOURCE, ILLINOIS-KENTUCKY FLUORSPAR DISTRICT, USA: IMPLICATIONS FOR GENESIS

Hicks Dome is a proposed source of fluorine in coarse crystalline fluorite (~7 Mt) produced from MVT deposits of the IL-KY fluorspar district. Recent work by Hicks Dome LLC has defined a 65.8 Mt area-wide, critical mineral resource with 15.8% fluorite, 4.8% barite, 1.3% TiO₂, 0.3% REY oxide, 0.2% Nb₂O₅, and 0.16% BeO, at a fluorite cut-off grade of 10%. Fine-grained, purple fluorite mineralization occurs in rock-flour-matrix, solution-collapse breccias in Ordovician to Silurian platform carbonate rocks at depths of 700-1000 m. The critical mineral resource is associated with 271 Ma lamprophyre dikes and diatremes, a Th radiometric anomaly, and an upper crustal magnetic anomaly.

Multielement analyses of drill core (N=3,176 samples; 6,700 m) and micro-XRF maps of 3 high-grade samples were used to identify element associations, characterize mass transfer, and investigate ore-gangue relationships. In comparison to average upper crust, Hicks Dome ore is enriched in Be, Ba, Th, U, Nb, Ti, Sr, Pb, Zn, HREE, F, P, Mg, and Ca and depleted in Mn, Fe, Al, and Si. Chondrite-normalized REE patterns mimic patterns typical for the main REE mineral, xenotime, with patterns decreasing from La to Nd and elevated concave patterns from Sm to Lu. Compared to lamprophyre dikes, the REE patterns of ore are enriched in Sm to Lu. Element associations relate to lithology and mineralization as follows: carbonate rocks (Ca, Mg, Sr), terrigenous detritus (Si, Al, K, Ti), lamprophyre (Si, Al, Fe, Mg), critical minerals (F, Ba, Sr, Ti, Nb, Th, Y, REE, and Be), and sulfides (Fe, S, As, Zn, Pb).

Our hypothesis is that more extensive mafic, alkaline magmas were emplaced beneath the Paleozoic stratigraphy and that degassing and condensation of acid volatiles into a basinal brine caused dissolution of carbonate rocks and formation of caverns and collapse breccias. Repetitive input of magmatic fluids containing critical elements preferentially replaced rock-flour-matrix breccias to form high-grade ore. The critical element assemblage may be related to a highly fractionated alkaline intrusive phase or carbonatite with further hydrothermal enrichment facilitated by the presence of specific ligands (Cl, F, PO₄, SO₄, HCO₃, CO₃).