Stable isotope constraints on the source of ore fluids for the Hicks Dome REE+Y-HFSE-fluorspar deposit

Hicks Dome is comprised of coarse crystalline Mississippi Valley Type deposits at shallow levels and an enigmatic, fine-grained fluorite, rare earth elements, Y, high field strength elements, Be, and Ba rich deposit at deeper levels. Phyllosilicates from a lamprophyre dike and a breccia from two Hicks Dome drill cores were sampled to resolve the fluid history of the entire deposit using light stable isotopes. Silicate fluorination coupled with isotope ratio mass spectrometry give δ18O values from +6.9 to +16.0 ‰ (Vienna Standard Mean Ocean Water). Temperature conversion elemental analyzer and gas chromatography-isotope ratio mass spectrometry give δ2H values from -54 to -33 ‰ (Vienna Standard Mean Ocean Water). Muscovite from metasomatized dikes and breccias are relatively enriched in 18O compared to phlogopite from lamprophyre. 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 carbonate host rocks or from magmatic fluids that mixed with basinal brines. Enrichment of deuterium in fluids calculated from muscovite suggest that fluids were derived from hypothesized carbonatites or were acidic. These data demonstrate that the Hicks Dome critical mineral resource is magmatic hydrothermal in origin.