Exploring Hydrothermal REE Mobilization in Ore Deposits: A Journey from Field Geology & Thermodynamics to Spectroscopy & Molecular Processes

Abstract: The rare earth elements (REE) are classified as critical elements because of their economic supply risk and strategic use in high-tech and green energy such as wind turbines, energy-efficient lights, computers, and hybrid cars. The formation of REE mineral deposits in geologic systems is controlled by magmatic-hydrothermal processes with aqueous fluids which can mobilize, fractionate, and enrich the REE during fluid-rock interaction processes. The stability of aqueous species, particularly the REE chloride, fluoride, sulfate, carbonate, and hydroxyl complexes plays a key role in the solubility of REE minerals like monazite and xenotime. However, the thermodynamic constants for many of these species have not yet been determined experimentally or only within a limited temperature and pressure range. In this study, we combine new experimental data measured at subcritical (25-350 °C) and supercritical (400-700 °C) conditions based on solubility experiments and in situ UV-Vis and Raman spectroscopy, and synchrotron based X-ray fluorescence. Thermodynamic equilibrium calculations and optimizations are conducted using the GEMS code packages and the MINES thermodynamic database (https://geoinfo.nmt.edu/mines-tdb/), in combination with the ThermoExp_REE experimental database. Using this approach, we extract new thermodynamic constants for modeling the stability of REE complexes in hydrothermal fluids. The REE phosphate solubility data indicate that the light and heavy REE can be fractionated by several orders of magnitude with increased temperature and pressure at supercritical conditions with important  implications for ore forming processes.