Hydrothermal fluorite solubility experiments and mobility of REE in acidic to alkaline solutions from 100 to 250 °C

Abstract

Fluorite is a common vein mineral in critical mineral deposits that are overprinted by hydrothermal processes. Its chemistry potentially reflects the mobilization conditions of rare earth elements (REE) in these hydrothermal fluids. However, the controls of temperature and fluid chemistry (i.e., pH and salinity) on fluorite solubility and coupled REE mobilization is still uncertain. In this study, batch-type experiments were conducted to investigate REE mobility in aqueous fluids controlled by the solubility of fluorite at temperatures between 100 and 250 °C. Natural fluorite crystals from Cooke’s Peak (New Mexico) were equilibrated with aqueous solutions of variable starting pH (2–10), salinities (0 and 15 wt.% NaCl), and initial REE concentrations (0 and 0.5 ppm). Reacted fluorite crystals display dissolution-controlled reaction textures (i.e., etch pits and dissolution channels along fluorite cleavage planes), which are most pronounced in experiments conducted in acidic fluids and at elevated temperature. Addition of NaCl considerably enhances fluorite dissolution, in both acidic and alkaline solutions, which is also reflected by the compositions of the reacted solutions displaying an increase in Ca, F, and REE concentrations. The overall fluorite solubility is controlled by the association behavior of HF0 at acidic conditions, and by the formation of Ca chloride, Ca hydroxyl, and Na fluoride complexes, which become enhanced by addition of NaCl, including in mildly acidic and alkaline solutions. Mass balance calculations were used to compare the REE stoichiometry of unreacted fluorite crystals with measured REE concentrations in the reacted solutions, which reveal several possible controls on REE mobility: i) stoichiometric release of REE upon fluorite dissolution; ii) precipitation of secondary REE-depleted fluorite; and/or iii) precipitation of secondary light REE fluorides, which were observed to form on fluorite crystals surfaces in the REE-doped experiments. Fluorite solubility constants (Ksp) were regressed with available low temperature solubility data using the equation logKsp = A + BT + C/T + Dlog(T) (T, temperature in K) valid between 25–250 °C. Fitted coefficients yield: A = 171.180, B = 0.01477, C = −7794.10, D = −64.6634. The experimental results have important implications for geochemical modeling of natural systems, where fluorine controls the mobility of REE, and can be enhanced by addition of NaCl in both acidic and alkaline solutions. Therefore, the salinity of magmatic-hydrothermal fluids plays a crucial role in mobilizing REE during the formation of hydrothermal fluorite veins.