Fingerprinting the Hydrothermal Mobility of Rare Earth Elements (REEs) in Ore Deposits from the Stability of Monazite-(Ce)

In this study, a series of hydrothermal solubility experiments were carried out using synthe�c monazite crystals (i.e., LaPO 4 , PrPO 4 , NdPO 4 , and EuPO 4 ) to assess the consistency of reported mineral calorimetric data and the thermodynamic data of the aqueous REE complexes. The solubility experiments were conducted in aqueous HClO 4 -H 3 PO 4 -bearing solu�ons at temperatures between 100° and 250°C and at saturated water vapor pressure. Equilibrium constants (K s0 ) for the dissolu�on reac�on of monazite end members were retrieved as a func�on of temperature and extrapolated to standard condi�ons of 25°C and 1 bar. Results indicate significant differences between the new solubility data and those reported in the literature. We demonstrate the impact of these new thermodynamic data in a series of fluid-rock interac�on models using the GEMS code package (h�p://gems.web.psi.ch) and the MINES thermodynamic database (h�p://tdb.mines.edu). The simulated monazite stability can be correlated to field observa�ons and allows for the predic�on of the behavior of REE in hydrothermal fluids and their associa�on to altera�on observed in ore deposits.