Thermodynamic modeling of REE behavior in oxidized hydrothermal fluids of high sulfate sulfur concentrations

Thermodynamic calculations using the HCh software were made for mineral equilibriums including REEs in the fluoride–sulfide–chloride–carbonate–sulfate–system in the presence of Na, Ca, and P with fluids of various acidities–alkalinities [11]. The obtained thermodynamic characteristics of thenardite allowed us to carry out the calculations for this phase under complicated hydrothermal conditions simulating the presence of oxidized fluids at 500–100°C and 2000–125 bar. Among other solid phases, REEs–fluorite, monazite, and REE–F–apatite were formed as CaF 2 –(Ln,Y)F 3 , LnPO 4 , and Ca 5 (PO 4 ) 3 F–(Ln,Y) 3 (PO 4 ) 3 ideal solid solutions, respectively, where Ln is La, Ce, Pr, Nd, Sm, Eu, and Gd. Xenotime, anhydrite, elemental sulfur, and calcite were found as well.