Thermodynamic model for REE complexation in the course of interaction between REE-fluorite and hydrothermal fluid

The latest thermodynamic data on REE speciation in solutions (Haas et al., 1995; Shock et al., 1997) were used in the development of a model for the physicochemical behavior of REE in CO2- and NaCl-bearing fluids at temperatures of 500-100degreesC and pressures of 2000-100 bar. By interpreting simultaneous dissolution curves for a mixture of quantitatively dominant fluorite and small amounts of eleven REE + Y fluorides, we explained the physicochemical causes of the typical fractionation directions of REE in fluorites that crystallized during distinct stages of the hydrothermal process. For instance, an increase in the concentrations of HREE in the late fluorite was illustrated by the examples of Lu and La as resulting from both a change in their complex speciation in an aqueous solution and a divergence of their bulk concentrations with the evolution of pH and other parameters of natural fluids.