log (mEu or mi /mj ), pH, Eh

Kolonin G.R., Shironosova G.P. Thermodynamic modeling of possible reasons of REE fractionation with participation of high temperature fluids of complicated composition.
Institute of Mineralogy and Petrography of SB RAS (IMP SB RAS), Novosibirsk, kolon@uiggm.nsc.ru key words [thermodynamic modeling solubility REE-fluorite fluid fractionation Eu-anomalies]

8 4 0 -4 -8 -12 -16 -20 -24 0,1 0,01 0,01 0,1 0,2 0,5 1
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pH Eh CO2/CH4 Eu++ Eu+++ Eu+++/Eu++ Eu(II) Eu(III) Eu(III)/Eu(II)

The data base of the stability constants of the complex forms of REE in the solutions of complicated composition at temperatures up to 500°C and pressures of up to 2 kbar [1] has been adapted on the basis of the parameters of HKF equation for simple and complex REE ions [2,3]. An extended thermodynamic modeling of possible behavior of REE elements during the evolution of the parameters of complex fluoride-chloride-carbonate fluids, saturated with REE-containing fluorite, has been conducted on the basis of this data base [4], using the software package “Hch” [5]. The peculiarities of complexing of trivalent and bivalent Eu, including the influence of acidity-alkalinity on the ratio of total concentrations of Eu(III) and Eu(II), have been also discussed [6]. Specifically, it has been shown that during the evolution of fluid compositions typical of rare-metal fluoritecontaining deposits of hydrothermal genesis [7], two REE groups, which differ in the specificity of the complex forms predominant in the fluid, can be recognized. In regard to the LREE (La, Ce, Nd and Pr), the LnF++ and LnF2+ (for reduced temperatures) fluoride complexes would be expected to dominate, if positively charged or neutral hydroxocomplexes are insignificant. As for MREE (Sm, Eu, Tb) and HREE (Ho, Yb and Lu), the anion or neutral hydrocomplexes with the subordinate concentrations of LnF++ and LnF2+ fluoride complexes are bound to dominate.

HCL

KOH

mol/kg H2O

Fig.