Home – Home – Jornals – Russian Geology and Geophysics 2018 number Неопубликованное

Thermodynamic calculations were carried out to model the process of interaction between fluid with different concentrations of fluorine and the mineral association calcite + monazite (in the presence of barite and celestine), in order to establish how fluorine concentrations in the fluid change the initial mineral association and the forms of REE concentration during the formation of carbonatite complexes. Thermodynamic calculations were carried out at temperatures of 500, 400, 300, 200 and 100°C and pressures of 2000, 1000, 500, 250 and 125 bar in the range of fluorine concentrations of 10^-4 – 1 mol/kg H₂O. It has been established that when monazite and calcite are exposed to an acidic (pH 3) fluoride-carbonate-chloride solution, increasing fluorine concentrations result in the formation of REE-containing fluorapatite, fluorite, and the fluorocarbonates – bastnasite and parisite. Fluorocarbonates begin to form in equilibrium with decreasing temperature, beginning with decreasing fluorine concentrations in the initial solution. Formation of parisite, according to the generalized formula CaLn₂(CO₃)₃F₂, requires twice as much fluorine as that for bastnaesite LnCO₃F, and under acidic conditions, it appears only at 200 and 100°C in the range of 0.01–0.1 m HF. At the highest concentration considered (1 m HF), REE-fluorapatite, along with parisite, also disappears from the equilibrium assemblage due to the maximum possible amount of the third calcium consumer, REE-containing fluorite. Lowering the temperature increases the stability of REE-fluorite by reducing the fluorine concentration required for its formation in the fluid. With increasing pH of the ore-forming environment, the formation of parisite at 200°C is observed even at low HF concentrations (0.0001 m), replacing bastnaesite under these conditions. Increasing pH is also accompanied by increased stability of REE fluorite and the appearance of strontium-bearing calcite and hydroxyapatite in fluorine-poor fluids. In an acidic fluoride-carbonate-chloride solution, after its interaction with the monazite+calcite assemblage, the total concentration of lanthanides (Ln _tot) decreases with decreasing temperature. Starting at 400°C, the Ln _tot curve exhibits a complex dependence on the initial HF concentration and reaches its minimum at maximum m HF. The main contribution to Ln _tot comes from the chloro- (at low m HF) and fluoro- (starting at 0.01 m HF with decreasing temperature) complexes of light REEs. The contribution of heavy REEs is approximately three orders of magnitude lower than that of light REEs.