ESTIMATING THE CONTRIBUTION OF CALCIUM IN THE FORMATION OF RARE-EARTH MINERALIZATION IN HYDROTHERMAL CONDITIONS ACCORDING TO THERMODYNAMIC MODELING

Thermodynamic calculations have been first carried out for the entire series of lanthanides (+Y) to determine the influence of Ca in the system on the formation of rare earth (REE) mineralization during the cooling of a hydrothermal fluid from 500 to 100 °C, affecting the monazite association with a variable amount of calcite. Results show that increasing the calcite introduced into the system and raising the pH of the solution lead to significant changes in the equilibrium mineral assemblage. Increasing the initial amount of calcite in the system is followed by an increase in the stability of parisite and REE-bearing fluorite. The transition from acidic to near-neutral conditions expands the range of parisite formation with a simultaneous decrease in the amount of equilibrium bastnaesite. In acidic fluids (pH = 3 and 4), REE-bearing fluorite forms under low-temperature conditions, whereas it can be stable in the range of 400–500 °C at pH = 6.6. Under near-neutral conditions, residual unconsumed calcite occurs in the equilibrium assemblage. With an increase in calcite introduced into the system under acidic conditions, the concentration of calcium in the equilibrium fluid also rises, with a simultaneous increase in the total equilibrium concentration of lanthanides in it. In this case, calcium-enriched acidic fluids can contribute to the removal of REE, and, conversely, calcium-depleted fluids provide the deposition of lanthanides, as well as an increase in the pH of the ore-forming environment.