EVALUATION OF THE ROLE OF CALCIUM IN THE FORMATION OF REE MINERALIZATION UNDER HYDROTHERMAL CONDITIONS BASED ON THERMODYNAMIC MODELING DATA
G.P. Shironosova1, V.O. Goryunova1,2, I.R. Prokopyev1,2,3
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia
sup>2Novosibirsk State University, Novosibirsk, Russia
3Tuvinian Institute for Exploration of Natural Resources, Siberian Branch of the Russian Academy of Sciences, Kyzyl, Russia
Keywords: REE, monazite, xenotime, bastnaesite, parisite, REE-fluorapatite, thermodynamic modeling
Abstract
Thermodynamic calculations of the influence of Ca on the formation of rare-earth mineralization during cooling of hydrothermal fluid from 500 to 100°C, which affected the association of monazite with a variable amount of calcite, were performed for the first time for the entire series of lanthanides + Y. It was found that both an increase in the calcite introduced into the system and an increase in the pH of the equilibrium fluid lead to noticeable changes in the equilibrium mineral association. An increase in the initial amount of calcite is accompanied by an increase in the stability of parisite and REE-containing fluorite. The transition from acidic to near-neutral conditions expands the range of parisite formation with a simultaneous decrease in the amount of equilibrium bastnasite. In acidic fluids (pH 3 and 4), REE-containing fluorite is formed under low-temperature conditions, whereas at pH=6.6 it can be stable, on the contrary, in the range of 400 - 500°C. In near-neutral conditions, residual unspent calcite appears in the equilibrium association. In acidic conditions, with an increase in calcite introduced into the system, the calcium concentration in the equilibrium fluid also increases with a simultaneous increase in the total equilibrium concentration of lanthanides in it. This means that calcium-enriched acidic fluids can promote the removal of REE and, conversely, calcium-depleted fluids can promote the precipitation of lanthanides, as does an increase in the pH of the ore-forming environment
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