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Rare earth element (REE) fluorcarbonates are found for the first time in the ore-hosting sequence of the Saf’yanovka Cu–Zn massive sulfide deposit, Middle Urals: bastnäsite, synchysite-(Ce), and synchysite-(Y) in an assemblage with fluorite, calcite, barite, pyrite, and sphalerite. Samples with fluorcarbonates were found at a depth of 262 m in quartz veinlets of an altered zone of pyrite–carbonate–kaolinite–sericite–quartz rocks at the contact with massive pyrite–chalcopyrite and pyrite–sphalerite ores. It is suggested that the REE fluorcarbonates formed from a hydrothermal fluid synchronously with fluorite and calcite. This event can be related to hydrothermal activity at the postcollision stage of the evolution of the Urals fold system. [ABSTRACT FROM AUTHOR]

Thermodynamic calculations on the impact of carbonate–bicarbonate fluids cooling from 500 to 100°C on a monazite + calcite association with a simultaneous decrease in pressure from 2000 to 125 bar are performed. It is shown that the REE carbonate–bicarbonate complexes gain importance only at low temperatures. In this case, the total concentration of lanthanides in the fluid turns out to be rather low. This indicates that, in the presence of even insignificant amounts of fluorine and calcium in the hydrothermal system, carbonate–bicarbonate fluids do not lead to the removal of REEs but contribute to their accumulation in the form of precipitating fluorocarbonates. Thus, the role of carbonate-bicarbonate hydrothermal fluids in the transport and deposition of rare earth elements has been assessed for the first time for a wide range of TP-parameters and for the entire series of lanthanides. [ABSTRACT FROM AUTHOR]