Physicochemical crystallization conditions of Al-F sphene in metasomatic rocks with ore mineralization at the Berezitovoe Deposit

Al-F sphene (grothite) was found in mineralized rocks at the Berezitovoe Deposit in the Russian Far East. The paper is devoted to the mineral assemblages and composition of the mineral and its thermodynamic crystallization conditions. The average Al and F concentrations (p.f.u., microprobe data) in the grothite are 0.45 and 0.42 in sample 1374, 0.32 and 0.32 in sample 1306, and 0.35 and 0.33 in sample 96. Grothite was found in the rocks in association with chlorite, ilmenite (pyrophanite), and magnetite, and this mineral assemblage was obviously overprinted on the primary garnet-biotite assemblages. We estimated the temperature of grothite crystallization at 400–500°C. With regard for available experimental data on the mineral equilibrium between Al-F sphene, fluid, and anorthite, a tool is proposed for evaluating F concentrations in fluids by the equilibrium of Al-F sphene with plagioclase, rutile, and F-bearing aqueous fluid. Our model simulations indicate that the maximum F concentration in fluid during the crystallization of Al-F sphene richest in F at the temperatures and pressures of metasomatic rocks at the Berezitovoe deposit could reach 300–500 mg per kg of the aqueous solution. The level of F concentration in the fluid during the crystallization of Al-F sphene at the deposit is comparable with the F concentration in fluid during the development of greisens and rare-metal pegmatites, but these high F concentrations were reached only during the final evolutionary stages of the deposit.