Numerical modeling of rare earth elements in fluorite: A monitor for REE mineralization in magmatic-hydrothermal systems

Fluorite, a common gangue mineral in magmatic-hydrothermal ore systems and often a major component of rare earth element (REE) deposits, provides valuable insights into melt and fluid evolution as well as mineralization processes. This study investigates REE distributions within fluorite in the Bayan Obo deposit, Inner Mongolia, China, the world’s largest known REE resource. Combining field observations, petrographic analysis, high-resolution geochemistry (TESCAN integrated mineral analysis, cathodoluminescence imaging, and laser ablation inductively coupled-plasma mass spectrometry) with numerical modeling (lattice strain and REE fractionation models), we classify fluorite into five mineralization stages and seven ore types. Stage I fluorite is observed in dolomite-, magnetite-, and aegirine-type ores, and is followed by Stage II fluorite in richterite-biotite type ore. Stage III fluorite is found in fluorite-REE vein-type ore, Stage IV is a late dark fluorite in veins, and Stage V consists of fluorite from quartz-fluorite vein assemblages. Modeling results indicate that REE patterns in fluorite are influenced by fractional crystallization caused by the precipitation of REE minerals simultaneously with fluorite, as well as decreasing temperature and salinity during later fluid evolution. Early-stage fluorite (Stage I) has the highest total REE concentrations, indicating primary REE mineralization associated with magmatic fluid source. The middle stages (II–III) have a decrease in REE in Stage II, and particularly in light REE in Stage III, in response to fractional crystallization and fluid evolution. Late-stage fluorites (IV and V) have low total REE concentrations associated with cooling due to dilution from Si-rich fluid convection, resulting in fluorite±quartz veins without REE mineralization. The results demonstrate the utility of fluorite as a dynamic tracer of magmatic-hydrothermal processes in the Bayan Obo deposit, providing new insights into REE mineralization in other fluorine-rich systems.