REE Daughter Minerals Trapped in Fluid Inclusions in the Giant Bayan Obo REE-Nb-Fe Deposit, Inner Mongolia, China

The Bayan Obo REE-Nb-Fe deposit hosts the world's largest known REE resource. The deposit consists of replacement bodies hosted in dolomite marble and of magnetite, REE fluorocarbonates, fluorite aegirine, amphibole, calcite, and barite. Three types of fluid inclusions have been recognized: two-phase aqueous liquid-vapor (L-V), two- to three-phase CO2 (C), and three-phase liquid-vapor-solid (L-V-S) inclusions. Microthermometry measurements indicate that the carbonic phase in C inclusions is nearly pure CO2. During heating experiments, hexagonal or irregular-shaped daughter minerals in L-V-S inclusions complete dissolution at temperatures of 420-480°C and recrystallize again at about 400-320°C. These show that daughter minerals in multiphase inclusions in mineralizing veins were crystallized from trapped fluids, and are real daughter minerals. REE-carbonates, halite, sylvite, barite, calcite, and pyroxene (?) have been identified on the basis of crystal habit (microscopic and SEM) and EDX analysis. By comparison with Raman spectra of reference REE-carbonate mineral crystals, hexagonal or irregular-shaped daughter minerals in L-V-S inclusions might be cebaite and bastnaesite. The presence of REE-carbonates as an abundant solid in the ore-forming veins shows that the original oreforming fluids were very rich in REE, and therefore had the potential to produce economic REE ores at Bayan Obo.