Fan and 6 co-authors reported in 2004 that 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. Micro-thermometry 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 Roman 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 ore-forming fluids were very rich in REE, and therefore had the potential to produce economic REE ores at Bayan Obo.
The authors concluded that:
1. SEM-EDX and Raman spectroscopy of rare-earth daughter minerals in fluid inclusions shows that these minerals might be bastnaesite and cebaite. Microthermometry measures show that REE daughter minerals in multi-phase inclusions in mineralizing veins were crystallized from trapped fluids, and are real daughter minerals.
2. The presence of REE-carbonates as an abundant solids in the ore-forming veins shows that the ore-forming fluids were very rich in REE, and therefore had the potential to produce economic REE deposits at Bayan Obo without requiring large-scale convective fluid. Energy-dispersive spectra (EDX) of daughter minerals in opened fluid inclusions. Note the Si from the host quartz and Au from plating. systems and high water-rock ratios typically envisaged for other magmatic-hydrothermal ore-forming environments.
3. Laser Raman spectrometry can be used as a reliable tool to identify unknown daughter minerals hosted in fluid inclusions when combined with mineral shapes and SEM-EDS analyses.
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