Aqueous-carbonic-REE fluids in the giant Bayan Obo deposit, China: implications for REE mineralization

Abstract

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 made up of magnetite, REE fluorocarbonates, fluorite, aegirine, amphibole, calcite and barite. Three types of fluid inclusions have been recognized: two phase aqueous liquid-apor (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 shape daughter minerals in L-V-S inclusions complete dissolution at temperatures of 420–480°C and re-crystallize 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, the hexagonal or irregular shaped daughter minerals in the 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 are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo.