Fluid unmixing/immiscibility as an ore-forming process in the giant REE-Nb-Fe deposit,Inner Mongolian,China:Evidence from fluid inclusions

摘要

The giant Bayan Obo REE-Nb-Fe deposit consists of replacement bodies hosted in dolomite marble made up of magnetite,REE fluorocarbonates,fluorite,aegirine,amphibole,calcite and barite.Two or three phase CO_2-rich,three phase hypersaline liquid-vapor-solid,and two phase liquid-rich inclusions have been recognized in mineralized fluorite and quartz samples.Microthermometry measurements indicate that the carbonic phase in CO_2-rich inclusions is nearly pure CO_2.Fluids involving in REE-Nb-Fe mineralization at Bayan Obo might be mainly of H_2O-CO_2-NaCl-(F-REE)system.Coexistences of brine inclusions and CO_2-rich inclusions with similar homogenization temperatures give evidence that immiscibility happened during REE mineralization.An unmixing of an original H_2O-CO_2-NaCl fluid probably derived from carbonatitic magma.The presence of REE-carbonates as an abundant solid in fluid inclusions 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.