Generally, the activity of various fluids in Earth’s interior has been commonly considered as the fundamental condition for the mineralization and metallogeny. With the advance in Earth sciences, the mantle fluid has been attracted attention for its special function. Recent years a mantle fluid replacement ore deposit in China was discovered by the authors.
    The Bayan Obo in Inner Mongolia, northern China is the world’s largest known rare earth elements(REE) ore deposit. In view of the overall low Sr isotopic ratios of REE minerals, the metallogenetic elements, REE and Nb etc. are thought to be derived from mantle source and a large involvement of older crustal material can be ruled out. The carbon and oxygen within the REE fluocarbonate minerals are composed of mantle C and O; meanwhile, the surphur in the associated surphides belong to the moteorites, according to their isotopic composition. It is suggested that the REE ore-forming solution was not the hydrothermal brine from crustal source, but would be the mantle fluid that has never been documented in the classical works of economic geology and traditional metallogeny. Hence, it might be a unique mantle fluid metasomatic REE ore deposit in the world.
    In this paper, the basic problems on the mantle fluid and related metallogeny have been summarized.