Distribution and mineralogical features of niobium in the Bayan Obo deposit: Implications for hydrothermal metasomatic Nb enrichment

The Bayan Obo Fe-REE-Nb deposit hosts the enormous niobium (Nb) resources in China, however, these resources have not yet been industrially exploited, due to its spatial distribution of Nb remaining unclear, occurrence and composition of Nb-bearing minerals lacking systematic studies, and the poorly understood formation mechanisms of Nb-bearing minerals. In this study, 583 samples from the Bayan Obo deposit were collected and analyzed to draw a map of the general spatial distribution of Nb anomalies across diverse orebodies. Using advanced mineral identification and characterization system (AMICS) and electron probe microanalysis (EPMA), we systematically analyzed the compositions of Nb-bearing minerals from the above Nb-enriched areas and then discussed their formation mechanisms. The results indicate that Nb is heterogeneously distributed in the Bayan Obo deposit: in the Main and East orebodies, Nb is mainly concentrated in ilmenorutile, pyrochlore and aeschynite in fluorite- and aegirine-type ores, whereas in the West orebody, Nb is predominantly enriched in columbite in various types of ores along the contact zone between dolomite- and mica-amphibole-type ores. These representative Nb-bearing minerals are consistently associated with alkali- or fluorine-rich gangue minerals, including aegirine, biotite, and fluorite, biotite, apatite and fluorocarbonates, indicating that the formation of ilmenorutile was primarily controlled by alkali-poor and F-rich hydrothermal metasomatism, whereas pyrochlore and aeschynite were mainly formed through F- and Na-rich alkaline hydrothermal metasomatism. In contrast, columbite in West orebody was closely related to biotitization. Overall, the columbite-dominated dolomite and mica-amphibole-type ores in the West orebody are the most favorable targets for the comprehensive utilization of Nb resources