Multistage ore formation in the world's largest REE-Nb-Fe deposit of Bayan Obo, North China Craton: New insights and implications

The Bayan Obo deposit in the northern margin of the North China Craton is a world-class REE-Nb-Fe deposit, the complex mineralization history of which remains unresolved. In this study, we employ SEM and Image J software combined with mineralogical data obtained by in-situ EDS and EPMA analyses to analyze petrographic images rapidly and to investigate the mineralogical assemblages of each type of rocks/ores from the Main Orebody in an attempt to understand the mineralization process and ore-formation mechanism of this deposit. We identify three metallogenic periods and six ore-forming stages based on the field occurrence of ores combined with published geochronological data. The Mesoproterozoic magmatic event comprises two stages, which were distinguished as the coarse-grained dolomite stage (stage 1) and the fine-grained dolomite stage (stage 2). Three stages were recognized in the Mesoproterozoic shear deformation-hydrothermal mineralization period, including the disseminated mineralization stage (stage 3), the banded mineralization stage (stage 4) and the massive mineralization stage (stage 5). The Paleozoic hydrothermal period involved the vein mineralization stage (stage 6). Based on detailed studies of the mineral assemblages and paragenesis, as well as the complex textures of the ores, we propose a model of the multi-stage metallogenic process as follows: (1) The REE minerals and rutile occurring as inclusions wrapped within magmatic idiomorphic dolomite grains indicate that the REE-Nb mineralization might have started during the magmatic period. (2) The Nb minerals, such as fergusonite and ilmenorutile, intergrown with REE minerals, such as monazite and bastna center dot site, indicate that the Nb and REE were transported by the same ore-forming magma or fluid and precipitated during the same metallogenic process. (3) From the fine-grained dolomite stage to the massive mineralization stage (from stage 2 to stage 5), the mineral assemblages become more complex, with a gradual increase in the degree of hydrothermal alteration suggesting that more hydrothermal fluid was involved in the ore mineralization. (4) The complex mineralogical assemblages and textural relationships indicate multiple formation mechanisms for the REE-Nb-Fe minerals. We propose that the unusually large volume of REE-Nb-Fe resources in the Bayan Obo deposit was the result of combined magma and hydrothermal fluid activities in the Mesoproterozoic (from stage 1 to stage 5), whereas the fluids only caused reactivation of the ore materials previously deposited without the addition of exogenous materials in the Paleozoic stage (stage 6).