Mineralogical and geochemical study of Nb mineralization in the giant Bayan Obo deposit

The Bayan Obo deposit is the world’s largest rare earth (REE) and second-largest niobium (Nb) deposit. It contains more than 70 % of China’s present Nb resources. Most published papers deal with the origin of the H8 ore-hosting carbonatite and its REE mineralization of the deposit. However, few studies focus on the Nb mineralization. The nature and ore-forming process of Nb mineralization, as well as its relationship with REE mineralization, are still poorly constrained. This study conducted petrographic, mineralogical, and geochemical work on representative Nb ores (i.e., primary calcite and dolomite carbonatite, veined calcite and dolomite ores) in the Bayan Obo deposit. The carbonatites are calcite and dolomite carbonatites. Their carbonates and apatites were characterized by high Sr contents and mantle-derived depleted ⁸⁷Sr/⁸⁶Sr_i (0.70272–0.70327) isotopes, confirming the carbonatitic origin. Pyrochlore and columbite are the main Nb minerals in carbonatites, and the former was marginally overgrown by the latter due to post-magmatic alterations. U-Pb dating suggests that the columbites were formed at 1321 ± 53 Ma, consistent with the reported carbonatite intrusion age. In veined calcite ores, aeschynite is the dominant Nb mineral, associated with bastnäsite, apatite, aegirine, and barite, and occurs as veinlets that infiltrate calcites. In veined dolomite ores, columbite aggregates occur along the edge of the monazite-fluorite veins. Various Nb (rutile-Nb, columbite, baotite, fersmite, etc.) and REE minerals intergrow with each other, occur as biotite-dominated veinlets or patches overprinted on the dolomites. These mineralogical textures of veined ores indicate the obvious hydrothermal origin of Nb mineralization. Based on the above results, we suggest that primary Nb mineralization at the Bayan Obo originated from the Mesoproterozoic carbonatite magmatism. Later, Nb was remobilized and transported via external F-bearing hydrothermal fluid alteration, and subsequently precipitated due to an ore-forming fluid-rock reaction, leading to the present complex Nb mineralization. The Nb mineralization was usually associated with REE mineralization, and the different nature of the metasomatic fluids could explain their decoupling.