Niobium and heavy rare earth element mineralization in the Bayan Obo deposit, China: Insights from aeschynite mineralogy, geochemistry, and geochronology

The Bayan Obo deposit in China is the largest light rare earth element (LREE) deposit worldwide, and also hosts substantial Nb and heavy REE (HREE) resources. However, the timing and enrichment mechanisms of the Nb and HREE mineralization are poorly constrained. Aeschynite is the most important Nb- and HREE-bearing mineral at Bayan Obo and was investigated in this study to reconstruct the history of Nb and HREE mineralization. In situ Lu-Hf isochron dating of two types of aeschynite yielded consistent ages of 435-420 Ma, which overlap with the Th-Pb isotopic ages of intergrown monazite (438-392 Ma). These results demonstrate that early Paleozoic is a significant episode for Nb and HREE mineralization. The aeschynite has a close spatiotemporal relationship with alkali silicates, suggesting that hydrothermal fluids were naturally rich in alkalis and dominantly exsolved from coeval carbonatitic magmas. Strontium isotopic compositions of aeschynite are relatively enriched and exhibit an increased trend from the type-1 (⁸⁷Sr/⁸⁶Sr = 0.7037-0.7064) to type-2 (⁸⁷Sr/⁸⁶Sr = 0.7072-0.7128), which was likely caused by heterogeneous crustal contamination during the evolution of hydrothermal fluids. Paragenetic aeschynite, calcite, fluorite and bastnäsite reflect that carbonate (CO₃^2-) and fluoride (F^-) anions played a key role for Nb mobilization. The unloading of Nb in aeschynite was usually related to the variation of Ti content in hydrothermal fluids. According to apparent correlation of REE, Th and Ca in aeschynite and developed ilmenite, Nb unloading was influenced by the processes of coupled substitution or ilmenite crystallization. The enrichment of HREE was ascribed to preferential LREE fractionation following gradual temperature decrease of CO₃^2--bearing alkaline hydrothermal fluids. Moreover, earlier crystallized LREE minerals is potential trigger for local elevation of HREE content.