In situ SIMS Pb-Pb dating of aeschynite reveals Early Paleozoic Nb mineralization at Bayan Obo

Niobium (Nb) is a strategically critical metal with geographically concentrated global reserves that are hosted mainly in carbonatite-related deposits. The Bayan Obo deposit in northern China is the world's largest rare earth element (REE) resource, and also represents one of the most significant global Nb reserves. However, the timing and origin of Nb mineralization are still poorly constrained due to the complex multi-stage geological evolution (>1 Gyr) and diversity of Nb minerals in the deposit. Here we present the first detailed in situ SIMS Pb-Pb isotopic dating study of aeschynite, which is the dominant Nb mineral among more than 30 Nb minerals and accounts for >30% of the Nb resource in the Main and East orebodies of the Bayan Obo deposit. Eleven of the 12 samples of various ore types and orebodies yielded consistent Early Paleozoic ages between 402 ± 43 and 449 ± 56 Ma (weighted mean = 429 ± 11 Ma), indicating a regionally widespread Nb mineralization event. Integration of these new aeschynite ages with petrographic observations and previously published geochronological data demonstrates that Nb mineralization in the Main and East orebodies at Bayan Obo was controlled mainly by Early Paleozoic carbonatite-derived hydrothermal fluids. The remaining aeschynite sample yielded an age of 291 ± 55 Ma, which is interpreted to record isotopic resetting related to Permian granitic plutonism rather than a distinct mineralization event.