The Th-Pb isotopic dating of bastnaesite from the Bayan Obo Nb-Fe-REE deposit and its implication

<p indent="0mm"> The Bayan Obo Nb-Fe-REE deposit has undergone a complex ore-forming process. Its initial mineralization time is generally determined to be approximately 1.3 Ga, but there remains significant controversy regarding the reworking ages of rare earth elements orebodies. This study systematically analyzed bastnasite from the deposit using scanning electron microscope, electron microprobe, electron backscattering diffraction, and Th-Pb isotopic analysis to investigate the effect of recrystallization of bastnasite on its Th-Pb isotopic dating system. Bastnasite grains from different locations within the same thin section or different parts of the same bastnasite grain exhibit three distinct crystallographic orientations, indicating that bastnasite may have undergone recrystallization after their formation. The ages of bastnasite with the same crystallographic orientation range from <sc>396 Ma</sc> to <sc>628 Ma</sc> and have positive correlations with ²³⁸U and Pb contents, suggesting that the superposition on the bastnasite by U-bearing fluids may lead to the local disturbance of its Th-Pb isotopic system. However, during the recrystallization of bastnasite, their contents of rare earth elements remained relatively stable, indicating that there is limited contribution of rare earth elements to these bastnasites in their reworking process. By combining the crystallographic orientation characterization with the isotopic analysis, this study provides an important methodological reference for the precise isotopic dating of other minerals.