Constraints of in situ chemical compositions and Rb-Sr age of biotite on hydrothermal overprinting in the Bayan Obo REE-Nb-Fe deposit, Inner Mongolia, North China

The Bayan Obo REE-Nb-Fe deposit in Northern China is famous for its large reserves of critical metals such as REE and Nb. Previous studies about ore genesis mainly focused on the timing and origin of primary REE and Nb mineralization. However, the timing of deformation and hydrothermal overprinting on different types of primary mineralization was not well constrained. Biotite slate is one of significant wall rocks and has been hydrothermally altered and mineralized. The biotite slate is composed of biotite and calcite, with minor pyrite, magnetite/hematite, ilmenite, and bastnäsite. Biotite exhibits curved bands and is partially replaced by chlorite and muscovite, indicative of post-crystallization ductile deformation and fluid-induced alteration. Biotite belongs to the Mg-rich variety with the crystallization temperatures of 672 °C to 773 °C. It was possibly originally magmatic in origin and was modified by hydrothermal fluids. Biotite has relatively low REE and high Nb and F contents, probably reflecting modification by Nb-F-rich hydrothermal fluids or Nb and F enrichment inherited from magmatic biotite. In situ Rb-Sr dating of biotite from two samples yields Rb-Sr isochron ages of 310 ± 11 Ma and 309.4 ± 9.3 Ma, respectively. Biotite has initial ⁸⁷Sr/⁸⁶Sr ratios between 0.7011 and 0.7054, similar to those of mantle reservoirs. Together with previous dating results, our study provides constraints on the Hercynian hydrothermal activity which likely contributed to Nb mineralization.