Origin of metasomatic fluids in the Bayan Obo rare-earth-element deposit

•Mineralogy and geochemistry of apatites are studied from a 1776-m-long drill core in Bayan Obo REE deposit.•Apatite can fingerprint the nature and evolution of metasomatic fluids.•Metasomatic fluids were obtained from Bainaimiao arc and sedimentary materials.•Metasomatic fluids show changes in the Na content and F/Cl ratio.