Superposed effect of Early Paleozoic alkaline magmatic-hydrothermal activity on the Bayan Obo deposit

• The Bayan Obo Deposit was subjected to superposed reworking during the Early Paleozoic. • Contemporaneous alkaline pegmatites have been identified in the deposit. • Alkaline pegmatites show consistent geochemical characteristics with vein-type ores. • The Early Paleozoic hydrothermal event was related to southward subduction of the PAO. The Bayan Obo Deposit is the world’s largest iron-niobium-rare earth element (REE) deposit. Following Mesoproterozoic mineralization, the deposit was subjected to intense multi-stage superposed mineralization during the Early-Late Paleozoic. Particularly, the Early Paleozoic hydrothermal event made a significant contribution to the mineralization process. However, the source of ore-forming fluids is still unclear. The Early Paleozoic alkaline magmatism contemporaneous with superposed mineralization event has been identified in this study in Bayan Obo deposit. The Zircon U-Pb dating of the riebeckite albitite range from 418 Ma to 420 Ma, and the monazite Th-Pb dating range from 408 Ma to 428 Ma, which are consistent with the metallogenic epoch of vein-type mineralization. Moreover, the eckermannites in riebeckite albitite share high similarity in geochemical characteristics with eckermannites in vein-type ores. This phenomenon provides compelling geochemical evidence for their consistent material sources. The Sr-Nb isotopes of minerals of vein-type ores and riebeckite albitite are also in good agreement. It is inferred that the ore-forming fluid of the vein-type ores in the Bayan Obo deposit was derived from Early Paleozoic alkaline magmatism, which was genetically related to the southward subduction of the Paleo-Asian Ocean in the Early Paleozoic.