The genesis of giant fluorite resources in Bayan Obo Fe-Nb-REE deposit, Inner Mongolia of China

• Sm-Nd isotopic dating reveals fluorite ages of 1318 ± 101 Ma and 380 ± 44.2 Ma. • Mantle-derived carbonate magma drove Mesoproterozoic fluorite mineralization. • Middle- to high-temperature fluids (250–350 °C) contributed to fluorite formation. • Ca, Sr, and Nd isotopic analyses link fluorite to mantle sources. • Early Paleozoic tectonics remobilized, but did not form new fluorite. A substantial quantity of fluorite resources are associated with the Bayan Obo Fe-Nb-REE deposit in Inner Mongolia, China, yet comprehensive studies on these resources remain limited. An integrated study combining Sm–Nd isotopic dating, trace element analysis, fluid inclusion microthermometry, and Sr, Nd, and Ca isotopic investigations was conducted on fluorites and paragenesis dolomite from the Bayan Obo deposit. The temperature of fluid inclusions in the fluorite range from 125.0 °C to 294.0 °C, Sm–Nd dating yielded ages of 1318 ± 101 Ma and 380.2 ± 44.2 Ma for the fluorite. The Sr-Nd isotopic signatures of fluorite constrain its primary mineralization to the Mesoproterozoic, with Early Paleozoic hydrothermal activity triggering subsequent remobilization of the ore-forming system ( 87 Sr/ 86 Sr < 0.7035, εNd(t) = − 18.06 ∼ − 18.08). Fluorites of different isotopic ages show similar Ca isotopic compositions, with δ 44/40 Ca ratios ranging from + 1.60 ‰ to + 1.97 ‰ and + 1.54 ‰ to + 1.86 ‰, respectively. Ca-Sr-Nd isotopic analysis suggest that the ore-forming materials of fluorite mainly originate from the mantle- derived carbonatitic magma and the early crystallized H8 dolomite, and it is also possible that a small amout of marine carbonates may be mixed in. In summary, the fluorite resources in the Bayan Obo deposit likely formed in association with the emplacement of carbonatitic magma during the Mesoproterozoic. The moderate- to high-temperature hydrothermal fluorite resources were initially formed during the Mesoproterozoic and subsequently underwent partial recrystallization, without new fluorite formation, during later hydrothermal activity in the Early Paleozoic.