Translithospheric carbonatite magmatic system of the world’s largest REE deposit, Bayan Obo, China

Critical rare earth elements (REEs) are often sourced from carbonatite worldwide. In some cases, the magmatic system at depth is not well depicted due to a lack of geophysical constraints. The world’s largest REE deposit, Bayan Obo of the North China craton, offers a template for tracing such a system using 2-D electrical resistivity imaging. We first restore modifications triggered by two newly discovered Mesozoic thrust faults. The thrust fault along Boluotou, Dongjielegele, and south West Pit displaced gneiss and the carbonatite, which was intruded by Permian granite, in the hanging wall from Kulue ∼30 km south of Bayan Obo, as evidenced by four similar left-step en echelon high aeromagnetic anomalies. The second Haoqin−North Jianshan fault thrust is another carbonatite at Boluotou, East-Main-West pits, and Dongjielegele and Paleoproterozoic metasediments over Paleozoic sediments, that correlate with the Shuiyuantou klippe to constrain a displacement of ∼14 km. After restoration, both carbonatites are revealed to be situated above two low resistivity zones. We propose a Mesoproterozoic translithospheric carbonatite magmatic system consisting of a frozen zone of partial melt and mantle sill complexes stemming from as deep as near the lithosphere−asthenosphere boundary at 60−70 km depth and two magma pathways, plumbing magma upward across the crust to generate two carbonatites with multiple branches at the surface. Our geophysical constraint on the magmatic system of this giant REE deposit can serve as a blueprint for understanding and exploring other existing and new REE deposits.