The newly constrained carbonatite magmatic system of the world’s largest Bayan Obo REE deposit, China

The world’s largest rare earth element (REEs) deposit at Bayan Obo, northern North China craton (NCC), is geologically complex and its genesis is still debated due to the lack of geophysical constraints at depth. In this abstract, we newly discover two Mesozoic thrust faults and trace ore-bearing rocks by magnetotelluric (MT) method to constrain a carbonatite magmatic system. The Haoqin-North Jianshan thrust fault (F₁) transported Boluotou-East-Main-West-Dongjielegele carbonatites (Ca1) and Paleoproterozoic metasediments over Paleozoic sediments, and it correlated the Shuiyuantou klippe, constraining a displacement of ~13 km. The Boluotou-Donjielegele-south West Pits thrust fault (F₂) transported the carbonatite (Ca2), which was intruded by Permian granite, and gneiss in the hanging wall from Kulue, 30 km south of Bayan Obo, evidenced by similar four left-step en echelon high aeromagnetic anomalies (200-1000 nT). By restoring these faults, the Ca1 and Ca2 carbonatites situated above low resistivity zones (<500 Ω·m) at ~13 km south of the Bayan Obo deposit and Kulue, respectively. Based on low resistivity from 60-70 km in depth, high aeromagnetic anomalies and mantle-derived Mg, Fe and O isotopes of carbonatites, a carbonatite magmatic system consisting of a magma chamber and sill complex near the lithosphere-asthenosphere boundary (LAB) and two conduits, pluming magma upward to generate the Ca1 and Ca2 carbonatites is established. Our MT result is first a geophysical constraint on the igneous processes of the giant REE deposit from the LAB.

In summary, a REE-bearing carbonatite magmatic system consisting of a magma chamber, sill complex and two conduits has been established . This system can compare to the active carbonatite magmatic system in South Africa and volcanic system in Hawai’i . It also constrains the magma of the world’s largest Bayan Obo deposit originating from deeper level at the LAB, rather than the Moho.