Paleozoic carbonatites controlled rare-earth-elements mineralization at Bayan Obo

The net-zero emission race drives the consumption of rare-earth-elements (REE) at an accelerated rate. With the demand of REE being met by a few giant deposits such as Bayan Obo, decoding controls of ore formation is vital. Yet, after nearly a century of study, the genesis of Bayan Obo remains debated. Here, we demonstrate that two stages of carbonatites emplaced at Bayan Obo at 1320 and 430 million years ago (Ma). The effusive nature of the 1320 Ma carbonatite suppressed brine/fluid exsolution; hence, mineralization at this stage is limited, but its subsequent deformation created an ideal environment for water-rock interaction induced mineralization. The 430 Ma carbonatites exsolved voluminous brine/alkaline fluids rich in REE, with mineralization being promoted as veins and along beddings of the deformed Mesoproterozoic carbonatites via metasomatism. This stage contributed >70% REE. Our study highlights that multistage carbonatites emplaced at one locality could be a controlling factor for REE mineralization and explains the scarcity of giant deposits.