The formation of the giant Bayan Obo REE-Nb-Fe deposit, North China, Mesoproterozoic carbonatite and overprinted Paleozoic dolomitization

The Bayan Obo ore deposit in Inner Mongolia, North China, the largest-known rare earth element (REE) deposit in the world, is closely associated with carbonatite dykes. Scarce zircon grains, with a wide range of ages and diverse origins, have been extracted from the Wu dyke, a REE-enriched calcitic carbonatite dyke 2 km from the East Ore Body of the Bayan Obo deposit. Three zircon populations were identified based on ages and trace element compositions: 1) Captured zircons with Paleoproterozoic and Archean ages. These zircons have REE patterns and moderate Th/U ratios similar to zircon with silicate inclusions from basement igneous rocks, which have been recognized as contaminants from wall rocks. 2) Carbonatite magmatic zircons with Mesoproterozoic ages. These zircons have high to extremely high Th/U ratios (13–1600), a characteristic signature of the Bayan Obo deposit. Two zircon grains yielded concordant ²⁰⁶Pb/²³⁸U ages (1.27 ± 0.11 Ga ∼ 1.42 ± 0.18 Ga) and ²⁰⁸Pb/²³²Th age (1.26 ± 0.20 Ga) with calcite inclusions, indicating that the Wu dyke was emplaced at ca. 1.34 Ga, which coincides with a worldwide generation of Mesoproterozoic kimberlites, lamprophyres, carbonatites, and anorogenic magmatism. 3) Hydrothermal zircons with Caledonian and Triassic ages. The Caledonian zircon has ²⁰⁶Pb/²³⁸U age of 381 ± 4 Ma and ²⁰⁸Pb/²³²Th age of 367 ± 14 Ma with dolomite inclusion. These evidences are consistent with multiple stages of mineralization, Mesoproterozoic calcite carbonatite magmatism interacted by protracted fluxing of subduction-released Caledonian fluids during the closure of the Palaeo-Asian Ocean, coupled with interaction with the mantle wedge and metasomatism of overlying sedimentary carbonate.