Carbonatite-related delicate REE mineralization processes revealed by fluorocarbonates and monazite: Insights from the giant Bayan Obo REE-Nb-Fe deposit, China

Fluorocarbonates and monazite are common economic REE minerals in carbonatite-related REE deposits, such as the largest known REE deposit at Bayan Obo, China. The geochemical evolution of REE minerals is a sensitive and effective indicator of delicate REE mineralization processes. In this study, the fluorocarbonates and mona- zites were classified into three generations, corresponding to three REE mineralizing stages. The first generation (Gen-1) of bastna center dot site and monazite are generally characterized by elevated La/Ce, La/Nd and La/Lu ratios, with right-declining chondrite normalized REE distribution patterns. They have low Th and radiogenic Sr content, with forming age of about 1.3 Ga and epsilon Nd(t) around 0, indicating a primary origin and orthomagmatic crys- tallization from carbonatite melts. The second generation (Gen-2) of REE minerals, formed at 1.25-1.2 Ga, normally constitute banded aggregates with associated hydrothermal gangue minerals (fluorite, aegirine, sodium amphibole etc.). The Gen-2 REE minerals are featured by moderate-low La/Ce, La/Nd, La/Lu ratios but slightly elevated87Sr/86Sr0 ratios, indicating a Mesoproterozoic hydrothermal origin related to carbonatitic fluids. Gen-2 bastna center dot site and monazite precipitated earlier in low-fluid/rock-ratio environment with right-declining LREE distribution patterns, while the subsequent Gen-2 parisite and huanghoite display convex LREE patterns and higher OH/F ratios, indicative of relative La-depletion and increased ambient fluid/rock ratios during progres- sive hydrothermal REE mineralization. The third generation (Gen-3) of REE minerals are generally megacrysts in the vein-type ores formed in the early Paleozoic REE mineralization stage (470-420 Ma). The Gen-3 REE min- erals are typified by significantly variable REE, Sr and Nd isotopic components, indicating extensive hydro- thermal metasomatism and intense contamination of crustal material. Induced by activities of early Paleozoic subduction-related fluids, Gen-3 REE minerals were formed through local REE re-enrichment within the ore - hosting dolomite, rather than by introduction of REE from external sources. In this case study, a comprehen- sive model of the delicate REE mineralization processes of the giant Bayan Obo REE-Nb-Fe deposit has been established. This study highlights the great potential of fluorocarbonates and monazites widely distributed in carbonatite-related REE deposits as universal geochemical indicators of late-stage evolution of carbonatite and delicate magmatic-hydrothermal REE mineralization processes.