Origin of metasomatic fluids in the Bayan Obo rare-earth-element deposit

Abstract

The Bayan Obo REE–Fe–Nb deposit is the largest light rare earth element (REE) deposit in the world, and it has undergone serious hydrothermal reworking processes. However, the origin and nature of metasomatic fluids in this deposit remain controversial. To determine the origin of metasomatic fluids, we conducted a detailed mineralogical and geochemical investigation on the apatite from the ∼ 1.8-km drill core in the Bayan Obo deposit. The textural and geochemical variations of the apatite could trace the hydrothermal metasomatism during REE mineralization. Four types of apatites were identified according to their cathodoluminescence characteristics, including turbid euhedral apatite (Ap-0), rounded pinkish–violet apatite veinlets (Ap-1), yellowish–green apatite aggregates (Ap-2), and apatite with a pinkish–violet core and a yellowish–green rim (Ap-3). According to the in-situ Sr isotopic compositions, these apatites were classified into two groups. Ap-0, Ap-1, and Ap-2 display constant and low 87Sr/86Sr ratios (0.7030–0.7035), whereas Ap-3 displays various Sr isotopic compositions (0.7047–0.7105). The in-situ Th–Pb age of the bastnäsite associated with the Ap-0 ranges from 390 to 425 Ma. The Sr isotopic compositions and geochronological study indicate that metasomatic fluids in the Bayan Obo deposit were obtained from different sources, including the Silurian Bainaimiao arc plutons and sedimentary materials. The four apatite types display various REE distribution patterns, which could account for hydrothermal fluid properties. F-rich and Na-poor fluids metasomatized the precursor Ap-0, releasing REEs from apatites and resulting in low REE contents of Ap-0. Ap-1 and Ap-2 were derived from the same fluid but were characterized by different trace element compositions, which could be attributed to the evolution of hydrothermal fluids with changes in the Na content and F/Cl ratio. The primary Ap-3 was derived from a REE-rich source and was altered by Na-poor and Cl-rich fluids. Metasomatic fluids with different origins and variable chemical compositions result in complex mineral assemblages and geochemical features in the Bayan Obo deposit.