Carbon and Oxygen Isotopic Characteristics of REE-Fluorocarbonate Minerals and Their Genetic Implications, Bayan Obo Deposit,Inner Mongolia, China

REE-fluorocarbonates as major REE minerals in the Bayan Obo deposit, the largestREE deposit in the world, were analyzed for their stable isotopic compositions. The δ13C andδ18O values of huanghoite, cebaite and bastnaesite from late-stage veins vary in the ranges of -7. 8 - - 4. 0‰ and 6. 7 - 9. 4‰ respectively. These data are relatively similar tO those ofbastnaesites from banded ores: δ13C - 5. 6 - - 5. 2‰ and δ18O 3. 6 - 5. 5‰ The REEfluorocarbonates from both late-stage veins and banded ores are characterized by lower δ13C andδ18O values, especially the δ18O values of bastnaesites from banded ores. Compared with them,the disseminated bastnaesits the dolomite-type ores possess rather high δ13C and δ18O values, i.e., - 2. 1 - - O. 4‰ and 8. 6 - 12. 9‰　respectively. The high values are typical of thesedimentary host dolomite rocks as well as of the dolomite-type ores. The carbon and oxygenisotopic characteristics of REE fluorocarbonate minerals provide new evidence for the hypothesison the origin of Bayan Obo deposit-epigenetic hydrothermal metasomatism.