『Abstract
　Trace element and isotopic compositions of carbonate from ore bodies, country rock which hosts the ore bodies (H8 dolomite), a carbonatite dyke exposed in Dulahala near Bayan Obo, and rare earth element (REE)-rich dolomite in Bayan Obo have been determined to understand the genesis of the Bayan Obo Fe-Nb-REE ore deposit, the world's largest resource of REE. The REE and trace element distribution patterns of samples from the REE-rich carbonatite dykes are identical to those of mineralized carbonate rocks, indicating a genetic linkage between the REE-rich carbonatite and mineralization in this region. By contrast, carbon and oxygen isotopes in the mineralized carbonate varied significantly, δ¹³C = -7.98‰ to -1.12‰, δ¹⁸O = 8.60-25.69‰, which are distinctively different from those in mantle-derived carbonatite. abnormal isotopic fractionations between dolomite and calcite suggest that these two minerals are in disequilibrium in the carbonatite dyke, ore bodies, and H8 marble from Bayan Obo. This isotopic characteristic is also found in mineralized sedimentary marine micrite from Heinaobao, 〜25 km southeast of the Bayan Obo Fe-Nb-REE ore deposit. These facts imply that the carbonate minerals in the Bayan Obo deposit have resulted from sedimentary carbonate rocks being metasomatised by mantle-derived fluids, likely derived from a REE-enriched carbonatitic magma. The initial Nd isotope values of ore bodies and carbonatite dykes are identical, indicating that ore bodies, carbonatite dykes and veins may have a similar REE source.』

1. Introduction
2. Geological setting and sampling
　2.1. Regional geology and sampling
　2.2. Age of Bayan Obo REE deposits
3. Analytical procedures
4. Results and discussion
　4.1. Trace elements and REE geochemistry
　4.2. Carbon and oxygen isotopes
　　4.2.1. Micrite
　　4.2.2. Carbonate rocks in the Bayan Obo ore deposits
　4.3. Models of ore genesis at Bayan Obo
　　4.3.1. Previous genetic models
　　4.3.2. A new model based on metasomatism
5. Conclusions
Acknowledgments
References