This paper is the result of mineral exploration engineering.
[Objective] The formation of the giant Bayan Obo REE deposit is closely related to carbonatitic magmatism. Near the ore district, southwest of West Ore Body, there are carbonate rocks with a certain scale covered by Mesozoic-Cenozoic sediments, the genesis of which can indicate the formation process of the well-known Bayan Obo deposit. [Methods] This contribution demonstrates that these rocks are actually igneous by the detailed study on petrology and mineralogy. [Results] The rocks were little affected by fluid metasomatism and can be divided into dolomite type and calcite type according to their mineral compositions. Bulk rock analysis shows that these carbonatites are rich in Sr (>4940×10^-6), Mn (>2150×10^-6) and Ba (>106×10^-6), and the average REE content are 938×10^-6, much higher than that of sedimentary carbonates. The δ¹³C_V-PDB and δ¹⁸O_V-SMOW of the rock samples are-3.7‰--4.2‰ and 6.7‰-7.7‰, respectively, typical of primary igneous carbonatite. They have relatively homogeneous Sr isotopic compositions (⁸⁷Sr/⁸⁶Sr=0.702815-0.703185), indicating the mantle-derived rocks were contaminated by the crust limitedly. In addition, the mineral chemical features of dolomite, calcite and apatite also indicate an igneous origin. [Conclusions] The identification of these carbonatites is of great significance for the comparative study on the Bayan Obo deposit and tectonic evolution of the area.