In this study, XRF and ICP-MS methods were used to analyze major and trace elements of dolomitite and surrounding slate combined with field investigation and indoor observation. Sulfur isotopic compositions were also conducted on the samples. The results of element analyses show that the dolomitic rock in the Bayan Obo Fe-REE-Nb formation exhibits geochemical characteristics of both igneous carbonate and sedimentary carbonate, indicating that the dolomite is neither typical igneous carbonatic rock nor sedimentary carbonatic rock. Sulfur isotopic analysis show that S isotopic compositions of whole rock is characterized by normal distribution model with two distinct peaks: one at around 0‰ indicating a deep-mantle source and the other around +8‰ higher than that of mantle-derived sulfur. The results suggest two possible origins: mantle-derived and sedimentary. Niobium in ore-bearing dolomite was enriched as REE mineralization took place, but Ta enrichment was weak. This suggests that ore-forming fluid was geochemically characterized by enrichment of REE and Nb and depletion of Ta. Our element and sulfur results demonstrate that the ore-bearing dolomitic rock was the product of metasomatism of sedimentary carbonate by mantle-derived carbonatic magma but not the product of direct eruption of igneous carbonatic magma. Deep mantle-derived carbonatic magma and its REE-enriched fluid migrated along regional deep fault and metasomatised with sedimentary carbonate, resulting in the formation of the unique, giant Fe-REE-Nb deposit in Bayan Obo.