In light of systematic microtexture and chemical composition studies of the dolomite minerals of the ore -bearing dolomites from the Bayan Obo deposit, it has been proposed that the ore -bearing dolomites were originally a carbonatitic sub-volcanics. Most ore bearing fine-grained dolomites are magnesio-carbonatite and ferro-carbonatite; they generally have high FeO, MnO and SrO contents and contrast with the typical sedimentary limestone and dolostone with very low FeO, MnO and SrO contents. The carbonatite magma may rise slowly in crust to fractionate and produce a evolved dolomite carbonatite magma with high alkali, REE and Fe components and high volatile (F, Cl, P, S) contents, which were lost to the stratigraphically upper portion of the Bayan Obo Group and concentrated at the contact zone between the dolomite carbonatite of the footwall and the overlying Bayan Obo Group, thus producing extensive fenitizing aureole consisting broad zoned various ore types rich in Fe, Nb, REE, F and alkali metals in ore bodies. In terms of a clear distinction between the aegirine and Na-amphibole ores in their REE contents and distribution patterns, this indicates that the fluids for forming aegirine ore and Na-amphibole ore may have clear difference in Xco(2) and halogen content. The formation of REE minerals is correlative to the oxidized fluids rich in CO2; the case may be responsible for the formation of ore with higher REE abundance like aegirine ore. If the fluids reducing in Xco(2) and increasing in XH2O, in falling temperature then the Na-amphibole ore may be formed and the case was unavailable for great enrichment of REE. Following the fenitization process the CO2/H2O ratios and oxygen fugacity and halogen content of the fluids may be changed, the case plays an important role in forming various fenitized ore types. Magnetites selected from the ore -bearing dolomite rock having oxygen isotope composition are similar with those of various ore types; this implies that the magnetites of various ore types have an origin consistent with that of the ore -bearing dolomite. The normalized REE patterns for ore -bearing dolomite and fenitizing ore types show strong LREE enrichment and strong HREE depletion indicating that LREE and HREE experienced extreme fractionation; however, there may be some large difference in their LREE contents for some fenitized ore types providing evidence for the greater mobility of LREE under fenitizing conditions. It is worth mentioning that the Main and East ore bodies are located the place where the ore -bearing dolomite is the thickest, the fenitization is the strongest and the Fe, Nb and REE resources are the richest. The case indicates that the fenitizing ore formation occurred at the place where a large amount of fluid escaped from the dolomite carbonatite magma was accumulated and the fluid metasomatism may be strong and widespread.
