The A-A´ profile located at the western margin of the East ore body. The profile is about 500 meters long passing through the dolomite of the footwall into the slate of the hanging wall, the slate is capping on the dolomite. There is a transitional zone between the dolomite and slate, in which the rocks are different from the dolomite and slate and mainly consist of banded ores containing REE minerals, magnetite and fluorite similar with the banded ores of the Main and East ore bodies. The dolomites of the profile are similar to the dolomites from the footwall of the ore bodies except the former has higher Fe content. The carbonatite magma from which the dolomite of A-A´ profile was derived may have a high alkaline, REE and Fe component and high volatile content, during the process of fractional crystallization of the carbonatite magma, these components were further enriched in the evolved residual magma and accumulated to the upper portion of the dolomites of the profile and producing the banded ore and massive thin iron layers rich in Fe, Nb, REE and F, which contact with the covering slates. However, in comparison with the Main and East ore bodies, the Na-alteration had been much weeker at the transitional zone of the profile, in which the aegirine ore and Na-amphibole ore did not occur. In terms of a clear distinction in mineralization scale between the ore bodies and the A-A´ profile, it has been suggested that the REE and Fe mineralization mainly occur at the contact zone between the dolomite and its wall rocks, however, when the dolomite shows a large scale and very rich in REE, Fe, Nb and volatile, the REE and Fe mineralization may reach a very large scale with strong Na-fenitization forming the Main and East ore bodies, however, if the dolomite occurs as small scale or at the margin of a large dolomite body, there would not occur a large scale of mineralization just like the case of A-A´ profile, in which the K and Ba metasomatism developed well in the slate and can be named potassium fenitization.
Wang Kaiyi Zhang Ji′en Hao Meizhen Zan Jingfen Hu Fuyou. Rare earth´elements geochemistry of the ore?bearing dolomite from the Bayan Obo deposit：Constraints on the REE mineralization of the Bayan Obo deposit[J]. Chinese Journal of Geology, 2020, 55(2): 439-458.