Bayan Obo “dolomite” is distributed mainly at the south wing of the Kuangou anticline in the northern margin of the North China Craton. It is not only host to world′s largest rare earth deposit but also parental rock for the large or very large niobium, iron, scandium and other minerals. The “dolomite” is not layered rock due to lacking no obvious bedding and fixed sequence, but a beded dolomite belt with varying sizes. The “dolomite” has the obvious relationship of invasion with surrounding rock, which is reflected by the “dolomite” cutting through quartz sandstone (H4), slate (H5), and granite, and by many dolomite veins intruding the quartz sandstone (H4) and slate (H5). Residual cover phase of quartz sandstone (H4) occurs in the “dolomite”. Meanwhile xenoliths of quartz sandstone and slate were also found in the dolomite, along with strong alteration of country rocks. Biotitization of slate, riebeckitization of quartz sandstone, and alkaline metasomatism are very common in the mining area. The “dolomite” contains a large amount of mineral crystals such as niobium, rare earths, thorium, etc., which are common in igneous rocks. The sulfur, carbon, oxygen, strontium and iron isotope composition have deepsource characteristics. All the features indicate that the dolomite is magmatic carbonate but not sedimentary carbonate.