Dolomite marble and K-rich slate are two main sorts of rocks that host the Fe-Nb-REE orebodies of the Bayan Obo deposit in Inner Mongolia. Nevertheless, their protolith rocks and petrogenesis remain problems of much controversy. With 18 microphotographs, the authors illustrate the autometasomatism of the rocks and, on such a basis, try to demonstrate their petrogenesis. It is held that the protolith of the dolomite marble is magmatic carbonatite and that of the K-rich slate is trachytic volcaniclastic rock or volcanic tuff.