Trachytic rock and associated fenitization in the Bayan Obo ore deposit, Inner Mongolia, China: Evidence for magmatic-hydrothermal mineralization related to a carbonatitic complex

Trachytic rock and its altered rock-fenite-in the Bayan Obo ore district, I nner Mongolia, China, were referred to as slate or feldspar rock before, an d identified by the authors for the first time (in 1992). In the paper the mineral assemblages, structures and textures and petrochemical compositions of the rocks, as well as the electron microprobe analysis of feldspars in the rocks are described. The Sm-Nd isochron age of the trachytic rock is 10 96 +/- 56 Ma, with I-Nd = 0.51100 +/- 4 (2 sigma) and epsilon(Nd)(t) = -4.4 +/- 0.7. Alterations of the trachytic rock, including microclinization, ri ebeckitization, aegirinization and biotitization, and accompanied rare elem ent and REE mineralizations are discussed. Based on the occurrence of the t rachytic rock and associated fenitization it is deduced that the Bayan Obo Fe-Nb-REE ore deposit is genetically related to magmatic-hydrothermal activ ity of an alkali carbonatite complex.