Mesoproterozoic carbonatitic magmatism in the Bayan Obo deposit, Inner Mongolia, North China: Constraints for the mechanism of super accumulation of rare earth elements

The Bayan Obo mine in North China contains the largest rare-earth element (REE) resources in the world. The mechanism of giant REE enrichment in such a restricted region has been the focus of several studies. Carbonatites are known to contain high concentrations of REE. Three types of carbonatite dykes occur around the Bayan Obo deposit, including dolomite, calcite and calcite-dolomite carbonatite varieties. The contact relations show that the intrusion of calcite carbonatite dykes post-date the dolomite dykes during the process of evolution of the carbonatite magma. The geochemical data show that the content of Sr and light (L) REE in these dykes gradually increases from dolomite [(La/Yb)N values range from 1.6 to 3.8], through calcite-dolomite [(La/Yb)N ranging from 51 to 57], to calcite type [(La/Yb)N ranging from 85 to 4617]. This evolutionary trend suggests the crystal fractionation of the carbonatite magma, which might have played a critical role for the giant REE accumulation in the Bayan Obo region. The εNd(t) values of the carbonatitic dykes show a tight cluster between − 0.47 and 0.65, whereas the initial Sr isotope values show a broad range from 0.703167 to 0.708871. The massive ore-hosting dolomite marbles show comparable element content and Nd isotope composition, and a Sm–Nd isochron age similar to that of the carbonatite dykes, implying a close relationship during their magmatic origin. This interpretation is also supported by the intrusive contact between the ore-hosting dolomite marble and Mesoproterozoic Bayan Obo group, as well as the presence of wall-rock xenoliths in the dolomite marble.