The Bayan Obo (China) giant REE accumulation conundrum elucidated by intense magmatic differentiation of carbonatite

The Bayan Obo deposit in China is endowed with the largest rare earth element (REE) resource in the world. The mechanism resulting in this REE enrichment has been the focus of many studies. Carbonatite is known globally as the most favorable carrier of REE ores. In the Bayan Obo deposit, REE ores are hosted in dolomites (including coarse-grained and fine-grained varieties), and many carbonatite dikes (ferroan, magnesian, and calcic) have been identified. All of the dolomites and carbonatite dikes appear to be broadly coeval and possess similar geochemical characteristics. The Sm-Nd isochron age of apatite (1317 +/- 140 Ma) from coarse-grained dolomite is consistent with the Th-Pb age of monazite (1321 +/- 14 Ma) from a calciocarbonatite dike. The epsilon(Nd(t)) values and initial Sr-87/Sr-86 ratios at 1.3 Ga of apatite from coarse-grained dolomite show a tight cluster between -2.5 and +1.0 and between 0.70266 and 0.70293, respectively. The delta O-18(VSMOW) values (relative to Vienna standard mean ocean water) of apatite also vary narrowly from 5.0 parts per thousand to 6.2 parts per thousand. These results are consistent with primary mantle-derived carbonatite and prove a magmatic origin for the ore-hosting dolomite. Furthermore, the rim and core texture of dolomite and calcite in the magnesian and calcic carbonatite dikes shows that carbonatite at Bayan Obo has an evolutionary sequence from ferroan through magnesian to calcic in nature. There is a clear negative correlation between the iron content and REE concentration in different stages of carbonatite. Intense magmatic differentiation of carbonatite is likely the critical factor for the giant REE accumulation.