Abstract The genesis of the Bayan Obo giant rare earth element (REE) deposit has been debated for several decades. Here, we report the isotopic effects of dynamic recrystallization in the H8 carbonatite, which is the principal ore carrier in the deposit. We studied fresh drill core to a depth of 1.78 km and documented the elemental and C-O-Sr isotope evolution of rock-forming dolomite during its deformation and reaction with fluids. The precursor dolomite and the products of its recrystallization differ in δ13CVienna-PeeDee Belemnite (V-PDB) (–1.09 to 2.37 vs. –3.59 to 0.79‰, respectively) and 87Sr/86Sr (0.70241–0.70394 vs. 0.70288–0.71409, respectively), and show a similar δ18Ovienna-standard mean ocean water (V-SMOW) range (10.3–16.9‰). The strong negative shift in δ13CV-PDB indicates that, locally, there was as much as 40% loss of CO2 from the precursor dolomite, although most of the recrystallized dolomite experienced decarbonation on a smaller scale. Clumped monazite grains associated with apatite in paragenetically similar samples yielded variable in situ Th-Pb dates (980–340 Ma), whereas those in monomineralic veinlets give a consistent age of ~400 Ma and consistent initial Nd isotope ratios. This indicates that the wide range of dates may not represent real REE depositional events and that the primary REE minerals deposited in the Mesoproterozoic underwent isotopic reequilibration and REE remobilization in the mid-Paleozoic. Recrystallization and decarbonation of dolomite in the H8 unit were facilitated by its reaction with subduction-derived silica- and halogen-rich fluid, genetically linked to plate-convergence processes along the northern margin of the North China craton, and did not require an influx of REEs from an external source.
|Number of pages||12|
|Publication status||Published - 17 Mar 2022|
Bibliographical noteFunding Information:
We thank A.E. Williams-Jones, Charles Beard, Zengqian Hou, and an anonymous referee for their constructive comments and suggestions to improve the manuscript. This research was financially supported by the National Natural Science Foundation of China (41825008, 92162219) and the Guangxi Natural Science Foundation (2020GXNSFGA297003). ARC acknowledges support from the Natural Sciences and Engineering Research Council (Canada). MS acknowledges support from the UK-RI Natural Environment Research Council grant NE/V008935/1. JK was supported by the Czech Science Foundation GACR EXPRO (grant number 19-29124X).
© 2022 Gold Open Access.
- Rare Earth Elements
- Ore Deposit
- Bayan Obo REE-Nb-Fe deposit