Mineralization of the Bayan Obo Rare Earth Element Deposit by Recrystallization and Decarbonation

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 δ13CVPDB 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.

A natural calcite reference material for microbeam Sr isotope analysis

“…9–12 Compared with the solution nebulizer (SN)-MC-ICP-MS technique which requires complex and time-consuming sample preparation, the microbeam analytical technique, notably the laser ablation (LA)-MC-ICP-MS with high sensitivity and spatial resolution, has been widely used to obtain the grain- or subgrain-scale Sr isotopic ratios. 9,12–16…”

“…[9][10][11][12] Compared with the solution nebulizer (SN)-MC-ICP-MS technique which requires complex and time-consuming sample preparation, the microbeam analytical technique, notably the laser ablation (LA)-MC-ICP-MS with high sensitivity and spatial resolution, has been widely used to obtain the grain-or subgrain-scale Sr isotopic ratios. 9,[12][13][14][15][16] In addition to the correction of isobaric interferences (Rb and Kr), 9,[17][18][19][20] doubly charged rare earth elements (Yb 2+ and Er 2+ ) 15,[21][22][23][24] and using a reactive gas in a collision/reaction cell (CRC) to allow Sr + to react but not Rb + , [25][26][27][28] a matrix-matched reference material is essential to reduce element fractionation and obtain high-quality Sr isotopic ratios during in situ Sr isotope analyses. The dri of 87 Sr/ 86 Sr ratios determined from reference values has been observed in previous studies, even though the samples have high Sr contents and low Rb/Sr ratios.…”

An Unusually Warm Upper‐Crust in the Late Paleozoic North China Continental Arc: Implications for the Thermal Modification of the Giant Bayan Obo REE Deposit

“…The other types of ores occur near the contact between the carbonate sills and host slate, which may be related to late hydrothermal alteration and recrystallization of the early REE‐bearing minerals (Figures 11b–11d, 11f, and 11g). Existence of abundant fluorite in the veined or banded ores (e.g., Deng et al., 2017; Fan et al., 2016; S. Liu et al., 2018, 2022; Shimazaki et al., 2008; Wei et al., 2022; X. Y. Yang et al., 2017; P. S. Zhang & Tao, 1986) indicates presence of F in the hydrothermal fluids, which would have enhanced the thermal stability and recrystallization of bastnäsite (Figures 11b, 11c, and 11h).…”

“…260 Ma (e.g., X.‐C. Li et al., 2021, 2022; Ling et al., 2013; Song et al., 2018; Wei et al., 2022; K. F. Yang et al., 2019; Y. H. Yang et al., 2019). This large spread in ages of REE‐bearing minerals resulted in confusion and intense controversy in interpreting the timing, genesis and episodes of REE mineralization in the Bayan Obo deposit (Fan et al., 2016; Ling et al., 2013; Smith et al., 2015; X. Y. Yang et al., 2017).…”

“…1,320 to ca. 260 Ma (e.g., X.-C. Li et al, 2021Li et al, , 2022Ling et al, 2013;Song et al, 2018;Wei et al, 2022;K. F. Yang et al, 2019;Y.…”