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.

中文翻译：

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白云鄂博稀土元素矿床重结晶脱碳成矿

摘要 白云鄂博巨型稀土（REE）矿床的成因争论了几十年。在这里，我们报告了 H8 碳酸岩中动态再结晶的同位素效应，它是矿床中的主要矿石载体。我们研究了 1.78 km 深度的新鲜钻芯，并记录了造岩白云岩在变形和与流体反应期间的元素和 CO-Sr 同位素演化。前驱体白云石及其再结晶产物在 δ13CVienna-PeeDee Belemnite (V-PDB)（分别为–1.09 至 2.37 对 –3.59 至 0.79‰）和 87Sr/86Sr（分别为 0.70241–0.70394 对 0.70288–0.71409）和)，并显示出类似的 δ18Ovienna 标准平均海水 (V-SMOW) 范围 (10.3–16.9‰)。δ13​​CVPDB 的强烈负移表明，在局部，尽管大部分重结晶白云石经历了较小规模的脱碳，但前体白云石的 CO2 损失高达 40%。在共生相似的样品中，与磷灰石相关的丛生独居石颗粒产生了可变的原位 Th-Pb 日期（980-340 Ma），而单矿物细脉中的那些则给出了一致的约 400 Ma 的年龄和一致的初始 Nd 同位素比。这表明，大范围的日期可能并不代表真正的稀土沉积事件，中元古代沉积的主要稀土矿物在中古生代经历了同位素再平衡和稀土再活化。H8 单元中白云石的重结晶和脱碳通过与俯冲衍生的富含二氧化硅和卤素的流体的反应而得到促进，