白云鄂博稀土成矿精细过程与富集机理

Bayan Obo REE-Nb-Fe deposit is the currently largest REE deposit worldwide. There has been over 30-years history of research about its metallogenesis, especially geochronology and petrogenesis of the ore-hosting dolomite. However, influenced by continental rift and plate subduction, the Bayan Obo deposit suffered complicated multi-stage magmatic-hydrothermal process. In recent years, some controversial questions have emerged, mainly about detailed ore-forming process and dominating ore-forming factors. And specifically, obscure topics include the origin of ore-forming fluids, detailed magmatic-hydrothermal-related ore-forming process, and REE enrichment mechanism. REE-bearing minerals are able to record REE mineralization process. However, they have not been systematically applied as ore-forming indicators in case of analytical limitation. This project targets Bayan Obo REE-bearing minerals, with application of in-situ geochemical analyses and thermodynamic calculations, aiming to conduct systematic research about petrography, composition-provenance evolution, and precipitating mechanism of REE-bearing minerals. According to in-situ element geochemistry based on petrography, the composition evolution of multi-stage REE-bearing minerals would be studied. The evolving origin of ore-forming fluids and provenance of REE would be further explored through in-situ Sr-Nd-Pb-O isotope of REE-bearing minerals. And with preliminary thermodynamic calculations, the precipitation of REE-bearing minerals and corresponding physico-chemical conditions would be discussed. Based on above research, this project could provide insights into critical ore-forming factors, detailed ore-forming process and REE behavior in hydrothermal system of Bayan Obo REE-Nb-Fe deposit, meanwhile summarizing metallogenic model for carbonatite-related REE deposits that had complex magmatic-hydrothermal-tectonic history.

白云鄂博REE-Nb-Fe矿床是迄今世界最大的稀土矿床，具有悠久的矿床成因研究历史，其成矿年代和赋矿白云岩成因已具有成熟的研究基础。然而该矿床经历过复杂的岩浆-热液活动、成矿先后受到大陆裂谷和俯冲带的影响。随着研究持续深入，近年在其成矿过程和成矿作用等科学问题上涌现争议，具体包括主要的成矿流体来源、岩浆-热液相关的稀土矿化精细过程、及稀土元素的富集机理。稀土矿物可以直接记录稀土的矿化过程，受到分析技术的局限一直未充分发挥其指示作用。项目拟以白云鄂博稀土矿物为研究对象，通过基于岩相学的原位微区元素和同位素（Sr-Nd-Pb-O）成分分析技术，研究多期次稀土矿物和成矿流体的成分-物源演化；初步应用地球化学热力学模拟稀土矿物的沉淀过程，约束成矿的物理化学条件。为深入了解白云鄂博稀土矿化过程、评估裂谷和俯冲作用对成矿的相对作用、探索复杂岩浆-热液-构造历史下的碳酸岩型稀土成矿模式提供新思路。

白云鄂博REE-Nb-Fe矿床是典型的碳酸岩相关稀土矿床，关于其成矿过程和成矿模式的研究将有力拓展对碳酸岩型稀土矿床的成矿理论认识。本项目以白云鄂博氟碳酸盐和独居石为研究对象，通过系统岩相学观察、原位主微量成分分析、原位Rb-Sr、Sm-Nd同位素体系成分分析、独居石Th-Pb年代学研究等，证实白云鄂博的主要稀土矿物（氟碳酸盐、氟碳钙铈矿、黄河矿-氟碳铈钡矿、独居石）可以分为三类，分别对应稀土矿化的三个阶段，即中元古代碳酸岩岩浆阶段、中元古代碳酸岩分异热液阶段、以及早古生代热液改造阶段。氟碳铈矿通常沉淀于类似于流体通道末端的低流体/岩石比环境，氟碳钙铈矿的沉淀指示更高的流体/岩石比。氟碳铈矿的沉淀总是早于同阶段的钙氟碳酸盐和钡氟碳酸盐。古生代稀土矿化阶段造成稀土元素在赋矿白云岩内部的再富集，无显著外源稀土引入。此外，本研究还应用地球化学热力学模拟计算稀土矿物的沉淀过程，约束稀土矿物沉淀时的物理化学条件，明确温度降低和白云石中和反应促进氟碳铈矿对独居石、以及Ca-氟碳酸盐对氟碳铈矿的替代，且稀土矿物共生组合指示成矿热液应为高于400℃的高温热液。上述研究内容提升对白云鄂博精细稀土成矿过程和稀土富集机理的认知，也证实稀土矿物地球化学和地球化学热力学模拟等技术路径在稀土成矿作用研究的巨大潜力。项目基本按计划完成研究目标和研究内容，也完成预定的学术论文成果目标和人才培养目标。