地幔交代和流体出溶对牦牛坪超大型REE矿床的控制作用

Abstract: Maoniuping REE giant deposit is located in the eastern margin of the Tibet. The genesis of the carbonatite-syenite complex, characteristics of the deposits, evolution of hydrothermal fluids and mechanism of the REE deposit have been systematically studied. However, the controls on REE enrichments are still unclear. Recently, based on the studies on the carbonatites in the world and their potentials for REE formation, two processes could be concluded：(1) metasomatism in carbonatited mantle;(2) dissolution of fluids from carbonatite-syenite complex melts. But detailes also needed to eluminate two processes. According to the studies obtained, B isotopes, noble gases isotopes and LA-ICP-MS for single fluid inclusion examination are employeed for further analysis focusing on the weakness of the previous studies. Also, carbonatite-syenite complex, minerals formed under balance fractional crystallization, gangues minerals and REE minerals will be sampled for multiple analysis. Then, origin of REE in the carbonatite-syenite complex and the process of metasomatism in carbonatited mantle could be speculated. Furthermore, REE tranfering and enrichement during the evolution from magma to hydrothermal fluids could be traced. Finally, based on all data and results obtained before with regards to the formation of Maoniuping REE deposit, a synthesis of mineralization styles with an integrated genetic model of carbonatite-syenite-hosted REE deposits in theMaoniuping doposit, the eastern Tibetan Plateau, southwestern China could be set up.

位于青藏高原东部的牦牛坪矿床是世界上最重要的超大型碳酸岩型REE矿床之一。前人对该矿床的碳酸岩-正长岩杂岩成因、矿床地质、流体演化及成矿等进行了系统研究，取得重要进展，但REE超常富集机制仍不清楚。我们最近对全球岩浆碳酸岩地球化学及含矿性的对比发现，牦牛坪稀土矿床的REE超常富集可能与两个关键过程相关: (1)岩浆源区地幔交代富集过程；(2)岩浆系统流体出溶过程，但详细过程尚未得到充分阐释。本项目将在前人研究基础上，针对以往研究的薄弱环节，拟采用B同位素和惰性气体同位素、单个流体包裹体LA-ICPMS原位直测技术等，选择牦牛坪矿区碳酸岩-正长岩杂岩体及平衡结晶矿物、岩浆-热液演化各阶段的脉石和矿石矿物，开展系统的多种同位素联测和化学成分原位分析，深入探索成矿碳酸岩REE来源与地幔源区交代富集特征，追踪岩浆流体出溶过程中REE元素迁移-富集过程及控制因素，为构建REE成矿模型奠定坚实基础。

本项目对牦牛坪超大型稀土矿床进行了系统研究。矿床中脉石矿物和矿石矿物的Sr-Nd-Pb同位素组成显示碳酸岩不仅可以来自于地幔柱，还可以形成于富CO2和REE的板片流体交代的富集岩石圈地幔。这些含矿碳酸岩还需要经历热液阶段才能形成高品位的稀土矿床。基于牦牛坪矿床1:5000的矿化-蚀变-构造填图在此基础之上，采用了多种研究手段相结合的办法，发现了牦牛坪矿床“三层楼”式的脉状矿化，并进一步明确了成矿流体的演化过程和REE的沉淀富集机制。针对成矿流体的研究，首先基于野外地质和矿物中流体包裹体岩相学特征划分了三个完整的热液演化阶段；其次，综合运用ICP-MS、IC、QMS，对不同阶段典型单矿物中群体包裹体液相和气相成分进行定量分析重塑了成矿流体的演化过程：F-、Cl-和SO42-在早阶段成矿流体中(~480 ℃，pH< 3.5)均能起到对REE络合迁移作用，其中以Cl络合物占主导，而在中阶段流体自然冷却(~350 ℃)和pH值增加(>3.5)，F-以沉淀REE为主，到晚阶段流体进一步冷却和大气降水混合(流体温度<260℃)促使Cl、SO42-的REE络合物不稳定和脉石矿物(萤石和重晶石)沉淀，致使REE大规模沉淀。根据牦牛坪矿床和冕宁-德昌REE矿带的研究发现：(1)大型和超大型矿床有伟晶岩阶段，并和成矿规模正相关；(2)成矿物质主要来自于碳酸岩和正长岩，并未受围岩混染；(3)脉状矿化系统中大规模成矿阶段主要集中在热液阶段的最晚期；(4)大气降水的加入、成矿温度的降低和矿物结晶导致的REE络合物的大量破坏是大规模成矿的重要机制。基于以上研究资助发表SCI论文12篇，获侯德封矿物岩石地球化学青年科学家奖、中国地质调查局优秀地质人才称号、受邀成为欧盟地平线计划顾问（2018-2019年）、《Ore Geology Reviews》副主编。2019年获得国家自然基金优秀青年基金。