CO2/碳酸盐在稀有金属花岗伟晶岩成矿中的作用——以川西典型稀有金属矿床为例

The roles of CO2 in the formation of hydrothermal ore deposits were paid much attention in recent years. However, in granitic pegmatite-type rare metal deposits, the roles of CO2 were ignored in some degree, as the formation of pegmatite was ascribed to the roles of H2O, F and other fluxes traditionally. In these years, many kinds of carbonates were distinguished within crystal-rich inclusions in rare metal granitic pegmatite. And our research found the silicates dissolved in the carbonate aqueous solutions during heating the crystal-rich inclusions. Furthermore, the granitic magma that pegmatite melt exsolved from were enriched with C-H-O components, which is different from the CO2-poor features of granite type rare metals deposits. These features indicate the importance of CO2 in the separation of pegmatite melts from granitic magma and the mechanism for silicate to dissolve in pegmatite melts. This preliminary conclusions were consistent with the enrichments of CO2 and carbonate in the typical pegmattie deposits in western Sichuan. But no enough research to supported the conclusions. Therefore, in this project, we plan to take the typical lithium deposits in western Sichuan as a case study, in order to investigate the roles of CO2 in the minerallization of pegmatite, through the research in petromineralogy, melt and fluid incusions and high temperature and pressure experiments. Finally, we will try to conclude the typomorphic characteristics of pegmatite rare metal deposits based on the roles of CO2, and develop the metallogenic theory of rare metal resources.

CO2是热液成矿系统中备受重视的挥发分，但在稀有金属伟晶岩中，因传统上多强调H2O和F等助溶剂，CO2的作用未得到足够重视和深入研究。近年来，多种碳酸盐子矿物被发现于稀有金属花岗伟晶岩中的富晶体包裹体。在均一实验中，这类包裹体的(稀有金属)硅酸盐子矿物溶解于碳酸盐水溶液中；而且，许多稀有金属伟晶岩的母花岗岩浆也富含C-H-O组分，不同于许多花岗岩型稀有金属矿床贫CO2的特点。这些特征暗示，CO2可能是伟晶岩熔体自花岗岩浆分异和容纳大量硅酸盐和成矿物质的关键组分，这与川西伟晶岩矿床富CO2和碳酸盐的特征一致，但该认识缺乏足够的地质资料和实验数据支持。本项目拟在川西选择典型伟晶岩矿床，在野外工作的基础上，开展“岩石矿物学-熔体和流体包裹体原位观测-高温高压实验”研究，揭示CO2在稀有金属花岗伟晶岩成矿中的贡献，建立基于CO2作用的稀有金属伟晶岩成矿和找矿标型特征，发展稀有金属成矿理论。

CO2是内生成矿系统重要的挥发分。在稀有金属伟晶岩中，因传统上多强调H2O和F等助溶剂，CO2的作用未得到足够重视和深入研究。近年来，多种碳酸盐子矿物被发现于稀有金属花岗伟晶岩中的富晶体包裹体；在均一实验中，这类包裹体的硅酸盐子矿物溶解于碳酸盐水溶液中。这些特征暗示，CO2可能是伟晶岩熔体自花岗岩浆分异和容纳大量硅酸盐和成矿物质的关键组分，具有重要的科学研究意义。为此，项目通过改进实验设备，以满足高温高压实验和实验目标；研究了富 CO2 花岗岩浆分异出伟晶岩熔体/流体的机制，及其在伟晶岩熔体/流体中的富集机制，剖析了研究富CO2伟晶岩熔体/流体溶解大量硅酸盐和成矿元素的物理化学机制。在研究中取得了以下方面的重要成果。（1）研制了HDAC-VII型新型热液金刚石压腔，新型压腔工作温度从-170 ℃到1000 ℃，极大的扩大了其工作条件，满足了本次研究的实验条件。（2）通过高温高压实验，发现在富CO2或碳酸盐溶液中锡石和锂辉石具有较大的溶解度，且Li+和Sn4+主要以离子形式迁移，而非络合物，提出CO2是稀有金属伟晶岩成矿的助熔剂和pH缓冲剂的创新认识。（3）通过熔体包裹体和高温高压实验研究，提出花岗岩浆液态不混溶作用是稀有金属伟晶岩熔体自花岗岩浆分异的重要机制，初始不混溶作用可以熔体包裹体尺度发生，进而不断聚集和伴随氧逸度升高，形成富CO的伟晶岩熔体。（4）在川西锂辉石伟晶岩的富晶体包裹体中，发现火成扎布耶石和原始结晶成因的锂绿泥石，提出伟晶岩中的富晶体包裹体为 伟晶岩矿物结晶系统；进而通过流体包裹体研究提出川西伟晶岩形成于一致的P-T条件，经历了早期的偏碱性、富碳酸盐的锂辉石结晶阶段；低盐度、弱酸性的盐水体系；偏碱性条件有利于锂辉石结晶。本项目成果，在世界上第一次系统阐述伟晶岩中CO2的形成机制及其成矿意义，对深入理解伟晶岩形成和指导锂资源找矿具有较重要的科学意义。