四川冕宁-德昌稀土矿带碳酸岩流体物理性质、化学成分的精确定量研究

英文摘要： Except calcite carbonatite, the Sr-rich carbonatite and fluorite rich carbonatite is confirmed recently, the additional field, petrographic and geochemical studies will be used in this study to investigate the intrusive sequence and genetic links for varied carbonatites in the Manning-Dechang REE belt. According to previous research, it has been concluded that melt-fluid inclusions in fluorite provide the best examples of primary carbonatite fluids, whereas the CO2-rich fluid inclusions in bastnaesite and quartz provide good samples for ore fluid investigation. Based on petrographic results, the proper melt-fluid and daughter phase-bearing CO2-rich fluid inclusions were selected for study by ultra-high resolution X-ray scanning Nano CT, FIB-SEM（Focused Ion Beam-Scanning Electron Microscopy）and 3D modeling. These techniques, together with SEM/EDS analysis, provide results for the solid phases in melt-fluid and CO2-rich fluid inclusions. They show that the volume percentage of phases varied and that the K and Na content can be estimated. According to the multi-composition phase-diagram and solubility product theory, the K and Na concentration in aqueous fluid can be estimated. This permits the K and Na concentration in the whole inclusion to be calculated. Previous and new LA-ICP-MS results (e.g. melt-fluid in fluorite and CO2-rich fluid inclusion in quartz and bastnaesite) will be used to estimate the quantity of REE and metallic elements in the inclusions. Based on the evolution path defined in previous research, the physical properties and chemical composition that have been confirmed for primary carbonatite and later CO2-rich fluids, the chemical composition of ore-forming fluids from different stages, REE and other metallic element behavior during the evolution, and the REE enrichment and precipitation mechanics are presented.

深化冕宁-德昌稀土矿带典型矿床的地质、地球化学研究，查明不同类型碳酸岩的侵位关系和成因联系；通过萤石中熔流体包裹体和石英及稀土矿物中含子矿物富CO2流体包裹体的岩相学、显微测温、固相组成的SEM/EDS、EPMA、包裹体的纳米CT分析、聚焦离子束-扫描电子显微分析、包裹体的三维模拟等，查明包裹体中各物相的体积百分含量、固相组成及固相中K、Na的含量；结合多组分相图、溶度积原理等化学理论，查明液相和整个包裹体中K、Na的含量及包裹体的密度。在前期工作基础上，进一步补充熔流体包裹体和含子晶富CO2包裹体的LA-ICP-MS分析，并以包裹体中K、Na为内标对包裹体中的主量和微量元素（特别是REE和主要成矿金属元素）含量进行定量计算。根据碳酸岩流体出溶、演化过程中流体性质的变化，探讨流体演化过程中稀土和其它成矿金属元素的地球化学行为、稀土迁移、富集及沉淀机理，重建稀土成矿过程。

碳酸岩型稀土矿床多与碳酸岩浆出溶的流体有关，精确厘定碳酸岩出溶流体的成分及演化特征是查明碳酸岩型稀土矿床稀土迁移、富集和沉淀机理的关键。碳酸岩流体由于其成分复杂，传统的包裹体成分定量分析方法由于缺少内标而无法奏效。.本项目在详细的包裹体岩相学研究基础上，综合运用包裹体成分的显微激光拉曼探针（LRM）、LA-ICP-MS、包裹体中固相成分的扫描电镜/能谱分析（SEM/EDS）、包裹体的聚焦离子束-扫描电子显微分析(FIB-SEM)等，对代表碳酸岩早期出溶流体的熔流体包裹体、代表碳酸岩流体演化晚期的富CO2流体包裹体中不同物相组成的成分分析、体积含量估算等，得到了包裹体中的主要元素组成与REE含量，初步实现了复杂体系熔流体、流体包裹体成分的定量分析。结果表明，碳酸岩早期出溶流体中富含CO2、SO42-、NaCl、REE和Ca、Sr、Ba、Fe，Mn、Mg等多种组分，其中稀土含量可达百分数。SO42-和Cl-稀土迁移的主要配体，硫酸盐熔体与水流体的不混溶过程中NaCl也随硫酸盐熔体一起进入硫酸盐熔体相，造成残余流体中Cl-、SO42-含量大大降低和稀土矿物的沉淀。熔流体包裹体中大量石盐的发现也表明早期出溶流体中除硫酸盐外，NaCl也是其重要组成部分，据此可推断碳酸岩浆中富硫酸盐和盐熔体，这一成果对理解碳酸岩的岩浆组成、地幔源区特征等提供了新的思路。.另外，通过水热金刚石压腔实验对硫酸钠-水、硫酸钠-水-SiO2体系进行了加温和淬火实验。结果表明，在有SiO2存在的条件下，硫酸盐可以在很低的温度（约330℃）下熔融，且这种富硫酸盐流体的稀土溶解能力很强，进一步证实富硫酸盐流体在稀土迁移中的作用。本项目还发现了白云鄂博稀土矿床萤石中的硫酸岩-碳酸盐熔体包裹体和多相硫酸盐熔体包裹体，表明白云鄂博稀土成矿流体也是富SO42-的，进一步证实了SO42-在稀土迁移中的作用。