西藏地热系统B、Li、Rb和Cs元素空间分布规律与超常富集机制

西藏地热资源异常丰富，属于地中海—喜马拉雅地热带的重要组成部分。地热系统最为典型的地球化学特征是大部分地热泉或与之相应的泉华沉积物超常富集B、Li、Rb、Cs等元素，具有重要的矿产资源意义。有关这些特征元素来源与水化学演化成因机制，长期存在争议，在理论上缺乏系统论述。本文基于多年野外考察与数据积累，以及对前人研究成果的系统总结，提出了西藏地热系统特征元素组合与富集成因模式，宏观板块构造与微观地球化学分布规律结合，揭示了其成因机制。数据综合分析表明，B、Li、Rb、Cs元素在空间上的分布富集规律总体具有同步性，主要集中分布于雅鲁藏布江缝合带与南北向断裂带交汇区的高温地热系统。B同位素、元素组合、水循环特征及大量地球物理证据表明物源受控于岩浆残余流体，水-岩作用难以作为唯一物源支撑地热系统如此大规模的特征元素超常富集。结论提出西藏地热系统超常富集的元素受控于板块俯冲碰撞、地壳局部重熔、重熔型岩浆流体分异演化、地下水深循环等一系列内外生地质耦合作用过程。本文对西藏地热系统元素超常富集成因机制的解释，有利于增强人们对高温地热系统除水资源与热能意义之外，也同时关注矿产资源价值，实现“水-热-矿”于一体的系统理解，也对未来正确评价地热水体或沉积物矿产资源价值有理论指导意义。

The spatial distribution law of B, Li, Rb and Cs elements and supernormal enrichment mechanism in Tibet geothermal system

The Tibet Plateau hosts the very typical geothermal resources in the world, which belongs to the main part of the Mediterranean-himalayan geotropics. The most typical characteristic is that these geothermal springs show unusual enrichment of many rare and dispersed elements such as B, Li, Rb, and Cs. Correspondingly, large-scale travertine or silica sinters are widely deposited in almost all geothermal fields. Some of the silica sinters show an unusual enrichment of Cs that formed a new type of Cs deposit. However, the origin of those enriched elements and their enrichment mechanism in geothermal water has remained unclear. This study based on the long observation in the field as well as accumulated datasets, and previous literature summarized for the geothermal system in the Tibetan Plateau, is in an attempt to provide new insights into the origin and mechanism of the enrichment of these typical elements. Geochemical datasets show an unusual and coincident enrichment of B, Li, Rb and Cs in the high-temperature geothermal springs as well as silica sinters along Yaluzangbu Suture in Tibet. Depleted B isotope and elemental association, groundwater deep circulation as well as much geophysical evidence indicate the dominant source likely originates from residual magmatic fluids derived from crustal partial remelting while water-rock interaction itself seems difficult to develop so large-scale enrichment of these elements. It can thus be concluded that the plate collision and thrust, crustal partial remelting and magmatic fluids differentiation and evolution during the upwelling and groundwater deep circulation synergistically play effects on the unusual enrichment of typical elements. This study will strengthen a comprehensive understanding of the unique geothermal system for both water resources-energy-minerals in Tibet, in particular, help people focus on special minerals dissolved in geothermal water. In addition, the study will also instruct to well assess the values of mineral resources of geothermal water or deposits.