锂同位素地球化学在地热流体水岩反应中的应用——以川西现代富锂热泉研究为例

Application of Li isotope in geothermal fluid-rock interaction:A case study of modern Li-rich geothermal water in western Sichuan

高温流体的化学组成及同位素特征是深部环境信息的重要载体。本文基于锂同位素地球化学方法系统分析了川西现代热泉地热流体的水化学特征、水岩反应过程、补给来源、水岩反应温度及循环深度。研究结果显示,茶洛热泉水化学相类型为HCO_(3)-Na型,与地表水和冷地下水的HCO_(3)-Ca型存在明显区别。利用锂同位素温标估算茶洛热泉的水岩反应体系温度为227.80±19.84℃(2σ,n=8),热循环深度为7348.08m±684.26m(2σ,n=8)。茶洛热泉水化学成分主要受水岩反应控制,处于非平衡向局部平衡过度。高K、Li、F、Cl等与花岗岩岩体有关的特征元素表明水岩反应中以与花岗岩岩体反应为主导,估算其贡献率约为75.34%。矿物饱和指数的计算,显示了热泉水中的白云石、萤石、霞石、方解石处于过饱和状态;钾盐以及大多数样品的石膏、硬石膏处于不饱和状态;石盐则趋近于平衡状态。锂元素浓度及锂同位素分析结果表明,热泉中的锂元素浓度远高于地表水和冷地下水,且富集较轻的锂元素。锂同位素质量平衡拟合估算与茶洛热泉水进行水岩反应的岩体的锂元素浓度为13.43±7.04×10^(-6)(2σ,n=13),δ^(7)Li _(rock)值为1.14±2.06‰(2σ,n=13)。岩体深度约6958~7450m。研究成果对于认识地热流体的富锂机理,拓展地热型锂资源的找矿思路均具有理论意义与实际应用价值。

The chemical composition and isotopic characteristics of geothermal fluids can be carriers of deep environmental information.Based on the lithium isotope geochemistry,this study systematically analyzes the hydrochemical characteristics,recharge source,fluid-rock interaction process,temperature and circulation depth of modern geothermal water in western Sichuan.Piper diagram shows that Chaluo geothermal water belongs to HCO_(3)-Na facies and both surface water and groundwater belong to HCO_(3)-Ca facies.The Li isotopes temperature scale calculation results show that the fluid-rock interaction temperature of Chaluo geothermal waters is 227.80±19.84℃(2σ,n=8),and the geothermal circulation depth is about 7348.08±684.26m(2σ,n=8).The chemical composition of geothermal water was controlled by fluid-rock interaction,in a state of transition from immature waters to partially equilibrated waters.The high concentration of K,Li,F,Cl and other characteristic elements related to the granite rocks shows that the fluid-rock interaction is dominated by the granite rocks,and its contribution rate is estimated to be about 75.34%.Mineral saturation index of the geothermal waters shows that dolomite,fluorite,aragonite,and calcite are in a supersaturated state;sylvite,gypsum,and anhydrite in most samples were in an unsaturated state;and halite was approaching an equilibrium state.Analysis of the lithium and lithium isotope shows that geothermal waters have high lithium concentration and low δ^(7)Li.Mass balance simulation figure out that the rock mass which balanced with geothermal waters has the lithium concentration of 13.43±7.04×10^(-6)(2σ,n=13) and δ_(7)Li=1.14±2.06‰(2σ,n=13).Based on the interaction progress,the depth of the rock mass is about 6958~7450m.