Hydrochemical Characteristics And Genetic Mechanism Of Geothermal Resource In The Yangbajing Area,Tibet

The Yangbajing geothermal field is located at the Yangbajing Town of the Dangxiong County,Tibet Autonomous Region.It belongs to the north-central part of the Yadong-Gulu rift fault zone,as significant constitution of the Mediterranean-Himalayan geothermal belt.The geothermal resources in the area are enriched,and the high-temperature hydrothermal display types include boiling springs,vents,hot springs,warm springs,etc.With its development and utilization for over half a century,the scale and quantity of the original geothermal display activities in the Yangbajing are gradually decreasing,which is not conducive to the sustainable development of local geothermal resources.Therefore,it is urgent to further improve the study of hydrogeochemical evolution and genetic model of the Yangbajing geothermal system and provide theoretical support for the sustainable development of the Yangbajing geothermal field.In this thesis,the Yangbajing geothermal field was taken as the research object.Based on the regional geological data and the hydrochemical data from field sampling,the hydrogeochemical and isotope methods were combined to reveal the water-rock interaction process and recharge source of the Yangbajing hydrothermal system,and the geothermal reservoir temperature was estimated.The conceptual model of geothermal genesis the Yangbajing was further improved on the basis of the previous work,and the scaling trends and environmental impacts of geothermal water were analysed.Five research results were achieved in the paper as follows:（1）The analysis of hydrochemical data shows that the hydrochemical types of the Yangbajing geothermal field are mainly classified as two classes:Class A of Na-HCO₃ geothermal water is mainly distributed in the geothermal area with high altitude,and the surface outcrop temperature is68-76°C;Class B of Na-HCO₃ geothermal water is mainly distributed in the central part of the study area,and the intensity of water-rock interaction is higher than that of type A.The surface exposure temperature is 69-84°C.Cold spring water and surface water are of Ca-HCO₃ type.（2）The characteristics of hydrogen and oxygen isotope data show that geothermal water is mainly recharged by atmospheric precipitation and snow-melting water from surrounding mountains.The recharge elevations of Class A and Class B geothermal water are 5213-5736m and 5173-5331m,respectively.There is an obvious"oxygen drift"phenomenon in the Yangbajing geothermal field,indicating that a strong water-rock interaction occurred during the formation of geothermal water.Ion ratio,saturated index and strontium isotope also shows that silicic rock is mainly involved in the hydrochemical process controlling the chemical composition of geothermal water.（3）The geothermal reservoir temperature is estimated by cationic geothermometer,silica geothermometer,silicon-enthalpy equation and silicon-enthalpy diagram.The results show that the shallow geothermal reservoir temperature of Class A geothermal display area is 120.50-122.25°C,the deep geothermal reservoir temperature is about 156.17-168.04°C,and the cool water mixing ratio is 53.45-61.85%;the shallow geothermal reservoir temperature of Class B geothermal display area is 137.88-177.19°C,the temperature of the deep geothermal reservoir is about 202.29-236.75°C,and the cold water mixing ratio is 64.43-66.72%.The geothermal water circulation depth is 1055.12-1592.68 m.（4）This thesis further improves the genetic model of the Yangbajing geothermal field:the geothermal display area has experienced intense tectonic activities,and the heat sources of class A and class B geothermal water are both local melts in depth.The atmospheric precipitation and snow-melting water infiltrated downward along the fault,heated by the tectonic mechanic heat,partial-melting heat and crust radioactive heat.During the circulation,geothermal water is interacted with the surrounding rock.The geothermal water migrated upward along the fault to the surface,and finally formed as geothermal spring.（5）The geothermal water in the study area is mainly used as medical hot mineral water,which is not easy to be used as drinking water and drinking natural mineral water.The geothermal water is accompanied by slightly corrosive water,and the scaling degree is small.The F,B and As elements exceed the surface water environmental quality and drinking water standards,which have an impact on the environment and human health.