Genesis of Geothermal Fluid in Typical Geothermal Fields in Western Sichuan,China

The hydrogeochemical characteristics of geothermal fluids can reveal the genesis of geothermal systems and act as important references for developing and using geothermal resources.This study presents hydrogeochemical processes and thermal cycle mechanisms of typical geothermal fields in Western Sichuan.Based on the geological conditions in Western Sichuan,29 hot springs in three geothermal fields in the Batang and Litang areas were selected for hydrochemical and isotopic(δD andδ18O)analyses.Furthermore,the temperature of the thermal reservoir was calculated and the upflow cooling process of the hot springs was analyzed.Most of the subterranean hot waters in Batang and Litang are of the HCO3-Na hydrochemical type.The ion variation in Batang is primarily affected by water-rock interactions.There is a strong positive correlation between Na+,B?,and Cl?in Litang,suggesting that they have the same material source.The Na+and metaboric acid content is relatively high,which indicates that the groundwater runoff in both areas is relatively long-lasting,with reduced flow velocity;moreover,the metasilicic acid content is relatively high,which supports this conclusion.Both hydrogen and oxygen isotopes plot near the atmospheric precipitation line,indicating that groundwater recharge is functionally obtained from precipitation.The calculated thermal storage temperatures in Batang and Litang were 88–199℃ and 96–154℃,respectively.The proportion of cold water mixing in Batang was 64%–67%,while that in Litang was 60%–68%.According to the calculated results,the initial thermal cycle depth of the Batang area(4540–4780 m)was greater than that of the Litang area(3150–3960 m).The enthalpy of the deep parental geothermal fluid in Batang was 1550 J/g with a Cl^(-)concentration of 37 mg/L,while that in Litang was 2100 J/g with a Cl^(-)concentration of 48 mg/L.