西藏朗久地热田及其温泉水化学特征研究

西藏朗久地热田位于喜马拉雅——冈底斯微板块北西缘,区内构造、岩浆与变质作用强烈,在构造活动与河流的共同作用下其热储盖层遭到严重破坏,导致冷、热水发生混合。区内碳酸盐岩广布,未完全冷却的花岗岩为地热田的热源,热储平衡温度在191.11~213.85℃之间。区内地表水为HCO3-Ca型,温泉水阴离子为Cl-HCO3型,阳离子为K-Na型。温泉水TDS为2 000~2 300mg/L,Na＋/Cl-〉1,其演化程度介于部分成熟水与未成熟水之间,水岩反应尚未达到平衡。几种典型热液矿物饱和度指数均大于零,区内热液蚀变作用对温泉水化学组成贡献较大;温泉水PCO2〉2 000Pa,与背景值形成较大的梯度差,会发生强烈的CO2脱气。

Study on hydrochemical features of hot springs in Langjiu geothermal field, Tibet, China

Langjiu geothermal field in Tibet is located in the northeast margin of Himalaya-Gangdisê plate.In this region,tectonics,magma activity and metamorphism are intensive,and the cap rocks of the geothermal reservoir are damaged during the tectonic and fluvial processes,which lead to the cold water mixing with the thermal water.Carbonate rocks are widely distributed in this region,and incompletely cooled granites become heat sources of the geothermal field.Equilibrium temperature of the thermal reservoirs is between 191.11 and 213.85℃.The surface water in the geothermal field is bicarbonate-calcium type.Thermal spring＇s water is chlorine-bicarbonate type in terms of anionic and kalium-sodium type in terms of cationic.The thermal springs TDS is 2 000～2 300mg/L with the value of Na＋/Cl-greater than 1,and the evolution degree of thermal springs is between mature water and immature water,so water-rock interaction has not reached equilibrium.From saturation indexes of some typical hydrothermal minerals are greater than 0,we can infer that hydrothermal alteration has much contribution to chemical composition of thermal springs.The partial pressure of carbon dioxide is greater than 2 000Pa,which forms great grads difference comparing to the regional background and will lead to massive carbon dioxide degassing.