西藏东部阿旺地下热水化学特征及其成因初探

HYDROGEOCHEMICAL CHARACTERISTICS AND GENESIS MECHANISM OF GEOTHERMAL WATER IN AWANG,EASTERN TIBET

本文基于西藏阿旺乡的地热地质背景,综合应用野外调查、水文地球化学、环境同位素方法,初步探究了区内出露地下热水的发育特征及其成因机制。结果表明,地下热水化学类型为HCO3-Na型,这与地下热水在径流过程中与围岩发生溶滤作用和阳离子交互作用有关。氢氧同位素分析显示地下热水补给来源为大气降水,并伴随有轻微的氧漂移现象,表明水岩作用较强烈,热储温度较高。采用同位素方法估算补给高程在4600~4800 m左右,推测地下热水的补给区为阿旺乡西北部山区。Na-K-Mg三角图判别法和矿物饱和度指数表明地下热水为未成熟水,其在上升过程中受到了浅表冷水的混合,冷水混入比为60%~70%,采用地球化学温标计算得到的深部热储温度为170~200℃,地下热水循环深度为4500~5300 m。阿旺地区地下热水成因模式为:大气降水自补给区入渗进入深循环,经大地热流加热形成热水,热水在地下循环过程中沿断层破碎带上升受到浅循环冷水混合后出露地表形成温泉。

Based on the geothermal setting of the Awang Township of Tibet,we analyzed the characteristics and genetic mechanism of geothermal water with the aid of field investigation,hydrogeochemistry,and environmental isotopes.The hydrochemical types of geothermal water belong to HCO3-Na type,resulting from leaching and cation interaction between geothermal water and surrounding rocks.The composition of hydrogen and oxygen isotope indicates an origin of meteoric water.The slight oxygen drift is produced by water-rock interaction and high temperature of geothermal reservoir.The recharge area with estimated altitude of 4600~4800 m is located in northwest mountain areas.The analyses of Na-K-Mg triangle graph and saturation index collectively show the geothermal water is mixed by shallow cold water with the proportion of 60%to 70%.The temperatures of deep geothermal reservoirs are 170~200℃suggesting by quartz and Na-K geothermometers.The circulation depth of geothermal water is 4500~5300 m.The genetic mechanism of geothermal water in Awang Township can be concluded as follows:the thermal water recharged from precipitation is heated by terrestrial heat flow,arises and exposes as geothermal springs along the fault zone.