四川乡城然乌温泉水文地球化学特征和成因分析

Hydrogeochemical Characteristic and Formation of the Xiangcheng Ranwu Hot Spring in Sichuan Province

以四川乡城地震观测温泉(乡城然乌温泉)为主要研究对象,通过采集观测泉及周边其他出露温泉点的水样开展了水化学主要离子组分、微量元素和氢氧同位素分析,研究温泉水循环特征和成因模式。结果表明,该温泉水化学类型为Na-HCO_(3),是中温、弱碱性、低矿化度的温泉,温泉水中含有B、 Li、 Ba等微量元素,温泉水离子来源主要为水-岩作用过程中对围岩硅酸盐、碳酸盐及石膏矿物的溶解;氢氧同位素分析表明,温泉补给来源主要为大气降水;硅-焓模型估算观测泉的深部热储温度为275℃,循环深度为5.3 km;采用硅-焓方程法计算得冷热水混合比例为84%~86%。乡城然乌温泉成因是山区大气降水入渗补给后,经历深循环受到大地热流加热后,沿乡城断裂带上升,热水在上升过程中与浅部冷水相遇,最终在河谷中出露形成中温温泉。采用“多阶段定位法”对研究区M_(L)≥1.0地震进行重新定位,结果表明,乡城然乌温泉附近区域地震震源深度基本都位于温泉循环深度以下,说明乡城然乌温泉水热活动对所在区域地震活动具有一定的控制作用。乡城然乌温泉属于深循环温泉,能够反映地壳深部活动信息,适合于进一步开展地震前兆地球化学观测。

Taking the observation spring(Xiangcheng Ranwu hot spring)in Xiangcheng,Sichuan as the research object,we collect water samples from observation spring and other nearby hot springs and analyze the main chemical ions compositions,trace elements and hydrogen and oxygen isotope compositions to study water cycle characteristics and forma-tion mechanism of the hot spring.The results show that,the hot spring hydrochemical type is Na-HCO_(3),and belongs to medium temperature,alkaline,low salinity hot spring and contains B,Li,Ba and other trace elements.Ions in hot springs are mainly dissolved from silicate minerals in surrounding rock during water-rock interaction.The stable hy-drogen and oxygen isotopes(δ18O and δD)composition indicates that the main source of the recharge for the hot spring is precipitation.According to the result of Si-enthalpy model,the geothermal reservoir temperature of the hot spring is 275℃ and circulation depth of hot spring is 5.3 km.The ratio of mixing cold water with hot water is about 84%~86% which calculated by Si-enthalpy equation method.The genetic model of Xiangcheng Ranwu hot spring is that groundwater receives recharge from infiltration of precipitation in the mountain area,undergoes deep circulation and obtains heat from heat flow and flows up to the surface along the Xiangcheng fault,at the same time mixed with shallow cold water.Finally,exposing in the river valley as a medium hot spring.We relo-cated earthquakes in the study area by the"multi-stage"method.The results suggest that the focal depth that located near the hot spring is deeper than geothermal water circulation depth,which indicate that the geothermal cycle can weaken the fault strength and control the seismic activity around the Xiangcheng hot spring.Based on above results,we can conclude that Xiangcheng Ranwu hot spring plays an important role in earthquakes forma-tion in this area and is suitable for further geochemical observation of earthquake precursor.