Abstract: In the past several years, a series of geologic, seismologic, geochemical and geophysical investigations have been conducted to understand the crustal structure, activity of active faults and seismic risk in the Yanhuai Basin. During the investigations, a large number of data about the pre-seismic, co-seismic and post-seismic behaviors of subsurface fluids have been obtained by the Seismic Precursory Monitoring Stations in the area. This paper deals with the geochemistry of groundwater and noble gases in this region. It is aimed at the understanding of the background of groundwater, which is sensitive indicator for both earthquake and active faults in the region. The study shows that geothermal water in this region originates mainly from meteoric rainfall, the circulation depth is 1 600～3 800m, the temperature of water at the depths is 57～112℃, and the residence time is 50～30 320yr. Groundwater in this region can be divided chemically into HCO₃-Na, HCO₃-Cl-Na, HCO₃-SO₄-Na-Mg and SO₄-Na types, the TDS and pH of which range from 527.6～938.0 ppm and 7.50～8.73, respectively, belonging to low TDS and neutral-alkaline thermal water. The results of water-rock equilibrium calculations by NETPATH 2.0 show that groundwater in this area is over-saturated with regard to quartz and in equilibrium with chalcedony. The noble gases of geothermal water originate from meteoric, crust and mantle sources, respectively. The anomalies of geothermal water and gases regime before the occurrence of Datong, Baotou and Zhangbei earthquakes were closely related to the faults and their activity, i.e., those anomalies occurred in the places where the circulation depth of the groundwater is great, or even the circulation depth is shallow but having an evidence of mantle gases releasing. The residence time of geothermal water and the discovery of mantle gases in this region are firstly reported in this paper. These results are of significance to further study of active faults, earthquake risk and groundwater utilization in this region.

Key words: geochemistry, geothermal water, noble gases, mantle, Yanhuai basin

摘要： 对在延怀盆地采集的14组(次)地下热、冷水和气体样品的分析结果表明,地下热水主要是大气降水成因的,其循环深度为1600～3800m,深部最高温度为57～112℃,滞留时间为50～30320a;热水中的稀有气体为大气、壳源和幔源成因;热水与气体在地震前的异常变化与热水所处的断裂构造及其活动性密切相关。关于该区地下热水的年龄和幔源气体的发现为首次报道,它对该区的断裂活动性、地震危险性研究和热水合理利用具有重要的意义。

关键词: 地球化学, 热水, 同位素, 幔源气体, 延怀盆地