Formation characteristics of carbonate thermal water controlled by fault in southeastern Chongqing

Southeast Chongqing is located in the mountain region at the edge of the basin between Wuling mountain and Dalou mountain. Municipally, it falls within two districts and four counties of Chongqing, which are Qianjiang District and Wu Long district, the Shizhu county, the Xiushan county, the Youyang county and the Pengshui county. Paleozoic and Mesozoic sedimentary rocks widely distribute in the area, with lithology dominated by carbonate and clastic rock types. A number of synclines and anticlines expose in a parallel manner in the area where develop many faults. The thermal reservoir studied is mainly composed of dolomites and limestones of the upper & middle Cambrian and the lower Ordovician. It’s overlaid by the stratum layers comprise the upper & middle Ordovician and the lower & middle Silurian, their lithologies are mainly shale, silt stone and inclusion limestone. Below the thermal reservoir is the layer of aquiclude consisting of Gaotai formation of the middle Cambrian, with its lithology dominated by shales. The geochemical characteristics and the formation of the fault-controlled thermal water in carbonate rocks in southeastern Chongqing City were investigated by using the methods of hydrochemistry and isotopic tracer.The result of Gibbs diagram shows that the geochemical characteristics of thermal water in the west and east of the Yushan fault region are affected by evaporation and rock weathering, respectively. The hydrochemical type of the water in the west of Yushan fault region is Cl-Na. Ratio of gNa+/gCl- is close to 1, indicating that high Cl- and Na+ concentrations of the thermal water are mainly dominated by the dissolution of halite in the stratum. The hydrochemical facies of the thermal water of east Yushan fault are SO4-Ca?Mg. Ratio of g(Ca2++Mg2+)/g(HCO〖_3^-〗+ SO〖_4^(2-)〗) of the thermal water is nearly equivalent to 1, indicating that the high Ca2+ and SO〖_4^(2-)〗 concentrations in thermal water are derived from the dissolution of the anhydrite. The δD and δ18O values of the thermal water range from -64.7‰ to -50‰ and from-9.17‰ to -7.89‰, respectively, falling around the local atmospheric precipitation line, which indicates that the recharging source is mainly atmospheric precipitation. The average recharge elevation of thermal water source is 1，278 m, which is likely to come from the recharge nearby karst middle-mountain. The thermal reservoir temperature is from 41℃ to 90 ℃ with an average value of 66 ℃. The circulation depth is in the range of 1,000-3,500 m with an average value of 2，300 m. After the atmospheric precipitation infiltration, it flows into ground under the effect of gravity (topography) and follows with the heat conduction into the upper earth. The thermal water flows from the northwest to the southeast along the fault zone, and is mixed with shallow karst groundwater.