The effect of granite fracture network on silica-enriched groundwater formation and geothermometers in low-temperature hydrothermal system

Abstract

Silica-enriched mineral groundwater has important commercial and health value and is most common in volcanic areas and high-temperature geothermal areas. But we found abundant silica-enriched mineral groundwater (dissolved SiO2 up to 51.57 mg/L) exposed from the granite fracture network in Suining, South China, where is no volcanoes and high-temperature hydrothermal system. Pumping test and hydrogeochemistry were employed to investigate the formation mechanism of silica-enriched mineral groundwater in granite fracture network. It is HCO3-Ca type and high content of dissolved silica, formed warm groundwater on the F86-1 fault and recharged by meteoric water with a close distance of 3.90 km. Pumping test results indicate that the hydraulic conductivity decreases from top to bottom in the granite fracture network (in order of 2.27 × 10−1 m/d, 5.42 × 10−2 m/d, 7.04 × 10−4 m/d), and they are much smaller than that of the F86-1 fault zone (0.751 m/d). The flow framework constructed by water chemistry suggest the groundwater has experienced the maximum temperature of 68–75 °C and reached the maximum depth of 1.93 km. And the average seepage velocity is about 2.8 × 10−3 m/d and 1.5 × 10−3 m/d in the horizontal and vertical directions, respectively. Water-rock interaction confirms that the granite fracture network provides long-term and sufficient water-rock interaction and dominantly contributes the formation of silica-enriched mineral groundwater because of the weatherability of granite. But the temperature of SiO2-geothermometer will be slightly underestimated for the deep warm groundwater continuously mixing with the shallow groundwater in the granite fracture network.