Fluid geochemistry and its implications on the role of deep faults in the genesis of high temperature systems in the eastern edge of the Qinghai Tibet Plateau

Abstract

Deep faults could provide fluent channels for geothermal water uplift so that it is one of the targets for high-temperature hydrothermal exploration in orogenic geothermal belt. In the eastern edge of the Qinghai Tibet Plateau, significant geothermal potential reflected by hot springs, fumaroles, and sinters, are indeed exposed along a series of lithospheric-scale faults, including the Jinshajiang, the Ganzi-Litang and the Xianshuihe faults. However, as the controlling fault of the eastern edge of the plateau, the Longmenshan Fault has few geothermal manifestations. In order to uncover the role of deep faults in the genesis of high-temperature geothermal systems, a comprehensive chemical and isotopic comparison of geothermal fluid between the Xianshuihe-Anninghe Fault and the Longmenshan-Minjiang Fault was investigated in this paper. According to FixAl modeling and cation geothermometric calculations, the reservoir temperatures of geothermal systems along the Longmenshan Fault are lower than 150 °C with circulation depth of geothermal water less than 4 km while those in the Xianshuihe Fault reaches up to 260 °C with geothermal water circulating as deep as 8 km. Compared to the Xianshuihe Fault, the low reservoir temperatures along the Longmenshan Fault are accompanied by two characteristics of geothermal fluid: (1) no distinctive oxygen shift occurs in stable isotopes of geothermal waters; and (2) little mantle-derived volatiles found in the gaseous components. We propose that, extensional fracture systems are locally formed in the strike-slip movement of the Xianshuihe and Anninghe faults, which not only act as conduits for deep-derived geothermal volatiles, such as metamorphic carbon dioxide and mantle helium, but also enhance the heat convection processes, resulting in the formation of high-temperature geothermal systems. In contrast, in the Longmenshan Thrust Fault, the shallow circulation of geothermal water in closed fracture systems accounts for its lower reservoir temperatures. Therefore, deep extensional fault is a crucial element in forming a high-temperature geothermal system in the eastern edge of the Qinghai Tibet Plateau.