Fluid Geochemical Genesis of the Rehai High Temperature Geothermal System in the Southeast Himalayas

Abstract

ABSTRACT: Western Yunnan is located in the southeast Himalayas, with well-developed faults and abundant high-temperature geothermal resources. In the western Yunnan geothermal fields, the hydrochemical and isotopic characteristics of geothermal fluids are studied and analyzed, elucidating the genetic model of magmatic high-temperature geothermal systems, and revealing the hydrochemical processes and genesis of geothermal water during the circulation process. The Rehai geothermal water type is mainly HCO3 Cl-Na. The high Cl content of geothermal water comes from the supply of magmatic water, while Na, K, and HCO3 mainly come from the leaching of feldspar minerals by mantle-derived CO2. Water hydrogen and oxygen isotopes indicate the source of atmospheric precipitation, while Rehai hot spring water shows an "18O shift". It is related to the mixing of magmatic water and the interaction between water and rock. According to the Na-K cation thermometer, among numerous geothermal fields of the western Yunnan region, the deep reservoir temperature of the Rehai geothermal field is the highest, reaching 260 °C. The Rehai geothermal water undergoes deep circulation and obtains deep heat after being recharged by atmospheric precipitation infiltration in the metamorphic rock area of the Gaoligongshan group located in the northeastern mountainous area. Rising along the fault, exposed as a hot spring at the surface. 1. Introduction Under the combined action of the collision between the Indian and Eurasian plates and the westward subduction of the Pacific plate, a Cenozoic orogenic belt belonging to the Tethys tectonic domain has developed in the southwest region, while a Mesozoic Cenozoic fault basin group belonging to the Pacific Rim tectonic domain has formed in the eastern region. It belongs to the Mediterranean Himalayan and Pacific Rim geotropics, respectively, which have shaped the highest heat flow in Southwest China and Taiwan, with higher heat flow in the east and lower heat flow in the central region. It controlled the enrichment of deep geothermal resources in China. The Indian Plate drifted northward and collided with the Eurasian Plate (Chen et al., 2015; Tang et al., 2017; Zhou et al., 2017). It subducted towards its lower part, causing the lower edge of the Eurasian Plate to be heated and melted, and to invade along the weak structural zone of the upper plate, which provided a very favorable tectonic background and heat source conditions for the formation of diverse geothermal activities in the western Yunnan region on the southeastern edge of the Himalayas. From the perspective of geothermal flow, western Yunnan is one of the most favorable regions for geothermal formation in China The average heat flow value in the southwestern region of Yunnan Province is 75 mW/m2, while the heat flow value in Tengchong County is 93 mW/m2, which is the highest in Yunnan (Figure 1).