Constraints of hydrochemical and geological controls in deep-circulation geothermal systems: Insights from Chengde, Dehua, and Tashkorgan geothermal fields in China

Controlled by the collision of the Indian–Eurasian Plate and the subduction of the Pacific–Philippine Plate into the Eurasian Plate, deep-circulation geothermal resources are widely developed on the Tibetan Plateau in western China, the Yanshan Mountains in northern China, and the coastal region in southeastern China. However, the hydrochemical and lithospheric thermal–structural responses under different geological and tectonic background conditions are currently unknown, which constrains our understanding of the factors controlling the genesis of low- to medium-temperature deep-circulation geothermal systems. In this study, the deep-circulation geothermal systems of Chengde in Northern China, Dehua in Southern China, and Tashkorgan on the Pamir Plateau in Northwestern China were investigated. Geothermal systems in the Pacific tectonic domain at the eastern margin of the Eurasian Plate have similar elemental geochemistry and hydroxide isotopic characteristics with differences in major anion types and reservoir temperatures, which may be attributed to differences in fluid circulation depths along the fracture networks in the geothermal systems. The geothermal systems in the Tethys tectonic domain, caused by the collision of the Indian and Eurasian Plates, differ significantly from those in the Pacific tectonic domain, with more complex hydrochemical types, higher crustal and reservoir temperatures, and hydroxide isotopes showing more significant magma–water drifts. These differences indicate that the regional tectonic domain may have first-order control over deep-circulation geothermal systems. Under the same tectonic domain, the background heat flow acts as a secondary controlling factor, affecting the reservoir temperatures and circulation depths of the geothermal system.