Fluid chemistry and circulation patterns from typical convective hydrothermal system on the northeastern Tibetan Plateau

Abstract

The northeastern Tibetan Plateau is abundant with intermediate to low-temperature geothermal resources, with hot springs varying significantly among locations and tectonic conditions. Understanding the causes of these differences is crucial. This research specifically examines the Wahongshan-Wenquan Fracture Zone (Area Ⅰ) and the Zhiduo-Yushu Mountainous Zone (Area Ⅱ) in Qinghai Province. Hydrochemical and gas isotope data were collected from convective hydrothermal systems in these distinct tectonic settings. A comparative analysis of geothermal fluid geochemical characteristics and sources was conducted using fluid geochemistry methods. Results show that hot water in the igneous rocks of Area Ⅰ is mainly of Cl-Na type, while in the carbonate rocks of Area Ⅱ, it is primarily of HCO3-Ca•Mg type. The salts in the former come from silicate mineral dissolution, while the solutes in the latter are primarily influenced by carbonate rock breakdown. Igneous thermal reservoirs have higher temperatures and greater fluid circulation depths than carbonate reservoirs. Geothermal gases in both regions are dominated by N2 of atmospheric origin. Most of He originates from the crust, with mantle contributions not exceeding 5%. High CO2 content (14%) in certain carbonate reservoirs is mainly of inorganic metamorphic origin. Both regions are medium-low temperature convective geothermal systems, primarily driven by crustal heat. However, isotopic analysis suggests that the carbonate reservoirs in the Zhiduo-Yushu Mountains have a higher mantle contribution than those in the Wahongshan-Wenquan Fracture Zone. This study summarizes the fluid circulation patterns in these two regions, revealing regional and tectonic influences on fluid sources and transport mechanisms. It provides a theoretical framework for developing and utilizing geothermal assets on the northeastern Tibetan Plateau.