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• Classification of complex geothermal systems in the Gonghe Basin. • Geothermal gas and isotope characteristics of the Gonghe Basin. • Deep heat source indication based on fluid geochemistry. The Gonghe Basin is located in the northeast of the Tibetan Plateau and is rich in geothermal resources. In recent years, Hot Dry Rocks have been discovered in the basin, and related studies have suggested that localized melts in the middle and lower crust provide an additional heat source. However, there needs to be more conclusive evidence. In this paper, geothermal water chemistry and gas isotope samples from the Gonghe Basin and its neighboring areas were systematically collected. Comprehensive fluid geochemical analysis was carried out in conjunction with the distribution and nature of the regional fracture to propose the circulation pattern of the complex geothermal system and the source of the gas components in the Gonghe Basin. The results show that the hot water in the Gonghe Basin and its surroundings is recharged by meteoric water and can be classified into three types according to its hydrochemical and isotopic characteristics: deep-fluid type (deeper circulation depth or deeper burial of the geothermal reservoirs), runoff type (shallower circulation depth), and mixed type (shallow layered geothermal reservoirs at the center of the basin), with a gradual decrease in the temperature of the geothermal reservoirs accordingly. N 2 of atmospheric origin dominates the hot water gas. Except for the relatively high CO 2 concentration (7.44%) in the deep geothermal reservoir in the center of the basin, most of the samples contain low CO 2 concentration, which is mainly derived from decarbonization of carbonate rocks and sediments, and the contribution of mantle carbon release is almost zero; some of the hot water is detected with a slightly higher content of CH 4 , which is inferred to be of organic origin. The distribution of deep fractures provides a channel for the uplift of deep mantle-sourced volatiles, which makes some hot springs and geothermal wells show some mantle-sourced diagenesis of He, which provides some corroboration for the existence of partial melts in the crust. This understanding is significant for deepening the understanding of geothermal genesis in the northeast Tibetan Plateau and guiding future geothermal exploration in the region. [ABSTRACT FROM AUTHOR]

The uplift of the Tibetan Plateau has caused the development of many high-enthalpy geothermal fields that are rich in exploitable rare and dispersed resource elements. However, the mechanism of the unusual enrichment of these resource elements is still unclear. From a geochemical viewpoint, including major chemical compositions, some rare and dispersed resource elements and trace elements in geothermal water and some river samples from the northern Lhasa block (saline lake area), the southern Lhasa block (Gangdise volcanic belt) and Tethyan Himalaya in Tibet, this study provides new insights into the mechanism of the hydrochemical evolution of the Tibetan geothermal system. The Cl-type geothermal waters in the Gangdise volcanic belt and Tethyan sedimentary area show similar chemical characteristics that are apparently different from that of surface cold waters. The concentrations of Sb, Tl, As, K, Cs, Li, Rb, Ga, B, Cl, Th, Sc, Mn, V, and Ti in Cl-type geothermal waters are at least one order of magnitude higher than those in surface cold waters, but the concentrations of Ca, Mg, Ni, Cr, Zn, Fe, Co, Cu, Pb, and U in Cl-type geothermal waters are slightly higher or even lower than those in surface cold waters. Some rivers and streams in Tibet also show high concentrations of toxic elements. These rivers and streams are mainly polluted by geothermal spring discharge and are unsuitable for drinking. Some ions and elements (such as Ca, Mg, Ba, Sr, SO and Mn) in HCO -type geothermal waters from sedimentary rocks are affected by the availability of soluble minerals such as calcite, dolomite and gypsum. However, the other dissolved elements in HCO-type geothermal waters show the characteristic of mixing Cl-type geothermal waters with surface cold waters. The origin of deep fluids in Cl and HCO-type springs is related, and this origin probably involves the contribution of crustal partial melting rather than single rock leaching. Thus, deep circulating groundwater mixes with residual magmatic fluids and evolves into the unusual enrichment of geothermal mineral resources. [ABSTRACT FROM AUTHOR]