Geothermal fluid chemistry and isotope for interpreting the formation of complex geothermal system in the Gonghe Basin, northeastern Tibetan Plateau

Abstract

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. N2 of atmospheric origin dominates the hot water gas. Except for the relatively high CO2 concentration (7.44%) in the deep geothermal reservoir in the center of the basin, most of the samples contain low CO2 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 CH4, 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.