Magmatic Origin of Geothermal Fluids Constrained by Geochemical Evidence: Implications for the Heat Source in the Northeastern Tibetan Plateau

The northeastern Tibetan Plateau (NETP) represents the growth front of the Tibetan Plateau (TP) system. This region has long been recognized as a key in understanding the topographic response and crustal thickening of the entire TP. A heat flow anomaly (Gonghe Basin, 102 mW/m(2)) was found in the NETP. However, the heat-generation mechanism and the heat source of the Gonghe Basin are still debated as it is unclear whether they are related to magmatic melting activities. Herein, we provide systematic hydrogeochemical data of two types of geothermal waters (type I and II: geothermal waters sampled from within the basin and the mountainous regions, respectively) found in the region. Type I high 8D (-85.0 parts per thousand to -59.0 parts per thousand), delta O-18 (-11.1 parts per thousand to -8.0 parts per thousand), Cl- (mostly range from 300 to 900 mg/L), and trace element, whereas type II has low delta D (-97.1 parts per thousand to -89 parts per thousand), delta O-18 (-13.0 parts per thousand to -11.8 parts per thousand), Cl- (30 to 180 mg/L) and trace elements. Furthermore, we identified the existence of a high-temperature parent geothermal fluid based on the chloride - enthalpy model, it was estimated to possess a temperature of 310 degrees C and a circulation depth of 6.8-7.8 km. The parent geothermal fluid originated from snowmelt water, which later mixed with a magmatic fluid. The helium (He) ratios of geothermal gas ranged from 0.01 Ra to 0.18 Ra and indicated that the source of He was primarily from a crustal source. In addition to the heat flow analysis and magnetotelluric (MT) data, we suggested that the magmatic nature of the geothermal fluid is caused by a partial melt zone, which is ubiquitous in the middle to lower crust and serves as the heat source in the NETP. Finally, a conceptual model was built to illustrate the occurrence of magmatic fluid and its genesis. The findings will help to improve the understanding of the uplift of the TP and reveals the important role of deep groundwater circulation in the formation of high-temperature geothermal resources.