Genesis model of geothermal fields in Yangmeichong, Guangxi

Hezhou in Guangxi is rich in geothermal resources with great development prospects and research value. However, there are problems to be addressed in terms of supply sources, cyclic evolution processes, and genesis mechanisms of geothermal resources in this region. Through geothermal geological surveys and analyses of geophysics, geochemistry, and environmental isotopes, this study has explored thermal storage characteristics, supply sources, circulation depths, and genetic models of the geothermal fields in Yangmeichong, Hezhou. A geological geophysical model and a convective geothermal model of uplift mountain faults have been preliminarily established, whose geological parameters such as temperature, composition, depth, and magnetism of geothermal resources are consistent with the understanding of geology. The thermal reservoir of geothermal fields in Yangmeichong is belt shaped and composed of the Yanshanian granite fracture zone. The Guposhan Fault (F₁) is the main water conducting and heat controlling structure in the geothermal fields in Yangmeichong. The source of geothermal heat flow in the geothermal fields is the heat from the upper mantle and deep crust (mantle heat flow), as well as the heat generated by the decay of radioactive elements in the shallow crust (crust heat flow), providing a heat source for the formation of deep circulating groundwater. Temperatures and geothermal gradients gradually increase from the western boundary (F_1-1) to the eastern boundary (F₁) and from the northern boundary to the southern boundary of the geothermal fields in Yangmeichong. In the vertical direction, geothermal gradients increase with the increase of depths. Temperatures within the depth range of 800–1,200 m are 53.5–73.0 ℃, and the geothermal gradient is 4.88 ℃/100 m. The deep thermal storage temperatures of the Yangmeichong geothermal fields have been measured by both silicon dioxide geothermal temperature scale and potassium magnesium geothermal temperature scale, and temperatures are 92.24 ℃ and 87.22 ℃, respectively. Accordingly, the depths of underground thermal mineral water circulation are 3,292 m and 3,069 m, respectively. The hydrochemical type of geothermal fluid in the geothermal fields in Yangmeichong is HCO₃-Na. Due to the leaching effect of deep groundwater on granite bodies, the contents of silicic acid and sodium ions in underground hot water are relatively high. The isotopic detection results indicate that the supply of geothermal water in the area comes from precipitation infiltration, and the tritium content of geothermal water is less than 2 TU. It is speculated that geothermal water in Yangmeichong was formed by atmospheric precipitation before 1960. The geothermal energy in Yangmeichong is a fault convection geothermal model. Geothermal water is directly supplied by atmospheric precipitation with fault zones and rock pores as water channels, and it flows deep by both hydraulic and thermal forces. After alternating water thermal convection, geothermal water is formed. Subsequently, the convection of geothermal water took place along the fault of Gupo mountain from deep to shallow and from north to south, which formed the fault convective geothermal model in Yangmeichong. The research results provide a theoretical basis for the exploration and rational utilization of geothermal resources in Yangmeichong, Guangxi.