Abstract: Hainan Island possesses abundant geothermal resources; however, the previous geothermal explorations has primarily focused on production, neglecting research into the origins of geothermal water chemistry, the water-rock interactions, multiple methods for evaluating thermal reservoir temperature, the formation mechanisms of geothermal fields, and so on. [Objective]Building upon the existing exploration data, this paper conducts a in-depth analysis to enhance our understanding of genetic mechanism of the geothermal field and to offer valuable insights for the development of geothermal fields.[Methods]By utilizing the ratios and correlations of major ions, piper diagrams, F- concentration distribution maps, silica-enthalpy and SiO2 mixing model graphs, silica-enthalpy equations, δD-δ18O of water along with GMWL curves, this study focuses on the geothermal water of Jiusuo to deeply analyze the sources of chemical components, the cation exchange processes, the origin of F-, the most probable reservoir temperatures, and the circulation depths of the geothermal water, thereby proposing a conceptual model for explaining genesis of the field.[Results]The findings show that the geothermal water hydrochemistry in the study area is mainly of the SO4·HCO3-Na type, with Ca2+ and Mg2+ replacing Na+ and K+ in the rock. The primary source of SO42- is sulfide oxidation in andesite and rhyolite. The dissolution of minerals such as mica, amphibole, and fluorite, ion exchange, and alkaline environment are the controlling factors for the concentration of F-.The chemical composition in the geothermal water is predominantly influenced by silicate mineral dissolution, ion exchange processes, as well as the degree of development of geological strata and structures. It is most likely that when mixed with cold groundwater, the temperature range of geothermal water in this area falls between 99 °C to 169 °C; with cold groundwater mixing accounting for 80% to 93%, while approximately 10% steam loss occurs before mixing cold groundwater. The depth at which geothermal water circulates ranges from 1.8 to 3.8 km. [Conclusion] The conceptual model of geothermal origin indicates that the geothermal water in the Jiusuo geothermal field originates from rainfall recharge and flows under the control of the Furongtian-Wangxia structural belt, Ledong-Xichang structural belt, and the Jiusuo-Lingshui deep fault. It flows from the granite area to the andesite and rhyolite areas, where it absorbs energy from radioactive decay in the granite and a small amount of possible mantle-derived thermal energy. This process leads to heating up, silicate mineral dissolution, cation exchange, sulfide oxidation, and resulting in increased concentrations of fluoride ions (F-), sulfate ions (SO42-), and other chemical components in water. These processes cause an evolution in water chemical type. Due to the increase in temperature, the density and viscosity of geothermal water decreases, pressure increases, so buoyancy is generated. Driven by both hydraulic gradient and buoyancy gradient, geothermal water ascends along the Jiusuo-Lingshui deep large fault zone and rock fractures in the subsurface. It then mixes with cold groundwater in the shallow subsurface of the study area before being buried in the Quaternary and Tertiary sedimentary cover layers, ultimately forming a geothermal field. Is there mantle-derived thermal energy in the deep region of geothermal fields in southwestern Hainan? Presently, there is insufficient evidence to substantiate this assertion. This is a scientific inquiry that warrants further investigation.
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