Fluid geochemical genesis analysis of geothermal systems in the uplifted mountain-type of Hainan Island

The Hainan region has developed faults, widely distributed hot springs, and abundant geothermal resources. By analyzing the isotopic and hydrochemical characteristics of 28 groups of geothermal water in Hainan Island, a genesis model of a mid-low temperature convective geothermal system in uplifted mountainous areas was established, revealing the hydrochemical processes and genesis of groundwater. The types of geothermal water are mainly Cl-Na and HCO3-Na. Based on 2H and 18O analysis, the geothermal water comes from atmospheric precipitation recharge in mountainous areas, with an elevation of 200-1250m. Using a SiO2 geothermal thermometer, the reservoir temperature of Hainan Lanyang, Guanxin, and Qixianling springs is calculated to be above 140°C, and the groundwater circulation depth is 4-5km. Based on the geothermal geological characteristics, atmospheric precipitation infiltrates into the central and southern Hainan Island, absorbing the rock heat around the deep Mesozoic granite reservoir, forming geothermal fluid, rising along the east-west and northeast deep fault, partially mixed with seawater in the shallow part, and discharged out of the surface at weak cover layers to form hot springs.