Characteristics and origin of the Denggeng and Mabu hot spring in Lushui County, Yunnan Province

Located in the Lushui County of Yunnan Province,the Denggeng hot spring and the Mabu hot spring emerge on the western bank of the Nujiang River valley. Sand of the Quaternary (Q₄²),basalt of the Upper Carboniferous Woniusi Group (C₃w) and limestone of the Lower Formation of Triassic Hewan Group (T₂h¹) underlie the study area. The temperature of hot springs range from 48.9 to 69.6℃,and TDS from 0.493 to 0.782 g/L. The hydrochemical types of hot water are of HCO₃·Ca-Mg type and HCO₃·Ca-Na type. The hot springs are of low to moderate temperature and weak acid. The F^- concentrations of the hot springs range from 0.78 to 2.13 mg/L,and the H₂SiO₃ concentrations from 41.0 to 70.2 mg/L. Li,Sr,Rb,Cs and Ba are relatively abundant in the hot water. The values of δ²H and δ¹⁸O of the spring water indicate that the hot springs in the study area are meteoric in origin. The elevation of the hot spring's recharge area is estimated to be from 1260 to 1435 m,the temperature of the recharge area varies in the range of 6.18-9.02℃,and the temperature of the geothermal reservoirs varies in the range of 100 -127.5℃. The content of Ca²⁺ and HCO₃^- in hot water is rich,and the percentages of milliequivalent concentrations of anions and cations are high,accounting for more than 60% and 82% respectively. Calcite and aragonite in the hot water are in the saturated state,the content of CO₂ in water is high and the pCO₂ in the hot water is much higher than atmospheric pCO₂. The hydrochemical and hydrodynamic conditions are favorable to CaCO₃ deposition. Travertines are precipitating near YLS3-1,YLS3-3 and YLS3-4 of the Denggeng hot spring and YLS4-1 of the Mabu hot spring. Thermal groundwater rises up to the land surface on the western bank of the Nujiang River valley after groundwater receives recharge from infiltration of precipitation in the recharge areas of the mountainous areas to the west of the Nujiang River,undergoes deep circulation and obtains heat from heat flow. The hot springs are of the deep infiltration-cycle type and are of low to moderate temperature. The hot water mixes with the shallow cold water when it rises to the land surface. The proportion of mixing cold water is about 60%-73%,and the circulation depth of the hot water ranges from 2375.2 to 3161.7 m.