Characteristics and formation of the Dakongbang hot spring in Yun county of Yunnan

Located in Yun county of Yunnan,Dakongbang hot spring lies in the Yunnan-Tibet geothermal belt.In the hot spring area,the geothermal reservoir is mainly composed of the Lincang Granite with well-developed fractures.There are five spring orifices with water temperature ranging [JP2]from 88.3 to 96 ℃. Most of the cations in the hot water are K+, Na+, Ca2and Mg2+, and most of the anions are Cl-, SO42- and HCO3-. The hot water is characterized by HCO3Na type in hydrochemistry,and TDS 0.625～0.702 g/L, pH 6.7～9, H2SiO3 161.1～261 mg/L and F-5.85～16.1 mg/L. According to analysis of stable isotope, the δ2H and δ18O contents of the hot water samples indicate that the hot spring originates from precipitation. The elevation of the recharge area is estimated at 1 800m.Temperature of recharge area is about 3.6～7.2 ℃, and that of geothermal reservoir 143～181 ℃.The circulation depths of the thermal groundwater are approximately 2 858～3 496 m.The formation of the hot spring can be summarized as that recharging by precipitation in the surrounding mountain areas,the groundwater flows to deep and large fractures,heated by heat flows during deep circulation,then upwells as geothermal water along [JP]the fracture zones and finally crops out after mixing with cold shallow groundwater as boiling springs and fountains of high temperature.

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