Hydrochemical And Isotopic Characteristics Of Geothermal Water In The Rehai Region In Tengchong Of Yunnan

The Tengchong area in western Yunnan province of China is located in the Yunnan-Tibet geothermal belt.The faults were well-developed and a lot of high-temperature hot springs occur in this area.This paper studies the hydrochemical and isotopic characteristics of the geothermal waters from the Rehai(RH)geothermal field in Tengchong and the nearby Hehua and Longwo hot springs to clarify the source of elements,the factors affecting the elements contents,the hydrochemical processes happening in the subsurface path and the genesis of the geothermal waters.The hydrochemical types of the geothermal waters are of Cl·HCO₃-Na,HCO₃·Cl-Na,HCO₃-Na,SO₄·Cl-Na and SO₄-Ca type.According to the hydrochemical types,the geothermal waters can be grouped into bicarbonate geothermal waters and sulfate waters.The bicarbonate geothermal waters in RH are of high contents of Na,Cl,HCO₃,Li,Be,F,Sc,Ti,V,Cr,Ga,Rb,Nb,Sb,Cs,W,Tl,Zr and Si,and low contents of Ca,Mg and SO₄.Due to the different mixing ratios between the cold and geothermal waters,the geothermal waters from the north side of the Zaotang River contain more elements than those of the south side of the river.Cl in most of geothermal waters in the study area is derived from leaching of the host rocks and the supply of the magmatic water.Ca,Mg and SO₄are mainly derived from dissolution of the host rocks,mixing with shallow cold groundwater and the oxidation of H₂S.Na,K and HCO₃are mainly derived from dissolution of the feldspar minerals by mantle-derived CO₂.The contents of trace elements in the geothermal waters are mainly related to the incongruent dissolution of the host rocks,the precipitation of minerals and the supply of magmatic water.The rare earth elements(REEs)in the geothermal waters inherit the characteristics of REEs in the host rocks to some degree.Negative Ce anomalies may result from precipitation of Ce O₂due to oxidative scavenging process of Ce and absorbed by Fe oxyhydroxides.Low p H value,high Temperature,low TDS and dissolution of Fe-bearing minerals is beneficial to the increase of the contents of REEs.Ln(CO₃)₂^-and Ln CO₃⁺are the main complextion speciation in the alkaline,weakly alkaline water,neutral water and weakly acidic geothermal water.In the strong acidic geothermal water,Ln SO₄⁺is the main speciation.In the high-temperature goethermal waters in RH,Banglazhang,Yangbajing and Yangyi geothermal fields,Na is the main cation and one or two or three of Cl,HCO₃and SO₄is the main anions.The Na-K-Mg triangular diagram indicates that most high-temperature geothermal waters have partially equilibrated with the host rocks.The high-temperature geothermal waters are mainly of meteoric origin,and the extent of“¹⁸O shift”is different due to the different altitudes and the supply of magmatic water.The deep geothermal reservoir temperature of RH is estimated as about 250?with Na-K geothermometer and that of the Hehua hot spring is 167.6/179.3?.The mixing ratios of cold water in the geothermal waters range from 12.91%-54.26%and the circulation depth is aproximately 1594 m.After receiving recharge from precipitation in the metamorphic rocks of the Gaoligong Mountain Group in the northeast mountainous area,the geothermal water in RH undergoes deep circulation and is heated by heat source from below.The geothermal waters ascend toward the land surface and discharges in the form of hot springs along the faults.