A Study Of The Hydrochemical And Isotopic Characteristics And Geneses Of Some Of The Hot Springs In The Xishuangbanna Area Of Yunnan

The Xishuangbanna district is located in the southwest of Yunnan Province and belongs to the geothermal belt of Yunnan and Tibet.It has a good geothermal background.The Lancangjiang Fault exists in this area.The subsurface geothermal zone occurs in the neutral-acid intrusive rocks of the Hercynian-indosinian Period.These conditions have laid a good geological and geothermal foundation for the formation of numerous hot springs.Studies of the characteristics of the hydrochemical and the genesis of the hot springs in the study area may enrich the geothermal knowledge and promote the development and utilization of geothermal resources in the area.This article deals mainly with nine hot springs,and a total of 16 hot spring water samples and one rainwater sample were collected.They are the Fula hot spring,the Mengman Geothermal field,the Manzhao hot spring,the Mengajie hot spring,the Sunhuan hot spring,the Mansa hot spring,the Dongfengnongchang Qifenchang hot spring,the Manbang hot spring and the Manmai hot spring.The temperature of the hot springs ranges from 43?to 85.5?,pH value from 6.5 to 8.1,and TDS from300 to 370 mg/L,belonging to low-temperature and low-TDS hot springs.The Eh value is relatively low,ranging from-388 mV to-257 mV,indicating that the groundwater is in a reducing environment.The AquaChem software was used to analyze the data of the spring water samples.Hydrochemistry types of all the samples are HCO₃-Na type.The main cause for this phenomenon is the large-area incongruent dissolution of the granite base rocks.Except for the abnormal points?the Mengman Geothermal field and the Manzhao hot spring?,the contents of F^-and H₂SiO₃ in the hot springs are generally high,with the contents of F^-of above 10 mg/L and the contents of H₂SiO₃ ranging between 40.6 mg/L and 130 mg/L.Both the contents of F^-and H₂SiO₃ are above the naming standards of Fluorine water and Silicon water.Minor components include Fe and HBO₂,and trace elements,Br,I,Li,Sc,Cs,W,Rb and so on.According to the chemical compositions of the hot springs in the study area,it can be seen that the hot spring water is not drinkable,but bathing with the hot spring water is of therapeutic value.The isotope analysis shows that the hot springs are meteoric in origin.The elevation of the hot spring's recharge area is estimated as about 870-1300 m,the temperature of recharge area,approximately 7?.The hot springs belongs to the deep-circulation spring and the circulation depth of the geothermal systems ranges from 2200 m to 3000m.Through the comprehensive application of the multi-mineral dissolution equilibrium curve,silica geothermometers,silicon germanium equation and silicon germanium graphical method,it is calculated that the temperature of the geothermal reservoirs is about 81-170°C,which belongs to the low-to-moderate temperature geothermal system.The proportion of the mixed shallow groundwater is estimated as 55-80%.The formation of hot springs in the study area can be summarized as:during deep circulation,the hot water from precipitation in the surrounding mountain areas is heated and rises along the fault zones and fractured zones.The hot water mixes with the shallow groundwater and continuously reacts with surrounding rocks when it rises to the land surface.