Chemical And Isotopic Characteristics Of Rehai Geothermal Field In Tengchong, Yunnan

The Rehai geothermal field lies about 14 km from the Tengchong Town, Southwestern Yunnan, China. The field is still very rich in active manifestations such as boiling spring, fumoroles and hydrothermal explosion and is of high significance due to its theoretical research sense.Cl, B and other elements of the spring water demonstrate very good linear correlation, implying that the springs share a same deep hot reservoir and get mixed with groundwaters at different levels. ¹⁸O and D isotopes of the hot springs form a line dipping towards to the "andesitic water", indicating an obvious contribution of mantle water. Also, with the inverse modeled ¹⁸O and D of the reservoir, we identify that the geothermal field is charged from the Northern and Eastern mountainous areas at altitude of 1600 m to 2300 m. The more enriched ³H in some hot springs than the annual mean ³H of the local precipitation should be noted. Greater part of the deep hot water may be recharged before 1951; otherwise, the reservoir may be very young, with very quick circulation of precipitation.¹³C of total dissolved carbon (TDC) in hot spring water and CO₂ diffused from the springs or fumaroles is also investigated to estimate the process subground. It can be concluded that ¹³C of TDC in water decreases with the progressively degassing of CO₂. The decreasing rate will be accelerated either due to continuing temperature dropdown or to larger extent of gas degassing. Also, we find that the springs with high ¹³C-TDC cover areas which are either distributed with deep faults or intruded by the acid volcanic rocks.³He/⁴He of the gas diffused from water or fumaroles demonstrate good spaticial correlation with faults and magma below. The deeper the faults reveals, or the nearer the spring is to the magma, the higher the ratio of ³He/⁴He is.Chemical and isotopic geochemistry of spring water and gas provide us a deep insight into the reservoir and the subsurface process. Together with geothermometries of Na-K, SiO₂, CO₂-CH₄ etc, we put forward a conception model of the hot reservoir. The reservoir can be recognized as two aquifers at different depth; the shallower one is about 250-320 m subground with temperature of about 220-230℃and very quick water circulation; the deeper one is distributed at depth more than 660-785 m with temperature of 270-280℃, and the deep fluid is about 400-450℃.