Chemical characteristics and circulation process of geothermal water beneath Longchuan basin, western Yunnan

A survey of geothermic geology has been conducted to clarify the distribution of geothermal resources beneath the Longchuan basin. Results show that geothermal sources are exposed at 11 sites in this basin, in which heat reservoirs are zonal and layered type structures. Of them, the layered type appears in the central part of the basin, and the edges of each side of the basin are of zonal type. In the northwest of the basin, the heat reservoirs are obviously controlled by faults, while in the southeast joints and cracks have profound influence on the geothermal sources. The host rocks of these thermal reservoirs include metamorphic rocks, granitic glutenite and fine sandstone of Neogene Manban formation. Hydrochemical analysis suggests that geothermal water is obviously affected by atmospheric precipitation, and its circulation depth is above 1，600 m, mostly between 1，800 and 2，400 m, mainly from the surrounding mountains, with recharge distance over 1.5 km. The Nanwanhe hot spring has the highest temperature and the deepest groundwater circulation, with the longest recharge distance up to 10 km. Overall the water temperature is relatively high and water circulation is deep in the northeast and west of the basin. The heat of the geothermal water comes from the deep thermal source. It is a mixed type of geothermal water that receives groundwater recharge from mountainous areas and surface water through faults, fissures and sand and gravel pores as upwelling channels for water diversion and storage.