Geochemistry of geothermal fluid with implications on circulation and evolution in Fengshun-Tangkeng geothermal field, South China

•Geothermal fluid from Fengshun Tangkeng area for power generation is recharged by precipitation from surrounding mountains.•Silicate minerals dissolution, cation exchange in the reservoir and volatile migration from mantle are dominant geochemical processes for geothermal fluid evolution.•About 5.6–12.2% He and Ne are from mantle through regional scale faults systems with orientations of NE and NW.•Mixing with shallow groundwater occurs during geothermal fluid ascending process.