An additional source for the hydrochemical formation of geothermal waters in granites

Abstract

The hydrochemical components mainly come from the dissolution of granite minerals for geothermal waters in granites as reservoir rocks. The hydrothermal alteration and albite dissolution are considered the main source of Na+ content. However, the average content of Na+ in medium-low temperature geothermal waters caused by the dissolution of sodium-bearing minerals in granites and derived from ion exchange are 42 mg/L and no more than 60 mg/L, respectively. The main hydrogeochemical approaches are hard to explain high Na+ contents of more than 115 mg/L abnormally existing in the medium-low temperature geothermal waters in granites. A case study is discussed in the Heyuan fault zone, South China. For the geothermal waters, the reservoir temperature is about 116.0–136.4 °C with a maximum circulation depth of 5.5 km. Albite dissolution can provide 33.03 mg/L Na+ for geothermal waters at 136.4 °C, but the true Na+ contents in deep geothermal waters range from 193.6 to 1107.3 mg/L. The reality is that granite minerals dissolution can only provide Na+ contents between 10.6 and 66.7 mg/L with an average of 31 mg/L, and cation exchange and seawater intrusion have a minor contribution. We propose that the deep parent geofluid for enhanced feldspar dissolution under high temperature and pressure in the deep below the geothermal reservoir significantly contributes to 84%-85% of high Na+ contents in geothermal waters. Therefore, the deep parent geofluid as an additional source for the hydrochemical formation of geothermal waters in granites as reservoir rocks is hardly ignored.