Geochemical studies in the drainage basin of the Rio Vouga (Portugal): II. A model for the origin of hydrothermal water in the Vouzela region

Abstract

The hydrothermal waters in the Vouzela region (Portugal) have a temperature of 68°C and are characterized by a high content of F, Li, B, bicarbonate, sulphate and silica. Ninety-three water samples, including two hydrothermal waters, were analyzed for major and trace elements in order to investigate the genesis of the hydrothermal water. In the Vouzela region two Older Hercynian Granites are exposed, the Abas Granite and the São Pedro do Sul Granite. The Abas Granite is intersected by the Ribama fault. Albitization is ubiquitous. Other alteration effects are tourmalinization, K-feldspathization and muscovitization. These processes and the associated Sn—W deposits were induced by the intrusion of the Younger Viseu Granite. In the present situation rainwater penetrates into the Abas Granite especially via the Ribama fault and the greisenization zone. It becomes part of a hydrothermal system, leaches Li, F, B and silica, and rises via the Termas fault in the hydrothermal springs of Termas de São Pedro do Sul. Geothermal equilibrium calculations yield a temperature of 205 ± 10°C for the thermal reservoir. The percentage of mixing with cold groundwater near the surface in the circulation system is estimated at 73%. The circulation system is possibly maintained by the heat generated by the decay of U, Th, K and their daughter products, elements which are encountered in extremely high concentrations in the São Pedro do Sul Granite.