Hydrothermal alteration of Hercynian granites,its significance to the evolution of geothermal systems in granitic rocks

We discuss geochemical and isotopic (¹⁸O/¹⁶O, ²H/¹H and ⁸⁷Sr/⁸⁶Sr) data recording the hydrothermal alteration of northern Portuguese Hercynian granites by Na–HCO₃–CO₂-rich mineral waters. Whole-rock samples from drill cores of Vilarelho da Raia granite have δ¹⁸O values in the +11.47 to +10.10‰ range. The lower values correspond to highly fractured granite samples displaying vein and pervasive alteration. In the pervasive alteration stage, which probably results from a convective hydrothermal system set up by the intrusion of the granites, the metamorphic waters are in equilibrium with hydrous minerals. In contrast, the vein alteration of these granitic rocks was caused by water of meteoric origin. The oxygen ratios between water (W) and rock (R), the so-called W/R ratios, obtained for the open system (where the heated water is lost from the system by escape to the surface) range between 0.05 and 0.11, suggesting that the recrystallization of the veins was influenced by a small flux of meteoric water. Stable isotope analyses performed on the cores show that the vein alteration stage relates to post-emplacement tectonic stresses acting on the granite, probably of late Hercynian age. Our results are consistent with the existence of two separate alteration events (pervasive and vein) caused by hydrothermal waters of different isotopic characteristics. The studies presented in this paper should be viewed as a natural analogue that uses the alteration features observed in a “fossil” geothermal system at Vilarelho da Raia to assess possible water–rock reactions presently occurring at depth in granitic rocks of the nearby Chaves area.