Hydrogeochemistry of granitic mountain zones and the influence of adjacent sedimentary basins at their tectonic borders: the case of the Spanish Central System batholith

The hydrogeochemical characteristics of springs in the granites of the Spanish Central System (SCS), a mountain range affected by cortical tectonic structures, are described, along with an investigation of the spring water origin. In springs with variable flow and where they are associated with minor alterations and fractures, water type is Ca-(Na)-HCO₃ with low total dissolved solids (TDS; 54–200 ppm) and a neutral or slightly acid pH. In springs that have continuous flow and association with relevant fractures, water type is Na-HCO₃, with higher pH and TDS (240–563 ppm). There are five springs with water type Na-(Ca)-Cl and high TDS (780–9,205 ppm) near the SCS’s southern tectonic borders. Within 5,000 years of apparent residence time, Ca-(Na)-HCO₃ water progresses to Na-HCO₃ type, slightly increasing HCO₃ content, losing Ca due to calcite precipitation and ionic exchange, and gaining S due to sulphate reduction. No changes are observed after 20,000 years of apparent age. The influence of internal CO₂ is compatible with Na-HCO₃ water type characteristics, facilitated by fracturing. Most of the springs yield cold water; thus, a flow depth up to 500 m can be deduced. Only the Messejana-Plasencia fault region shows Na-HCO₃-type thermal springs. Chloride type water is a result of the influence of the Tertiary basins’ water moving towards the southern border of the SCS. The reverse fault in such contact, together with the subvertical structures affected by it, are responsible for the springs emerging in the granite. The northern border of the SCS shows the opposite behaviour.