The Sao Pedro do Sul granite, Portugal: hydrogeochemistry related to the exploration of uranium

The Sao Pedro do Sul granite in the north-central part of Portugal is highly radioactive but is not affiliated with any known uranium mineralization. This setting therefore provided an interesting case study for the hydrogeochemistry of uranium. Spring, tap and stream water samples were collected and analysed for pH, Ec, Na, K, Ca, Mg, Cl, SO4, HCO3, H4SiO4, PO4, NO3, DOC, F, U, Sr and Ba. Univariate and multivariate statistical techniques were applied to gain an understanding of the important factors that influence the water composition. Evaporation appeared to dominate the chemistry of the waters. Faults and the deeper weathering of the granite are also reflected in the water composition, especially in the behaviour of the pH, HCO3, F and H4SiO4. The U concentrations are low and do not show any interesting groupings. The combined results concur with the absence of known outcropping mineralizations, and also indicate that the presence of hidden ore bodies is unlikely. (orig.)