Abstract

Water–rock interactions are one of the main factors that control water quality in rivers worldwide. Hydro-geochemical analyses from mountain rivers of the North of the Sierra de Velasco (Argentina), in combination with petrological studies of the granitic rocks, suggest that A-type plutons are one of the main fluorine suppliers in surface waters that drain these granitic terrains. In the rivers of the northern sector of the study area, the dissolved F− is above 1.72 mg L−1 and reach up to 3.65 mg L−1, while in the southern sector the values range between 0.27 and 1.05 mg L−1. Conversely, there are no significant differences in the major hydrochemistry of the studied rivers, since all the waters are diluted (TDS: 36–116 mg L−1), slightly alkaline (pH: 7.50–8.16), of bicarbonate-calcium type, and with no seasonal influence of precipitation. The elevated values of F− in waters spatially match the F-rich A-type strongly peraluminous granite of La Costa Pluton. Fluorine in this unit is five to ten times higher than in the S-type peraluminous granite from the southern sector, and is mainly hosted in micas and fluorapatite. The higher dissolved trace elements and REE in the northern sector, also match the different lithological sources. Therefore, the identification of A-type peraluminous granites in a catchment area could be used as a petrological indicator to predict a possible high fluoride content in river waters. Additionally, high fluoride values in surface waters could be a valuable proxy of the presence of A-type plutons in the nearby region.