Water-rock Interaction Ascribed to Hyperalkaline Mineral Waters in the Cabeço de Vide Serpentinized Ultramafic Intrusive Massif (Central Portugal)

Abstract

The conceptual model of Cabeço de Vide mineral waters, issuing at the contact metamorphism zone developed by the local mafic/ultramafic pluton, is described. These mineral waters discharge with a temperature between 17 and 20ºC, show a very alkaline pH (up to 11.8), and have: Na-Cl/Ca-OH facies; dry residuum ≈ 200 mg/L; Ctotal < 3.0 mg/L; and extremely low Mg2+ and silica concentrations (< 0.3 and 6.5 mg/L, respectively). CH4 isotopic composition (-24.4 o/oo < δ13C-CH4 < -14.0 o/oo and -285 o/oo < δ2H-CH4 < -218 o/oo) falls in the range of typical abiotic gas. In the case of Cabeço de Vide mineral waters, the understanding of the resource will help to preserve the mineral water quality, creating public interest.