Springs and streams of the Taro–Ceno Valleys (Northern Apennine, Italy): Reaction path modeling of waters interacting with serpentinized ultramafic rocks

Abstract

In the area of the Taro–Ceno Valleys (Northern Apennine, Emilia-Romagna region, Italy), waters of meteoric origin interact with ophiolite rocks of the External Ligurides. Fresh water springs issuing from basalts have a Ca–HCO 3 composition, whereas freshwater springs from ultramafites vary in composition from Ca–HCO 3 or Mg–HCO 3 to Na–OH or Na–SO 4 types and in pH values from 7.3–8.8 up to pH 11, respectively. In addition, the boron content of the alkaline waters is up to 13 mg/L, which is unusually high for freshwaters in general and ultramafites that have undergone oceanic serpentinisation in particular and gives a boric alkalinity to the waters. The springs waters show evidence of recent low-temperature continental serpentinisation and the process is modeled by reaction paths using an updated geochemical thermodynamic database, consistent with the local primary and secondary serpentinite paragenesis. For the model, bicarbonate waters evolve to alkaline waters supersaturated in Ca–(Mg)-carbonate, based on the assumption that the dissolution of serpentinite results in supersaturation with respect to kaolinite, ferrihydrite, vermiculite, Fe 2+–Mg 2+-saponite, and poorly crystalline serpentine. The alkaline composition and the chloride content of the waters suggest a prolonged interaction with the rocks at depth that led to dissolution of albite and leaching of olivine-hosted fluid inclusions. A similar evolution is also proposed for the more developed springs issuing from the ultramafic rocks of the Voltri Group (Liguria region), where solutions are supersaturated in brucite and are in equilibrium with enstatite and/or chlorite.