Massive silicification of Miocene sandstones cropping out at the Montjuı̈c mountain in Barcelona has been analyzed in order to constrain paleoflow systems, which may have contributed to the diagenetic reactions. The section consists of five units with alternating sandstone–marl units. The principal diagenetic features are observed in strongly silicified sandstone units. Alunite mineral precipitation indicates presence of saline fluids and low pH. Salinity is most probably derived from marine seawater and low pH may be due to oxidation of pyrite. A quantitative thermodynamic modeling is applied to characterize the percolating fluids and to constrain the hydrodynamic system.