This study focuses on the mineralogical and chemical analysis of sediments overlying basaltic basement through which seawater circulates (ODP Leg 168, flank of the Juan de Fuca Ridge). A SEM and TEM-EDX study of the authigenic clays and zeolites allowed the characterization of the hydrothermal phases and the determination of their mechanism of formation. Silicate authigenesis related to hydrothermal alteration of sediment was observed in the sediment layer just above basement at sites located >30 km from the ridge axis. This sediment alteration is particularly intense at Site 1031 where authigenic formation of Fe-Mg-rich smectites (montmorillonite and saponite) and zeolites occur, linked to the dissolution of biogenic calcite. Vertical advection of basement fluid through the sediment column is required to produce this alteration. This process is still active at Site 1031, based on systematic variations in porewater profiles and temperatures obtained from stable isotopic data on calcium carbonates, and the nature of authigenic minerals.