This study presents a detailed study of the dimensions, geometry, textures and breccias of a well-exposed epithermal vein system, the Kestanelik gold deposit in the Biga Peninsula, NW Turkey, and investigates the permeability enhancement mechanisms in epithermal gold deposits. Here mineralization is associated with quartz veins up to 13.6 m thick. Vein textures and breccia components indicate repeated sealing and subsequent brecciation of wall rock and pre-existing vein infill. Field and petrographic analyses characterize east–west-trending veins as left lateral faults, whereas NE–SW-trending veins are extensional (Mode I) fractures. Cataclasite and tectonic breccia of wall rocks and early quartz, hydrothermal crackle breccias, and matrix-supported chaotic breccias of pre-existing vein infill, all of which are cemented by late iron-oxide-bearing quartz, indicate that co-seismic rupturing and hydraulic fracturing are two major permeability enhancement mechanisms. In addition, transient variations in local stress direction, caused by syn-mineralization dyke intrusion, may have enhanced permeability on misoriented surfaces and at locations where the dip changes. This study emphasizes the importance of understanding structural geology and kinematics as controls on the location of boiling and mineralization mechanisms in epithermal gold deposits.