The relationship between hydrothermal fluids and fault development is studied through a petrographic analysis of faulted veins in an open-pit copper mine, Radomiro Tomic (RT), northern Chile. Brittle deformation in RT was initiated with the formation of veins, disrupting low-strain crystal-plastic deformation. Following cooling, shear fractures propagated along these veins with associated fluid influx, leading to phyllic alteration. Hydrolysis converted all plagioclase to kaolinite within the damage zones and all orthoclase to illite within fault cores and the less deformed rocks surrounded by faults. In turn, precipitation of quartz and pyrite locally cemented gouge in the fault cores. Concentration of clays around faults and veins weakened the rocks and promoted shear strain along them. These processes are expected to occur in any porphyry-related hydrothermal system and can combine during the development of the system. Although cementation tends to strengthen fault cores, the overall strength of the faults is expected to be reduced by hydrolysis of plagioclase, reducing the friction of the faults and their capacity to accumulate stress. These findings may have strong implications for understanding the evolution of ore deposit, the rheology of the crust, earthquake generation, and natural hazard assessment.