Quartz mylonites derived from felsic volcanic rocks and granite are a component in the early Paleozoic ductile shear zone at Clarence Stream in the Appalachians of southern New Brunswick. Vein-hosted gold mineralization and the deformed granitic rocks are related to the adjacent late Silurian-early Devonian Magaguadavic Granite. Quartz veins and granitic minor intrusions formed in the ductile shear zone during an early phase of right-lateral strike-slip motion. Veins and their related alteration effects formed in response to localized ductile-brittle transitions during overall ductile deformation, characterized by brittle fracturing (with cataclastite dikes and pseudotachylite formation). Such veins then proceeded to influence subsequent deformation. Vein-fillings and wall-rock alteration changed the pattern of strain partitioning within the shear zone (especially in the quartz mylonites). Fabric overprinting relationships indicate that deformation was strongly domainal, and that ductile-brittle transitions occurred repeatedly during this overall ductile deformation history. Alteration assemblages, preserved as relics, indicate an overall cooling trend, from early zoisite ± amphibole dominated assemblages to quartz ± epidote. These brittle features represent transient permeability, and probably the most important mechanism permitting fluid movement through this shear zone.