Fault zones serve as pathways for large quantities of hot fluids that may change rock composition and structure, generating ore deposits and geothermal fields. This typically leads to quartz precipitation reflecting cyclicity of deformation and fluid-flow. The dextral strike-slip Pfahl shear zone (Germany) represents a fault zone with frequent pulses of deformation and fluid-flow. Polarizing microscopy and cathodoluminescence analyses reveal several stages of fluid flow, quartz crystallization and fragmentation: (i) At least three early stages of silicification and kaolinization of granitoid basement rocks result in µm-sized quartz matrices with variable amounts of kaolinite. The matrices are transect by an isotropic network of µm-thin quartz veins and show quartz overgrowth. (ii) Fragmentation during shear-zone activity and fluid flow leads to mm-cm wide veins roughly parallel to the fault zone and filled with blocky 100-500 µm-sized quartz grains. (iii) Brittle deformation in a central fault-parallel zone causes a massive quartz dyke with complex patterns of fragmentation and quartz overgrowth. (iv) Continued dextral shearing produces a set of steep, N-S oriented and cm-dm spaced fractures that locally form mm-cm wide quartz veins with voids, indicating decreased fluid flow. (v) µm-thin quartz veins with overgrowth textures represent the final stage of fragmentation coupled with silicification. The Pfahl shear zone, characterized by brittle deformation during fluid flow, represents long-term activity of a large-scale hydrothermal system. It is an excellent example of cyclic stress and strain-rate variation, fluid flow and mineralization. Fig. 1: Overview photograph of the study area (Waschinger quarry 1 km W of Regen/Germany), with mainly massive white quartz in the center and brownish altered wall rocks on the right side
