FLUID FLOW AND QUARTZ CRYSTALLIZATION IN THE PFAHL SHEAR ZONE: DEVELOPMENT OF LARGE-SCALE FLUID PATHWAY TO A FOSSIL HYDROTHERMAL SYSTEM

During the post-Hercynian activity of the Pfahl shear zone a 150 km long and 30-100 m thick almost continuous quartz dyke formed along the main fault zone, with a lenticular and partly symmetric structure. The main quartz dyke is surrounded by sheared granitoids and gneisses which are, to a large extent, transformed by metasomatic processes to kaolinite, chlorite and phyllosilicates.

Four subsequent stages of quartz crystallization can be detected. (1) A homogeneous µm-sized, dark grey or red quartz mass with mm- to cm-sized angular wall-rock fragments, completely altered to kaolinite. (2) A µm-sized quartz mass with light grey to pink color, which contains fragments of the first quartz type. Both quartz generations form a mosaic texture with random crystallographic orientation and partly intricate fluid inclusion structures, suggesting formation during two (or several) fragmentation episodes and from silica gel precursors that underwent recrystallization after precipitation. (3) A set of mm- to dm-wide quartz veins roughly parallel to the trend of the Pfahl zone, cross-cutting the two first generations of fine-grained quartz groundmass, in connection to intense fracturing and multiple fluid injection. (4) Steep, roughly N-S oriented mm- to dm-thick quartz veins, oblique to the general trend of the Pfahl and cross-cutting the earlier quartz masses and veins. The veins are partly open, due to incomplete quartz precipitation, and accompanied by cm- to dm-spaced fractures of the same orientation, with subhorizontal striations that locally indicate dextral shear sense, coherent with the kinematics of the Pfahl shear zone.

The different types of quartz masses and veins allow to investigate the evolution of an important hydrothermal system that developed during the latest movements of the Pfahl shear zone. Early quartz formation by precipitation from supersaturated silica solutions was followed by quartz formation from decreasing saline hydrothermal fluids. Brecciation was favored by overpressure stages of fluid flow, leading to fracturing. Later en-echelon veins indicate evolution to less overpressured fluids with still less silica content.