Abstract
Fault systems are zones of crustal weakness and are used as pathways for ascending hydrothermal fluids. One impressive example is the more than 150 km long Bavarian Pfahl system at the SW boundary of the Bohemian Massif. Dextral ductile shearing of Moldanubian basement has shaped a several hundred meter wide zone of mylonites and ultramylonites. This setting was reactivated under brittle conditions and infiltrated by fluids resulting in a multi-phase quartz lode. Structural analysis using scanning electron microscopy cathodoluminescence (SEM–CL) images has been carried out in combination with trace element and oxygen isotope analysis on quartz along the system. Three successive quartz phases have been distinguished due to their structural details and trace element contents. Cryptocrystalline quartz fabric in the first quartz phase results from crystallization from a silica gel, whereas in phases II and III, quartz precipitation is characterized by repeated fragmentation and sealing, visible due to strongly varying CL intensities. The δ18O of Bavarian Pfahl quartz decreases from 13.8‰ in the NW to 8.5‰ in the SE, whereas quartz from the close-by-situated Wölsendorf fluorite deposit shows δ18O between 15.9 and 18.5‰ (vs. SMOW). Flat shale-normalized rare earth element patterns point towards an upper crustal origin of the hydrothermal fluids. The southeastern part of the Bavarian Pfahl is interpreted to represent a deeper crustal level with higher mineralization temperatures (about 350 °C) compared to the rest of the Pfahl (< 250 °C).
Highlights
Hydrothermal veins are abundant both in the continental and in the oceanic crust of the Earth and indicate the flow of hydrous fluids through the crust
According to Gerler (1990), the proportions of element contents in Bavarian Pfahl quartz stored in fluid inclusions lie below 15% for most elements that were determined in this study
Bavarian Pfahl mylonites formed the pathway for ascending fluids
Summary
Hydrothermal veins are abundant both in the continental and in the oceanic crust of the Earth and indicate the flow of hydrous fluids through the crust. Some fluids apparently are enriched in metals and other elements so that the precipitation from these fluids leads to the formation of hydrothermal ore deposits of Cu, Au, Pb, or Zn and of other economically important minerals such as barite or fluorite (Wilkinson et al 2009; Seward et al 2013). International Journal of Earth Sciences (2019) 108:521–543 the transport of large fluid volumes to the upper crust. Geochemical analysis of hydrothermal vein minerals is, a powerful tool to understand fluid evolution and vein formation. A detailed knowledge of the structural relationship of succeeding mineralization phases is required to result in a realistic interpretation of geochemical analysis
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