Chemistry of hydrothermal quartz in the post-Variscan “Bavarian Pfahl” system, F.R. Germany

Abstract

Chemical analyses of hydrothermal quartz by means of instrumental neutron activation, coulometric, dewatering, and decrepitation analysis are combined with previously published microthermometric and isotopic data in order to unravel the processes involved in the formation of the post-Variscan hydrothermal system “Bavarian Pfahl”. The Bavarian Pfahl is interpreted as a fossil zone of tectonic activity and large-scale channelized fluid movement in the upper continental crust. The predominance of K relative to Ca and Na in quartz as well as in fluid inclusions strongly supports the hypothesis that incongruous dissolution of feldspar during wall-rock alteration and K-metasomatism during fluid ascent along the shear zone accompanied by intense silicification are characteristic features of important fluid pathways in the crust. The element contents detected in quartz along a 70 km long profile can be correlated with variable wall-rock compositions. High concentrations of Ba, As, Rb, Sr, and Cs are found in the vicinity of sedimentary basins at the western margin of the Bohemian Massif. Elevated element contents in this part of the profile probably result from intense water-rock interaction within the adjoining sedimentary basin. High Ba-contents in the vicinity of the fluorite-barite mining district of Nabburg/Wölsendorf indicate a genetic relationship between both mineralizations. Low Cl/Br-ratios can be generated either by interaction with carbonaceous sediments in these basins or by interaction with graphite in basement rocks. The calculated salinities of the fluid inclusions vary widely, but correlate well with microthermometric data, and indicate mixing of basement or sedimentary brines with high salinity and meteoric water with low salinity. The effects of polyphase cataclasis, a result of seismic pumping, on the fluid inclusion population and the mechanical properties of quartz have been studied by combined dewatering and decrepitation analyses. Despite minor regional variations in the chemical composition of quartz along the profile, the overall uniform element pattern supports homogeneity of the hydrothermal system over distances of at least 70 km and probably over the total length of about 150 km. The fluid volume required for the formation of the Bavarian Pfahl is estimated to correspond to the volume of the Baltic Sea (approx. 23,000 km 3). In analogy with hydrothermal springs in the vicinity of recently active shear zones and faults the lifetime of the hydrothermal system “Bavarian Pfahl” is estimated to be about 160,000 years.