Hydrothermal veins linked with the Variscan structure of the Prague Synform (Barrandien, Czech Republic): resolving fluid-wall rock interaction

Abstract

Variscan syntectonic hydrothermal veins of the Prague Synform are important traces of small-scale fluid migration in Lower Palaeozoic sedimentary rocks -- a process induced by late Variscan tectogenesis. Two main structural types of Variscan syntectonic calcite veins were recognised during fieldwork. Veins of Type I have an irregular or sigmoidal shape and are often arranged in en echelon arrays. A shearing regime during the formation of this type is deduced. Veins of a second structural type (Type II) have a more regular and straight shape relative to those of Type I and in some places form a dense network. The structural position of the Type II veins is related to structural elements of Variscan folds. Veins were formed due to interlayer-slip combined with fold-related fracturing that gave rise to the infilling of dilational structures. A tensional regime also permits growth of the fibrous veins. Two principal directions were distinguished within the Type II veins. The first one is NNW-SSE and the second one shows a perpendicular ENE-WSW orientation. These directions seem to be parallel and/or perpendicular to the nappe architecture of the Prague Synform. Variscan syntectonic veins crystallised in a relatively closed, rock-buffered system. Extraction of chemical components from surrounding rocks is indicated by a combined microprobe/cathodoluminescent study and by isotope geochemistry. The carbon isotope values of hydrothermal calcites reflect the carbon isotope composition of the host rocks. The d13 C values of vein calcites and their host Silurian rocks are between -0.29 and -1.98 PDB. The same relationships were found between the veins and the host Devonian limestones (from 1.72 to 2.52 PDB). Samples close to the Silurian/Devonian boundary show transition values between 0.25 and + 1.16 PDB. The Sr-isotopic signature supports a genetic link between the calcite veins and the host rocks. The 87 Sr/86 Sr ratio in calcites ranges between 0.708619 and 0.708738 and in wall rocks between 0.708755 and 0.709355. Aqueous and hydrocarbon-rich fluid systems have been found in fluid inclusions. Liquid hydrocarbons show mostly a light blue fluorescence suggesting the presence of higher hydrocarbons. They are more abundant in dark Silurian rocks, which are rich in organic matter. Lower salinities (0.5-8.9 eq. wt.% NaCl) and homogenization temperatures with a maximum around 140°C are typical for the aqueous (H2O-NaCl) system. The oxygen isotopic composition of fluids ranges between -2.80 and +3.33 SMOW. This indicates that transformed formation waters interacted with the host rocks and/or deeply circulating isotopically depleted meteoric waters. Intersections with the isochore specify border trapping temperatures between 127 and 160°C and pressures from 300 to 1070 bars.