Conditions for veining in the Barrandian Basin (Lower Palaeozoic), Czech Republic: evidence from fluid inclusion and apatite fission track analysis

Abstract

The interplay between fracture propagation and fluid composition and circulation has been examined by deciphering vein sequences in Silurian and Devonian limestones and shales at Kosov quarry in the Barrandian Basin. Three successive vein generations were recognised that can be attributed to different stages of a basinal cycle. Almost all generations of fracture cements host abundant liquid hydrocarbon inclusions that indicate repeated episodes of petroleum migration through the strata during burial, tectonic compression and uplift. The earliest veins that propagated prior to folding were displacive fibrous “beef” calcite veins occurring parallel to the bedding of some shale beds. Hydrocarbon inclusions within calcite possess homogenisation temperatures between 58 and 68 °C and show that the “beef” calcites originated in the deeper burial environment, during early petroleum migration from overpressured shales. E–W-striking extension veins that postdate “beef” calcite formed in response to Variscan orogenic deformations. Based on apatite fission track analysis (AFTA) data and other geological evidence, the veins probably formed 380–315 Ma ago, roughly coinciding with peak burial heating of the strata, folding and the intrusion of Variscan synorogenic granites. The veins that crosscut diagenetic cements and low-amplitude stylolites in host limestones are oriented semi-vertically to the bedding plane and are filled with cloudy, twinned calcite, idiomorphic smoky quartz and residues of hardened bitumen. Calcite and quartz cements contain abundant blue and blue–green-fluorescing primary inclusions of liquid hydrocarbons that homogenise between 50 and 110 °C. Geochemical characteristics of the fluids as revealed by gas chromatography–mass spectrometry, particularly the presence of olefins and parent aromatic hydrocarbons (phenonthrene), suggest that the oil entrapped in the inclusions experienced intense but geologically fast heating that resulted in thermal pyrolysis of its hydrocarbons. This implies that the organic fluids in the fractures may have been partly influenced by heating associated with igneous intrusions that are hidden below the surface. Subvertical N–S-striking veins represent the most recent fracturing event(s). Some of these veins are only a few millimeters thick and sparsely mineralised with thin leaf-like quartz crystals that contain tiny blue and yellow–orange-fluorescing hydrocarbon inclusions. Most of the N–S veins, however, occur as thick calcite veins that generally crystallised at 70 °C or less from H 2O–NaCl solutions of variable salinity with admixture of petroleum. The origin of these fluids is interpreted in terms of deeply circulating meteoric waters that partially mixed with deep basinal fluids. Wider structural considerations combined with fission-track analysis of adjacent host sediments suggest that N–S veins formed during post-Mesozoic uplift of the area, probably in response to major Tertiary Alpine deformations transmitted far into the Bohemian Massif.