Precipitation of fracture fillings and cements in the Buntsandstein (NW Germany)

The relationship between fracturing and fracture filling in opening-mode fractures in the Triassic Buntsandstein in the Lower Saxony Basin (LSB; NW Germany) has been studied by an integration of petrographic and structural analysis of core samples, strontium isotope analysis and microthermometry on fluid inclusions. This revealed the relationship between the timing of the fracturing and the precipitation of different mineral phases in the fractures by constraining the precipitation conditions and considering the possible fluid transport mechanisms. The core was studied from four different boreholes, located in different structural settings across the LSB. In the core samples from the four boreholes, fractures filled with calcite, quartz and anhydrite were found, in addition to porefilling calcite cementation. In boreholes 2 and 3, calcite-filled fractures have a fibrous microstructure whereas in borehole 1, fractures are filled with elongate-blocky calcite crystals. Anhydrite-filled fractures have, in all samples, a blocky to elongate-blocky microstructure. Fractures that are filled with quartz are observed in borehole 2 only where the quartz crystals are ‘stretched’ with an elongated habit. ... mehrFluid inclusion microthermometry of fracturing-filling quartz crystals showed that quartz precipitation took place at temperatures of at least 140°C, from a fluid with NaCl–CaCl 2–H 2O composition. Melting phases are meta-stable and suggest growth from high salinity formation water. Strontium isotopes, measured in leached host rock, indicate that, in boreholes 2 and 3, the fluid which precipitated the calcite cements and calcite-filled fractures is most likely locally derived whereas in borehole 1, the 87 Sr ⁄ 86 Sr ratios from the pore-filling cements and in the elongate-blocky calcite-filled fracture can only be explained by mixing with externally derived fluids. The elongate-blocky anhydrite-filled fractures, present in boreholes 1, 3 and 4, precipitated from a mixture of locally derived pore fluids and a significant quantity of fluid with a lower, less radiogenic, 87Sr ⁄ 86 Sr ratio. Taking into account the structural evolution of the basin and accompanying salt tectonics, it is likely that the underlying Zechstein is a source for the less radiogenic fluids. Based on the samples in the LSB, it is probable that fibrous fracture fillings in sedimentary rocks most likely developed from locally derived pore fluids whereas elongate-blocky fracture fillings with smooth walls developed from externally derived pore fluids.