Veins in the Buntsandstein (NW Germany)
This is an overview map of the CEBS with the different structural elements and location of our study area. On the enlarged map you see the location of the boreholes where we studied Buntsandstein cores and sampled veins.
We have chosen these 4 boreholes because they are located in a different structural position. This is illustrated by profiles through the different boreholes. Borehole 1 is in an inversion structure, borehole 2 is located on top of a salt pillow, borehole 3 is again in an inversion structure but in this area the inversion was stronger, compared to the first borehole. Borehole 4 is located on top of a salt pillow and next to a salt dome.
The first surprise in this study was that although the cores are located in different structural position and are quite far apart, the mineralogy of the vein fill and the microstructures are quite similar. We observed mainly 2 types of veins. The first type are fibrous calcite veins, which are oriented normal to bedding. This vein has a straight wall rock interface in the upper part of the picture, where the host rock is composed of clay. Where the host rock becomes more coarse-grained in the lower part, the fracture is delocalized and the vein splits up into many thin branches. The second type of vein is again oriented normal to bedding but contains blocky anhydrite. The vein to wall rock interface is much more regular and these veins can have thicknesses of a few centimeters.
The microstructures in the veins contain information about the fracturing and the precipiation conditions of the vein minerals and a microstructural study is therefore a very useful tool to evaluate vein formation. This is a typical microstructure of our fibrous calcite veins. The fibres are normal to the vein wall. On the left the vein microstructure is asymmetric. This indicates that this is an antitaxial vein which grew outwards towards the wall rock, but more rapidly on one side than the other. The fibrous crystals indicate that the growth velocity of the crystals was on average equal to the opening rate of the vein and that there was no growth competition. This means that there was no open fracture during vein growth and no large increase in bulk permeability when these veins were formed. These veins could have formed as a a response of a change in the far field stress but it is also possible that the crystals pushed the walls apart by the force of crystallization. The microstructure on the right shows how sometimes these veins incorporate fractured fragments of the vein wall. However, the crystals are still fibrous and parallel with each other.
Similar microstructure are observed at the contact of mudstone and a sandstone. The fracture does not cross the sand grain, indicating that the rock was not yet sufficiently consolidated to allow transgranular fracturing. So, in summary, the calcite veins formed before the rock was strongly consolidated and this vien growth event was not associated with open fractures and strong increase in permeability
The microstructures in the anhydrite vein are quite different. Here you see elongate-blocky anhydrite crystals with clear evidence for growth competition. This indicates that the opening velocity was faster than crystal growth and there was an open fracture and significant increase in permeability when the crystals precipitated.
Sometimes one can see the relation between the anhydrite veins and the calcite veins. In this picture it is clear that the anhydrite grows in a fracture in the calcite: showing that the calcite vein was fractured and blocky anhydrite precipitated in reactivated fracture.
Stable isotopes were measured in the calcite veins and in carbonate cements in the host rock. In this plot the carbon isotope data is plotted against the oxygen isotope data. Values range from -11.98 to -6.11 ‰ (PDB) for d18O, and between -2.81 and 1.87 ‰ for d13C. Values from cements are similar as those from the veins: this indicates that the fluid that precipitated in the veins is the same as in the cements. Additional arguments come from CathodoLuminescence: this image shows that vein and cement have the same luminescence colour. The carbon isotope values around 0 indicate that the carbon is derived from initial pore water and is not derived from organic material. . From the oxygen isotopic composition we can estimate the temperature at which the calcite precipitated if we know the fluid compostion. If we assume that the original pore water was evaporitic meteoric water, we can take a negative d18O of -5 ‰ SMOW. This would however mean precipitation temperatures which are very low. We think that it is more likely that the initial pore fluid mixed with more positive connate fluids during the early burial and therefore we propose a fluid with a higher d18O of 0 to 4 SMOW. This corresponds to a precipitation temperature between 55 and 122°C.
In the large blocky crystals of the anhydrite veins, we found clear fluid inclusions in growth zones. Homogenisation temperatures are between 120 & 250°C with some exceptions around 105°C. The highest frequency is around 150°C - we interpret this value as the minimum precipitation temperature in the anhydrite veins. The large range may be explained by a long period of anhydrite precipitation under changing temperature. But perhaps it is more likely that the fluid inclusions were reequilibrated or stretched when the veins were increased in temperature during further burial. An additional argument for this interpretation is that often we find a wide range of homogenization temperatures in the same crystal. The fluid has water-sodiumchloride-calciumchloride composition and based on measurement of melting temperatures of ice, hydrohalite and halite daughter, we can say that the fluid has very high salinities of 3 wt% NaCl & 20 wt% CaCl2.
To find the source for the anhydrite, we measured sulfur isotopes. Values vary between 11.9 and 13.4. If we compare these with the values for seawater sulfate during the Lower Triassic, we see that these values are higher. If we compare the values in the veins with the Permian signature, we see a much better correspondance. Therefore the Zechstein evaporite is the most likely source for sulfate-rich fluids thess could have formed when the primary gypsum transforms to anhydrite.
This is a characteristic subsidence curve for southern part of LSB from Petmecky et al. We can use this diagram to find the depths where the veins precipitated when we know the precipitation temperature. All the veins we observed are oriented normal to bedding and are extensional veins. This means that they were formed with the largest principle stress being vertical, during subsidence of the basin. Veins formed during the inversion would be horizontal or more oblique if they were reactivated. We know from the oxygen isotopes that the calcite veins precipitated between 55-122°C, this means that they precipitated at depth between 1.5 and 2.5 km. The fluid inclusions indicated that the anhydrite veins precipitated around 150°C or higher temperatures. This means that they formed at min. depth of 3.2 km.
Now that we know the depth of the vein formation and the orientation of sigma one, we may be able to say a bit more about stress conditions during vein growth. This can be illustrated by a Mohr diagram. To form extensional veins, we need a small differential stress because the Mohr circle has to touch the failure envelope in the tensile regime. This implies that the differential stress is less than four times the tensile strength of the rock. For our extensional veins, formed during subsidence, the vertical stress is the maximum principal stress and is equal to r (density of the rock) multiplied by g (gravitation) multiplied by the depth, which is for 3.2 km equal to 78MPa. The minimum principal stress is equal to –T. A typical value for the tensile strength of Buntsandstein mudstones is around 10 Mpa. In this case, the vertical effective stress has to be reduced by an increased fluid pressure to allow the left foot of the Mohr circle to touch the enveloppe.
One way to calculate the fluid pressures is with the brittle-failure-mode diagram from Sibson, where we can read the overpressure for a certain failure mode, which is for us extensional, for given tensile strength T at the depth of interest. If we assume that the tensile strength is 10MPa, in the Bunter mudstones, the differential stress must be less than 40 MPa and therefore the min fluid pressure we need is 18MPa above hydrostatic for this depth to create extensional fractures
Summarizing once more, we can conclude that both generation veins in the Buntsandstein are extensional veins which formed during subsidence and not during inversion. This is a regional trend, observed in four wells far apart and in different structural settings. The calcite veins formed at relatively shallow depth (max. 2.5 km), without the presence of open fractures. The anhydrite veins probably formed between 3-5km. During their formation, it is likely that the Buntsandstein was slightly overpressured.7th oct. 2004 sofie