English abstractThis thesis was written on the purpose of identifying and characterising a hydrothermal vent complex, associated with sill complexes at lower stratigraphic levels. The studied hydrothermal vent complex, which was called "Witkop I", was situated in the Karoo Basin in South Africa. Field work and geochemical analysis, and petrographic studies as well, were done in order to figure out whether this conduit-structure had been influenced by hydrothermal fluids and whether the complex can have acted as a migration pathway for fluids in the basin. Seismic velocity-analysis were done on a few samples to characterize elastic properties of the hydrothermal vent.
From the field it was obvious that Witkop I was consistent with mainly cream-yellow sandstone, which contained non-horizontal cracks following sedimentary structures within the ventfill. Sedimentary structures indicate in situ deposition. The sidewall of the hydrothermal vent complex consists of horizontally layered altering sedimentary units of cream yellow sandstone and red siltstone from the uppermost part of the Elliot Formation. Hydrothermal vent complexes are usually associated with layers dipping towards the vent complex as a result of a collapse of the complex. Horizontally layered sidewall may indicate little or none vertical movements of the complex due to collapse. Observations such as a breccia-dike, comprising of fragments of sandstone and a matrix of the very same material as the fragments, indicates phreatic eruptions after the deposition of the ventfill. A calcite-cemented pipe-like structure can indicate calm migration of fluids. The calcite was tested for 13C and 18O isotope content to conclude the origin source for C and the precipitation-temperature, in case it could reveal a hydrocarbon source or hydrothermal activity.
From petrographic analysis from Scanning Electron Microscope it was concluded that the autigenic minerals present in the studied lithologies are probably due to normal burial diagenesis. None hydrothermal minerals are present. The geochemical analysis reveals a precipitation fluid, which is normal for meteoric waters and which is non-hydrothermal. Seismic velocities are lower in the vent complex compared to the surrounding strata due to lower porosity. The lower porosity in the vent complex can be due to higher content of pertite lamellas in Potassium-feldspars, which are easily weathered away, making secondary porosity. This can also be the reason why Witkop I stands out as a positive erosion structure in the field.
Compared to other hydrothermal vent complexes in the Karoo Basin, Witkop I has low content of brecciated materials and little or none vertical movements of the complex. Other complexes such as Witkop II and II described by Svensen et al. (subm.) have been hydrothermal influenced and more brecciated. The explanation for this can be that the local build up of overpressure which causes phreatic eruptions and brecciation and the formation of a vent to the surface, is due to collapse of the structure. Since the sidewall of Witkop I is horizontally layered, and due to its low content of brecciated material, it is suggested that hydrothermal vent complexes with similar characteristics are results of build up of local pressure in the underground due to little or none collapse of the complex. Brecciated materials can be a result of local build-up of pressure mainly caused by collapse of the structure. Maturing of organic rich materials could also cause a build-up of pressure causing brecciation, though there are no strong indications of this from the geochemical analysis. It is proposed that hydrothermal vent complexes with similar characteristics represent the uppermost level of the vent complex. At lower levels, the vent complex could be presented different; with an outer zone comprising of inward dipping strata; consisting of hydrothermal minerals and, perhaps, containing magmatic materials from the sill complex.