FISCHBECK, REINHARD R., OTTO BORNEMANN, and GERHARD MINGERZAHN
Halite-carnallite veins were observed at a depth of 355 m when an exploration mine shaft was sunk in the Gorleben salt dome. Gorleben is located in NW Germany near the Elbe river. The salt dome is being considered as a site for permanent storage of radioactive waste materials. The long-term integrity of a salt dome can best be assessed by the study of mineral reactions and mineral substance transport which occurred primarily during the formation of the dome and in the geological past. Harmful substances can be transported to the biosphere from a permanent repository in a salt dome by aqueous solutions only. Therefore, understanding how the halite-carnallite veins were formed in the geological past is important for the evaluation of the future integrity of the salt dome.
We propose a model for the formation of the halite-carnallite veins which is based on petrographic examinations and the fact that the bromine content of the chloride salt minerals of the veins can be used as an indicator of their genetic origin and the rubidium content can be used as an indicator for the potassium-bearing salt minerals. Halite was formed when solutions rich in MgCl from within the salt dome mixed with NaCl solutions from the fringe of the salt dome or from outside. The MgCl solutions are of metamorphic origin or remnant solutions from the Zechstein sea. The fractures were not completely filled by the crystallization of hypidiomorphic halite. Later, the 1 to 2 cm thick sulfate layers which permeate the wall rock acted as pathways for MgCl solutions rising from the hot lower regions of the salt dome. Carnallite then crystallized in the remaining voids due to the drop in temperature.