Formation of Rotliegend Ca-Cl brines in the North German Basin compared to analogues in the geological record

Abstract

The formation of terrestrial Ca-Cl brines is controlled by two major processes: chemical reactions between halite brines and igneous and metamorphic rocks and/or their debris in sedimentary basins, and the abstraction of H2O by evaporation, freezing or formation of hydrous minerals. Albitization dominates in environments of low H2O activity, i.e., halite saturated systems. Chloritization, serpentinization and formation of smectites seems to be related to less saline systems, i.e., enhanced water activity.The Rotliegend brines of the North German Basin evolved by interaction of infiltrating halite brines with volcanic debris of the Permian acid volcanic rocks by albitization and chloritization of plagioclase. At present their temperatures are between 130 and 150°C at 3000–4000m depth. Although they developed in a different environment they chemically and isotopically resemble the basinal brines from the Mississippi Salt Dome basin or even the deep brines from the Red Sea. All these brines have in common that halite brines interacted with plagioclase (An+SiO2+2Na+⇔2Alb+Ca2+) or possibly with clay minerals (Ca-clay+2Na+⇔Na2-clay+Ca2+) at enhanced temperatures. The trends of stable isotopes show slopes in the δD-δ18O diagram which are typical for hydrothermal reactions of evaporation brines with rock or sediments. The genesis of Rotliegend brines differ from Ca-Cl brines in the Canadian Shield, Siberian Platform, the Dry Valleys in Antarctica and even the local brines from the Chilean salars and the Californian Bristol Dry Lake. The Antarctic Dry Valley brines seem to be dominated by melting of ice-cements of their permafrost sections. The Ca-Cl brines of springs in the Chilean salars and the Bristol Dry Lake ascend due to thermal convection along tectonic faults. Although the Canadian Shield brines chemically resemble those of the Siberian Platform their stable isotope fractionation suggest that they developed by different processes of water abstraction. Exotic stable isotope values in brines are gained by freezing or ultrafiltration processes in the precursor sediments of the present crystalline rocks of the Canadian Shield and by small volume ratios of liquids and metamorphic rocks such as of KTB/VB.