Origin of salinity of deep groundwater in crystalline rocks

Deep groundwater in fractured crystalline basement has been reported from d eep mines and from scientific deep wells. Highly saline brines have been de scribed from several km depth in the continental basement of the Canadian, Fennoscandian and Ukrainian shields and elsewhere in the world. The origin of salinity is unknown and many different possibilities have been presented , We compare the compositional evolution of deep waters in the Black Forest basement, SW Germany, with those of other deep crystalline waters, and use halogen systematics (e.g, Cl/Br ratios) and other parameters of the waters to deduce the origin of their salinity. In the Black Forest the compositio n of deep thermal waters results from chem ica I interaction of surface wat er with the rock matrix (mainly weathering of plagioclase and mica) and fro m mixing of the reacted water with stagnant saline deep water. Here we show by Na/TDS-and Cl/TDS-investigations, by molality-ratios of the Na and Cl c oncentrations, and by Cl/Br systematics that these deep saline waters have a marine origin. The Cl/Br ratios in deep crystalline waters are very close to normal marine ratios (Cl/Br = 288 ppm basis). In contrast, Cl/Br ratios of other possible sources of salinity show distinctly different Cl/Br rati os: water derived from dissolved Tertiary halite deposits of the rift valle y is in the order of Cl/Br = 2400 and water from dissolved Muschelkalk hali te deposits has values of about Cl/ Br = 9900, Leaching experiments on crys talline rocks, on the other hand, show that the average Cl/Br ratio of crys talline rocks is far below Cl/Br = 100.