The composition of brines in the early diagenetic mineralization of the Permian Kupferschiefer in Germany

The Kupferschiefer is an approximately 0.5 m thick black marly shale of Lower Zechstein age in Germany. If one includes some of its footwall and hanging wall, it contains 300 Mt Cu, 800 Mt Zn and 300 Mt Pb. The regional pattern of metal distribution demonstrates its relationship to Variscan and Permian tectonic structures. Faults and the topographic relief of the basement apparently controlled the uprise and lateral migration of reducing and slightly acid hot brines from deeper crustal levels to supply the metals for the mineralization of the Proto-Kupferschiefer. Deep fluids are mainly richer in Zn than Pb and richer in Pb than Cu. Mixing of such slightly acid fluids with slightly alkaline formation waters (seawater) caused a gradient in pH from about three to eight and in sulfide concentration. Most of the sulfide came from dissolved pyrite which was very light in sulfur isotopes. This gradient controlled the sequential precipitation of bornite, chalcopyrite, (chalcocite), galena and sphalerite, which is observed in a lateral and vertical zoning of these sulfides. The fluids experienced a fractionation of the metals during migration over meter to kilometer distances from the tectonically controlled vents within the unconsolidated Proto-Kupferschiefer. Close to the vents the sulfide deposits attained concentrations up to 2% Zn + Pb + Cu. The migration of the metals over large distances took place in unconsolidated sediment. Thus the major mineralization of the Kupferschiefer has to be classed as an early diagenetic process.