Genesis of Kupferschiefer Cu-Ag deposits by convective flow of Rotliegendes brines during Triassic rifting

Abstract

Extensive and rich Cu-Ag sulfide mineralization occurs in Poland across the contact between the Upper Permian Zechstein restricted marine sequence and the Lower Permian Rotliegende continental volcanic and clastic sequence. Geologic evidence suggests that the mineralization was formed during late diagenesis when metalliferous brines migrated through the Rotliegende, leaching metals from the volcanic detritus, and up the flanks of basement highs, possibly along fracture porosity, to the pyritic Kupferschiefer and Zechstein limestone above. Thick evaporites in the lower Zechstein preclude a vertical flow-through model, but metal zoning attitudes suggest that the brines overturned where the Rotliegende pinched out against the highs and moved laterally along the base of the Zechstein toward the basin centers, presumably to sink back down into the Rotliegende, completing a convection cell. A Middle Triassic paleomagnetic age for the metal zoning and Kimmerian attitudes of dilatant sulfide veinlets indicate that the mineralizing event coincided with continental rifting associated with the opening of the Tethys ocean.Slow unicellular convection may be commonplace in sandstone basins where small lateral temperature gradients, delta T, exist, but greater velocities are necessary to form large orebodies. An anomalous tensional and thermal event, such as rifting, can increase velocities by increasing permeability, slope angle, or delta T. The paleothermal structure of southwestern Poland was determined by modeling the conductive heat flow in 14 one-dimensional geologic sections which described the evolution of the basin architecture for 10 m.y. in the Early Triassic. Rifting was simulated by increasing the lower boundary condition from 500 degrees to 1,000 degrees C at 25 km. This thermal pulse produced a surprisingly high delta T of 25 degrees C across the Lubin ore district solely from differences in thermal conductivities between the basement high (4.2 W/m degrees C), the sandstone (2.5 W/m degrees C), and the shale basin center (1.25 W/m degrees C). Unicellular convection patterns would be induced by the lateral boundary conditions of warm basement highs and cold shale centers and by the greater horizontal permeability. These cells were 15 to 20 km long, 400 m high, and subhorizontal.A slope angle of 2 degrees and a permeability of 1 D produces a convection velocity of 13 cm/yr which, with a copper solubility of 1,000 mg/kg in 20 to 30 percent Ca-Na-Cl brines in equilibrium with hematite, can form the Lubin deposit in less than 6 m.y. Using a fracture permeability of 2 D and a solubility of 300 mg/kg, the time needed is less than 10 m.y. The Konrad mine can be formed in similar time periods. Without the continuous recycling of the brine inherent in convective flow (20 times or more), the metal solubilities needed to form the Lubin deposits by a flow-through model would be unreasonably high.Natural gases likely migrated along with the metalliferous brines and helped convection by creating secondary porosity and increasing the buoyancy of the fluids. Because the fluids are recirculated and not expelled, convection provides a way in which secondary migration of methane and petroleum can occur effectively in solution as well as in separate phases.