Copper mineralization of the Polish Kupferschiefer: A proposed basement fault-fracture system of fluid flow

Fluid flow models previously proposed have been unable to account for the 350 Mt of copper metal associated with the Kupferschiefer shales in the southwest part of the Polish basin without an unreasonably high concentration of Cu in the brine transport system. A new conceptual model is presented, on the basis of more detailed knowledge of the structural, stratigraphic and thermal evolution of the Polish basin, that involves a localized high heat flow anomaly present at the time of the main phase of mineralization in the Early Triassic. The mechanism proposed involved injection of hot brine from a deep-seated basement fault-fracture system into Rotliegend eolian sandstone and then updip flow through this porous formation to the Lubin area, where chemical conditions in the immediately overlying Kupferschiefer shale allowed precipitation of the metals from the brine. The copper mineralization of the Lubin area is suggested to have resulted from a large number of short pulses of fluid, and flow was triggered by coseismic strain associated with normal faulting. It is proposed that, within the basement, a network of fractures with high aspect ratios opened within each interseismic period under lateral tension and filled with brine. At the time of a normal-fault-related earthquake, when fault rupture occurred, coseismic strain resulted in the closure of fractures in the vicinity of the fault and brine was expelled. Calculations suggest that, with fluid pressure generated by the coseismic strain close to lithostatic, the force from hot brine injected into the Rothegend sandstone was sufficient to drive the brine through the aquifer at up to 63 m/yr to the site of copper deposition in the Kupferschiefer shale. If flow from each earthquake lasted, on average, for just over a year, then approximately 0.07 km(3) of fluid would have been discharged. If earthquakes and normal faulting occurred at 100- to 200-yr intervals, averaging 7,000/m.y., then 6,000 km(3) of fluid could have been mobilized in a period of 12 m.y. Assuming that the fluid was a brine containing 60 ppm dissolved copper, this volume would have been sufficient to account for the observed quantity of copper in the southwest part of the Polish basin.