The Kupferschiefer Cu–Ag ore deposits in Poland: a re-appraisal of the evidence of their origin and presentation of a new genetic model

The Kupferschiefer stratiform copper deposits in central Europe have long been considered a classic example of syngenetic mineralization. However, metal zoning and host-rock relationships determined during exploration around the recently discovered Lubin district orebodies in southwest Poland suggest that the ore may instead be late diagenetic. The mineralization occurs as thin, extensive blankets of sulphides in the pyritic, organic-rich basal units of the Late Permian Zechstein restricted marine sequence and in the eolian Weissliegendes sandstone at the top of the Early Permian Rotliegendes continental rift sequence of bimodal volcanic rocks and redbeds. It is directly underlain and controlled by irregular, oval zones of Rote Fäule (RF), a barren oxidized and reddened portion of the normally dark grey, pyritic basal Zechstein rocks. In plan view, the metals occur as successive belts enriched in copper (+ silver), lead, and zinc, which developed laterally from and encircle the RF zones. The ore transgresses the depositional strike of the basal Zechstein and can occur in any sedimetary facies. In section, the ore cuts across sedimentary layering from the Zechstein limestone down through the Kupferschiefer shale (Ks) into the sand stone below. Framboidal, presumably very early diagenetic, pyrite is the dominant sulphide above and lateral to the ore zones; over most of the basin, the Ks is pyritic and not anomalously high in metals compared with similar shale elsewhere. In the ore bodies, Cu, Ag, Pb, and Zn are significantly above the typical background contents, implying that these metals have been introduced. Metal contents in the RF are within the range of background contents of the Ks, suggesting that the metal source is external to the Ks. The RF–copper zones are coincident with underlying buried basement highs and occur only above Rotliegendes sediments with underlying volcanic rocks, indicating a possible genetic connection. The metal zones generally dip away from the highs toward the basin centres, suggesting convective, rather than throughgoing, fluid flow. Sulphides, as disseminations and streaks, commonly replace earlier calcite cement and lenses, lithic fragments, and quartz grains. Horizon tal and vertical dilatant veinlets of calcite and copper sulphides appear to have formed after lithification and contemporaneously with the disseminations and streaks, but not by remobilization of existing sulphides. The veinlet orientations are believed to be Kimmerian age (mid-Triassic to Late Jurassic), and the orebodies are cut and displaced, but not remobilized, by fractures and dikes of inferred Alpine (Cretaceous) age. Published sulphur-isotope ratios indicate that the copper sulphides were not formed in isotopic equilibrium with the pyrite, but adequate sulphur- and lead-isotope information on the ore deposits is not available to support an early or late diagenetic timing. The paleomagnetic age of the stable chemical remanence of the RF is Middle Triassic, supporting a late diagenetic origin of the ore coincident with a second rifting event. On account of these asso ciations and controls, it is proposed that the RF–ore systems were formed by converting Na–Ca–CI Rotliegendes formational brines. The metals were leached from the Rotliegendes volcanic detritus and carried in solution as chloride complexes through the redbeds up the flanks of the buried basement highs to the reduced pyritic Ks above. The fluids oxidized the original Ks pyrite to form the Rote Fäule, and the ore sulphides were precipitated on the far side of this subhorizontal oxidation–reduction front. This late diagenetic mineralizing event likely occurred during the Triassic, when rapid burial and increased heat flow, associated with the opening of the Tethys ocean to the south, generated natural gases and vertical fractures in the Ks and increased convective fluid velocities in the Rotliegendes basins.