Evidence of oxidation of the Kupferschiefer in the Lubin-Sieroszowice deposit, Poland: implications for Cu-Ag and Au-Pt-Pd mineralisation

In the western part of the Lubin-Sieroszowice mining district, processes of secondary oxidation of the Kupferschiefer sediments have led to the formation of the Rote Fäule hematitic footwall alteration and resulted in a unique Kupferschiefer profile clearly comprising reduced, transitional and oxidised rocks. Redox zones were identified by petrographic, geochemical and stable isotope studies of selected core and mine sections. The vertical petrographic and geochemical zonation of the Kupferschiefer sections implies that this variation is the result of an ascending flow of hydrothermal oxidising fluids through the basal part of the Zechstein sediments. The upward, cross-formational flow and water-rock interaction resulted in the oxidation of the initially reduced Kupferschiefer shales that led to the destruction and leaching of unstable components, leaving only refractory and immobile constituents behind within the Rote Fäule. The oxidised rocks are characterised by an abundance of ferric Fe oxides, the presence of gold in association with hematite, high concentrations of aromatic hydrocarbons and asphaltenes, the lowest delta13 C and delta18 O values in carbonates and the highest delta 34 S values in disseminated sulfides. The residual organic matter is significantly depleted in bitumen and hydrogen, and characterised by the absence of alginite and sporinite, low collinite and bituminite contents, and a high relative proportion of solid bitumen. A considerable loss of pyrite and base metals coincides with organic matter degradation. The association of Au-Pt-Pd mineralisation with the oxidised rocks, and Cu-Ag ores with the reduced sediments implies that the processes forming the Rote Fäule were paralleled by sulfide mineralisation. The Rote Fäule/ore system developed as post-sedimentary event caused by large-scale flow of metal-bearing fluids from the underlying Rotliegend aquifer.