Mineralogy, crystallinity and stable isotopic composition of illitic clays within the Polish Zechstein basin: implications for the genesis of Kupferschiefer mineralization

The Permian siliciclastic Kupferschiefer of Poland is the host for economically important Cu-Ag deposits. Within the Kupferschiefer of the Polish Zechstein basin, mineralogical and detailed stable isotopic analyses of clay minerals, quartz and calcite show that the metal enrichment processes are associated with significant changes in oxygen and hydrogen isotope composition of the illitic clays, with a decrease in illite crystallinity and an increase in the percentages of neoformed illite (1M/1Md polytypes) in the clay size fractions. D/H and 18 O/16O ratios of the minerals in the host rock have been affected by the mineralizing process. The magnitude of the isotopic shift in 18O/16O (518O of illite decreases of ∼4‰) and D/Hof illitic clays (δD increases of ∼ 40‰) coeresonnds with diatance to “Rote Fa¨ule” and Cu-mineralized zones within the Kupferschiefer. The isotopic zonation of fine-grained illite is explained by fluid-rock interaction with ascending, oxidizing solutions, which are thought to have been responsible for base metal deposition and formation of Rote Faule. Published microthermometric fluid inclusion data and the degree of maturation of organic matter imply a maximum temperature between 100° and 150°C during host rock alteration and ore formation. The characterization of the fluids as basinal brines were confirmed by the published fluid inclusion studies. The estimated isotopic composition of the mineralizing brines (δ18O between +2 and +7%0; δD between –3 and –23%o) rgue for its origin from meteoric waters by salt dissolution and evaporation in the sedimentary basin of the Rotliegendes. A participation of heated geopressured brines from Zechstein salt-bearing rocks and, therefore, an origin from evaporated seawater cannot be excluded. The results, together with previously published K-Ar data on illite, argue for a post-depositional origin of base metal mineralization within the Kupferschiefer of Poland. The metals were most probably derived by leaching of Rotliegendes volcanics with the saline waters. Upward migration and penetration into the Kupferschiefer were restricted to zones of enhanced permeability at the margins of the sedimentary basin and adjacent to paleohighs.