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

The Permian siliciclastic Kupferschiefer of Poland is the host for economic ally important Cu-Ag deposits. Within the Kupferschiefer of the Polish Zech stein basin, mineralogical and detailed stable isotopic analyses of clay mi nerals, quartz and calcite show that the metal enrichment processes are ass ociated with significant changes in oxygen and hydrogen isotope composition of the illitic clays, with a decrease in illite crystallinity and an incre ase in the percentages of neoformed illite (1M/1M(d) polytypes) in the clay size fractions. D/H and O-18/O-16 ratios of the minerals in the host rock have been affected by the mineralizing process. The magnitude of the isotop ic shift in O-18/O-16 (delta(18)O Of illite decreases of similar to 4 parts per thousand) and D/H of illitic clays (delta D increases of similar to 40 parts per thousand) corresponds with distance to "Rote Faule" and Cu-miner alized zones within the Kupferschiefer. The isotopic zonation of fine-grain ed illite is explained by fluid-rock interaction with ascending, oxidizing solutions, which are thought to have been responsible for base metal deposi tion and formation of Rote Faule. Published microthermometric fluid inclusi on data and the degree of maturation of organic matter imply a maximum temp erature between 100 degrees and 150 degrees C during host rock alteration a nd 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 (delta(18)O between + and +7 parts per thousand; delta D between - 3 and -23 parts per thousand) argue for its origin from meteoric waters by salt dissolution and evaporation in the sed imentary basin of the Rotliegendes. A participation of heated geopressured brines from Zechstein salt-bearing rocks and, therefore, an origin from eva porated 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 Rtliegendes volcanics with the saline waters. Upward migration and penetration into the Kupferschiefer ware restr icted to zones of enhanced permeability at the margins of the sedimentary b asin and adjacent to paleohighs. (C) 2000 Elsevier Science B.V. All rights reserved.