A stable isotope and organic geochemical study of the relationship between the Anthracosia shale and Kupferschiefer mineralization (SE Poland)

There are at least two mineralized horizons that occur in association with the Rotliegendes, red-bed sediments of Central Europe. The first is the well-known Kupferschiefer at the top of the Rotliegendes, the second is the Upper Anthracosia shale of the North Sudetic Depression — in the middle part of the red-bed section. In the present study, Upper Anthracosia shale has been investigated by use of various geochemical methods and results have been compared with those obtained from previous studies of the Kupferschiefer. Both mineralized horozins show the same organic geochemical phenomena and isotope variations interpreted as a result of post-depositional migration of mineralizing fluids, originating from red-bed sediments. Both organic-rich horizons acted as reducing traps for migrating metals. Therefore, the processes that led to the base-metal emplacement in the ore-grade areas of the Kupferschiefer are shown to have an universal character, also with regards to the Upper Anthracosia shale. The patchy occurrence of Cu mineralization in the Anthracosia shale horizons is related to the fault-controlled movement of mineralized fluids. The Cu-dominated composition of mineralization is explained by the simplicity of the base-metal source, preferentially Rotliegendes formation waters of mostly meteoric origin. New data on the isotopic compositions of C in organic materials and of C and O in carbonates, combined with an organic geochemical study of a non-mineralized section through the Lower Anthracosia shale, provide information on the sedimentary environment and conditions during diagenesis. The changes during sedimentation of the Lower Anthracosia shale are related to subsidence (the thickness of water column), sedimentation rate, redox potential and paleoclimatic conditions. In the center of the profile from the Lower Anthracosia shale the amounts of calcite and organic carbon concomitantly increase, which can be explained by the development of anoxic bottom waters. Due to enhanced subsidence of the basin the water column might have temporarily increased, resulting in increased biomass production and better preservation of organic material. In the lower and the upper parts of the profile the biomass production was significantly lower due to increased rate of sedimentation and probably decreasing water column.