Differences in the depositional environment of basal Zechstein in southwest Poland: implication for base metal mineralization

Abstract

Base metal enrichment in the Permian basal Zechstein sediments occurs in two major areas in southwest Poland, the North-Sudetic Syncline and the Fore-Sudetic Monocline. Two profiles of Kupferschiefer and adjacent limestones from the Konrad mine and from the Rudna mine were studied by petrographic and geochemical methods in order to recognize similarities, and differences, in the comparative depositional environment of both areas. In the North-Sudetic Syncline basal Zechstein was deposited in a shallow marine environment under slightly oxidizing sedimentary conditions. It is composed of 1.2m thick mergelkalk, which might be the equivalent of Werrakalk (Ca1). In the Fore-Sudetic Monocline the basal Zechstein is a black shale, the so-called Kupferschiefer (T1) with a thickness of 0.2–0.5 m. Microscopical investigations have shown that a substantial contribution of higher continental plant material is observed only in the North-Sudetic Syncline because of its location near the Zechstein sea-shore. Pyrite is present in small amounts only. In the Fore-Sudetic Monocline, apart from the former shoreline of the Zechstein sea, the organic matter is preferentially composed of degraded algae due to its deposition under deeper anoxic bottom waters. Abundant pyrite is also present. Differences in the chemical composition of the respective kerogens are reflected in the results obtained from Rock-Eval analyses. Hydrogen indices (HI) are significantly higher in the T1-section of the profile from the Rudna mine, in comparison to the basal Zechstein of the Konrad mine profile. A depletion of hydrogen is only observed in the highly mineralized section and in Rote Fäule of the Konrad mine. This indicates that hydrogen equivalents from organic matter participated in metal sulfide precipitation by thermochemical sulfate reduction (TSR). This is further confirmed by the occurrence of pyrobitumen, saddle dolomite and calcite spars. In contrast, in the mineralized section of the Rudna mine profile a depletion of hydrogen is not observed, pyrobitumen is missing and saddle dolomite and calcite spars are very rare. Instead, abundant pyrite originating from bacterial sulfate reduction (BSR) could have been the source of reduced sulfur species, available for base metal precipitation after deposition of the sediment (pyrite replacement). The results allow one to conclude that in both areas base metals were precipitated from ascending solutions due to the interaction with the reducing Zechstein barrier. Several observations argue for a post-depositional mineralization process at a temperature of less than 130°C in both areas.