Geochemical and isotopic composition of organic matter in the Kupferschiefer of the Polish Zechstein basin: relation to maturity and base metal mineralization

Abstract

Drill core samples from the Kupferschiefer of Poland were collected throughout the Zechstein basin. The samples included oxidized Kupferschiefer from Rote Fäule zones, adjacent Cu-mineralized Kupferschiefer of southwestern Poland, and drill cores from the central and northern parts of the Zechstein basin. The Kupferschiefer samples reflect differences in base metal mineralization and in burial depth (630±5067 m). The organic matter of the Kupferschiefer is characterized by Rock-Eval and GC-MS analyses. Classification of kerogen by hydrogen and oxygen indices (HI, OI), correlations of T max vs the present depth of the Kupferschiefer, soluble organic matter (SOM) yields, and relative proportions of saturated and aromatic hydrocarbons of the SOM provide evidence for an oxidative alteration of organic matter in highly mineralized Kupferschiefer samples near the Rote Fäule zones. This is confirmed by differences in the composition of the saturated and aromatic hydrocarbon fractions of the soluble organic matter: Saturated hydrocarbons from Rote Fäule samples are dominated by short-chain n-alkanes and higher abundances of pristane and phytane relative to heptadecane (n-C17) and octadecane (n-C18), respectively, compared with samples more distant to the Rote Fäule zone. Compositional changes of the aromatic hydrocarbon fractions with decreasing distance to that zone are characterized by the occurrence of polycyclic aromatic hydrocarbons and elevated ratios of phenanthrene to methylphenanthrenes that are attributed to demethylation reactions and resulted in a decrease of the methylphenanthrene index (MPI 1). Kupferschiefer samples from the barren zone of the Polish Basin do not show these alteration patterns. The observed variations in organic matter composition with burial depth are consistent with changes due to increasing thermal maturation. Maturity assessment is achieved from MPI 1 and the methyldibenzothiophene ratio (MDR). From the relationship between the maturity of organic matter in terms of vitrinite reflectance values and depth of the Kupferschiefer strata, a continuous increase in reflectance of vitrinite is obtained within the Polish Basin. The alteration pattern of organic matter related to base metal mineralization of the Kupferschiefer corresponds to changes in the isotopic composition of organic carbon and calcite. Kerogen within, or close to, Rote Fäule zone is enriched in 13 C caused by the preferential release of isotopically light organic compounds through progressive degradation of organic matter. The opposite tendency towards lower d 13 C and d 18 O values of calcite provides evidence for isotopic exchange between carbonate and the oxidizing, ore-bearing solutions and for organic matter remineralization. In contrast, organic matter and calcite from the Kupferschiefer do not show regular trends in d 13 C with increasing thermal maturation.

Correlation between hydrogen isotope ratios of lipid biomarkers and sediment maturity

“…Both basins should help clarify the influence of different maturation in areas with a low geothermal gradient (30°C/km) (Bechtel et al, 2000a), such as Poland, and in a contactmetamorphic zone related to a magmatic intrusion during the Lower Cretaceous, Hils syncline of the "Vlotho Massiv" .…”

“…The geological setting of the KS of Northern Poland is described in Bechtel et al (2000a). The total KS deposition of Ͻ 100,000 yr has a mean sedimentation rate of 1.5 to 5 cm/ka.…”

“…The Lower Permian Basal Zechstein "Kupferschiefer" horizon (KS, Kupferschiefer, Wuchiapingian) of the barren-zone, NE Poland Zechstein basin (Bechtel et al, 2000a) and the extensively studied Jurassic Posidonienschiefer (PS, Lower Toarcian), Northern German basin were used for this study. Both basins should help clarify the influence of different maturation in areas with a low geothermal gradient (30°C/km) (Bechtel et al, 2000a), such as Poland, and in a contactmetamorphic zone related to a magmatic intrusion during the Lower Cretaceous, Hils syncline of the "Vlotho Massiv" .…”

Phenyl derivatives of polycyclic aromatic compounds as indicators of hydrothermal activity in the Silurian black siliceous shales of the Bardzkie Mountains, Poland

“…On the contrary, values of the methylphenanthrene index (MPI1) are low and differ greatly from all other maturity parameters. That is due to the significant decrease of methylphenanthrene isomers in relation to phenanthrene, and was described in detail for the hydrothermally-altered Rote Fäule zone in the Permian Kupferschiefer (Bechtel et al, 2000(Bechtel et al, , 2001Püttmann et al, 1990;Sun, 1998).…”

Timing and genesis of base-metal mineralisation in black shales of the Upper Permian Ravnefjeld Formation, Wegener Halvø, East Greenland

“…In Poland and Germany, important Cu deposits are found together with Pb and Zn in the Kupferschiefer (e.g. Jowett et al 1987aJowett et al , 1987bVaughan et al 1989;Pu¨ttmann et al 1990;Oszczepalski 1999;Bechtel et al 2000), and in eastern England the time-equivalent Marl Slate and overlying carbonates of the Cadeby Formation contain abundant occurrences of Pb, Zn, Cu, Fe, Ba and F (Turner et al 1978;Vaughan and Turner 1980;Harwood and Smith 1986). Since the first discoveries of copper, lead and silver on Wegener Halvø, East Greenland, by members of the Danish Three-Year Expedition to East Greenland between 1930and 1934(Eklund 1944Koch 1955), Upper Permian carbonate rocks in central East Greenland have been known to be pervasively mineralised with base metals, Ag and Ba.…”