Variable alteration of organic matter in relation to metal zoning at the Rote Fäule front (Lubin-Sieroszowice mining district, SW Poland)

Abstract

Drill core samples of the Lower Zechstein strata in the Lubin-Sieroszowice Cu-mining district (SW Poland) were collected at different distances to the oxidized zone (Rote Fäule), developed around the Zary Pericline. The sampled intervals include Kupferschiefer, underlying Weissliegendes sandstone and overlying Zechstein Limestone from Rote Fäule zones, adjacent Cu-mineralized strata, and drill cores from the Pb/Zn-mineralized zone more distant to the Rote Fäule front. The Kupferschiefer samples show differences in base metal and noble metal contents, accompanied by variations in the molecular composition of hydrocarbons. Organic matter is characterized by the soluble organic matter (SOM) yields, and relative proportions of saturated and aromatic hydrocarbons of the SOM. Our results provide evidence for an oxidative alteration of organic matter in Kupferschiefer samples within and adjacent to the Rote Fäule zones. The extent of this alteration is reflected by differences in the composition of the saturated and aromatic hydrocarbon fractions of the soluble organic matter. In contrast to Kupferschiefer from Pb/Zn-mineralized areas, short-chain n-alkanes dominate the saturated hydrocarbons of oxidized samples. With decreasing distance to the Rote Fäule zone, compositional changes of the aromatic hydrocarbon fractions are characterized by the increase in concentration of polycyclic aromatic hydrocarbons and elevated ratios of phenanthrene to methylphenanthrenes, resulting in a decrease of the methylphenanthrene index (MPI 1). In highly oxidized samples, the increase in the ratio of phenanthrene to methylphenanthrenes concentrations due to demethylation reactions appears to be reversed, probably caused by degradation of phenanthrene as well as the methylphenanthrenes. Organic matter of noble metal-bearing Kupferschiefer from the Rote Fäule front is characterized by high abundances of polycyclic aromatic hydrocarbons (PAHs) and sulfur- and oxygen-bearing PAHs. This result might argue for a concomitant transport of the PAHs and the noble metals in ascending oxidizing solutions, and a contribution of metal-organic complexes in mobilisation and transport of gold. The regional base metal zonation in the Kupferschiefer inside the mines, is explained by redox controlled metal precipitation from oxidizing brines through decreasing Eh-conditions during its infiltration into the reduced Kupferschiefer. Short-scale zonation in Au and Pt/Pd-contents at the oxidized-reduced interface is consistent with that model of ore formation. Organic geochemical maturation parameters argue for maximum temperatures during Kupferschiefer diagenesis between 100 and 120°C. Comparable temperatures are assumed for the ore fluids responsible for Cu mineralization near Rote Fäule.