Signals of low grade organic matter alteration in the Upper Permian Kupferschiefer (Spremberg area, Eastern Germany) – A by-product of copper mineralization?

Abstract

Organic matter (OM) is known to be an important reductant in many sediment-hosted base metal deposits. However, the precise nature of the interactions between OM and hydrothermal fluids is still debated, as is just how such interconnected reactions develop over geological timescales. We investigated the Kupferschiefer mineralization in two drill cores from the Spremberg-Graustein field in Eastern Germany focusing on the inventory of organonitrogen, -sulfur and -oxygen (NSO) compounds, which are most affected by organic–inorganic interactions with mineral surfaces, metals and aqueous fluids. Rather homogeneous deposition of type II marine organic matter, prevailing photic zone euxinia and an early oil window thermal maturity were documented using conventional hydrocarbon analysis. Furthermore, a slight level of hydrothermal alteration was detected in the lower part of drill core 121 h/72, in proximity to the epigenetic hematitic alteration zone known as ‘Rote Fäule’. A minor loss of extractable organic matter and especially long-chain n-alkanes was accompanied by significant enrichment in oxygenated compounds and increased molecular oxygen content detected by ultra-high resolution mass spectrometry (FT–ICR–MS). A strong enrichment of deuterium observed in the n-alkanes (up to −36‰) and isoprenoids (up to −20‰) indicates that hydrogen exchange with an evaporitic fluid occurred in the Spremberg Kupferschiefer over geological timescales at maximum temperatures not exceeding 110 °C. We therefore conclude that FT–ICR–MS, combined with determination of stable hydrogen isotope compositions, can provide valuable information about interaction time scales and the origin of mineralizing hydrothermal fluids.