Microbial life in deep-seated selenide veins reflected by extreme δ³⁴S fractionation of framboidal pyrite

Microbial life can be found in the continental subsurface down to depths of several kilometers, where it may affect mineral-forming and reaction processes. Sulfur isotopes provide an important tool for discriminating between deep microbial activity and abiotic processes. Here, we report the sulfur isotopic composition (δ³⁴S) of pyrite in selenide-rich domains of hematite-carbonate ± selenide veins hosted in black shales from Tilkerode, Harz Mountains (Germany). Framboidal pyrite (PyII) occurs in voids in clausthalite (PbSe) formed by hematite and carbonate dissolution, but is also enclosed in anhedral pyrite (PyIII). Results of secondary-ion mass spectrometry yielded extreme δ³⁴S values ranging from ‒8.5 to + 92.5‰ for PyII, and from ‒40.1 to + 79.1‰ for PyIII. Extreme positive values suggest microbial sulfate reduction (MSR) under closed-system conditions with limited sulfate availability and slow metabolism, and negative values a switch to open-system conditions. All MSR processes most likely occurred after fast uplift of the vein system from about 5 km to 1-2 km depth during Cretaceous-Tertiary reverse faulting, accompanied by cooling from 220 °C to 30-60 °C, as indicated by fluid-inclusion microthermometry and U‒Pb carbonate dating. The results demonstrate that microbial activity at great depth of 0.8 to 1.8 km is possible even in normally toxic Se-Pb-Ag-Hg-rich environments.