Early diagenetic versus hydrothermal signals in pyrite from ancient metamorphic sediment-hosted massive sulfides – implications for the stability of sulfur and iron isotope records in deep time

Abstract

Stable isotope compositions in pyrite are widely employed for tracing microbial sulfur and iron cycling through geological time. In hydrothermal sulfide systems, however, sulfur and iron pools can be affected by both microbial and abiotic processes, limiting the applicability of the respective stable isotopes as biosignatures. Moreover, the diagenetic and metamorphic stability of sulfur and iron isotope signatures in pyrite under hydrothermal conditions is insufficiently understood. Here, we employed coupled in-situ Secondary Ion Mass Spectrometry (SIMS) triple sulfur (δ³⁴S and Δ³³S) and iron (δ⁵⁶Fe) isotope analysis on morphologically diverse pyrite in ∼390 Ma sediment-hosted massive sulfides to better understand biosignature preservation in hydrothermal systems. Petrographic analysis reveals recrystallized or cemented framboid-like pyrite that was locally overgrown by a secondary generation of subhedral pyrite. δ³⁴S and Δ³³S signatures of the pyrites (−15.13 to +18.77 ‰ and − 0.21 to +0.26 ‰, respectively) can be explained by either microbial or thermochemical sulfate reduction. However, the isotopically lightest δ³⁴S value in framboid-like pyrite (−15.13 ‰) most likely represents a mixed signal of early diagenetic microbial sulfur cycling and later sulfidic hydrothermal fluids driving recrystallization or cementation. The same pyrites show highly variable δ⁵⁶Fe compositions (−1.30 to +2.19 ‰), indicating precipitation from hydrothermal Fe(II) at varying rates and/or pyritization of a diagenetically fractionated iron pool. The lower median δ⁵⁶Fe value in framboid-like versus subhedral pyrite points to a greater expression of kinetic and equilibrium fractionation in the former. This may reflect differences in precipitation rates between early diagenetic (microbial) processes and hydrothermal overprint of the system, consistent with textural evidence for framboid recrystallization or cementation, and overgrowth. Nevertheless, the likely presence of microbially formed pyrite and the incomplete equilibration with hydrothermal fluids highlight that signatures of early diagenetic redox cycling can be preserved in hydrothermal sulfides despite alteration by sulfidic fluids or greenschist metamorphism. Our study stresses the challenges and potentials of coupled textural and in-situ stable isotope analysis for tracing microbial sulfur and iron cycling in hydrothermal sulfide systems through Earth's history.