Chalcopyrite (CuFeS₂) is the main source of copper worldwide and is usually formed at high temperature. Its occurrence at low temperature is poorly documented, and the mechanisms controlling its formation remain uncertain. We found evidences of chalcopyrite formation in acidic pit lake sediments at ~ 12 °C. Using high-resolution electron microscopy and synchrotron-based spectroscopy, we observed aggregates of nanoscale crystals with the composition and structure of disordered chalcopyrite. Laboratory incubations at 20 °C and geochemical modelling suggest that microbial activity may contribute to chalcopyrite formation under these conditions. Particularly, crystal growth was associated with hollow structures resembling microbial cell surfaces, and on the membranes of eukaryotic-like cells, providing nucleation sites. Our findings suggest that microbial processes, including the production of hydrogen sulfide and the presence of organic surfaces, promote chalcopyrite formation at low temperature. This has implications for understanding copper and sulfur cycling and its potential biotechnological application in sulfidic environments.

Hydrogen sulfide (H₂S) in environments with temperatures below 100 °C is generally assumed to be of microbial origin, while abiotic H₂S production is typically restricted to higher temperatures (T). In this study, we report an abiotic process for sulfidogenesis through the reduction of elemental sulfur (S⁰) by hydrogen (H₂), mediated by pyrite (FeS₂). The process was investigated in detail at pH 4 and 80 °C, but experimental conditions ranged between 40 and 80 °C and pH 4–6. The experiments were conducted with H₂ as reducing molecule, and µm-sized spherical (but not framboidal) pyrite particles that formed in situ from the H₂S, S⁰ and Fe²⁺ present in the experiments. Fe monosulfides, likely mackinawite, were identified as potential pyrite precursors. The absence of H₂ production in controls, combined with geochemical modelling, suggests that pyrite formation occurred through the polysulfide pathway, which is unexpected under acidic conditions. Most spherical aggregates of authigenic pyrite were composed of nanometric, acicular crystals oriented in diverse directions, displaying varying degrees of organization. Although it was initially hypothesized that the catalytic properties were related to the surface structure, commercially sourced, milled pyrite particles (< 50 μm) mediated H₂S production at comparable rates. This suggests that the catalytic properties of pyrite depend on particle size rather than surface structure, requiring pyrite surfaces to act as electron shuttles between S⁰ and H₂.