Microbial mediation in chalcopyrite formation at low temperature

Abstract Chalcopyrite (CuFeS 2 ) is an important source of copper, typically found in high-temperature ore deposits. Its precipitation at low temperature has been rarely documented and remains a topic of debate. In this study, we report the transformation of copper sulfide to a Cu-Fe phase with chalcopyrite-like stoichiometry in acidic pit lake sediments at 12 °C. Scanning/Transmission Electron Microscopy (STEM) revealed aggregates of acicular nanocrystals, and X-ray Absorption Spectroscopy indicated characteristics of disordered chalcopyrite. Incubation experiments at 20 °C suggest that microbial activity may play an important role in chalcopyrite formation at low temperature, also supported by geochemical modelling. Remarkably, STEM analysis showed chalcopyrite crystal formation around hollow structures resembling coccoidal cells (external molds). Additionally, cryo-Transmission X-ray microscopy identified chalcopyrite precipitation on the membranes of eukaryotic cells. We propose that microbial processes, including bacterial production of H 2 S and the availability of nucleation sites on outer cellular surfaces, promote chalcopyrite formation at low temperatures.