Arsenic substitution and galvanic coupling in pyrite trigger uranium mineralization in sandstone

Abstract Sandstone-hosted uranium (U) deposits account for the majority of global U production. In these deposits, pyrite is widely considered one of the most common reducing agents playing a crucial role in ore formation. However, it has been shown that pyrite is not simply consumed (dissolved) during U mineralization as is expected if pyrite is used as a reducing agent; instead, pyrite (specifically arsenian pyrite) is also precipitated during U mineralization. Here, we propose a model that can explain this dynamic dissolution-reprecipitation process based on micro- to nano-scale analysis of pyrite and associated U minerals using multiple micro-analytical techniques, including electron probe microanalysis (EPMA), Nano secondary iron mass spectrometry (NanoSIMS), Raman spectroscopy, transmission electron microscopy (TEM) and aberration-corrected scanning TEM (STEM). In the model, an oxidizing fluid carrying arsenic (As 3+ ) and U 6+ reacts with deep-derived fluids to form arsenian pyrite. Incorporation of arsenic into pyrite induces lattice expansion and creates a galvanic couple with preexisting As-poor pyrite, thereby enhancing localized reduction capacity. The newly formed As-rich pyrite acts as an anode and supplies electrons, while the preexisting As-poor pyrite plays the role of a cathode and accepts them. This galvanic couple gives pyrite an enhanced redox potential and facilitates the reduction of U 6+ adsorbed on the pyrite surface to U 4+ , resulting in the formation of numerous pitchblende nanoparticles, which aggregate to form U minerals. The dynamic electrochemical engine induced by As doping in pyrite thus provide a self-organized mechanism triggering and sustaining sandstone-hosted U mineralization.