Phosphorites as Bio-mediated Critical Raw Material (BCRM) A Crossdisciplinary Approach for Unveiling Microbial Phosphogenesis

This study supports the critical role of microorganisms in phosphogenesis by both direct and indirect evidence, fostering the inclusion of phosphorites in the new, here-proposed, group of Bio-mediated Critical Raw Materials (BCRMs). Notably, the presence of microbial signatures as the mineral nanostructure and composition, fossil traces of microbial activity and remains of extracellular polymeric substance (EPS), imply the presence of a microbial community and its decisive role in the precipitation of Ca- and F-rich carbonate fluorapatite (CFA). Microbial activity also facilitated the post-depositional alteration of bone and shell fragments, and even the already formed phosphatic grains, which often served as nucleation sites for further phosphate precipitation. This is evidenced by microboring morphologies with curved, elongated, and unbranched filamentous shapes, indicative of endolithic, primarily photosynthetic bacteria. These established pH gradients, enabling simultaneous dissolution and precipitation of further localized CFA mineralization processes within microborings. The findings suggest that the origin and/or post-depositional alteration of the studied phosphorite deposits occurred in a shallow-water setting during the middle-late Toarcian, when the paleogeographic context of the External Rif region (NW Africa) provided optimal conditions for phosphorite formation such as localized riverine discharge, channelization of upwelling currents, and anoxic bottom waters extending into the shallow shelf. However, sedimentological features indicate that their accumulation occurred in deep-sea environments via diluted currents rather than in-situ precipitation. This implies at least one episode of erosion, winnowing, and redeposition in a different sedimentary environment.