Microbialites in muddy waters? Evidence for greenalite and silica mineralization of a 1.88 Ga stromatolitic iron formation

The identification of microfossils in stromatolitic chert from the 1.88 Ga Gunflint iron formation (North America), including forms likened to Cyanobacteria, prompted early models linking the deposition of iron formations to biologically mediated iron oxidation. However, the exact relationship between microbial processes, iron deposition, and stromatolite growth remains murky. Here we show that stromatolites in 1.88 Ga iron formations (Sokoman Formation) from the Labrador Trough, Canada, comprise laminae of dusty chert interbedded with dusty chert clasts and sand-sized Fe-silicate grains. High-resolution transmission electron microscopy and analysis show that the dust particles comprise nanometer-sized crystals of greenalite and apatite. The presence of clasts of dusty chert that closely resemble surrounding laminated chert suggests that they are rip-up clasts, indicating penecontemporaneous silica cementation of greenalite-rich muds. Our observations suggest that the stromatolites grew during tranquil conditions by microbial trapping and binding of suspended nanoparticles accompanied by in situ silica precipitation. Our results also provide potential insights into the origin of granular iron formations, suggesting a link between the deposition of greenalite nanoparticles and the production of Fe-silicate grains by accretion on muddy substrates or erosion of silica-cemented greenalite muds. We speculate that greenalite nanoparticles were not only a major primary phase in banded iron formations (e.g., Hamersley Group, Western Australia) but also an important building block of granular iron formations, representing a key ingredient in the formation of sand-sized grains during sedimentary reworking in high-energy, shallow-water environments.