Microbial activity preserved in 3.4 Ga colloform pyrite: A micro–scale sulfur isotope analyses

Microfossil-bearing black cherts from the ca. 3.4 Ga Strelley Pool Formation in the Goldsworthy Greenstone Belt, Pilbara Craton, offer critical insights into early Archean microbial ecosystems. In this study, we analyzed concentric colloform pyrite grains in these cherts to investigate their formation mechanisms and potential microbial involvement. The examined chert samples contain abundant organic matter and microfossils. The colloform pyrite grains, typically less than 10 µm in diameter, exhibit high sphericity and concentric internal growth structures. Their spatial association with diagenetic silica veins suggests pyrite formation prior to lithification. In situ sulfur isotope analyses using nanoscale secondary ion mass spectrometry (NanoSIMS) revealed large intra-grain δ 34 S variations, up to 28.6‰. δ 34 S values reach as low as –20.1‰ (VCDT) in the innermost cores and progressively increase toward the outer layers, where they average approximately +4‰. These trends are consistent with Rayleigh-type isotope fractionation under sulfate-limited conditions, likely mediated by heterotrophic microbial sulfate reduction. Secondary ion mapping of 12 C − and 12 C 14 N − revealed alternating layers of organic matter and pyrite. The close spatial association between pyrite and organic matter with significant 12 C-enriched carbon isotope signatures, further supports a biogenic origin. Collectively, these findings provide new microscale geochemical evidence for microbial sulfur metabolism in early Archean sedimentary environments