Sulfidization of 3.48 billion-year-old stromatolites of the Dresser Formation, Pilbara Craton: Constraints from in-situ sulfur isotope analysis of pyrite

This study reports in–situ sulfur isotope analyses (³²S, ³³S, ³⁴S and ³⁶S) of pyrite in strongly sulfidized stromatolites from the ~3.48 billion–year–old Dresser Formation, Pilbara Craton, Australia. These data shed light on sulfur reservoirs and sulfide precipitation processes and provide clues for the contribution of sulfur–cycling microbes to sulfidization. Sulfur isotope signatures derived from mass dependent fractionation (MDF; monitored by δ³⁴S) and mass independent fractionation (MIF; here Δ³³S and Δ³⁶S) of pyrite in stromatolites, and of microscopic pyrite within associated barite, allow for the identification of distinctive sulfur sources: i) magmatic–hydrothermal sulfide (H₂S) with δ³⁴S and Δ³³S ~ 0%; ii) magmatic–hydrothermal sulfate (SO₄ ^{2 −}) with a MDF signature (MDF–SO₄ ^{2 −}; δ³⁴S ~ 10‰ and Δ³³S ~ 0‰; iii) photochemically–derived sulfate with a MIF signature (MIF–SO₄; δ³⁴S ~ −6‰ and Δ³³S ~ −3.0‰); iv) photochemically–derived elemental sulfur (S⁰) with δ³⁴S ≪ 0 and Δ³³S ≫ 0‰. The sulfur isotope data suggest that sulfidization was largely driven by reduction of intermixed MDF–SO₄ ^{2 −} and MIF–SO₄ ^{2 −} (bulk signature of δ³⁴S ~ 5‰ and Δ³³S ~ −1.4‰), and dilution of produced H₂S (δ³⁴S ~ −12‰ and Δ³³S ~ −1.4‰) by native H₂S in magmatic–hydrothermal fluids. The δ³⁴S shifts (up to ~17‰) generated by sulfate reduction are consistent with both thermochemical reactions and influence of sulfate–cycling microbes, the latter which may have facilitated rapid pyrite precipitation and preservation of microbial remains that are entombed within the petrogenetically earliest pyrite generation of stromatolites. Collectively, our data are consistent with ancient stromatolite growth in proximity to shallow marine hydrothermal vents, where hydrothermal fluids contributed to sulfidization that may have been further influenced by sulfur–cycling microbes.