Earliest Seafloor Hydrothermal Systems on Earth: Comparison with Modern Analogues

Recent developments in multiple sulfur isotope analysis of sulfide and sulfate minerals provide a new tool for investigation of ore-forming processes and sources of sulfur in Archean hydrothermal systems, with important implications for the Archean sulfur cycle, the origin and impact of various microbial metabolisms and the chemistry of surface waters. In the current study we show that most of the sulfides and sulfates in the 3.49 Ga Dresser Formation and 3.24 Ga Panorama Zn–Cu field of Western Australia have non zero Δ³³S values that indicate variable proportions of seawater sulfate and elemental sulfur of UV-photolysis origin were incorporated into the deposits. Our results show that the multiple sulfur isotope systematics of the Dresser Formation sulfides and sulfates mainly reflect mixing between mass independently fractionated sulfur reservoirs with positive and negative Δ³³S. Pyrite occurring with barite is depleted in ³⁴S relative to the host barite that has been interpreted as evidence for microbial sulfate reduction. We note, however, that the reported quadruple sulfur isotope systematics of pyrite-barite pairs are equally permissive of a thermochemical origin for this pyrite, which is consistent with inferred formation temperatures for the chert-barite units in excess of 100°C. The variably positive Δ³³S anomalies of the Panorama VHMS deposits, disequilibrium relations among sulfides and sulfates and general trend of increasing sulfide Δ³³S with stratigraphic height in individual ore systems most likely reflects temperature evolution and fluid mixing through the life of the hydrothermal system. The absence of sulfides with significant negative Δ³³S anomalies suggests that volcanic sulfur, not seawater sulfate, was the dominant sulfur source for the Panorama mineral system. The data presented here require Paleoarchean seawater to be at least locally sulfate bearing.