Atmospheric and hydrothermal sulfur isotope signatures recorded in Neoarchean deep marine sedimentary pyrites from the Yilgarn Craton, Western Australia

Abstract

We report in-situ multiple sulfur isotope analyses for pyrites from deep marine sediments that are interbedded with ∼ 2.7 Ga greenstone lava flows in the Kalgoorlie-Kambalda area of the Eastern Goldfields Superterrane, Western Australia. Two endmember sediment types are recognised: shale and chert, with transitional chert as an intermediate. Petrologic studies are consistent with both the pyrite and pyrrhotite having a syngenetic/diagenetic origin. Pyrites from the shales and transitional cherts have positive Δ33S, whereas those from the cherts have Δ33S ∼ 0. We suggest that the principal sources of S are atmospheric photolytic S8, with Δ33S > 0, and nanoparticulate sulfides from hydrothermal seafloor vents, with neutral Δ33S. In an anoxic Archean ocean, nanoparticles of pyrite and/or pyrrhotite, issuing from black and white smokers, were dispersed through the ocean by currents. S8, together with nanoparticulate sulfides and fine detrital particles, rained down slowly and accumulated on the sea floor to form the shales. During diagenesis, pyrrhotite reacted with available S to form pyrite until all the S was consumed, with unreacted pyrrhotite remaining in the shale. Variations in Δ33S in the sedimentary pyrites are therefore attributed to variations in the relative proportions of pyrite derived directly from black and white smokers, and pyrite formed by the diagenetic reaction between nanoparticulate pyrrhotite and photolytic S8. The cherts are interpreted to have formed close to hydrothermal vents where rapid accumulation of amorphous silica and pyrite from white smokers negated the influence of slow S8 rain.The Δ33S isotopic trend across individual sedimentary layers, can be explained by variations in the hydrothermal flux as local volcanic activity waxed and waned. The marked global increase in Δ33S in sedimentary pyrites at ca. 2,650 Ma is attributed to the emergence of several cratons above sea level at that time, associated with a marked increase in sub-aerial felsic volcanism.