For much of the Precambrian era, the bulk ocean was anoxic and iron(II)-rich (ferruginous) with the exception of late Archean shallow ocean oxygenation and the first development of temporally and spatially restricted sulfide-rich waters (euxinia) along productive continental margins, which prevailed throughout much of the remaining Precambrian. There is little detail pertaining to transition zones between ferruginous, euxinic and oxic seawater over the continental shelf that presumably played an important role in shaping the composition of the underlying sediment. Here we present spectroscopic data on the iron- and sulfur-mineralogy in the Arvadi Spring, a proposed analogue for such conditions. Our study reveals green rust, ferrihydrite and lepidocrocite as the main iron minerals. Since the reactivity of green rust differs from ferric hydroxides and Fe(II) sulfides, it is important to understand its role in the transfer of metals and nutrients from seawater to underlying sediments, if those sediments are to be used as chemical archives of paleo-seawater. We observed elemental sulfur as the dominant sulfur precipitate and found indications for its role in pyrite formation, implying S0 could have controlled Precambrian deposition of pyrite-poor or pyrite-rich sediments.