PROTEROZOIC SEDIMENTARY EXHALATIVE (SEDEX) DEPOSITS AND THEIR LINKS TO EVOLVING GLOBAL OCEAN CHEMISTRY

Sedimentary exhalative Zn-Pb-sulfide mineralization peaked during the Proterozoic. Metal sulfides in most deposits have yielded broad ranges of predominantly positive S isotope data traditionally attributed to bacterial or thermochemical sulfate reduction. But rather than the global controls outlined below, heavy isotopic signatures are often ascribed to fractionation within closed or partly closed local reservoirs isolated from the global ocean by rifting before, during, and after the formation of Rodinia.

Stratigraphic S isotope variation is common in the ore bodies and host shales and can span 10s of per mil over only 10s to 100s of meters. Up-section isotopic enrichments are often explained as progressive reduction of seawater sulfate in restricted settings; however, the isotope data also show up-section depletions in S-34, and a single basin volume of seawater is generally inadequate to account for the quantity of metal sulfide present. Also, Proterozoic sulfate preserved within gypsum and carbonate rocks can display similar S isotope variability.

The ubiquity of heavy S isotope ratios and the patterns of rapid variability suggest that global seawater sulfate was increasing but remained substantially lower than today, implying that levels of atmospheric oxygen fell between the deficiencies of the Archean and the abundances of the Phanerozoic. Given these limitations, deep-water anoxia may have persisted well into the Proterozoic in the presence of a growing sulfate reservoir, which promoted widespread euxinia. Collectively, these observations suggest that the Proterozoic SEDEX maximum and the absence of Archean deposits reflect a critical threshold in the accumulation of oceanic sulfate and thus sulfide within anoxic bottom waters, which favored both the production and preservation of sulfide mineralization on the seafloor. Consistent with this idea, Proterozoic SEDEX deposits in northern Australian formed from relatively oxidized fluids that required reduced conditions at the site of mineralization. By contrast, the generally more oxygenated Phanerozoic ocean may have only locally and intermittently favored the formation and preservation of exhalative mineralization, and most Phanerozoic deposits formed from reduced fluids that carried some sulfide to the site of ore precipitation.