A SURVEY OF SULFUR ISOTOPE TRENDS IN PROTEROZOIC SEDEX DEPOSITS AND INFERRED LINKS TO GLOBAL OCEAN CHEMISTRY

Stratiform, sediment-hosted Zn-Pb-sulfide mineralization occurred throughout the Proterozoic. Despite genetic differences among these sedimentary exhalative (SEDEX) deposits, the metal sulfides are similar in their broad S isotope ranges and the predominance of positive values. These S isotope ratios have traditionally been interpreted as products of bacterial sulfate reduction in local reservoirs isolated from the global ocean by rifting before, during and after the formation of Rodinia. Systematic stratigraphic variations in S isotope ratios in the ore bodies and host shales are common and can span up to 10s to 100s of meters. While such trends are often attributed to progressive evolution of seawater sulfate in restricted settings, our mass balance approach suggests that a single basin volume of seawater is often inadequate to explain the stratigraphic extent of the trends. For example, Fe sulfides in the Mesoproterozoic lower Belt Supergroup, northwestern U.S., show isotope values that increase and decrease systematically by up to 45 per mil over 10s to 100s of meters. Similar S isotope trends have been observed for carbonate-associated sulfate in limestones of the middle Belt Supergroup and in carbonates and gypsum of the Mesoproterozoic Bylot Supergroup, northeastern Canada. Rift-related isolation and extensive sulfate reduction and reduced S burial--coeval with the carbonate/gypsum accumulation--are unlikely in the two basins.

Collectively, our observations suggest that the heavy S isotope ratios and the frequently observed stratigraphic isotopic variability observed in Proterozoic SEDEX systems reflect a low sulfate ocean wherein the S isotope ratio varied more rapidly than during the Phanerozoic. Sulfate limitations may have been exacerbated by inhibited exchange between the ocean and the sites of mineralization. In such settings, quantitative sulfate reduction would result in sulfide S isotope enrichment that paralleled the isotopic evolution of sulfate in the global ocean. This presentation will explore this model in light of Phanerozoic analogs, the paucity of SEDEX deposits in the Archean and recent arguments for widespread euxinicity in the deep Proterozoic ocean.