INVESTIGATING THE MIDDLE ORDOVICIAN (DARRIWILIAN) REDOX STRUCTURE OF THE ARGENTINE PRECORDILLERA WITH SULFUR ISOTOPE AND IRON SPECIATION PROXIES
New Fe-speciation and trace metal data from the Gualcamayo black shale provide further constraints on Darriwilian environmental conditions. Highly reactive iron phases (FeHR) including pyrite, iron carbonate, iron oxides, and magnetite were determined via sequential extraction techniques. Fe-speciation data indicates the shales of the Gualcamayo Formation were deposited under dominantly anoxic (ferruginous) conditions through the lower Eoplacognathus suecicus conodont Zone. However, anomalously low concentrations from the pyrite fraction (<0.05 wt %) and high concentrations of Fe-oxides (up to 3 wt. %) suggest the potential for pyrite oxidation in these outcrop samples. To evaluate the potential for alteration we determined Mo concentrations, which are generally resistant to depletion during oxidative weathering. Mo concentrations fall within the range of intermittently euxinic (14-100 ppm) in the modern ocean, but below the accepted threshold for ancient euxinic environments (25 ppm). Combined, these data suggest that the Gualcamayo was deposited in non-euxinic waters with sulfide restricted to sedimentary pore-waters, that Mo drawdown occurred elsewhere, or that the sulfate/sulfide reservoirs were small enough to limit Mo drawdown under anoxic conditions.