C-S-Fe relationships and the isotopic composition of pyrite in the New Albany Shale of the Illinois Basin,U.S.A.

The relationship between pyritic sulfur content (S_pyr) and organic carbon content (C_org) of shales analyzed from the New Albany Group depends upon C_org. For samples of <6 wt.% C_org, S_pyt, and C_org are strongly correlated (r = 0.85). For C_org-“rich” shales (>6 wt.%), no S_pty-C_org, correlation is apparent. The degree of Fe pyritization (DOP) shows similar relationships to C_org. These C-S-Fe relationships suggest that pyrite formation was limited by the availability of metabolizable organic carbon in samples where C_org < 6 wt.% and by the availability of reactive Fe for samples where C_org > 6 wt.%. Apparent sulfur isotope fractionations relative to contemporaneous seawater sulfate (Δ³⁴S) for pyrite formation average ?40% for non-calcareous shales and ?25%. for calcareous shales. Δ³⁴S values become smaller with increasing C_org, S_pyt, and DOP for all C_org-“poar” (<6 wt%) and some C_org-“nch” (<6 wt.%) shales. These trends suggest that pyrite formation occurred in a closed system or that instantaneous bacterial fractionation for sulfate reduction decreased in magnitude with increasing organic carbon content. The isotopic trends observed in the New Albany Group are not necessarily representative of other shales having a comparable range of organic carbon content, e.g. Cretaceous shales and mudstones from the western interior of North America (GAUTIER, 1986). Δ³⁴S values in the remainder of the C_org-rich New Albany Group shales are relatively large (?38 to ?47%.) and independent of C_org, S_pyr, and DOP, which suggests that pyrite in these shales formed mostly at or above the sediment-water interface by precipitation from an isotopically uniform reservoir of dissolved H₂S.