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

Abstract

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 (Δ 34S) for pyrite formation average −40% for non-calcareous shales and −25%. for calcareous shales. Δ 34 S 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). Δ 34 S 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 H 2S.