Abstract

The Devonian Appalachian Basin provides a unique window to the unusual geochemical conditions required to form and preserve detrital metal sulfide deposits during the oxygen-rich Phanerozoic Eon. The Devonian Leicester Pyrite Member, a marine transgressive lag, occurs in the northern Appalachian Basin of western New York and is a reworked and redeposited acid-resistant residuum dominated by a wide range of pyritic clasts in the absence of carbonate grains. These iron sulfide grains include 10⁻² m nodules, framboids, and pyritized fossil material (e.g., cephalopods, burrows, and steinkerns). A previous model for the formation of the Leicester Pyrite Member requires establishment of a strong and persistent density and chemical stratification in the Appalachian Basin. Although this model permits erosion under the low-oxygen conditions required for pyrite preservation, little geochemical evidence has been provided in corroboration, and the chemical conditions responsible for the formation, deposition, and preservation of the pyrite lag have remained speculative.

Using a combined geochemical and petrographic approach focusing on the pyritic fraction and emphasizing sulfur isotopes and pyrite framboid size distributions, the specific redox conditions responsible for the formation of the Leicester Pyrite Member are addressed. The wide ranges of textural and isotopic data observed for the Leicester Pyrite Member, with δ³⁴S values that range from −22 to +54% at a single location, are consistent with the general idea of lag concentration of pyritic debris formed under diverse diagenetic and possibly water-column (syngenetic) conditions. Energy propagation, possibly storm related, along the redox transition of the pycnocline and chemocline would have favored pyrite preservation, while still permitting enough oxidation and concomitant acid production to dissolve associated hydrodynamically equivalent carbonate debris. Intermittence in the stability of water-column stratification would have enhanced this oxidation, although an association with the overall oxygen deficiency that characterized the deposition of the overlying Geneseo Shale Member is demonstrated by the occurrence of microlaminated, Geneseo Shale Member-like clasts within the Leicester Pyrite Member.

Our geochemical results confirm that during times of atmospheric oxygenation, pyrite reworking and lag formation require high-energy but low-oxygen conditions at pycnocline depths in a subaqueous marine setting. While not unique to the Leicester Pyrite Member, the specific sets of environmental conditions required for pyrite preservation differ from those of the fluvial settings in which Archean detrital pyrite and uraninite deposits formed. We suggest, therefore, that the presence of Phanerozoic pyrite lags such as the Leicester Pyrite Member does not compromise the utility of Archean detrital pyrite as a paleoredox proxy delineating early deficiency of atmospheric oxygen.