GEOCHEMICAL CONSTRAINTS FOR THE DEPOSITION OF THE DEVONIAN LEICESTER PYRITE MEMBER, WESTERN NEW YORK: IMPLICATIONS FOR THE ACCUMULATION OF DETRITAL SULFIDE DEPOSITS BENEATH AN OXIC ATMOSPHERE

The Devonian Leicester Pyrite Member of western New York is a detrital pyritic lag associated with a discontinuity at the base of the Geneseo Shale Member (Genesee Formation) and overlying the Windom Member (Moscow Formation). It has been suggested that the lag formation resulted from internal waves traveling along a pycnoclinal surface, within a highly stratified anaerobic basin, which had sufficient energy to rework and redistribute the pyritic lag. The range of diagenetic and syngenetic environments and the extensive reworking represented by the Leicester Pyrite are evident from both the large range in the sulfur isotope compositions (-17.0 to +43.4‰) often observed within individual grains and framboid size distributions within the Leicester Pyrite. Though the Leicester Pyrite is the result of extensive reworking in an oxygen-depleted marine setting, essential questions remain pertaining to the mechanistic and environmental controls that formed such a spatially significant detrital sulfide deposit despite an oxygen-rich surficial ocean and atmosphere.

Although the S isotope data are relatively consistent with the proposed model for the deposition of the Leicester Pyrite, fundamental questions remain. First, is the inferred extreme water-column stratification and oxygen depletion supported by independent evidence? Second, what environmental variables ultimately exhumed pyrite from the underlying consolidated Windom Shale while concurrently minimizing both sulfide oxidation and the presence of calcium carbonate bioclasts? Finally, what environmental conditions can be characterized based upon trace metal partitioning within the lag deposit? Addressing geochemical conditions responsible for the formation of the Leicester Pyrite will provide insight into the formation of other geologically significant detrital sulfide deposits.

Constraining depositional controls responsible for the formation of the Leicester Pyrite will assess the validity of applying detrital pyritic lags as proxies for low atmospheric oxygen concentrations. Understanding the fundamental controls governing the formation of metal sulfide deposits will have important implications for the global redox cycling of sulfur and atmospheric oxygen throughout the Proterozoic and Phanerozoic.