SULFUR, OXYGEN AND CARBON ISOTOPES IN THE GAGARYAH BARITE DEPOSIT, FAREWELL TERRANE, INTERIOR ALASKA, AND COMPARISONS TO THE ISOTOPIC RECORD OF MARINE SULFUR IN DEVONIAN TIME

The Gagaryah barite deposit, located in the Farewell terrane of the western Alaska Range, was discovered by Bundtzen and Gilbert in 1989 (Alaska Div Geol Geophys Surv Prof Rpt 111, 1991) and was resampled by USGS geologists in 2011. The barite layers occur in a 39 m interval of stratigraphic section and are exposed for 640 m along strike. The interval is underlain by gray shale and overlain by gray to blue-gray shale and siltstone; individual barite layers are interbedded with black shale that locally contains barite nodules. Frasnian brachiopods have been identified 50–60 m stratigraphically above the barite, which supports other evidence for a Middle to Late Devonian age for the barite.

The δ³⁴S and δ¹⁸O values for nine barite samples range from 35.8 to 49.6‰ and 15.0 to 19.0‰, respectively. Total organic carbon (TOC) contents of the barite layers range from 0.3 to 0.7%, and associated shales are as high as 3.1%. δ¹³C values for TOC increase up section from –30.2 to –29.4‰ and are consistent with a marine origin for the organic matter.

The barite isotopic compositions resemble those of bedded deposits on the modern seafloor in which nodular and rosette forms have high δ-values due to isotopic enrichment of sulfate during diagenesis, and finely laminated and massive forms preserve lower values closer to the basin sulfate values. On a plot of δ³⁴S vs δ¹⁸O, the Gagaryah data display a concave upward shape that suggests basin sulfate had a δ³⁴S value near 35‰. This is significantly higher than the 23‰ value that has been inferred for Middle-Late Devonian marine sulfate from time-averaged data for evaporites and carbonate-associated sulfate (Kampschulte & Strauss 2004).

The high δ³⁴S values for Gagaryah laminated barite may reflect local effects: basin restriction, sulfate reduction by organic matter or methane at seafloor vents, or remobilization of ³⁴S-rich sulfur from underlying strata. Alternatively, the high values could reflect barite formation at a time when the global ocean experienced a brief enrichment in ³⁴S, independent evidence for which has been reported by other workers near the Frasnian-Famennian boundary and in the early Frasnian. A review of isotopic data for other Middle-Late Devonian barite deposits suggests that some deposits—including Gagaryah—may indeed have formed at times of high global ³⁴S.