Depositional setting of the Late Archean Fe oxide- and sulfide-bearing chert and graphitic argillite in the Shaw Dome, Abitibi greenstone belt, Canada

Abstract

Interbedded chert-rich exhalite and graphitic argillite are the only sedimentary rocks deposited in deep-water settings during long-lived hiatuses in mafic to ultramafic volcanism within the Hart area of the Shaw Dome in the Late Archean Abitibi greenstone belt in Canada. The Fe oxide- and sulfide-bearing, but predominantly cherty, exhalite lithological unit in the Hart area can be traced laterally to iron formation elsewhere in the Shaw Dome. Whole-rock as well as Fe and S isotope geochemistry suggest that the exhalite unit was formed as a result of direct precipitation from seawater, distally from hydrothermal centres. Fractionation of Fe isotopes through the precipitation of iron oxyhydroxides in a neutrally buoyant hydrothermal plume removed the heavier isotopes of Fe, resulting in the negative δ56Fe values observed in the exhalite in the Hart area. Archean seawater is generally considered to be anoxic, but moderate Mn enrichments (up to 1.87 wt% MnO) in exhalite along with negative Fe isotope values resulting from partial Fe(II) oxidation suggest the presence of oxygen in the upper part of the water column along the pathway of hydrothermal plumes from their source to the depositional site in the Abitibi greenstone belt. In contrast, the graphitic argillite contains abundant pyrite nodules and bands that exhibit systematic negative Fe isotope values, but does not show Mn enrichment. This unit likely formed in a zone of upwelling of nutrient-rich waters from deeper parts of the basin resulting in high organic productivity. Both exhalite and graphitic argillite have negative Δ33S values, suggesting that sulfur was derived from seawater sulfate, which is consistent with an anoxic atmosphere with sulfate aerosols produced by photochemical reactions. Combined, our data indicates disequilibrium between anoxic atmosphere and partially oxygenated upper part of the water column during periods of volcanic quiescence in the ∼2.7 Ga Abitibi greenstone belt supporting the existence of oxidized oases within the Archean ocean.