Sulfur and carbon isotopes in Archean banded iron formations: Implications for sulfur sources

Abstract

Sulfur isotope ratios of pyrite from two Late Archean banded iron formations were used to constrain possible sources of sulfur. The iron formations studied were the Temagami iron formation which hosts the Sherman Mine near Temagami and the Boston iron formation including the Adams Mine south of Kirkland Lake, both in Ontario. In both cases Algomantype oxide-facies magnetite-chert ores predominate. The sulfur studies were supplemented by limited carbon isotopic data for the Sherman Mine. Iron formation in the Sherman Mine occurs on both limbs of a regional synformal structure. Each iron formation unit caps a thick calc-alkaline andesite-dominated volcanic sequence with one band hosted in sedimentary rocks of turbidite origin. The Adams Mine includes four major iron formation bands interbedded with mafic and ultramafic volcanic rocks and greywacke of turbidite origin. The sulfur isotope statistics for averages of pyrites from these three units are: Sherman Mine volcanic-hosted suite, δ34S= -1.7 ± 4.6‰. (n=14); Sherman Mine sediment-hosted suite, δ34S=+1.4±1.6‰ (n=43); and Adams Mine, δ34S=-1.8±2.9‰ (n=17). These averages differ by <2‰ from juvenile sulfur, and together with their limited dispersions, suggest a predominantly unfractionated source of sulfur. It is argued that this sulfur has been derived from the associated volcanic piles. Most of the pyrite occurs as disseminated grains in oxide-facies iron formation and interbedded clastic sediments. A distinctive nodular pyrite in the Sherman Mine sediment-hosted suite occurs in cherty graphitic sulfide-facies iron formation. While the sulfur isotopic properties of this subset (δ34S=0.0±0.8‰) are not suggestive of biogenic sulfur, the geological setting and carbon isotopic compositions of associated graphite (δ13C=-34‰) imply that anaerobic bacterial processes were active in this reducing environment. The apparent lack of biogenic sulfide is rationalized by models suggesting that Archean marine basins differed from Phanerozoic analogues in various parameters which influence sulfur isotopic characteristics. It is argued that biogenic sulfide is probably a significant component of the sulfur in Sherman Mine nodular pyrite. © 1994.