Sulfur isotopic studies of Archean slate and graywacke from northern Minnesota: evidence for the existence of sulfate reducing bacteria

Sulfur isotopic studies of pyrite from metasediments in the >2.6 Byr old Deer Lake green-stone sequence, Minnesota, have been conducted in order to evaluate the possible importance of sulfate reducing bacteria in sulfide formation. Pyrite occurs as ovules up to 2 cm in diameter within graphitic slates, and as fine disseminations in metagraywacke units. SEM studies indicate the pyrite is framboidal in morphology.$\text{δ}{}^{34}\text{S}$ values of pyrite from the Deer Lake sediments range from ?2.3 to 11.1‰, with a peak at ~ +2‰ Isotopic data are consistent with either high temperature inorganic reduction of circulating seawater sulfate, or low temperature bacterial reduction. However, the lack of sulfide bands or massive occurrences in the sediments, the restriction of pyrite mineralization to the sediments, and the absence of evidence for hot spring activity suggest that a diagenetic origin of pyrite is more feasible. Sulfide in such an environment would be produced principally by the action of sulfate reducing bacteria.Results of the study are in agreement with those of Goodwinet al. (1976) who suggest that dissimilatory sulfate reduction was operative in the Archean ocean some 2.75 Byr ago.