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A study of minerals from Proterozoic and Archean series of various regions has been carried out. Sulfate and sulfide sulfur, having doubtlessly passed through the cycle of isotopic fractionation, has been determined in Archean rocks of the Baikal region. In the lazurite deposit of Malaya Bystraya, the country rocks are Precambrian stinking (fetid) dolomites which contain ancient hydrogenlsulfide with ~s4S ~rom+ 19.0 to + 33.40/00, and sulfate with ~s4S = + 44.90/00. Carbonate rocks have served as sources o{ sulfide and sulfate sulfur for anumber of minerals (lazurite, scapolites, apatite, pyrrhotite, pyrite, a. o.). A connection between the enrichment of sulfides of metamorphic strata in the a2S isotope or the 84S isotope and the presence in rocks of increased amounts of graphite --the product of metamorphism of the organic matter of ancient sediments --is established. A conclusion has been drawn regarding the isotopic fractionation about 3"109 y. ago.