New Evidence of the Organic Origin of Carbonaceous Matter in Archean Banded Iron Formation of the Kostomuksha Greenstone Belt of Karelia, Russia

The results of studies of carbonaceous matter (CM) and the S isotopic composition of associated sulfides in metapelites of the Neoarchean banded iron formation of the Kostomuksha greenstone belt of Karelia (Karelian Craton of the Fennoscandinavian Shield) are presented. The petrographic observations show that the CM occurs inside and between silicates, in sulfides, or at the grain boundaries between the sulfide crystals and biotite or amphibole. The results of scanning electron and atomic force microscopy showed the presence of several CM types distinct in structure and C content. The analysis of Raman spectra of CM revealed the presence both of well-ordered graphite and weakly structured kerogene in samples. The isotopic composition of the total organic carbon is typical of biogenic processes. The δ 13 C org values from –27.9 to –30.6‰ are consistent with the fixation of carbon by photo- or chemoautotrophs. The S isotopic composition of associated sulfides is characterized by a positive Δ 33 S anomaly (up to +0.94‰) and negative δ 34 S values (from –2.06 to –4.1‰). The positive Δ 33 S values indicate a genetic link with photochemical elemental sulfur (S 8 ) from the atmosphere, whereas the negative δ 34 S values reflect the fractionation of isotopes during bacterial-related processes. Based on these observations, we suggest that the primary CM mostly has a biogenic origin.