New evidence for organic nature of carbonaceous substance in Archean banded iron fomation of the Kostomuksha greenstone belt, Karelian Craton, Russia

Abstract

The paper considers the results of a study of particles of carbonaceous substance and sulfur isotopes of associated sulfides in metapelites of the Neoarchean banded iron formation (BIF) of the Kostomuksha greenstone belt of Karelia (Karelian Craton of the Fennoscandian Shield). According to the petrographic observations, the carbonaceous matter occurs within and between silicate minerals’ grains, inside sulfides or at the grain boundaries, separating sulfide crystals from biotite or amphibole. Scanning electron (SEM) and atomic force (AFM) microscopy revealed the several types of the carbonaceous material varying in structure and carbon content. Raman spectra approved both well-structured graphite and weakly structured graphite (kerogen) components of the carbonaceous substance. The isotopic composition of total organic carbon is typical for biogenic processes. The obtained δ13Corg value within the range of –27,9– –30,6‰ is consistent with carbon fixation by photo- or chemoautotrophs. The sulfur isotopy of the associated sulfides is marked by positive Δ33S anomaly (up to +0.94‰) and negative δ34S values (–2.06‰ ─ –4.1‰). Positive Δ33S values indicate sulfur genetic association with photochemical elemental sulfur (S8) from the atmosphere, while negative δ34S values reflect isotope fractionation in bacterial-mediated processes. Based on these observations, we believe that the initial carbonaceous substance has mainly organic origin.