Banded Iron Formations of the Main Ore Sequence of the Kostomuksha Greenstone Belt, Karelian Craton: Geochemistry, Petrography, and Formation Conditions

Banded iron formations (BIFs) are metamorphosed chemical sedimentary rocks that accumulated in the Early Precambrian. There are two types of BIFs: Algoma and Superior. One of the occurrences of the Algoma-type BIF is the Kostomuksha greenstone belt, where four BIF associations are distinguished, of which the Neoarchean (BIF-3) has the greatest thickness and extent. The distribution features of major, rare, and rare earth elements are a reliable indicator of the conditions under which these formations accumulated. The chemical composition of the samples was determined using an S8 Tiger X-ray fluorescence spectrometer (Bruker AXS GmbH, Germany) at Voronezh State University. Minor and rare elements were determined by the induction-coupled plasma with mass spectrometric end analysis (ICP-MS) at the Analytical and Certification Test Center of the Institute of Microelectronics Technology and High-Purity Materials, Russian Academy of Sciences. The Neoarchean BIFs (2.76–2.74 Ga) (BIF-3) occur in the Kostomuksha greenstone belt, Karelian craton. BIF-3 has a dark color and banded structure due to the alternation of layers enriched in quartz and magnetite. The sum of SiO2 and Fe2O3tot is 83–98%. BIF-3 has an increased content of K2O (0.12–2.9 wt %), while the content of other petrogenic oxides is comparable to other Archaean BIFs of the Algoma type. Although the HFSE contents are very low and variable, elevated concentrations of Rb (1.4–111 ppm) and Ba (6.54–799 ppm) are noted. The enrichment in HREEs relative to LREEs ((La/Yb)SN = 0.21–1.17), positive La/La* and YSN anomalies, and the superchondrite Y/Ho ratio (31–42) suggest accumulation of BIF-3 in marine conditions. Positive Eu/Eu* anomalies indicate a hydrothermal injection as the main source of Si, Fe, and Mn in BIFs. The absence of negative Ce/Ce* anomalies (0.9–1.04) and low U concentrations suggest that the accumulation occurred in the absence of oxygen in the atmosphere, i.e., before the Great Oxidation Event (GOE ~ 2.4–2.2 Ga). The low chromium content, low Ni/Fe molar ratios (0.04–0.57 × 10–4), lack of correlation between MgO and Cr, and weak correlation between MgO and Ni indicate the absence of ultrabasic and basic compositions in the provenance area, while strong positive correlations between Al2O3 and Zr, that of REE, TiO2 and Zr, and that of Hf and Zr point to the predominance of the felsic composition in the provenance areas. The formation of BIF-3 occurred in a marine basin lacking in oxygen, suggesting that it formed before the Great Oxidation Event. The main components, Si, Fe, and Mn, came from high-temperature hydrothermal vents, the proportion of which was more than 90%. The distribution patterns of Al2O3, TiO2, Zr, and Hf suggest that felsic rocks predominated in the source areas.