Banded iron formations of the main ore sequence of the Kostomuksha greenstone belt of the Karelian craton: geochemistry, petrography and formation conditions

Abstract

Introduction: Banded iron formations (BIF) are metamorphosed chemogenic sediments, the accumulation of which occurred in the early Precambrian. There are two types of BIF: Algoma and Superior. One of the places of manifestation of the Algoma-type BIF is the Kostomuksha greenstone belt, in which four BIF associations are distinguished, of which the Neoarchean (BIF-3) are characterized by the greatest power and extent. The distribution features of petrogenic, rare and rare earth elements are a reliable indicator of the conditions under which these formations accumulated.

Methodology: The chemical composition of the samples was determined using the S8 Tiger X-ray fluorescence spectrometer (Bruker AXS GmbH, Germany) at Voronezh State University. Small and rare elements were determined by induction-coupled plasma with mass spectrometric end analysis (ICP-MS) at the ASIC IPTM RAS.

Results and discussion: Neoarchean BIF (2.76–2.74 Ga) (BIF-3) occur in the Kostomuksha greenstone belt, Karelian craton. BIF-3 has a dark color and striped texture due to the alternation of layers enriched with 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 algome. Although the contents of highly charged elements are very low and variable, elevated concentrations of Rb (1.4–111 ppm) and Ba (6.54–799 ppm) are noted. The enrichment of HREE relative to LREE ((La/Yb)_SN=0.21–1.17), positive La/La* and Y_SN anomalies and an exceeding chondrite Y/Ho ratio (31–42) indicate the accumulation of BIF-3 in marine conditions. Positive Eu/Eu* anomalies indicate hydrothermal injection as the main source of Si, Fe and Mn in BIF. The absence of negative Ce/Ce* anomalies (0.9–1.04) and low U concentrations indicate that accumulation occurred in the absence of oxygen in the atmosphere, that is, before the Great Oxidative Event (GOE~2.47 billion years ago). Low chromium contents, low Ni/Fe molar ratios (0.04–0.57 x 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 sources of rock demolition, and strong positive correlations between Al₂O₃ with Zr and REE, TiO₂ and Zr, as well as Hf and Zr indicate the predominance of acidic composition in the sources of rock demolition.

Conclusions: The formation of BIF-3 occurred in the marine basin in the absence of a significant amount of oxygen, which suggests their formation before the Great Oxidative Event. The main components – Si, Fe and Mn – were supplied from high-temperature hydrothermal vents, the proportion of which was more than 90 %. The distribution patterns of Al₂O₃, TiO₂, Zr and Hf indicate that acidic rocks predominated in the sources of demolition.