Geochemistry and Formation Conditions of Mesoarchean Banded Iron Formations (BIF-1) from the Kostomuksha Greenstone Belt, Karelian Craton

Three variably old groups of banded iron formation (BIF) are known in the Kostomuksha Greenstone Belt (KGB) of the Karelian Craton. This paper deals with the earliest of them, Mesoarchean (2.87–2.81 Ga) BIF-1. BIF-1 occurs among the komatiite–basalt unit of the KGB. BIF-1 consists mainly of quartz and magnetite, with varying amounts of amphibole, biotite, and garnet; the variations of SiO₂ (48.3–58.6 wt %) and \({\text{F}}{{{\text{e}}}_{{\text{2}}}}{\text{O}}_{{\text{3}}}^{{\text{T}}}\) (21.34–33.82 wt %) suggest that the rocks are BIF. BIF-1 of the KGB, as well as most Archean BIFs, contain high \({\text{F}}{{{\text{e}}}_{{\text{2}}}}{\text{O}}_{{\text{3}}}^{{\text{T}}}\) concentration, display a contrasting positive Eu anomaly, lack of Ce anomaly, and the depletion of LREE relative to HREE. However, they differ from other BIFs in the higher Al₂O₃, TiO₂, MgO, K₂O, Cr, Ni, Zr, Ba, Cu and Zn concentrations. BIF-1 was formed in a marine basin at an anoxic atmosphere due to hydrothermal fluids, the proportion of which varies from 20 to 80%, and a terrigenous component derived mainly from basalts, komatiites, and dacites of host rocks. Mesoarchean BIF-1 of the KGB was accumulated in a small rift structures within an oceanic volcanic plateau, the formation of which is associated with the influence of a mantle plume on the oceanic lithosphere.