Lithostratigraphy, metal contents and depositional environment of the Algoma-type Porkonen banded iron formation, North Finland

The Porkonen Formation, exposed within the Porkonen–Pahkavaara metallogenic area in northern Finland, hosts one of the best-preserved Paleoproterozoic banded iron formations in the European continent. The iron ore forms a ~ 5 metres thick horizon comprising alternating diagenetically formed magnetite (±siderite) and chert bands having an average grade of 30% Fe. The Porkonen BIF displays all typical features of Algoma-type BIFs within greenstone belts, including close association with submarine tholeiitic volcanics and komatiites, sulfidic shales enriched in base and precious metals, and phosphorites. The stratigraphy is characterized by fine planar lamination, lack of shallow-water features, and gradual shift from cherty BIF through manganiferous carbonate horizon to sulfidic black shale. Rare earth element signature of authigenic siderite in the BIF, particularly heavy REE enrichment, elevated Y/Ho ratios, and pronounced negative Ce anomalies indicates precipitation from an oxygenated surface ocean above an anoxic, iron-rich water column and thus preserves a unique record of the ocean chemistry and atmospheric oxygenation that followed the Great Oxidation Event. Redox-sensitive metal ratios in the overlying black shale units document a shift from oxic to anoxic-euxinic conditions, with framboidal pyrite and metal enrichments confirming basin restriction, increased organic matter and detritus supply in a deep water stratified column.