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.出露于芬兰北部波尔科宁-帕赫瓦瓦拉成矿带内的波尔科宁组，赋存着欧洲大陆保存最为完好的古元古代条带状铁建造（banded iron formation, BIF）之一。该铁矿床形成厚度约5米的地层，由成岩作用形成的磁铁矿（±菱铁矿）与燧石条带交替组成，平均铁品位达30%。波尔科宁条带状铁建造展现出绿岩带内阿尔戈马型BIF的所有典型特征，包括与海底拉斑玄武质火山岩、科马提岩密切伴生，富集贱金属与贵金属的硫化物页岩，以及磷块岩。其地层学特征表现为细密平面层理、缺乏浅水沉积特征，且从燧石质BIF经含锰碳酸盐岩层逐渐过渡为硫化物黑色页岩。该BIF中自生菱铁矿的稀土元素（rare earth element, REE）特征——尤其是重稀土富集、升高的Y/Ho比值，以及显著的铈负异常——表明其沉淀于缺氧富铁水柱上方的氧化表层海水中，因此保留了大氧化事件（Great Oxidation Event）之后海洋化学与大气氧化作用的独特记录。上覆黑色页岩地层中的氧化还原敏感金属比值记录了从有氧环境向缺氧-硫化环境的转变，草莓状黄铁矿与金属富集现象证实了盆地封闭性增强，以及深水分层水柱中有机质与碎屑补给量的增加。