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.

芬兰北部波尔科宁-帕赫卡瓦拉成矿带（Porkonen–Pahkavaara metallogenic area）内出露的波尔科宁组（Porkonen Formation），赋存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。该铁矿床构成厚度约5米的层位，由成岩作用形成的磁铁矿（±菱铁矿）与燧石条带交替互层组成，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条带状铁建造的全部典型特征，包括与海底拉斑玄武质火山岩、科马提岩的密切伴生关系，富集贱金属与贵金属的硫化物页岩，以及磷块岩。其地层序列以细密平面层理为典型特征，缺乏浅水沉积标志，且呈现出从燧石质条带状铁建造经含锰碳酸盐岩层逐渐过渡至硫化物黑色页岩的变化过程。该条带状铁建造中自生菱铁矿的稀土元素（rare earth element，REE）特征——尤其是重稀土元素（heavy REE，HREE）富集、升高的Y/Ho比值以及显著的铈负异常——指示其形成于缺氧富铁水柱之上的氧化表层海水环境，由此保留了大氧化事件（Great Oxidation Event，GOE）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沉积环境从氧化环境向缺氧-硫化物环境的转变，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供给量的增加。