The late Neoarchean, ~2.53 to 2.51 Ga Dagushan banded iron formation (BIF), is a typical Algoma-type BIF located in the northeast part of the North China craton. Despite having undergone upper greenschist to lower amphibolite facies metamorphism, the Dagushan BIF retains evidence of varied depositional facies, making it an ideal archive to evaluate the paleomarine environment and the paragenesis of the ore minerals. A transition from oxide to silicate to carbonate facies BIF is evident in a northward direction. The mineralogical composition shifts from magnetite and quartz in the south through a magnetite-quartz-cummingtonite/stilpnomelane assemblage in the transition zone to magnetite-siderite in the north. Such a distinct distribution of mineralogical facies correlates well with the depositional environment of the BIF. The carbonate facies BIFs formed in a near-shore, proximal environment, whereas the oxide and silicate facies BIF assemblages formed in deeper waters, distal to the paleoshoreline. The BIF samples display characteristic seawater-like rare earth element + yttrium (REE + Y) profiles with positive La and Y anomalies and heavy REE enrichment relative to the light REEs when normalized to post-Archean Australian shale. Positive Eu anomalies suggest a high-temperature hydrothermal contribution to the BIF. The absence of a negative Ce anomaly in nearly all samples, coupled with positive δ⁵⁶Fe in magnetite in all mineralogical facies, indicates a dominantly anoxic water column contemporaneous with deposition of the BIF. At ~2.53 Ga in the Anshan area, seawater was mostly anoxic and rich in ferrous iron. Dissolved ferrous iron in upwelling hydrothermal fluids was oxidized and precipitated as Fe(III) oxyhydroxides in the photic zone leading to BIF formation. Proximal to hydrothermal vents, magnetite formed via the reaction of Fe(III) oxyhydroxides and aqueous Fe(II) supplied from the hydrothermal fluids and microbial dissimilatory iron reduction (DIR) coupled to organic carbon oxidation. Proximal to a paleoshoreline, siderite formed through DIR, as evidenced by the depleted δ¹³C values and the presence of graphite. Silicates, such as stilpnomelane and cummingtonite, are considered to be the metamorphic products of early diagenetic silicates (e.g., nontronite) that formed in the water column from admixtures of Fe(III) oxyhydroxides and amorphous silica.

中文翻译：

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新太古代晚期大孤山矿床（华北克拉通鞍山-本溪地区）的沉积和环境约束：Algoma型条带状铁地层

新太古代晚期~2.53～2.51 Ga大孤山带状铁地层（BIF），是位于华北克拉通东北部的典型Algoma型BIF。尽管经历了上绿片岩到下角闪岩相变质作用，大孤山 BIF 仍保留了各种沉积相的证据，使其成为评估古海洋环境和矿石矿物共生作用的理想档案。从氧化物到硅酸盐再到碳酸盐相 BIF 在北向的转变是明显的。矿物成分从南部的磁铁矿和石英转变为过渡带的磁铁矿-石英-明矾石/硅铁矿组合，再到北部的磁铁矿-菱铁矿。这种独特的矿物相分布与 BIF 的沉积环境密切相关。碳酸盐岩相 BIF 形成于近岸、近端环境，而氧化物和硅酸盐相 BIF 组合形成于古海岸线远端的较深水域。BIF 样品显示出特征性的类似海水的稀土元素 + 钇 (REE + Y) 剖面，当归一化为澳大利亚太古代后页岩时，具有 La 和 Y 正异常和相对于轻 REE 的重 REE 富集。正 Eu 异常表明高温热液对 BIF 的贡献。几乎所有样品中都没有负的 Ce 异常，加上正的 δ BIF 样品显示出特征性的类似海水的稀土元素 + 钇 (REE + Y) 剖面，当归一化为澳大利亚太古代后页岩时，具有 La 和 Y 正异常和相对于轻 REE 的重 REE 富集。正 Eu 异常表明高温热液对 BIF 的贡献。几乎所有样品中都没有负的 Ce 异常，加上正的 δ BIF 样品显示出特征性的类似海水的稀土元素 + 钇 (REE + Y) 剖面，当归一化为澳大利亚太古代后页岩时，具有 La 和 Y 正异常和相对于轻 REE 的重 REE 富集。正 Eu 异常表明高温热液对 BIF 的贡献。几乎所有样品中都没有负的 Ce 异常，加上正的 δ⁵⁶ Fe 在所有矿物相的磁铁矿中，表明与 BIF 沉积同时存在的主要是缺氧水柱。在鞍山地区~2.53 Ga，海水大多缺氧，富含亚铁。上升流热液中溶解的亚铁在光带中被氧化并沉淀为 Fe(III) 羟基氧化物，导致 BIF 形成。在热液喷口附近，磁铁矿是通过 Fe(III) 羟基氧化物和由热液提供的 Fe(II) 水溶液和微生物异化铁还原 (DIR) 与有机碳氧化反应形成的。靠近古海岸线，通过 DIR 形成菱铁矿，耗尽的 δ ¹³证明了这一点C 值和石墨的存在。硅酸盐，例如芪白质和 Cummingtonite，被认为是早期成岩硅酸盐（例如绿脱石）的变质产物，这些硅酸盐在水柱中由 Fe(III) 羟基氧化物和无定形二氧化硅的混合物形成。