The Banded Iron Formations (BIFs) have been invaluable archives for the Precambrian eons, providing evidence for the paleoenvironment conditions, including the atmospheric oxygen levels. This study explores the geological origin and depositional mechanism of BIF-like Carboniferous Heiyingshan (HYS) and Mesoproterozoic Pilot Knob hematite (PKH) formations, characterized by hematite-rich (Fe-rich) layer and quartz-rich (Si-rich) jasper layer alternations within felsic volcanic rocks. Through comprehensive mineralogical analyses and geochemical modeling, we present evidence supporting their formation in intra-caldera hot spring lakes, where episodic inputs of Fe(HSiO)-bearing fluids mixed with ambient lake water. Experimental simulations and PHREEQC modeling corroborate a mechanism where ferrihydrite and amorphous silica precursors sequentially precipitate, possibly driven by the oxidation and decomposition rates of Fe(HSiO) complexes. The sharp or gradational change in the Fe- and Si-rich layering transition can be caused by varying atmospheric oxygen levels. The Carboniferous HYS specimen exhibits a sharp FeSi boundary that is in accordance with modeling under high atmospheric O condition. In contrast, the Mesoproterozoic PKH sample contains both sharp and gradual change in the FeSi transition, implying the O lies between 10 and 10. This chemical precipitation model of BIF-like iron formations could provide insights into the FeSi source and banding mechanism of BIFs, and Precambrian O levels.

中文翻译：

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长英质火山活动与类 BIF 铁岩层相关：它们的起源及其对 BIF 的影响

带状铁地层（BIF）是前寒武纪时期的宝贵档案，为古环境条件（包括大气中的氧气水平）提供了证据。本研究探讨了以富赤铁矿（富铁）层和富石英（富硅）碧玉层为特征的类BIF石炭系黑鹰山（HYS）和中元古代先导旋钮赤铁矿（PKH）地层的地质成因和沉积机制。长英质火山岩内的交替。通过全面的矿物学分析和地球化学建模，我们提供了证据支持它们在破火山口内的温泉湖中形成，其中含有 Fe(HSiO) 的流体与周围的湖水混合。实验模拟和 PHREEQC 建模证实了水铁矿和无定形二氧化硅前体依次沉淀的机制，可能是由 Fe(HSiO) 络合物的氧化和分解速率驱动的。富铁和富硅层状转变的急剧或渐变变化可能是由大气中氧气水平的变化引起的。石炭纪 HYS 样本表现出尖锐的 FeSi 边界，这与高大气 O 条件下的建模一致。相比之下，中元古代 PKH 样品在 FeSi 转变中既包含急剧变化又逐渐变化，这意味着 O 位于 10 到 10 之间。这种类似 BIF 铁形成的化学沉淀模型可以为了解 FeSi 来源和 BIF 的带带机制提供见解。和前寒武纪 O 水平。