冀东司家营条带状含铁建造铁矿石铁同位素特征及其地质意义

司家营条带状含铁建造（Banded iron formation，BIF）型铁矿床是冀东地区规模最大的铁矿床，前人对其进行了大量的年代学、岩石学和元素地球化学工作，但目前尚未对其进行Fe同位素研究。本文通过Fe同位素和主微量、稀土元素相结合的方法对司家营BIF的成矿物质来源和形成背景提出了有效制约，同时对司家营BIF的锆石U- Pb年龄数据进行补充。锆石U- Pb年代学显示，司家营BIF形成于2537~2531Ma。地球化学数据显示司家营BIF矿石主要由TFe2O3和SiO2组成，具有较低的Al2O3和TiO2含量，富集Fe重同位素（δ56Fe=0.341‰~0.525‰）；稀土元素配分模式呈现轻稀土亏损、重稀土富集的特征，具有明显的Eu、Y、La正异常，Y/Ho比率较高（Y/Ho=34.96~45.84）。这些特征表明司家营BIF是基本无碎屑物质参与的化学沉积岩，稀土元素来源于高温热液和海水的混合溶液，铁质来源于海相热液流体。司家营BIF缺乏真正的Ce负异常和Fe同位素组成均为正值指示其形成于缺氧环境。综合对比世界上其他地区太古宙BIF的Fe同位素特征，本文认为新太古代时期地球海洋含氧量逐步上升，此时海洋总体属于缺氧环境，但部分地区氧气含量较高。

Iron isotope characteristics and geological significance of Sijiaying BIF- type iron deposit in eastern Hebei

The Sijiaying banded iron formation (BIF)- type iron ore is the largest iron deposit in eastern Hebei, for which a great deal of chronological, petrological, and elemental geochemical work has been carried out, but no Fe isotope study has yet been done. Methods:This article proposes effective constraints on the ore- forming material sources and formation background of Sijiaying BIF through the combination of Fe isotopes, major elements, trace elements, and rare earth elements. At the same time, it supplements the zircon U- Pb age data of Sijiaying BIF. Results:Zircon U- Pb geochronology shows that the Sijiaying BIF was formed at 2537~2531 Ma. Geochemical data shows that Sijiaying BIF ore is mainly composed of TFe2O3 and SiO2, with lower Al2O3 and TiO2 contents and enriched in Fe heavy isotopes (δ56Fe=0.341‰~0.525‰); the rare earth element distribution pattern shows the characteristics of light rare earth depletion and heavy rare earth enrichment, with obvious positive anomalies of Eu, Y, La, and high Y/Ho ratio (Y/Ho=34.96~45.84).Conclusions:These characteristics indicate that Sijiaying BIF is a chemically sedimentary rock with minimal involvement of detrital materials. Rare earth elements are derived from a mixed solution of high- temperature hydrothermal and seawater, while iron is derived from marine hydrothermal fluids. The lack of true negative Ce anomalies and positive Fe isotope compositions in Sijiaying BIF indicates that it was formed in an hypoxic environment. By comprehensively comparing the Fe isotope characteristics of the Archean BIF in other regions of the world, this article believes that the oxygen content in the Earth''s oceans gradually increased during the Neoarchean period. At this time, the oceans generally belonged to an anaerobic environment, but some areas had higher oxygen content.