鞍山新太古代大孤山条带状铁建造相分带精细剖析及形成环境研究

Banded iron formations (BIFs) are characteristic iron-rich marine chemical sediments that widely occur in Precambrian terranes. BIFs are not only regarded as the most principal source of iron for the global steel industry, but also the REE and isotopic compositions of BIFs have been commonly used as proxies for understanding the chemical composition and redox state of the ancient oceans and atmosphere. BIFs are also characterized by various sedimentary mineral facies composed of different mineral assemablages，which are products of a complex interplay between geological and ancient environmental processes. The scientific problem of this fund is to decipher the effects paleoenvironments exert on these diagnostic BIF mineral facies.The Neoarchean Dagushan BIF situated in the Anshan area is one of the most representative Algoma-type BIFs in the North China Craton. In contrast to other Algoma-type BIFs worldwide, relative intact sedimentary facies (oxide, silicate, and carbonate facies) are clearly recognized in this BIF, which has been subjected to greenschist to lower amphibolite facies metamorphism. Therefore, this BIF offers a good possibility to conduct a detailed and systematic research on the formation environment of sedimentary facies. In this regard, based on field occurrence, mineral and textural arrangements of the Dagushan BIF, we will present analyses of a variety of elemental and isotopic (e.g., C, O, Fe and Cr) of samples derived from isochronous and diachronous geological profiles. The objectives of our study are: (1) to reconstruct the rules of both distribution and zonation of various facies; (2) to deduce the mineral compositions of precursor sediments; (3) to reveal the environmental conditions during BIF deposition; and (4) to ultimately establish the depositional model of various facies of the Dagushan BIF.

条带状硅铁建造（BIF）作为前寒武纪特有的海相化学沉积岩类，富含铁质且分布广泛，故不仅是重要的铁矿石资源，也是古海洋和大气氧化状态的指示物。不同含铁矿物组合常构成BIF特色的沉积矿物相，它们是地质演化与古环境耦合作用的产物。如何有效揭示古环境对BIF分相沉积的影响与控制作用，是本申请需要解决的科学问题。鞍山地区新太古代大孤山BIF作为Algoma型BIF典型代表之一，变质程度较低（绿片岩相-低角闪岩相），且与国内外同类BIF相比沉积矿物相（氧化物相-硅酸盐相-碳酸盐相）发育相对完整，是理想的研究对象。本申请拟通过详细的野外地质调查和室内岩相学观察，厘定大孤山BIF各沉积相分带及空间展布形式；结合顺层(等时)和穿层(穿时)地质剖面上全岩和特征矿物的元素和同位素（如C、Fe和Cr）组成，进一步恢复BIF原始矿物组成，揭示BIF分相沉积的古环境条件；最终建立大孤山BIF沉积相的形成演化模式。

条带状铁建造（Banded Iron Formation，简称BIF）是前寒武纪特有的由富铁和富硅条带组成的化学沉积岩，其全铁含量一般大于15wt%。目前，BIF的沉积矿物相及形成环境一直是众多学者探讨的热点，其中关于BIF矿物特征及沉积相展布形式、原始矿物组成及演变规律和沉积时的古环境条件三个方面的研究尤为重要。当前较为成熟的BIF分相沉积模式均是基于Superior Lake型BIF的研究，而对于Algoma型BIF的分相模式却知之甚少，主要原因为Algoma型BIF一般变质程度较高，且沉积相分布局限。.大孤山BIF位于华北克拉通东北缘的鞍山-本溪地区，为典型的Algoma型BIF，与国内其他BIF不同，其整体经受的变质程度较低（绿片岩相-低角闪岩相）且沉积相保存较完整（氧化物相-硅酸盐相-碳酸盐岩相），存在磁铁矿、菱铁矿、黑硬绿泥石、绿泥石和镁铁闪石等多种特征性矿物，这为系统探讨BIF矿物成因及细化分相成因机制提供了条件。因而，本次研究选取大孤山BIF作为研究对象，在前人研究基础上，通过详细的野外地质调查和室内岩相学观察，应用岩石地球化学、锆石年代学及矿物Fe-C-O同位素等研究，厘定了大孤山BIF沉积时代为2.53-2.51 Ga，判定了其沉积构造背景—弧后盆地环境，分析了BIF各沉积相分带展布，确定了主要含铁矿物成因，估算了原始有机碳沉淀量，推测大孤山BIF原始矿物组成主要为三价铁的氢氧化物和未定形的硅质，认为当时的古海洋的氧逸度整体较低，处于贫氧还原状态。最终建立了BIF分相形成模式：在古海洋整体低氧还原环境下，由于富Fe2+的热液上涌，在透光区氧化形成Fe(OH)3沉淀，在远岸侧，形成的Fe(OH)3沉淀与微生物或热液中的Fe2+反应形成磁铁矿，构成氧化物相；而由于近岸处大量有机质与Fe(OH)3共同埋藏，因此在早期成岩作用过程中会在近岸形成大量菱铁矿，构成菱铁矿相；最终形成远岸氧化物相，近岸碳酸盐相的相分布。影响沉积相分布的主要因素为氧化还原结构、碳质来源、热液作用以及微生物作用等。这些结果揭示了大氧化事件前夕的古海洋氧化状态，为Algoma型BIF成因机制及形成环境提供翔实资料和研究基础。