北祁连元古宙镜铁山BIF沉积环境研究

Precambrian banded iron formation (BIF) of different age were deposited in different environment, the depositional environment of the small scale BIF with mid-Proterozoic age is therefore inexplicit. The Jingtieshan BIF were formed in the period of absence of BIF (Mesoproterozoic), and were subjected to low grade metamorphism. Hence, the Jingtieshan BIF are a typical example to understand the depositional environment. This application will take the BIF and its associated hosting rock series as a research subject. Based on detailed field geological investigation, a comprehensive research method including sedimentary petrology, petrography, mineral chemistry, X-ray diffraction analyses, total organic carbon abundances analyses, geochemistry and Fe isotope will be employed to recover the protolith of the hosting rock and sedimentary faces, and understand the spatial distribution and the original mineral assemblage of various faces of BIF, as well as the water depth, the redox state, and evolvement of the depositional environment of the ocean. The characteristic of the depositional environment of the Jingtieshan BIF will be synthetic studied when compared with the globe BIF. Consequently, the redox state and chemical compositions of the ocean in the Jingtieshan area will be reconstructed. This project provides significant scientific contributions to reconstruction of the evolution of Precambrian oceanic conditions, as well as the complementary theory of the BIF, and also provides important support to further prospecting of iron deposit in North Qilian area.

前寒武纪不同时代BIF沉积环境不同，对于分布范围有限的元古宙中期BIF沉积环境目前缺乏深入研究。北祁连镜铁山BIF形成于前寒武纪BIF缺失期（中元古代），变质程度低，是恢复BIF沉积环境的理想研究区。本申请以镜铁山BIF及赋矿岩系为研究对象，在查明成矿地质特征的基础上，采用沉积岩石学、岩相学、矿物化学、X射线粉晶衍射、有机碳含量分析、地球化学和铁同位素测试等方法，分析赋矿岩系原岩类型和沉积相，查明不同BIF沉积相空间分布规律和原始沉积矿物组合，重点研究沉积水体深浅和氧化还原条件及沉积环境的演化过程。与全球BIF进行系统对比，综合探讨镜铁山BIF沉积环境的特点，从而重建镜铁山地区古海洋氧化还原状态和化学组成。本研究为揭示前寒武纪海洋环境演化、丰富BIF成矿理论具有重要科学意义，同时可以为北祁连地区的铁矿找矿预测提供科学依据。

前寒武纪条带状铁建造（简称BIF）是早期地壳的重要组成部分和地球演化特定阶段的产物，同时也是世界上最重要的铁矿资源类型，资源储量和开采量均居世界首位。本项目以北祁连西段元古宙镜铁山BIF及其赋矿岩系为研究对象，采用沉积岩石学、岩相学、矿物化学、地球化学、碳、氧和铁同位素测试等方法，取得如下主要成果：（1）系统建立了镜铁山BIF空间分布规律，重晶石层主要赋存于氧化物相BIF顶部，而菱铁矿主要出现在BIF的中下部，碳质千枚岩赋存于BIF底板。重晶石显示显著的Ce负异常，而氧化物相BIF、碳酸盐-氧化物混合相和碳酸盐相BIF中菱铁矿、BIF底板碳质千枚岩未见Ce异常。据此，创新性提出13亿年左右为氧化还原分层海洋，海水表层氧化，深部缺氧、富铁，为海洋环境演化研究提供重要约束。（2）前人在FeV矿体及其底板岩石中发现了铜矿床，规模达中型，本次研究提出镜铁山BIF底板铜矿为后期热液改造成因，成矿时代为584 Ma左右。（3）将镜铁山铁矿与西天山海相火山岩型赤铁矿矿床开展对比研究，提出我国西天山海相火山岩型赤铁矿矿床为罕见的晚石炭世BIF型铁矿，可能为拉恩-迪尔型铁矿，为我国首次报道的该类型铁矿床，为找矿勘查部署提供了科学依据。同时提出即使大气氧浓度很高的晚古生代，强烈的海底岩浆作用和热液活动不仅提供铁质，还可以改变海水的氧化还原状态。本研究为揭示古海洋环境演化、丰富BIF成矿理论具有重要科学意义。项目发表国际SCI论文6篇，核心论文1篇。培养博士后一名、硕士研究生一名、本科生三名。