华北克拉通南缘早前寒武纪赵案庄铁矿床的成因

The Zhaoanzhuang iron deposit in the Paleoproterozoic Taihua Group of the southern margin of the North China Craton consists of abundant serpentine-magnetite ores. This high-Mg and low-Ti ore type is unique and has not been found in other Precambrian iron ore deposits worldwide. It remains unclear that whether the original material of this type of ores sourced from ultramafic rocks or Mg-rich chemical sedimentary rocks. This application presents integrated studies on ore bodies and wall rocks, in an attempt to provide a comprehensive understanding of protolith reconstruction and origin of high-grade ores. Systematic petrology, whole-rock and mineral geochemistry, and stable isotope geochemistry will be carried out to ascertain the protolith of serpentine-magnetite ores, amphibololites, actinolitites, serpentinites and phlogopite-rich rocks, and then to identify the source of their original materials. Detailed petrographical observation on internal structure, external morphology, and textural relationship with other minerals of zircons and monazites will be carried out to determine their origin. A variety of micro and in-situ dating analyses of zircons and monaiztes formed at different stages will be conducted to obtain age constraints on formation of original materials, metamorphism and hydrothermal process. The application can provide important support both to the theory of high-grade iron mineralization and to further prospecting of high-grade ore deposits in southern North China Craton.

产于华北克拉通南缘古元古代太华群中的赵案庄蛇纹石-磁铁矿型铁矿床，其高镁、低钛的特征在国内外早前寒武纪铁矿中十分独特，但其初始物质来源于超基性岩浆岩还是富镁化学沉积岩尚没有充分依据。本申请以该矿床的铁矿体和近矿围岩（蛇纹岩、金云母岩、角闪石岩等）为研究对象：通过矿相学和岩石学、岩石化学、矿物化学研究和稳定同位素示踪，开展铁矿石及近矿围岩的原岩恢复，查明其初始物质来源，探讨其形成的特殊地质环境；通过锆石和独居石外部形态、内部结构及与其它矿物的相互关系的系统岩相学观察，查明其成因，采用光薄片原位高精度测年手段，获取不同期次年代学信息，探讨初始物质形成作用、变质作用和热液改造作用的时代，并为原岩恢复提供年代学证据；综合研究这几种地质作用对本区铁成矿尤其是高品位铁矿成矿的贡献，总结主要控矿因素，丰富富铁矿成矿理论，为华北克拉通南缘高品位铁矿找矿预测提供科学依据。

产于华北克拉通南缘古元古代太华群中的赵案庄蛇纹石-磁铁矿型铁矿床，其高镁、低钛的特征在国内外早前寒武纪铁矿中十分独特，但其初始物质来源于超基性岩浆岩还是富镁化学沉积岩尚没有充分依据。为厘定该铁矿床成因，本项目以铁矿体和近矿围岩（蛇纹岩、金云母岩、角闪石岩等）为研究对象，对该矿床进行了原岩恢复、成岩成矿时代和富铁矿成矿作用研究，取得如下成果：（1）铁矿石主要由低钛磁铁矿（TiO2约为0.1%）、富镁蛇纹石（Mg#=92.42−96.55）以及少量硬石膏、磷灰石、白云石、金云母和角闪石组成，与基性岩浆成因矿物组合不符；进一步S（δ34Sv-CDT=19.8%~22.5‰）、Mg（δ26Mg=−1.20‰~−0.34‰）和B（δ11B=−0.2‰~3.6‰）同位素示踪表明赵案庄铁矿床的原岩为化学沉积岩，形成于海相蒸发环境。（2）锆石年代学研究限定赵案庄铁矿床初始成矿物质的沉积时代为古元古代早期（2.46~1.96 Ga之间，很可能在2.3 Ga左右），之后于~1.96 Ga经历了强烈的热液改造。（3）建立了赵案庄铁矿床的成矿模式，提出赵案庄铁矿床的成矿过程经历了富铁碳酸盐沉积、富镁碳酸盐沉积、变质和晚期热液流体改造三个阶段。本项目研究表明，赵案庄蛇纹石-磁铁矿型铁矿床为沉积变质成因，而非岩浆成因。这一独特类型矿床成因的厘定，丰富了我国前寒武纪富铁矿成矿理论，为华北克拉通南缘同类型富铁矿深边部找矿提供了科学依据。