华北克拉通南缘铁山庙组BIF的精细矿物学和微区地球化学研究

The BIF in the Tieshanmiao Formation from Wuyang area, Henan province, is the significant component of the Precambrian BIF in North China Craton. In the preliminary study, there are both quartz-magnetite and pyroxene-magnetite distributed in the BIF of Tieshanmiao Formation from Wuyang area, Henan province, and both of them show repeating occurrences. The genesis of pyroxene banding and its relationship between quartz and magnetite bandings greatly restraint the realization of the genesis, diagenesis, evolution, and rules of metallogenesis of BIF of Tieshanmiao Formation in Wuyang area. So, the modern testing technologies, with the feature of microarea and microbeam analysis, are proposed to analyse the fine characteristics of mineralogy and micro-geochemistry of the BIFs in this project. In order to open out the microfabric, typomorphic characteristicses of mineralogy and microarea geochemistry, the microarea characteristics of the Si, Fe bands, minerals, and the polymers of minerals of the BIF are treated as the studying priorities to extract information. In doing this, the rules of primary sedimentary environment, material sources, processes of sedimentation, diagenesis, metallogenesis and the upper reconstruction can be opened out by the characteristicses of mineralogy and micro-geochemistry. There will be great significances to explore the rules of forming conditions, diagenesis, evolution and metallogenesis of the Precamrian BIF from the southern margin of North China Craton. This project application introduces both micro-analysis and in situ analysis in the study, in mineralogical scale, on the BIFs, and it will give an example in studying the formation, evolution and their geological significances of the Precambrian BIFs.

河南舞阳地区铁山庙组BIF是华北克拉通前寒武纪BIF的重要组成部分。申请者前期的初步研究表明，该BIF同时发育石英-磁铁矿和辉石-磁铁矿两个铁建造端元，它们在空间上呈韵律出现。辉石条带的成因及其与石英条带、磁铁矿条带的关系制约了人们对该BIF成因、成岩演化及成矿规律的认识。本项目拟通过微区微束为特色的现代测试技术，分析河南舞阳地区铁山庙组BIF的精细矿物学及微区地球化学特征，重点对硅条带、铁条带、矿物晶粒及其聚合体的微区信息进行提取，以揭示其矿物组构、矿物标型、微区地球化学等特征，进而反演该BIF的原始沉积环境、物质来源、沉积成岩过程和遭受的改造作用。这将对探索华北克拉通南缘前寒武纪BIF的形成条件、成岩演化和成矿规律具有重要意义。本申请将微区及原位分析的思想引入至BIF矿物尺度的研究中，将为研究前寒武纪BIF的形成、演变特征及其意义提供一种新思路和研究案例。

BIF建造是富铁和富硅（或贫铁）条带交替产出的一类化学沉积含铁岩系。本研究选择华北克拉通内舞阳、霍邱和水厂及西昆仑赞坎地区四个典型BIF矿床为对象，开展了全岩XRF、ICPMS和GC-MS分析及微区LA-ICPMS、XRD、SEM、EDS、CL、Raman和EPMA分析，同时与我国的典型热水沉积硅质岩开展对比研究。研究涉及了全岩地球化学特征和微区形貌、结构及成分特征，它们记录了BIF沉积过程、形成环境及后期演化等信息。研究所取得的主要成果包括：（1）厘定了华北克拉通内舞阳、霍邱和水厂的“辉石-磁铁矿型BIF（简称PMB）”和“石英-磁铁矿型BIF（简称QMB）”两种亚类，PMB主要包含有辉石、石英和磁铁矿，QMB则主要由磁铁矿、石英和少量的碳酸盐矿物组成。（2）揭示了两种BIF亚类的磁铁矿成因均为“沉积变质成因”。QMB中磁铁矿的XRD分析晶胞参数呼应了纯化学沉积磁铁矿，其磁铁矿单矿物相对富集SiO2、Na2O、Al2O3、CaO、K2O、MgO及Sc、Co、Y、Nb、Ta、U等；PMB中磁铁矿的晶胞参数吻合遭受了火山活动影响的化学沉积磁铁矿，该类型磁铁矿单矿物以相对富集TiO2为特征。（3）建立了两种BIF亚类的形成过程。地壳拉张背景下引发了大规模的热水对流，QMB是在火山喷气作用下引发的陆源风化Fe和Si质热水体系两种物源共同沉积的产物，PMB则属于火山喷气作用下引发的陆源风化Fe、火山碎屑和硅质热水体系三种物源共同沉积的产物，火山喷发和火山碎屑物影响的周期性变化控制了QMB和PMB的交互产出，而火山活动的缺失则导致了高纯度SiO2沉积并形成硅质岩。（4）揭示了磁铁矿重结晶和铁条带汇聚归并过程中的元素迁移规律，即BIF中磁铁矿的汇聚归并过程伴随了K、Na和P元素的逃逸。BIF中的磁铁矿颗粒的粒径远大于生物合成磁铁矿，而碳酸盐似条带的碳酸盐矿物υs振动模及其FWHM值的无规律突变进一步反演了BIF体系内部的脆塑性流变，汇聚归并后的磁铁矿颗粒中Na2O, P2O5, K2O的含量明显降低。