鞍山—本溪地区前寒武纪条带状铁建造铁矿时代、物质来源与形成环境  

Age,Material Sources and Formation Setting of Procambrian BIFs Iron Deposits in Anshan-Benxi Area

作　　者：张连昌[1] 代堰锫[1] 王长乐[1] 刘利[1] 朱明田[1] 

机构地区：[1]中国科学院地质与地球物理研究所矿产资源研究重点实验室,北京100029

出　　处：《地球科学与环境学报》2014年第4期1-15,17,共15页Journal of Earth Sciences and Environment

基　　金：国家重点基础研究发展计划("九七三"计划)项目(2012CB416601);国家自然科学基金项目(41372100)

摘　　要：前寒武纪条带状铁建造（BIFs）蕴含丰富的铁矿石资源,记录了早期地球演化的重要信息.鞍山—本溪地区发育巨量的太古宙 BIFs.鞍山—本溪地区 BIFs 由互层的硅铁条带组成,金属矿物以磁铁矿为主,含少量赤铁矿、黄铁矿及菱铁矿等.围岩与BIFs呈整合接触,具有变火山岩的岩石学特征及地球化学特征,表明BIFs属于阿尔戈玛型,其变火山岩围岩年龄可大致代表BIFs沉积时代.锆石U-Pb 年代学显示,鞍山—本溪地区至少发育约3．10 Ga及约2．55 Ga两期BIFs 沉积成矿事件,后者在华北克拉通广泛发育.鞍山—本溪地区 BIFs 地球化学具有以下特征：①低的TiO2与Al2 O3含量及高场强元素负异常,表明沉积过程中陆源碎屑物质加入量很少；②PAAS 标准化REE配分模式显示重稀土元素富集,La、Y 及 Eu 正异常,暗示 BIFs 沉积自海水与海底高温热液的混合溶液；③接近同时代亏损地幔的εNd （t）值（3．0~4．7）,表明 Fe 可能来源于海底热液活动淋滤洋壳；④BIFs中仅发育少量黄铁矿,基本不显示 Ce负异常且富集铁的重同位素（δ（57 Fe）值为（0．13~2．73）×10^-3）,暗示古海洋整体处于低硫、缺氧环境,BIFs 的沉淀可能与富 Fe2＋溶液遭遇环境突变有关.元素地球化学分析表明,大孤山与陈台沟围岩具有弧火山岩特征,南芬与歪头山围岩类似于弧后盆地玄武岩,因此,鞍山—本溪地区BIFs沉积于火山弧相关构造背景.Precambrian banded iron formations （BIFs）contain large amounts of iron ores and record some important information about the evolution of early earth.Anshan-Benxi area has huge occurrences of Archean BIFs,which are helpful to enrich the theories of BIFs metallogenesis and advance the understanding of early evolution of North China Craton.These hosting rocks all have parallel occurrences to BIFs,and they show petrological and geochemical affinities with meta-volcanics.Therefore,the BIFs belong to the Algoma type and their formation ages can be reflected by the ages of wall rocks.It is proposed that there are at least two-stage BIFs deposition at about 3.10 Ga and about 2.55 Ga in Anshan-Benxi area,and the latter is widely distributed in North China Craton.The geochemical features of these BIFs include：①low concentrations of＆amp;nbsp;Al2 O3 ,TiO2 and high field strength elements,suggesting that terrigenous detrital materials contributed insignificantly to the chemical precipitation;②enrichment in heavy rare earth elements with positive La,Eu and Y anomalies on PAAS-normalized REE patterns,indicating their precipitation from marine seawater with a high-temperature hydrothermal component;③εNd （t）values （3.0-4.7 ） close to those of the coeval depleted mantle,implying that the iron is most likely derived from the leaching of oceanic crust by marine hydrothermal activities;④BIFs have only a few pyrites within them,and most of them show no Ce anomaly with enrichment ofδ（57 Fe）（（0.13-2.73）×10^-3 ）, indicating that Archean ocean is in low abundances of S and O2 .It is proposed that the deposition of BIFs is associated with environment variation during the transport of Fe2＋-rich solutions. Geochemical analyses show that Dagushan and Chentaigou wall rocks have the characteristics of arc volcanics,and Nanfen and Waitoushan rocks are similar to back-arc basin basalts.Therefore, the BIFs in Anshan-Benxi area are deposited in the volcanic arc setting.

关 键 词：条带状铁建造 太古宙 物质来源 形成机制 构造背景 鞍山-本溪地区 华北克拉通 

分 类 号：P618.31[天文地球—矿床学]