辽宁弓长岭铁矿二矿区条带状铁建造地球化学特征及成因探讨

摘要

本文以弓长岭铁矿二矿区磁铁石英岩、磁铁富矿和蚀变围岩样品为研究对象,进行了主量元素、微量元素、稀土元素和Fe同位素的测试.结果表明:磁铁石英岩主要由TFe2O3和SiO2组成,Al2O3和TiO2质量分数较低,微量元素质量分数和稀土元素质量分数均较低;经澳大利亚后太古界平均页岩(PAAS)标准化的稀土配分模式呈现出轻稀土亏损和重稀土富集,La、Eu和Y的正异常明显,Ce的异常不明显,Y/Ho值较高;富集Fe的重同位素,且与海底喷发热液经过氧化沉淀后的Fe同位素特征一致.磁铁富矿与磁铁石英岩的地球化学特征有很好的一致性和继承性,但磁铁富矿的REE和Eu质量分数较高,且较磁铁石英岩富集Fe的轻同位素,范围更大,与蚀变岩的Fe同位素组成相近.弓长岭铁矿的磁铁石英岩是陆源物质加入很少的古海洋化学沉积岩,为喷出的海底热液与海水的混合条件下氧化沉淀形成的.磁铁富矿推测为富Fe的轻同位素热液对磁铁石英岩进行改造,经过去硅富铁作用形成的.%In this paper, major elements, trace elements and Fe isotope composition of the magnetite quartzite, magnetite-rich ore and altered rock are presented.The average bulk compositions of the magnetite quartzite are characterized by high total Fe2O3 and SiO2, and low contents of Al2O3 and TiO2.The overall contents of trace elements and ΣREE in magnetite quartzite are very low.In the Post Archean Australian shale(PAAS) normalized REE patterns, samples display LREE depletion, distinct positive anomalies of La, Eu, Y, unobvious Ce anomalies, high Y/Ho values and heavy Fe isotope enrichment(relative to IRMM-014), which consists with the Fe isotope of oxidized and precipitated hydrothermal fluid.The geochemical charateristics of magnetite-rich ores are similar with those of the magnetite quartzite.However, ΣREE and Eu contents in the magnetite-rich ore are higher.Compared with magnetite quarterite, the magnetite-rich ore is characterized by the enrichment of light Fe isotope with larger range, which is similar to the altered rock.The magnetite quartzite is paleo-ocean chemical sedimentary rocks with devoid of terrestrial sediments, and experienced the oxidation and precipitation induced by the mixing of the submarine hydrothermal solution and the seawater.It can be deduced that the high grade iron ore was formed by the reworking of the low grade iron ore by the hydrothermal system with light Fe isotope enrichment, which resulted in Fe enrichment and Si depletion.