BIF微量稀土元素分析方法及其在冀东司家营铁矿中的应用

以硅铁条带交替出现为特征的条带状铁建造(BIF)是世界上最重要的铁矿资源类型,精确分析磁铁矿的化学组成具有重要意义。本文开展了磁铁矿样品不同溶样方法分析结果的比对,并详细分析和讨论了冀东司家营铁矿磁铁矿与燧石单条带的微量及REE元素分析的地球化学特征。分析结果表明,对于磁铁矿样品,常规HF+HNO₃溶样法与HBr+HNO₃组合溶样法具有一致的溶样效果;司家营BIF的Zr,Sc,Th含量极低,表明未受陆源碎屑的污染;铁质与硅质具有低LREE、高HREE、La和Y正异常的海水REE特征,同时具有Eu正异常的热液REE特征;Ce负异常的缺乏,说明当时的古海洋是一个缺氧的环境。研究发现富铁条带的稀土总量大于富硅条带的稀土总量,这可能与硅、铁沉积物的地球化学习性相关,铁质沉积物更易吸收稀土元素。富矿和普通矿石具有原生的热液与海水的混合来源,部分富矿受到后期流体的强烈扰动,甚至表现出热液流体的特征。

Characterisics of rare earth elements, trace elements and geological significations of BIF from Sijiaying in eastern Hebei

Banded iron formation (BIF), consisting of alternating layers of iron bands and silicon bands, are the most important iron resource in the world. This paper briefly presents two methods of magnetite sample preparation, and focuses on the trace elements and REEs for individual Fe-rich band and Si-rich band. The conventional dissolving method (HF+HNO₃) obtains the same effect with the HBr+HF method for the magnetite samples. For the studied samples, crustal contamination can be ruled out on the basis of the generally low concentrations of Th, Zr, Sc. Both the Fe-rich bands and the Si-rich bands display depleted light REE relative to heavy REE, combined with positive La, Y anomalies, suggesting characteristics of sea water. Meanwhile, hydrothermal fluids are identified as major sources because of the positive Eu anomalies. The absence of negative Ce anomalies indicates relatively anoxic condition in the local water. The Fe-rich bands show total concentration of REE higher than the Si-rich bands, which may be related to the characteristics of silicon and iron sediment, the iron sediment is more likely to absorb REE. The high-grade ore has mixed source of hydrothermal and seawater, which has the same origin with the normal ore. Part of high-grade ore are strongly disturbed by the later hydrothermal fluid, even show the characteristics of hydrothermal fluids.