河北迁安杏山铁矿床地球化学特征及其对成矿物质来源的指示

GEOCHEMICAL CHARACTERS OF BAND IRON FORMATIONS FROM XINGSHAN IRON DEPOSIT IN QIAN'AN AREA,HEBEI PROVINCE:IMPLICATION FOR THEIR ORIGIN

不同学者曾对迁安地区铁矿床的前寒武地质、岩石学和地球化学等方面进行了深入的研究,但是其成矿物质来源至今没有进行深入探讨。危机矿山勘察在迁安杏山矿床中发现了富大铁矿体,但其成因不明。本文通过对迁安富矿和普通矿石的主量、微量元素和稀土元素研究,结果表明它们的化学成分主要由Fe2O3(T)、SiO2组成,并且Al2O3和TiO2具有较低的含量,指示其形成时几乎没有碎屑物质的加入。而经PAAS标准化后,稀土元素的配分模式表现轻稀土亏损、重稀土富集的特征,无论是富矿还是普通矿石,都具有Eu正异常、其Co/Zn和Ni/Zn比值与热液类似的特征,表明形成时有高温热液加入;其Y/Ho>44、Y的正异常表明其有海水的成因;La/La*表明其没有陆源碎屑加入;LaN/YbN<1,表明既有海水特征,又有热液特征,所有这些数据都显示了迁安铁矿矿石的物质来源为海水和热液,与其他地方BIF铁矿物质来源一致。由于富矿和普通矿石的物质来源一致、主微量及稀土元素含量和分布类似、铁矿物主要为磁铁矿、原始沉积条带明显,推断富矿可能是火山—沉积建造原始沉积时由于局部富铁环境而形成的。

The origin of Qian＇an BIFs remains unclear, although plenty of efforts have been done in this area （especially on the Precambrian geology, petrology and geochemistry ）. In the early stage of ＂Exploring Mines Facing A Resourse Crisis＂ project, large and high-grade iron ores were discovered. The major and trace element analyses reported in this paper provide new insights on the origin and formation environment of Qian＇an BIFs. The average bulk compositions of the BIFs are rich in total Fe and SiO2, and very low in Al2O3 and TiO2 contents, indicating that a minor terrigenous component input. Their PASS-normalized REE patterns show strongly enriched HREE and positive anomalies of La, Eu and Y. All of the geochemistry fingerprints, in combination with Co/Zn, Ni/Zn, Y/Ho （ 〉 44）, La/La^＊ and LaN/YbN （ 〈 1 ） ratios, strongly suggest a combined origin of hydrothermal water and marine for Qian＇an BIFs, consistent with BIFs otherwhere. There are no distinct differences between High-Grade and normal ores, which have apparently similar original PASS-normalized REE pattern and Fe-bearing sedimentary belts, in major and trace element content, mineral assemblage （mainly magnetite）, demonstrating that they might have the same origin. It is suggested that the high-grade ores were probably formed at locally Fe-rich environment during volcanic-sedimentary activity.