山西五台山和冀东迁安地区条带状铁矿的岩石化学特征及其地质意义

摘要

Petrological and petrochemical characteristics of samples from four Neo-Archean banded iron ore deposits of Baiyuli, Baizhiyan and Ekou in Wutai Mountain area and Qian' an in eastern Hebei Province are reported in this paper.Some comparisons are also made with Anshan iron deposit in Liaoning, Hanwang iron deposit in Shandong and some other iron deposits of the same kind abroad.The authors have found that these iron deposits have highly consistent characteristicss in the following four aspects: all of the samples are depleted in Th, U, La, Ce, P and Sm and enriched in K, Nb, Sr, Hf, Er and Ti; as one of the characteristics of Archean marine sediments, they all have very low REE content, with the average values of the iron deposits in the four areas being 27.65× 10-6, 14.56× 10-6, 19.03×10-6 and 16.46×10-6 respectively; all of the iron deposits in the four areas are depleted in light rare earth elements and enriched in heavy rare earth elements, with Pr/Yb ratios in the range of 0.81～1.84, 0.35～1.21, 0.76～1.33 and 0.95～1.92 respectively; they have obvious positive anomalies of Y and Eu, with ratios of Eu/Eu* being 2.55～3.36, 2.07～2.25, 1.44～2.56 and 1.43～ 3.77 respectively, suggesting characteristics of sea water and high temperature seafloor hydrothermal solution.Therefore, the Fe-Si formations were probably formed in a hot sea water environment.In addition, as the values of δ34S of pyrite associated with BIF are close to zero, the authors hold that the sulfur might have come from the mantle.It is thus concluded that the studied BIF are chemical-sedimentary ore deposits formed in a high temperature sea water environment after the derivation of silicon and iron from mantle-derived volcanic eruption or exhalation.%详细报道了五台山地区白峪里、柏枝岩和峨口(又名山羊坪铁矿)3个新太古代条带状铁矿床和冀东迁安地区条带状铁矿样品的岩石学和岩石化学特征,并与辽宁鞍山和山东韩旺以及国外同类矿床进行了对比.五台山地区和冀东迁安地区条带状铁矿的微量元素和稀土元素的含量和配分特征与国内外同类矿床十分一致:4个地区条带状铁矿样品均富集Th、U、La、Ce、P、Sm等元素,亏损K、Nb、Sr、Hf、Er、Ti等元素;稀土元素总量均较低,是太古宙海洋沉积特征之一,轻稀土元素轻微亏损,重稀土元素稍富集,具有明显的Eu的正异常,部分具有Y正异常.Y的异常通常代表了海水的特征,Eu的正异常指示了高温海底热液的特征,由此可判断铁硅质建造形成于热海水环境.五台山地区与条带状铁矿伴生的黄铁矿的δ34S值在零附近,表明其来源于地帽.由此可知所研究条带状铁矿床是慢源的火山喷发或火山喷气带来的硅铁质溶于海水后在特定条件下经化学沉积而成.