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


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Age, Material Sources and Formation Setting of Procambrian BIFs Iron Deposits in Anshan-Benxi Area
ZHANG Lian-changDAI Yan-peiWANG Chang-leLIU LiZHU Ming-tian
Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
banded iron formations Archean material source formation mechanism tectonic setting Anshan-Benxi area North China Craton
Precambrian banded iron formations (BIFs) contain large amounts of iron ores and record some important in formation 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 Al2O3, 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 of δ(57Fe) ((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 arcvolcanics, 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.




Last Update: 2014-12-16