Petrological and geochemical features of the Jingtieshan banded iron formation (BIF): A unique type of BIF from the Northern Qilian Orogenic Belt, NW China

摘要

The Jingtieshan banded iron formation (BIF) is located in the Northern Qilian Orogenic Belt (NQOB) in NW China. The BIFs are hosted in Mesoproterozoic Jingtieshan Group, a dominantly clastic-carbonate sedimentary formation, and was metamorphosed to lower greenschist facies. The Jingtieshan BIFs include oxide-, carbonate- and mixed carbonate-oxide facies, and consist of alternating iron-rich and silica-rich bands. The BIFs are composed essentially of specularite and jasper, with minor carbonate minerals and barite. The SiO2 + Fe2O3 content is markedly high in the oxide facies BIF, followed by FeO, CO2 and Ba, with the other elements usually lower than 1%, suggesting that the original chemical sediments were composed of Fe, Si, CO32- and Ba. The positive correlation between Al2O3, TiO2 and Zr in the BIFs indicates that these chemical sediments incorporate minor detrital components. Oxide fades BIF shows low HFSE, low Sigma REE and low Y/Ho. The Post Archean Australian Shale-normalized REE patterns for Jingtieshan BIFs are characterized slight LREE depletion, strong positive Eu anomalies and lack of significant negative Ce anomalies. Siderite in the carbonate- and mixed carbonate-oxide facies BIF shows negative delta C-13 values varying from -8.4%0 to -3.0%, and delta O-18 values show a range of -16.6%0 to -11.7%. The geochemical signatures and carbon-oxygen isotopes suggest origin from high-temperature hydrothermal fluids with weak seawater signature for the sediments of Jingtieshan BIFs. The absence of negative Ce anomalies and the high Fe3+/Sigma Fe ratios of the oxide facies BIF do not support ocean anoxia. In contrast to the three main types (Algoma-, Superior- and Rapitan-type) of global BIFs, the Jingtieshan BIFs represent a unique type with features similar to those of sedimentary-exhalative mineralization. (C) 2015 Elsevier Ltd. All rights reserved.