江西新余铁矿的地球化学特征及其与华北BIFs铁矿的对比

Geochemical characters of Banded Iron Formations from Xinyu and North China

本文报道了新余和华北BIF铁矿的主量元素、微量元素、稀土元素的测试结果:新余和华北地区BIF铁矿的化学成分均主要由Fe2O3T和SiO2组成,它们的页岩标准化稀土元素配分曲线均呈现轻稀土亏损、重稀土富集的分馏模式。与华北BIF相比,新余铁矿具有更高的Al2O3和TiO2含量、微量元素含量、稀土总量,以及更低的Eu异常和Y/Ho比值,这些特征均表明,与华北地区相比,高温热液对新余地区BIF的贡献不明显。铁矿无明显的Ce负异常和Fe同位素组成为正值的特征暗示了铁矿沉淀时的海水为低氧逸度环境。

Banded Iron Formations （BIFs） is exposed among the Precambrian rocks in the Xinyu and North China area. The Neoproterozoic BIFs from Xinyu and Neoarehean BIF from North China area belong to the oxide facies and are formed with ahernating Fe-rich and chert-rich bands. The elemental geochemistry systematic has long been used as tool to understand the origin of Fe formations and corresponding chemistry of the ancient ocean. The results of major, trace and rare earth element compositions of BIFs from Xinyu and North China area are reported. The average bulk compositions of BIFs from Xinyu and North China are characterized by high total Fe2O3 and SiO2, and their Shale-normalized REE patterns display LREE depletion relative to heavy REE. Relative to those from North China, BIFs from Xinyu area are characterized by higher contents of Al2O3, TiO2, and ∑REE, and lower Eu anomalies and Y/Ho values. These features show that BIF from Xinyu and North China are chemical sediments, but relative to BIF in North China, the contribution of high-temperature hydrothermal fluids to the BIF in Xinyu is less important and the terrigenous component increase. The lack of Ce negative anomalies of BIF from Xinyu and North China and the heavy Fe isotope enrichment of contemporaneous BIF demonstrate that the oxygen fugacities in the depositional environments were very low, and that BIFs from Xinyu area is the result of Neoproterozoic ＇ Snowball Earth＇ condition when ice cover is begin to melt early and the ocean and atmosphere not fully exchange the material.