冀东、五台和吕梁地区条带状铁矿的稀土元素特征及其地质意义

Characteristics of Rare Earth Elements and Geological Significations of BIFs from Jidong,Wutai and Lüliang Area

详细报道了冀东、五台和吕梁地区条带状铁矿全岩样品的稀土元素分析结果。结果表明,研究区BIF具有非常相似的特征:稀土总量均较低;经页岩标准化的稀土元素配分模式均呈现轻稀土亏损、重稀土富集的特征;Y/Ho比值较高;具有明显的Eu、Y、La的正异常,且这些特征表明研究区BIF的稀土元素来源于火山热液和海水的混合溶液。虽然BIF均显示Eu正异常,但不同类型、不同沉积年龄BIF的铕异常程度不同:与吕梁地区Superior型铁矿相比,冀东和五台地区的Algoma型铁矿显示了更大的Eu正异常;并且自中太古代—新太古代—古元古代,BIF的铕正异常逐渐减小,这可能反映了随着BIF沉积年龄的减小,进入到该地区海水中的高温热液流体逐渐减少;同时,研究区BIF缺乏明显的Ce负异常,可能暗示在BIF沉积时海水的氧化还原状态为缺氧环境。

The results of rare earth elements of banded iron formations samples from East Hebei Province,Wutai and Lüliang of Shanxi Province are reported in this article.Analytic results show that the BIFs have the similar characteristics： REE concentrations in BIFs are very low;the shale-normalised REE＋Y patterns display depletion of light REE relative to heavy REE,high Y/Ho values and distinct positive anomalies of Eu,Y,and La.These demonstrate that the BIFs are the products of chemical sedimentation from paleo-seawater with significant input of volcanic hydrothermal fluids.Although BIFs are characterized by positive Eu anomalies,BIFs of the different type and different age show different size of positive Eu anomalies.Compared with superior BIF from Lüliang area,Algoma BIF from Jidong and Wutai shows the higher positive Eu anomalies.The Eu anomalies decreased from Middle archean-Neoarchean to Paleoproterozoic BIFs and this decrease in the positive Eu anomaly with depositional age might argue for a decrease of high-temperature hydrothermal activity.The lack of negative Ce anomalies in almost of BIFs samples indicates relatively low redox levels in the local seawater.