鄂尔多斯盆地南部延长组富铀凝灰岩地球化学特征及形成机制

本文系统地研究了鄂尔多斯盆地南部延长组凝灰岩元素和同位素地球化学特征。结果表明:凝灰岩主量元素具有高K、高Al、Si中等至高、低Na的特征;REE总体特征为轻稀土富集、重稀土亏损,∑REE在48.57～402.12μg/g之间变化,Eu呈现出负异常,在0.34～0.82内变化,δCe范围在0.80～2.08之间;微量元素中U含量在3.12～144μg/g之间,Ba、U、Th、Hf和Ce呈明显正异常,Nb、P、Ti和Rb呈现负异常。详细的分析表明:U的富集和一些微量元素如Ta、Dy、Lu等相关,Th与∑REE存在明显的正相关性,这些变化可能和盆地中铀矿的富集有关;凝灰岩的硫同位素总体呈现深源特征。根据以上的研究对凝灰岩的形成原因及形成环境进行了探讨。鄂尔多斯盆地南部延长组凝灰岩来源于秦岭造山带火山作用,其中的铀元素富集是由于砂岩型铀矿中铀向凝灰岩迁移而被还原所致。本研究对盆地砂岩型铀矿的铀运移和富集机理认识具有一定指导意义。

Geochemical study on U-rich tuffs in Yanchang Group in the southern Ordos Basin:Implications to their forming mechanism

On the basis of analyses of major element、trace elements and REE from the tuff in the southern Ordos Basin，preliminary study on the genesis and formation environment of the tuff. The result indicates that：the tuff is enriched in K，Al and Si，lack in Na. The result of the sulfur isotopic shows the characteristic of deep sourceThe distribution patterns of the sandstone type uranium samples show characteristics of light REE enrichment and high REE depletion，the total REE amount ranges from 4857 to 40212 μg/g. The results also indicate the abnormity of Eu between 034～082 and the abnormity of Ce between 080～208. The U abundances are 312～144 μg/g，the characteristic of trace element shows positive abnormity on Ba，U，Th，Hf，Ce and minus abnormity on Nb，P，Ti and Rb. Our results also shows some strong correlation between U and the related elements such as Ta、Dy and Lu. In addition，Th abundance is correlated with ∑REE. The formation environment of tuff is discussed based on gechemical studies，indicating that tuff in the southern Ordos Basin originated from volcanism of the Qinling orogenic belt during Triassic，where uranium migration from sandstone type uranium deposit moved to tuff layers and then being reduced resulted in the uranium enrichment in tuff. This study presents a geochemical basis for understanding both uranium source and migration as well as its formation environment.