Geochemical study on Urich tuffs in Yanchang Group in the southern Ordos Basin：Implications to their forming mechanism
Lai Xiaodong1,2 Yang Xiaoyong1,2 Gao Peng1 Wu Bolin3 Liu Chiyang3 Sun Weidong1,4
(1.Research Center for Oil and Natural Gases|School of Earth and Space Sciences|University Science and Technology of China|Hefei 230026； |2.State Key Laboratory of Geological Processes and Mineral Resources|China University of Geosciences|Wuhan 430074； 3.Department of Geology|Northwest University|Xian 710069； | 4.Guangzhou Institute of Geochemistry|Chinese Academy of Sciences Guangzhou 510640)
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 sourceThe distribution patterns of the sandstonetype uranium samples show characteristics of light REE enrichment and high REE depletion，the total REE amount ranges from 4857 to 40212 μg/g. The results also indicate the abnormity of Eu between 034～082 and the abnormity of Ce between 080～208. The U abundances are 312～144 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 sandstonetype 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.