Causes of anomaly of rare earth elements of seam 2 of Yan'an Formation in the northern margin of Ordos basin and its geological significance

The cause of rare earth elements(REE) anomalies in No.2 coal seam of the Yan'an Formation in the northern margin of the Ordos basin has been controversial. This paper takes Dahaize coal mine, Yulin area with well developed No.2 coal as an example to analyze REE, mineralogical characteristics of coal and partings, and to reveal the causes of anomaly of REE by Inductive Coupled Plasma Mass Spectrometry(ICP-MS) and Scanning Electron Microscopy(SEM). The results show that the total rare earth element(∑REE) concentration in the coal samples ranges from 3.71 to 46.21 μg/g. Light rare earths elements(LREE, La-Eu) are more abundant than heavy rare earth elements(HREE, Gd-Lu). The distribution patterns of their normalization show that a few of samples have positive anomalies. Samples with high REE content and samples with positive Eu anomalies mainly appeared in the analyzed coal seam close to the partings. It is indicated that the distribution of REE in coal is affected by its partings. There are many zircons, apatites, sanidines and anatase with better crystal form found in the partings, which are direct evidence of the volcanic activity during the peat formation. This suggests that there was a volcanic activity during the No.2 peat formation of the Yan'an Formation in Ordos basin. The volcanic ash fallen over and covered the swamp and affected the coal accumulation and the abnormal distribution of REE in the coal. The results explain the reason of REE anomaly in the coal and provide data support or theoretical basis for the future industrial use of REE in the coal in this area.