Rare Earth Element Geochemistry of Late Palaeozoic Coals in North China

Abstract

Abstract: Instrumental Neutron Activation Analysis (INAA) was done to determine the abundances of rare earth elements (REE) of 58 samples of Late Palaeozoic Carboniferous‐Permian coals and related rocks in North China. Detailed study of REE geochemistry shows that the ɛREE of most coals studied in this paper is in a normal range between 30×10−6 and 80×10−6 with a mean of 56×10−6. The REE in the Taiyuan Formation in the northern part of North China are much richer than those in the southern part. This is due to the shorter distance to the source area in the north. Moreover, the ɛREE is in positive correlation to coal ash, especially closely related to the content of clay minerals <2 μm in size. This reveals that most REE were carried by terrigenous clastic materials, especially fine clay minerals. In the coals the light REE (LREE) are much richer than the heavy REE (HREE), and the LREE/HREE ratio in coals generally varies from 2 to 8. The LREE/HREE ratio of high‐ash, low‐sulphur coals is higher than that of low‐ash, high‐sulphur coals, and is even higher in the roof and floor rocks, reaching 12 with the highest (up to 21) in pyrite concretions, which shows that the HREE have a stronger affinity to organic matter in a reducing environment influenced by seawater. Furthermore, Eu is generally depleted in coals. The REE chondrite‐normalized distribution patterns are very similar in coals of the whole of North China. From the REE geochemical characteristics it can be concluded that during the formation of Late Palaeozoic Carboniferous‐Permian coals in North China, the supply of terrigeneous materials remained quite stable. The ɛREE in low‐ash, high‐sulphur coals is relatively low and the REE are mainly carried by fine‐grained minerals and organic matter and a certain percentage of REE are adsorbed by organic matter; while the ɛREE in high‐ash, low‐sulphur coals is higher and the REE are mainly present in detritus. The ɛREE of magmatism‐influenced coals is the highest, which suggests that the introduction of magmatic substances may increase the ɛREE, thus causing the REE distribution patterns to show an abnormal feature. Moreover, some harmful elements such as U, W and As usually increase when the coals are influenced by magmatism.