Rare earth element geochemistry of Late Palaeozoic coals in North China

Instrumental Neutron Activation Analysis (INAA) was done to determine the a bundances of rare earth elements (REE) of 58 samples of Late Palaeozoic Car boniferous-Permian coals and related rocks in North China. Detailed study o f REE geochemistry shows that the CREE of most coals studied in this paper is in a normal range between 30x10(-6) and 80x10(-6) With a mean of 56x10(- 6). The REE in the Taiyuan Formation in the northern part of North China ar e much richer than those in the southern part. This is due to the shorter d istance to the source area in the north. Moreover, the CREE is in positive correlation to coal ash, especially closely related to the content of clay minerals <2 mu m in size. This reveals that most REE were carried by terrig enous elastic materials, especially fine clay minerals. In the coals the li ght 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-a sh, low-sulphur coals is higher than that of low-ash, high-sulphur coals, a nd 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 stronge r affinity to organic matter in a reducing environment influenced by seawat er. Furthermore, Eu is generally depleted in coals. The REE chondrite-norma lized distribution patterns are very similar in coals of the whole of North China. From the REE geochemical characteristics it can be concluded that d uring the formation of Late Palaeozoic Carboniferous-Permian coals in North China, the supply of terrigeneous materials remained quite stable. The Sig ma REE in low-ash, high-sulphur coals is relatively low and the REE are mai nly carried by fine-grained minerals and organic matter and a certain perce ntage of REE are adsorbed by organic matter; while the Sigma REE in high-as h, low-sulphur coals is higher and the REE are mainly present in detritus. The Sigma REE of magmatism-influenced coals is the highest, which suggests that the introduction of magmatic substances may increase the Sigma REE, th us causing the REE distribution patterns to show an abnormal feature. Moreo ver, some harmful elements such as U, W and As usually increase when the co als are influenced by magmatism.