REE Geochemistry of the Cretaceous lignite from Wulantuga Germanium Deposit, Inner Mongolia, Northeastern China
摘要
The Wulantuga Germanium Deposit (WGD), hosted in coal seams with Ge resources up to 1600 Mt, is located in the Shengli Coalfield in Xilingol, Inner Mongolia, China. Forty-two channel samples of Ge-bearing lignites of the No. 6-1 coal seam (Lower Cretaceous Bayanhua Formation) were collected and their REE compositions were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The modes of occurrence of REE in selected lignite samples were studied by scanning electron microscope with energy-dispersive X-ray spectrometer (SEM-EDX) and statistical methods. The distribution of REE and Y in all 42 samples from WGD are in accordance with the log normal distribution, the total rare earth element content (∑REE) of most lignite samples from WGD range from 9.02 to 118 ppm, only four samples show relatively higher (149–533 ppm) ∑REE concentration. The average REE contents of 42 lignite samples from WGD are slightly higher than those of the USA coals, world wide coals, sandstone from WGD, and the contemporary Ge-barren lignite from Hongqi Coal Mine. The lignite samples from WGD have shale-like Chondrite or NASC-normalized REE patterns similar to those of sandstone overburden and Ge-barren lignite from Hongqi Coal Mine. REE contents of most lignite samples are highly positive correlated with ash yields, indicating a mainly detrital source and association with syngenetic mineral matter. REE-rich phosphates (monazite) were identified by SEM-EDX in some medium-to high-ash (ash yields range from 23.58 % to 38.18 % ) lignite samples with high ∑REE Y contents from WGD. REE can be concentrated in the upper and lower proportions of the No. 6-1 coal seam. The vertical profiles of REE mainly follow that of TiO2, and the average REE/TiO2 ratio of lignite sample in three sections is very close to that of the sandstone from WGD or lignite from Hongqi Coal Mine. It was determined that most REE in the lignite from WGD were derived from a detrital source and associated with syngenetic mineral matter. However, when normalized to the overlying sandstone, some lignite samples from WGD display a small but distinct enrichment in middle-to heavy-REE, indicating small quantity of middle-to heavy-REE may be added and/or retained during epigenesis. The lignite samples from WGD are characterized by higher ratios of LREE/HREE and Eu/Eu*, and horizontal NASC-normalized REE patterns, and distinguished from those of the Ge-bearing lignites from Lincang Germanium Deposit, Yunnan, China and Ge deposits of Russian Far East. There is no distinct correlations between Ge and any individual REE or ∑REE Y content of lignite from WGD.