Modes of occurrence of rare earth elements and yttrium in the subbituminous coal of the Jungar Coalfield, Ordos Basin, North China

Abstract

Modes of occurrence of rare earth elements (REE; if including Y, REY) in coal have attracted much attention owing to the extraction potential of REY from coal fly ash. Although previous studies have explored the associations of REY in coals with different ranks and the affinities of light, medium, and heavy REY associated with organic matter, challenges remain due to the limitations of indirect or in-situ analytical techniques. This study focused on the in-situ distribution of REY in the selected subbituminous coals from the Jungar Coalfield as well as in their associated mudstone and tonstein partings and the widely distributed calcite veins inside the coal seam, using LA-ICP-MS spot and mapping analysis. The results showed that in the subbituminous coal, La and Ce as well as other light REY (LREY) are concentrated in the minerals such as bastnasite and monazite. Small amounts of Pr, Nd, and Sm can be associated with organic matter including vitrinite and liptinite. Medium REY (MREY) and heavy REY (HREY) are highly enriched in vitrinite in the subbituminous coal. In terrigenous mudstone of the subbituminous coal, La and Ce are concentrated in monazite. Other LREY are present in Ti-oxides, while HREY tend to occur in zircon. MREY can be associated with both Ti-oxides and zircon. The calcite veins in the coal seam are classified into three types based on their morphology and occurrence features: bedding calcite veins in the upper part of the coal, vertical calcite veins in the middle, and vertical calcite veins with a high Sr value in the lower part of the coal. Polarization and crystal size of calcite crystals in these veins exhibit multistage growth. LA-ICP-MS spot analysis shows that the bedding and vertical calcite veins are characterized by positive Y and Gd anomalies, suggesting their formation under meteoric water leaching. The vertical calcite vein in the lower part of the coal seam displays a positive Gd anomaly and high Sr concentrations, indicating formation under seawater injection. The concentration of REY in all epigenetic calcite veins increases with depth. For calcite veins with multi-stage growth, the REY content in the late-stage calcite is consistently higher than in the early stages. It is proposed that leaching of meteoric water and seawater on the coal seams is the key factor for the association of REY with organic matter, influenced by the chemical composition and pH value of the leaching fluids. In particular, MREY and HREY are preferentially associated with organic matter, mainly vitrinite, and the over-leached MREY and HREY in coal were concentrated in epigenetic minerals, with calcite being the primary phase in this study.