Mode of occurrence of critical metals in the Jurassic coals, northern Xinjiang, China: Based on the sequential chemical extraction procedure

Abstract

In the Jurassic coals from northern Xinjiang, China, the enrichment of critical metals has been continuously discovered. However, there is limited research on their mode of occurrence and a lack of quantitative analysis of their presence in different components. This study used a sequential chemical extraction procedure combined with field emission-scanning electron microscopy to investigate the mode of occurrence of critical metals in the Early Jurassic Taliqike Formation coals from Kubai coalfield and the Middle Jurassic Xishanyao Formation coals from Tuokexun and Heshituoluogai coalfields. The mineral substance includes quartz, calcite, pyrite, dolomite, anatase, clays, and minor zircon, florencite and monazite. Additionally, significant amounts of sphalerite were found in Heishan coals in Tuokexun coalfield. The Xishanyao coal has a generally higher content of major element oxides (except for CaO). Apart from the abnormal enrichment of Ge in Heishan coal (44.76 µg/g) and Zr in Sulahema coal (232.96 µg/g), other critical metals are slightly enriched or normal. Zr(Hf) in coal primarily occurs in zircon and anatase, while Nb(Ta) mainly exists in aluminosilicate. They are also potentially adsorbed as ions in the clay interlayers, encapsulated in clays as carbonate minerals, or present in pyrite and phosphate minerals. Gallium in coal generally exists in clays and phosphate minerals, but Ga in Sulahema coal primarily occurs in inertinite. Germanium primarily occurs in inertinite, whereas Ge in Heishan coal is mainly associated with sulfide. Germanium exhibits a strong positive correlation with both zinc and sulfur, indicating that Ge is potentially in sphalerite and associates weakly with pyrite and organic matter.