Distribution, occurrence, and enrichment of gallium in the Middle Jurassic coals of the Muli Coalfield, Qinghai, China

Gallium (Ga) is known as the backbone of the electronics industry, but its independent mineralization is rarely found in nature. The discovery of large Ga-bearing coal deposits suggests that the coal-bearing series can be used as the main source for the future development and utilization of Ga, and the organic matter of coal may play an important role in the aggregation of Ga. Based on the analysis of geological prospecting and study of coal samples, this paper discussed the distribution, occurrence, and enrichment of Ga in the Middle Jurassic coals of the Muli coalfield. The results demonstrate that coals from eastern mines are enriched in Ga, where the Ga concentration reaches the propositional cut-off grade (30 mu g/g). The Ga concentration increases along with an increasing distance from the sediment source; the areas with the highest Ga concentration are the deposit-center and the location with the highest content of mudstone. Sediments were derived from the acid and intermediate magmatic rocks of the Tuolai Mountain in the northeastern Muli coalfield, partly from the crust weathering of the Upper Triassic formation. Ga was first transported in the form of colloids or solutions and finally deposited in authigenic kaolinite. In addition, the positive correlation between the Ga concentration and the inertinite constituents of coal indicates that fibrous tissue might be the dominant component absorbing Ga and that organic matter had a positive impact on the migration of Ga to peat swamps. Ga is prone to deposition in partially oxidized, acidic coal-forming environments with strong hydrodynamic forces. Hydrothermal activity might also stimulate the enrichment of Ga in coal.