Discovery of the superlarge gallium ore deposit in Jungar, Inner Mongolia, North China

Mineralogy and geochemistry of No.6 Coal from the Heidaigou Mine of the Jungar Coalfield of North China were studied using instrumental neutron activation analysis, inductively coupled-plasma mass spectroscopy, X-ray fluorescence spectrometry, and scanning electron microscope equipped with an energy-dispersive X-ray spectrometer. The results show that gallium concentration is as high as 44.8 μg·g⁻¹ in the whole coal-channel sample, and varies from 30.1 μg·g⁻¹ to 76.0 μg·g⁻¹ (mean 51.9 μg·g⁻¹) in the main minable benches of No.6 Coal. Such high concentration of Ga in the coal is far higher than the industrial grade (30 μg·g⁻¹). The thickness of the main minable benches accounts for 81.9% of the whole coal bed. The laboratory high-temperature ashes (550°C) of the main minable coal benches also contain a high concentration of Ga, varying from 62.2 μg·g⁻¹ to 178 μg·g⁻¹, with an average of 89.2 μg·g⁻¹. The boehmite significantly enriched in No.6 Coal is the main carrier of the high Ga in the coal. Average concentration of Ga in boehmite is 0.09%. The average content of boehmite is 6.1% in the whole coal and 7.5% in the main minable benches. Boehmite is derived from the bauxite in the weathered crust of the underlying Benxi Formation in the north of the basin during the peat accumulation. The colloidal idrargillite had been shortly transported from the weathered crust to the peat mire, and owing to the compaction of the overlying strata during the peat accumulation and early diagenesis, the idrargillite colloid had begun to be dehydrated, leading to boehmite formation. A preliminary estimation showed that the ensured and prospected reserves of Ga in No.6 Coal are up to 6.3 × 10⁴t and 8.57 × 1⁵t, indicating a superlarge gallium ore deposit. The particular paleogeography of the Jungar Coalfield and the peculiar carrier of Ga in coal suggest that this Ga ore deposit is unique in the world. Rare earth elements are also enriched in coal and laboratory ashes. The weighted average concentration of the total rare earth elements is 255 μg·g⁻¹ and 830.36 μg·g⁻¹ in the main minable benches and their laboratory ashes, respectively. Because Al is the main composition of boehmite, aluminum and rare earth elements in No.6 Coal are also the available and valuable resources.