Resources of critical metals in coal-bearing sequences in China: Enrichment types and distribution

Abstract

Coal is a particular sedimentary organic ore and has a huge resource worldwide. Coal is a resource primarily used for electric power generation, and currently supplies 41% of global electricity needs. Significant quantities of coal are also employed in metallurgical processes, gasification, cement industries, and as raw materials for activated carbon and many common and industrial chemicals, as well as for heat for the wallboard, aluminum, and cement industries. A number of coals can also be considered as an economic source of some critical metals, and thus are coal-hosted ore deposits. The critical metals in coal and coal-bearing sequences occur at concentrations comparable to or even higher than those in conventional economic deposits. China is, and in the coming decades should continue to be, the largest producer and user of coal in the world. The high volume of coal usage in China has led to attention not only on the toxic elements that may be released from coal combustion but also on the critical metals that may occur in the coal and coal-bearing sequences, as well as coal combustion by-products. Critical elements (e.g., Ge, Ga, U, rare earth elements and Y, Nb, Zr, Se, V, Re, Au, and Ag, as well as the base metal Al) in a number of coals (or coal ashes) and some coal-bearing strata in China occur at concentrations comparable to or even higher than those in conventional economic deposits. This paper reviewed the distribution and ore-forming mechanism of some typical coal-hosted ore deposits in China, including Ge, Ga-Al, rare earth elements (REE), and Nb-Zr-REE-Ga. Additionally, other critical metals such as Be, Sc, Li, Mg, V, and Re in coal and coal bearing sequences have also been briefly reviewed. Several geological factors are responsible for the deposition of coal-hosted critical metals, including injection of hydrothermal fluids during peat accumulation or later epigenetic stages, deposition of syngenetic alkali volcanic ashes in peat swamp, input of terrigenous mineral matter into the coal-forming environment, leaching of partings by groundwater, and mixed processes involving both hydrothermal solutions and volcanic ash. Mixed processes involving both hydrothermal solutions and volcanic ash are also important geological factors that contribute to the deposition of coal-hosted critical metals. The critical metals in Chinese coal and coal-bearing sequences may be associated with either the organic matter or minerals. For example, germanium exclusively occurs in the organic matter; gallium mainly occurs in boehmite and kaolinite, and to a lesser extent, in the organic matter; rare earth elements and Y occur as carbonate minerals (e.g., florencite and parisite), phosphate minerals (e.g., rhabdophane, silico-rhabdophane, and xenotime), and in part are associated with the organic matter. Some critical metals (e.g., Ge, Al, and Ga) have been successfully extracted at an industrial scale from Chinese coals, and others have significant potential for such extraction. Major challenges remaining for coal scientists include the development of economic extraction methods from coal ash, and the control of toxic elements released during the metal extraction process to protect human health and to avoid environmental pollution.