Enrichment of Trace Elements in the Early Carboniferous Coals from the Machala Mine in Eastern Qiangtang Basin, Eastern Tibet, SW China

The Machala Mine, which is located in the Eastern Tibetan Plateau, was one of the highest coal mines in the world, with an elevation of 5200 m. The Machala coals are rich in a variety of critical and harmful elements. However, the occurrence and enrichment mechanisms of these elements are not well understood, which hinders the assessment of their environmental impacts and recycling potential. Furthermore, the enrichment mechanism identified in the Machala coals is expected to extend our understanding of the simultaneous enrichment of multiple elements in coal. A total of 18 coal samples were collected from 14 coal (or carbonaceous mudstone) seams at the Machala open-pit mine for systematic analyses, including macerals, coal quality, minerals, trace, and major elements of coals. The results suggested that the coal seams in the Machala Mine were dominated by low-sulfur (mainly less than 1%), medium-to-low-ash (average: 18.15%), low-volatile (average: 16.01%), and medium-high-to-high-calorific (average: 27.23 MJ/kg) coals. The coal macerals were predominantly vitrinite, with the collodetrinite being the most abundant, followed by collotelinite and vitroderinite, while telinite was present in low amounts. The mineral compositions were mainly quartz and kaolinite, with average contents of 37.8% and 48.2%, respectively, which were followed by illite and smectite-mixed layers, with average contents of 4.7% and 3.3%, respectively. Other minerals, including plagioclase, pyrite, siderite, chlorite, and potassium feldspar, were present in low quantities. However, C3 coal seam was an exception, with a high pyrite content of 23.2% (low-temperature ash sample). The Machala coal seams were characterized by varying degrees of enrichment in Li, As, Pb, Hg, and Sn. Correlation analyses and energy spectrum analyses indicated that clays were their major host minerals. The enrichment of Li in the Machala Coal Mine was mainly attributed to volcanic ash during the sedimentation period, while As, Pb, Hg, and Sn were primarily sourced from hydrothermal fluids in the late stages.