Minerals and potentially hazardous trace elements in the Late Triassic coals from the Qiangtang Basin, China

• Content of trace elements in the Late Triassic coal;
• Evaluate potentially hazardous trace elements in the Late Triassic coal;
• Determine mineralogical phases and the mode of occurrence of trace elements;
• Explore the enrichment sources of trace elements in the Qiangtang Basin coal.

The Tumen coal mine has the largest coal resources in Tibet, which have been developed and utilized as an important energy source in the last decades. It has raised some health problems during the coal exploration and utilization in Tibet. Fifty Late Triassic coal (or coaly) samples were picked up from the Qiangtang Basin (Tumen mine, Woruoshan and Hongshuihe areas) to determine the minerals, potentially hazardous elements and their mode of occurrence and possible sources. Coal samples from the Qiangtang Basin have medium- and high-ash yields (15.20–47.88%) with low or medium-high total sulfur content (0.04–4.86%). Minerals in Qiangtang Basin coal include clay minerals, dolomite, quartz, pyrite, siderite, and hematite, and trace amounts of halite, feldspar, anhydrite, barite, chromite, and galena. Potentially hazardous trace elements in coal samples include As, Hg, Pb, and Se. Arsenic is controlled mainly by aluminosilicate minerals. Mercury occurs mainly as an organic-bound form. The organically bound Pb is dominant. Selenium is controlled mainly by Fe-bearing (probably pyrite). A proportion of the elevated concentrations of As, Hg, Pb and Se in Qiangtang Basin coal may be related to underlying shale bed. These elements might have been eroded or leached from the shale bed, which were subsequently transported and accumulated syngenetically in the coal-forming peat swamps. The enrichment of As, Hg, Pb, and Se in Qiangtang Basin coal, however, is also partly to be related to magmatic/hydrothermal fluids.