Mineral matter and potentially hazardous trace elements in coals from Qianxi Fault Depression Area in southwestern Guizhou, China

摘要

Mineralogy, coal chemistry and 21 potentially hazardous trace elements (PHTEs) of 44 coal samples from the Qianxi Fault Depression Area (QFDA) in southwestern Guizhou province, China have been systematically studied. The major minerals in coals studied are quartz, kaolinite, illite, pyrite, calcite, smectite, marcasite and accessory minerals, including rutile, dolomite, siderite, gypsum, chlorite, melanterite, apatite, cellophane and florencite. The SiO_2 content shows a broad variation (0.8-30.7%). A high SiO_2 content in Late Permian coals reflects their enrichment in quartz. The Al_2O_3 content varies from 0.8% to 13.4%, Fe_2O_3 from 0.2% to 14.6%, CaO from <0.1% to 11.9% and the contents of other oxides are relatively low. The relationship between the major oxides and the ash content of coals from high to low is in the order of Si>Al>K>Ti>Na>Mg>Ca>Fe>S. A comparison with World coal averages shows that the Late Permian coals in QFDA are highly enriched in As, Hg, F and U, and are slightly enriched in Mo, Se, Th, V and Zn. The Late Triassic coals in QFDA are highly enriched in As and Hg, and are slightly enriched in Mo, Th and U. The concentrations of As, Hg, Mo, Se, Tl and Zn in the QFDA coal are higher than other Guizhou coal and Liupanshui coal nearby. The QFDA is an area strongly affected by the low-temperature hydrothermal activity during its geologic history (Yanshanian Age, about 189 Ma). The coals in QFDA are enriched in volatile PHTEs, including As, Hg, Se, Sb, Mo, among others. The regions where the coals are enriched in As, Hg and F have been mapped. The regions of coals enriched in volatile PHTEs overlap with the regions of noble metal ore deposits. These coals are located in the cores of anticline and anticlinorium, which are connected with the profound faults through the normal faults. Coals are enriched in volatile PHTEs as a result of the low-temperature hydrothermal activity associated with tectonic faulting.