Maceral composition, coal quality and depositional environments of the middle Permian Ukhaakhudag coal deposit, South Mongolia

Abstract

The Ukhaakhudag coal deposit is located c. 560 km SSE of the city of Ulaanbaatar in the northeastern sector the South Gobi Basin. The coal-bearing strata is part of the middle Permian Tavantolgoi Formation. This study aims to determine the coal petrographic composition and depositional environments of five coal seams in the lower part of the deposit based on petrographic and chemical analyses of 106 composite samples. Vitrinite ranges from 12 to 64 vol.%, and inertinite varies between 9 and 68 vol.%. Liptinite ranges from 1 to 7 vol.%. Microlithotype analyses indicate that coals are primarily vitrinertite, few coals are classified as trimaceralic microlithotypes duroclarite and clarodurite. The inorganic fraction in the studied coals is mainly composed of clay, small amounts of silica, carbonate, and pyrite minerals. The vitrinite random reflectance values of the samples vary between 0.93-1.16 %. Volatile matter content varies from 26.10 to 41.48 wt.% (dry ash free basis). Ash, moisture, and sulfur contents vary between 11.20-44.76 wt.%, 1.63-6.03 wt.% and 0.49-1.67 wt.%, (air dried basis) respectively. Based on random vitrinite reflectance values and volatile matter content, the studied coals are classified as coking (Ch4), fat (Ch5), 1/3 coking (Ch6), gas fat (Ch7) based on the Mongolian system and medium to high volatile bituminous coal when using the ASTM system. The Gelification Index and Tissue Preservation Index of the studied samples suggest most seams accumulated in wet forest swamps with a high tree density. The Middle Permian peats from this location accumulated in mostly alternate oxic and anoxic mire conditions. The climate was drier during the peat accumulation of Seam 0, but it became warmer, and humidity increased starting from Seam 3 accumulation.