[SCG55-03] Geochemistry of Middle Permian Tavantolgoi sandstone: Implications for paleoclimate and provenance
キーワード：sandstone geochemistry, Middle Permian, Tavantolgoi, Mongolia
The Tavantolgoi deposit, which has more than six billion tons of coal reserves, is situated in southern Mongolia. In 2018, more than 22Mt coal was produced from the deposit. During the exploration program completed in 2017, eighteen sandstone and two andesite samples for petrographical and geochemical analysis were collected from the upper part of coal-bearing Middle Permian Tavantolgoi Formation, which was intersected completely by 960 m deep bore hole.
Petrographically, the sandstones are classified as litharenite reflecting relatively weak weathering intensity in source area, probably due to arid or semiarid climate conditions. Provenance analysis indicates that the sandstones are sourced from transitional arc and undissected arc that were probably formed during the closure of Paleoasian ocean between southern Mongolia and North China block.
Geochemistry of major oxide and trace element result are summarized as below. CAI values from major oxides of the studied samples vary between 66.7 and 81.5 with an average of 77.0, reflecting weak to moderate intensity of weathering in source area. The plotted data on A-CN-K diagram displays that sandstones were sourced mainly from early Permian andesite and dacite, which are widely distributed around Tavantolgoi. It is also supported by Al2O3/TiO2 (avg. 19.6), TiO2/Zr (avg. 57.7) and petrographical study.
C-value, indicator of paleoclimate, in the sandstone ranges from 0.15 to 1.3 with an average of 0.50, suggesting semiarid and semiarid to semimoist condition. However, some samples accumulated during the short period of arid and moist conditions. Rb/Sr and Sr/Cu of the studied samples are consistent with those of C-value. In addition, Sr/Ba (avg. 0.56), Th/U (avg. 2.8) and Ni/Co (avg. 2.0) of the samples suggest that the sandstones were deposited in paleoenvironment with fresh and oxic water.
Petrographically, the sandstones are classified as litharenite reflecting relatively weak weathering intensity in source area, probably due to arid or semiarid climate conditions. Provenance analysis indicates that the sandstones are sourced from transitional arc and undissected arc that were probably formed during the closure of Paleoasian ocean between southern Mongolia and North China block.
Geochemistry of major oxide and trace element result are summarized as below. CAI values from major oxides of the studied samples vary between 66.7 and 81.5 with an average of 77.0, reflecting weak to moderate intensity of weathering in source area. The plotted data on A-CN-K diagram displays that sandstones were sourced mainly from early Permian andesite and dacite, which are widely distributed around Tavantolgoi. It is also supported by Al2O3/TiO2 (avg. 19.6), TiO2/Zr (avg. 57.7) and petrographical study.
C-value, indicator of paleoclimate, in the sandstone ranges from 0.15 to 1.3 with an average of 0.50, suggesting semiarid and semiarid to semimoist condition. However, some samples accumulated during the short period of arid and moist conditions. Rb/Sr and Sr/Cu of the studied samples are consistent with those of C-value. In addition, Sr/Ba (avg. 0.56), Th/U (avg. 2.8) and Ni/Co (avg. 2.0) of the samples suggest that the sandstones were deposited in paleoenvironment with fresh and oxic water.