Geochronology, Mineralogy, and Geochemistry of the Tonsteins from the Permo–Carboniferous Benxi Formation, Ordos Basin, North China Craton

Abstract

Tonstein layers are found worldwide in the Permo‐Carboniferous coal‐bearing strata. This study investigates the geochronology, mineralogy, and geochemistry of four tonstein samples from the Permo‐Carboniferous Benxi Formation, Ordos Basin, North China Craton (NCC). The typical features of the studied tonsteins include thin beds, lateral continuity, angular quartz grains, and euhedral zircons with similar U–Pb ages, indicating a significant pyroclastic origin. In addition, the tonstein samples have low TiO2/Al2O3 ratios (< 0.02) and rare earth elements and yttrium (REY) concentrations with obvious negative Eu anomalies, indicating that the tonsteins have a felsic magma origin. Moreover, compared with the mean composition of clay shale, the studied tonsteins are characterized by high concentrations of the elements Nb and Ta, which may affect the concentration of the corresponding elements in surrounding coal seams. The zircon U–Pb ages of the tonsteins (293.9–298.8 Ma) provide a precise chronological framework on the Benxi Formation in the Ordos Basin, constraining the Gzhelian‐Aselian stages. The tonsteins were probably sourced from arc volcanism along the western margin of the NCC during the Early Permian, implying that the Alxa Terrane had not amalgamated with the NCC at that time.

Cooperative Exploration Model of Coal–Gallium Deposit: A Case Study of the Heidaigou Coal–Gallium Deposit in the Jungar Coalfield, Inner Mongolia, China

“…The enrichment of Ga in the top and bottom plates of the coal seams indicates that the Ga-containing carriers in the clay minerals entered the coal seams through the surrounding rocks under the action of groundwater, showing the influence of catagenesis, which indicates that the catagenesis leaching action and the activity of groundwater are other factors contributing to the enrichment of Ga in the Heidaigou coal-Ga deposit [20]. Moreover, in the clayey conglomerate of the Heidaigou coal series, significant quantities of volcanic clasts and volcanic ash have been identified [67,[77][78][79]. Earlier research on the Taiyuan Formation revealed thin layers of volcanic tuff [80], indicating that these volcanic tuff layers and clasts might have originated from volcanic activities along the orogenic belt at the boundary of the North China and South Mongolia Plates.…”

Cooperative Exploration Model of Coal–Lithium Deposit: A Case Study of the Haerwusu Coal–Lithium Deposit in the Jungar Coalfield, Inner Mongolia, Northern China

“…Due to the stable structural conditions, it was conducive to the formation of bauxite, and the moyite clastic materials of the Yinshan ancient land provided direct provenance for the study area [14,19]. Bauxite residue within the weathering crust was a source of Li, and the input of volcanic ash produced by volcanic activities affects the enrichment characteristics of Li [55,[58][59][60][61]. During the formation of Haerwusu Li-rich coal, the chemically altered detrital material of moyite from the Yinshan ancient land was initially enriched in the weathering crust of the Upper Carboniferous Benxi Formation after long-term surface efflorescence, denudation, and transport.…”

Controls on Gas-Reservoir Formation in the Benxi Formation in the Ganquan–Fuxian Area of the Ordos Basin, China

“…The average porosity and permeability of the gas-bearing Benxi Formation sandstones in the Ganquan-Fuxian area are 2.9% and 0.8141 × 10 −3 µm 2 , respectively, indicating extra-low porosity and permeability [9,11]. The lower limit of effective reservoir porosity is 4%, and that of permeability is 0.07 × 10 −3 µm 2 [12]. Previous studies have investigated the reservoir characteristics and diagenetic evolution of Benxi Formation sandstone using various experimental techniques and established a quantitative model for pore evolution [9].…”