Strontium isotopes in high- and low-Ge coals from the Shengli Coalfield, Inner Mongolia, northern China: New indicators for Ge source

Abstract

The coal-hosted Ge deposit (Early Cretaceous) at Wulantuga, Inner Mongolia, northern China, is one of the largest Ge deposits in the world. Although the origin, modes of occurrence, and spatial distribution of Ge in this deposit have been investigated by several studies, the provenance and pathways of Ge from the source to the deposit are largely unknown. In this study, we investigated the 87Sr/86Sr values in the high-Ge coal of the Wulantuga deposit and the stratigraphically-equivalent low-Ge coal of the No. 1 Pit, both from the Shengli Coalfield, Inner Mongolia, northern China. In addition to an organic association, Sr and Ca in the high-Ge and low-Ge coals also occur in gypsum. The water-soluble Sr (i.e., in gypsum) in the high-Ge coal (Ge concentrations: 45–1170 ppm, 273 ppm on average, whole coal basis) has 87Sr/86Sr values in the range of 0.710411 to 0.710633 (with an average of 0.710478). In contrast, the low-Ge coal (Ge concentrations: up to 4.23 ppm, 0.45 ppm on average, whole coal basis) has lower 87Sr/86Sr values in two sub-groups, a low range of 0.709896–0.71005 (with an average of 0.709736) and a high range of 0.710173–0.710547 (with an average of 0.710329 and dominantly in the range of 0.710173–0.710406). The higher 87Sr/86Sr values in the high-Ge coals indicate that the highly-elevated concentration of Ge is associated with a local source of radiogenic Sr. The enrichment of Ge in the coal is due to several consecutive processes. Surface waters penetrated deeply into granite and dissolved at least partially more soluble phases of the granite. The Ge-rich solution flowed over ground or underground into the Shengli Coalfield and Ge precipitated in the peat, the precursor of the coal investigated in the coalfield. The 87Sr/86Sr values of the water-soluble fraction, the elemental compositions of high-Ge and low-Ge coals, and the geomorphological setting of the coalfield point to the Hercynian granodiorite intrusion on the nearby SE margin of the Shengli Coalfield as the primary source of Ge in the Wulantuga coal-hosted Ge deposit. In addition to serving as an indicator of the mode of formation of the coal-hosted Ge deposit, the 87Sr/86Sr values and their spatial distribution can provide an indication for the hydrological pathways of the mineralising fluid.