The Sr isotope signature of Wuchiapingian semi-anthracites from Chongqing, southwestern China: Indication for hydrothermal effects

Abstract

The strontium (Sr) concentration and 87Sr/86Sr ratios in leachates extracted from Upper Permian (Wuchiapingian) coals from the Tianjia Mine, Chongqing, southwestern China, were investigated using a selective leaching sequential extraction technique including deionized water, dilute HCl, and a mixture of HF and HNO3. The results show that the Sr concentrations in water-leachates (mean value of 12.9 ppm) and HCl-leachates (78.4 ppm) are lower than HF/HNO3-leachates (131 ppm). The water-leachates had more radiogenic (0.707182–0.707642) 87Sr/86Sr ratios than HCl-leachates (0.707111–0.707609), while the 87Sr/86Sr ratios in the HF/HNO3-leachates had a wide range (0.706972–0.707807), and the highest 87Sr/86Sr ratios. These variations in Sr concentration and 87Sr/86Sr ratios are most likely related to the differential solubility of the different Sr-bearing minerals in the coals, in addition to the Sr associated directly with the organic components of the coals. While the Sr/Ca ratios of the HCl-leachates (median = 11.3 mmole/mole) were similar to the ratio of marine carbonate dissolution, the 87Sr/86Sr values in the HCl-leachates were higher than the expected 87Sr/86Sr range of the contemporaneous seawater (0.70700–0.70725) recorded in the Wuchiapingian marine carbonate rocks. This indicates that the carbonate minerals in the coals are not of a syngenetic marine origin, despite the indications that the coal seams were influenced by seawater. The textural properties of the cell and fracture-filling carbonate minerals provide evidence that the carbonate minerals in the Tianjia coals are of late diagenetic or epigenetic origin. The combination of isotopic evolution trends of the coal seam, the petrological characteristics (high vitrinite reflectance and bireflectance) and mineralogical assemblage of NH4-illite-paragonite and kaolinite-paragonite as well as authigenic minerals such as calcite, ferroan dolomite, chamosite, quartz, anatase, and florencite suggests that the Tianjia coals was likely affected by diagenetic processes such as sediment dewatering with increasing burial depth (i.e., upward migration of pore fluids from the mudstones interbedded with the coal seams). These conclusions agree with the regional geological history marked by large-scale uplift and denudation of the crust following regional tectonic evolution (i.e., Dongwu Movement, 258–260.5 Ma).