Geochemical characteristics of illite clay rocks from the Shihezi Formation in the Hanxing mining area and its sedimentary environment

We studied the geochemical characteristics of illite clay rocks and their importance from the Shihezi Formation of Late Permian in the Hanxing mining area by means of ICP-MS and sequential X-ray fluorescence spectrometry. The results show that the amount of SiO₂ is between 53.37% and 61.58% (by weight) and that of Al₂O₃ 22.40% and 31.31% (by weight). The ratio of SiO₂/Al₂O₃ lies between 1.71 and 2.75. The amount of K₂O ranges from 1.11% to 2.56% (by weight). The amounts of Fe and Ti are higher than the theoretical values in illite clay rock would indicate. The amounts of some trace elements, such as Ga, As, Ba, Cu, Th and U are higher than their Clark values, while that of another 23 trace elements are found to be close to their Clark values. The amounts of REE range from 22.59 to 570.54 μg/g, with an average of 163.23 μg/g. The ratios of LREE/HREE range from 5.41 to 21.82, with an average of 8.87. These characteristics show that LREE are much richer in content than HREE. The REE distribution patterns of our samples were characterized by clearly negative Ce and Eu anomalies. We analyzed the sedimentary environment of the Hanxing mining area in Late Permian by the characteristic element ratio method. The ratios of Mn/Fe range from 0 to 0.0168, which are lower than those in a marine sedimentary environment. The ratios of Sr/Ba (0.20–0.41) are less than 1. These are all indications that the sediments of the Hanxing mining area in Late Permian form largely a continental sedimentary environment. The sedimentary water is freshwater, a conclusion reached on the basis of the ratios of Th/U (2.66–6.62) and of Ca/(Ca + Fe) (0.01–0.059); the average ratio of Fe²⁺/Fe³⁺ is 4.8. The sedimentary water condition is weakly acidic and weakly oxidative-weakly reductive, a conclusion reached on the basis of ratios of Fe²⁺/Fe³⁺ (4.8) and of Ce_anom (−0.08).