GEOCHEMICAL AND MINERALOGICAL FEATURES OF GYPSUM OF THE KUNGURIAN STAGE (Lower Permian) IN THE SOUTHERN CIS-URALS

Abstract

The geochemical and mineralogical features of gypsum of the Kungurian Stage in the southern Cis-Urals have been studied. Elevated contents of Sr (up to 1.6 wt.%) were detected, and its sedimentary genesis has been established. The main Sr-containing mineral in gypsum is сelestine of two generations differing in morphologic features and chemical composition. The presence of early diagenetic celestine in pelitomorphic zones of gypsum is evidence for lagoon sedimentation in the Kungurian. Barium-containing celestine of late generation often develops in assemblage with magnesite in recrystallized zones of gypsum. The weak positive correlations of Sr with Fe, Mg, and К indicate the enrichment of magnesium chloride and calcium–sodium sulfate–chloride brines with these elements during their interaction with rocks, including argillaceous ones, during halocatagenesis. During diagenesis, nodular and microcrystalline pyrite 1 with impurities of Cu, As, Co, and Ni formed. During catagenesis, the recrystallization of gypsum resulted in pure coarse-crystalline pyrite 2. The rocks near the boundary of the Kungurian and Ufimian stages are characterized by elevated Cu contents (up to 283 ppm). The catagenetic celestine is characterized by low Sr/Ba ratios, which, along with the data on the isotopic composition of sulfur of celestine (δ34S = 24.8‰) and with the presence of pyrite inclusions in gypsum, indicate widespread sulfate reduction processes during catagenesis. The increase in the share of 32S in gypsum and anhydrites of the southern Cis-Urals (δ34S = 7.4–13.4‰) is due to the increasing role of continental runoff in the Permian salt basin.