The chemical composition of fluid inclusions brine in halite of evaporite deposits in Wenkou Depression (PRC) in the context of salt mineralogenesis

The chemical composition of brines of fluid inclusions of different genesis has established the peculiarities of mineralogenesis of the Wenkou Depression of the Dawenkou Basin. The content of K+, Mg2+ and SO42− in sedimentary brines ranged from 27.6 to 32.9, 41.5 to 32.7, and 66.6 to 33.3 g/l, respectively. The obtained data on the chemical composition of sedimentary brines and the values of δ34S (+10.9...+35.7 ‰) and δ18O (+14.7...+19.4 ‰) of anhydrite do not exclude the influence of marine transgressions on the continental halogenation of the Dawenkou Basin. The main source of sulfate in the Basin was ancient evaporites in the Yi-Meng Mountains of Shandong Province, which were eroded by surface waters. At the post-sedimentation stage, the salt strata were heated to temperatures of about 45 and 63 ℃. Halite recrystallization occurred at elevated pressure, which was tens of times higher than normal atmospheric pressure. Weakly mineralized waters enriched in Ca(HCO3)2, which entered the basin, led to the precipitation of gypsum or glauberite. Taking into account the high potassium content in the brines, which is close to the beginning of sylvite deposition, one can expect polyhalite (due to newly formed gypsum) mineralization to be found in the sediments. The chemical composition of post-sedimentation brines is characterized by a wide fluctuation in the content of the main ions. In the salt-bearing stratum, brines of various compositions circulated: a) with a sharply increased (relative to the sedimentary) content of potassium, magnesium and sulfate; b) with an increased content of potassium, magnesium and a reduced content of sulfate; c) with a reduced content of potassium and an increased content of magnesium and sulfate; d) with a significantly reduced concentration of all ions. In the XZK-101 well under investigation, no other salt minerals were found except halite, mirabilite, and glauberite. However, according to the data from the study of the chemical composition of brine inclusions in halite, in the immediate vicinity of its location in the Wenkou Depression, one should expect the detection of kieserite, langbeinite, and other salt minerals in salt deposits. The formation of langbeinite was facilitated by elevated temperatures and pressure. The brines found with an abnormally high magnesium content are apparently residual brines (reaction products) during the formation of langbeinite due to unstable sedimentary hexahydrite and sylvite. According to the obtained data on the chemical composition of brines of inclusions in halite, the boundaries of both the halite and potassic facies on the existing facies maps of the basin are subject to revision.