Role of evaporitic sulfates in iron skarn mineralization: a fluid inclusion and sulfur isotope study from the Xishimen deposit, Handan-Xingtai district, North China Craton
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Mineral resource classification
Tourmaline
δ34S
The origin of Middle Jurassic evaporites in the Qamdo Basin is still controversial because palaeontological studies have reported that they have both marine and continental characteristics. The 87Sr/86Sr ratios of the gypsum in the Middle Jurassic Dongdaqiao Formation in the Qamdo Basin range from 0.707602 to 0.708163, which are higher than that of contemporaneous seawater. Model calculations suggest that continental water prevailed over seawater during the precipitation of these evaporites. However, the majority of the gypsum samples have δ34S values of 15.3‰ to 16.3‰, which are consistent with that of contemporaneous seawater. This range of values (15.3‰ vs. 16.3‰) was likely caused by S isotope fractionation during evaporation because the δ34S values and Sr contents are negatively correlated. The δ34S values of the other three gypsum samples are 20.0‰, 20.5‰, and 20.8‰, which are significantly higher than that of Middle Jurassic seawater. The trace element compositions and scanning electron microscopy (SEM) observations indicate that these elevated δ34S values were caused by bacterial sulphate reduction (BSR). The Sr and S isotope systematics of the gypsums from the Dongdaqiao Formation demonstrate that the parent brines from which the evaporites precipitated were marine based with a large quantity of continental input. A comparison of the lithologies and Sr isotope compositions of the Middle Jurassic sequences in the Qamdo and Qiangtang Basins revealed that the Qiangtang Basin was mainly recharged by Jurassic seawater, while the Qamdo Basin was primarily recharged by continental water with some seawater-derived overflow from the Qiangtang Basin.
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Halite
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Anhydrite
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Evaporites are widely distributed within continental “red beds” in the Lanping–Simao Basin, west Yunnan, China. Sr (Strontium), S (Sulfur), and O (Oxygen) isotope compositions have been measured on 54 sulfate or/and sulfate-bearing samples collected from Lanping, Nuodeng, Jinggu, Mengyejing, Baozang throughout the Lanping–Simao Basin. The 87Sr/86Sr ratios of all samples (0.708081 to 0.710049) are higher than those of contemporaneous seawater, indicating a significant continental contribution to the drainage basin. Sulfates in the Lanping Basin have higher 87Sr/86Sr ratios (0.709406 to 0.710049) than those (0.708081 to 0.709548) in the Simao Basin. Nevertheless, the δ34S values of gypsums (13.4‰ to 17.6‰) in Lanping and Baozang fall within the range of Cretaceous seawater. Gypsums from a single section in Baozang have trends of decreasing δ34S values and increasing 87Sr/86Sr ratios from base to top, indicating continental input played an increasingly significant role with the evaporation of brines. High δ34S values (20.5‰ to 20.7‰) of celestites in Lanping are probably caused by bacterial sulfate reduction (BSR) process in which 34S were enriched in residual sulfates and/or recycling of Triassic evaporites. The reduced δ34S values of gypsums (9.5‰ to 10.4‰) in Nuodeng could have been caused by oxidation of sulfides weathered from Jinding Pb-Zn deposit. The complex O isotope compositions indicate that sulfates in the Lanping–Simao Basin had undergone sulfate reduction, re-oxidation, reservoir effects, etc. In conclusion, the formation of continental evaporites was likely derived from seawater due to marine transgression during the Cretaceous period. Meanwhile, non-marine inflows have contributed to the basin significantly.
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Dolomitization
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Early Triassic
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Background. Although the Kuhilal deposit of Mg skarns has been thoroughly studied, tourmaline in their composition has never been considered [5, 6, 16]. Uvite, a tourmaline variety, was mentioned in the list of rare minerals only with its crystallochemical formula [17]. L.N. Shabynin, a great connoisseur of skarns, noted the rarity of tourmaline in Mg skarns of various deposits. Tourmaline of this type is characterized by a strong variation in ferruginosity and a significant content of Ca. At the same time, Ca:Na varies from 2:3 to 20:1 [15]. According to N.N. Pertsev, another major specialist in skarns, tourmaline is a rare mineral of Mg skarn deposits [11]. The boron-iron ore deposit of Tayozhnoe, Aldan shield, is an exception. This deposit belongs to skarns, where a widespread development of tourmaline was noted [12]. Tourmaline (dravite) of this deposit was found only in silicate metamorphic rocks containing marbles and Mg skarns. Aim. To determine the paragenesis, crystal morphology, chemical composition, and genetic characteristics of tourmaline from Kuhilal Mg skarns. Materials and methods. Large aggregates (more than 10 cm across) and crystals of apple-green tourmaline from the Mg skarns of the Kuhilal deposit were investigated. Samples of Mg skarns containing tourmaline were collected and documented by the authors during fieldwork research at the Kuhilal deposit in 2017. The samples were examined by mineralogical and petrographic methods, X-ray diffraction analysis using a DRONE-3M (analyst A.V. Fedorov, Sergo Ordzhonikidze Russian State University for Geological Prospecting), microprobe analysis using a Cameca SX 100 in 15 kV 30 nA shooting mode (analyst V.I. Taskaev, IGEM RAS), and X-ray fluorescence analysis using an AXIOS advanced spectrometer with an X-ray tube equipped with a 3 kW Rh anode and Philips PW-2400 with a sensitivity of 10-4% (analyst A.I. Yakushev, IGEM RAS). The majority of studies were conducted at the Department of Mineralogy and Gemology of the Sergo Ordzhonikidze Russian State University for Geological Prospecting. Results. Tourmaline from the Mg skarns of the Kuhilal deposit has been studied for the first time. This mineral is rare for skarn rocks. Two tourmaline mineral associations were identified: with spinel and forsterite, and with chlorite and serpentine. In terms of chemical composition, tourmaline isattributed to fluorine-containing uvite with a close to zero ferruginosity. Uvite is characterized by a lenticular shape with a rarely observed, underdeveloped prism. Its mineralogical properties, chemical composition, and formation conditions were analyzed. Tourmaline can be used to make inexpensive jewelry inserts. Conclusion. Tourmaline crystallization occurred under the conditions of granulite facies of regional metamorphism. For boron formation, an apo-sedimentary, evaporite source is assumed.
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Geological observations coupled with geochemical, isotopic and thermobaric-geochemical research of danburite ores of skarn-borosilicate deposits unique in their reserves in the Sikhote-Alin (Dalnegorskoe) and in the Pamir (Ak-Arkhar) indicated lagoonal strata and evaporites in the form of atoll fragments within the Mesozoic subduction mélange as the most probable boron source, which preceded the skarn formation. At the Dalnegorskoe deposit, the fact that boron is mobilized by skarn-forming solutions from evaporite accumulations is directly confirmed by isotopically heavier composition of boron (δ11В = +17.7 ‰) corresponding to marine evaporites, and a typical enrichment of fluid in lithium.
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This study investigates for the first time the subsurface Miocene evaporite facies (Gachsaran Formation) in Abu Dhabi, United Arab Emirates. Forty-five evaporite rock samples were selected for petrographic, mineralogical, and geochemical investigations and stable isotope analyses to decipher their origin and constrain their age. Secondary gypsum with anhydrite relics dominates the investigated evaporitic rocks, with minor amounts of clays, dolomicrite, Fe/Ti oxides, and celestite. These samples are characterized by their excellent purity and low variability in geochemical composition. The distribution of trace element concentrations is significantly influenced by continental detrital intake. The main focus of the study is to determine the strontium, sulfur, and oxygen stable isotope compositions. The measured 87Sr/86Sr values of 0.708411–0.708739 are consistent with Miocene marine sulfates and indicate ∼21.12–15.91 Ma (Late Aquitanian-Burdigalian). The δ34S and δ18O values are 17.10‰–21.59‰ and 11.89‰–19.16‰, respectively. These values are comparable to those of Tertiary marine evaporites. The relatively low values of δ34S suggest that non-marine water possesses little influence on S distribution. The geochemical composition and Sr, S, and O isotope distributions of the Abu Dhabi gypsum facies from the Gachsaran Formation reveals that their source brines were marine (coastal saline/sabkha) with subordinate continental input.
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