Isotopic and geochemical study of the conditions of tin ore formation of Solnechnoye deposit (Far East of Russia)

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Physical chemical characteristics of the Solnechnoye tin deposit hydrothermal system have been studied. Ores of cassiterite-tourmaline type was formed in a vertically dipping fracture zone, extended up to 8 km in a metamorphosed mass of sandstone-shale rocks. This rock mass transfers in its root part to a granitoid massif (quartz diorites-monzonites). The evolution in composition of the mineral-forming solution was studied by gas-chromatographic analysis of gaseous phase of fluid inclusions. The analysis of aqueous extracts from fluid inclusions (AAS, potentiometry, ICP and ionic chromatography) was also performed. It was found that as the process proceeded from the preore quartz-tourmaline stage to productive quartz-cassiterite, the values of a_HCO-3, a_(SII), CO₂/CH₄, HCO₃/Cl and pH increased. The value of f_O2 corresponded to the F-Q-M buffer on the preore stage and increased up to that of the Ni-NiO buffer during the formation of the main part of cassiterite ores. Study of organic compounds showed that a greater degree of oxidation in comparison to the preore stage is typical for the quartz-cassiterite and the main sulfide stages. It means that organic compounds participated in redox reactions which took place in the solutions during the precipitation of these mineral assemblages. The δ¹⁸O values of fluids calculated from the δ¹⁸O values obtained for the minerals show a distinct trend of decrease toward later stages. The δD values for the waters from the inclusions are close to those found for contemporary meteoric waters (–120‰). A transport model of non-isothermal isotope exchange between water and arbitrary number of minerals along the fluid flow paths was used for the interpretation of these isotope data as well as of the data on the isotope composition of the wall-rocks. Descending flow of meteoric waters was found to interact with sandstone-shale rocks and with granitoids before entering into the discharge zone. Comparison of the chemical and isotopic data which characterize the evolution of the mineral forming environment of the Solnechnoye tin deposit allows us to conclude that mixing of acid, reducing, high temperature solutions, genetically related to granitoid melt, with alkaline-excessive, more oxidized, less saline meteoric waters, which have undergone interaction with wall rocks and granitoids, is the necessary condition for the redox reactions, resulting in cassiterite ore formation.