ISOTOPIC AND GEOCHEMICAL STUDY OF THE CONDITIONS OF TIN ORE FORMATIONOF SOLNECHNOYE DEPOSIT (FAR-EAST OF RUSSIA)

Physical chemical characteristics of the Solnechnoye tin deposit hydro thermal 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 trans fers in its root part to a granitoid massif (quartz diorites-monzonite s). The evolution in composition of the mineral-forming solution was s tudied by gas-chromatographic analysis of gaseous phase of fluid inclu sions. The analysis of aqueous extracts from fluid inclusions (AAS, po tentiometry, 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 alpha(HCO3)-, al pha((SII)), CO2/CH4, HCO3/Cl and pH increased. The value of f(O2) corr esponded to the F-Q-M buffer 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 oxid ation in comparison to the preore stage is typical for the quartz-cass iterite and the main sulfide stages. It means that organic compounds p articipated in redox reactions which took place in the solutions durin g the precipitation of these mineral assemblages. The delta(18)O value s of fluids calculated from the delta(18)O values obtained for the min erals show a distinct trend of decrease toward later stages. The delta D values for the waters from the inclusions are close to those found for contemporary meteoric waters (-120 parts per thousand). A transpor t model of non-isothermal isotope exchange between water and arbitrary number of minerals along the fluid flow paths was used for the interp retation of these isotope data as well as of the data on the isotope c omposition of the wall-rocks. Descending flow of meteoric waters was f ound to interact with sandstone-shale rocks and with granitoids before entering into the discharge zone. Comparison of the chemical and isot opic data which characterize the evolution of the mineral forming envi ronment of the Solnechnoye tin deposit allows us to conclude that mixi ng of acid, reducing, high temperature solutions, genetically related to granitoid melt, with alkalne-excessive, more oxidized, less saline meteoric waters, which have undergone interaction with wall rocks and granitoids, is the necessary condition for the redox reactions, result ing in cassiterite ore formation.