Alexandrovskoe gold deposit (Eastern Transbaikalia): Sources of rock and ore substance

The relevance of the study is in the need to expand the mineral resource base of Russia. Eastern Transbaikalia is one of the oldest gold mining regions of the country. Nevertheless, even in such a well-studied region, the issue of search and discovery of new ore deposits is acute. For the successful completion of such a task, the data are needed from a detailed study of already known ore deposits, which can be used to develop scientifically based criteria for search for ore deposits and mineralization forecast. Such data include determination of sources of ore-bearing magmatic melts, age and conditions for gold mineralization formation of the Alexandrovskoe gold deposit. The aim of the study was to prove participation in formation of several magma chambers with different characteristics, using the results of studying the distribution of rare-earth elements in igneous rocks and ores, the isotopic composition of oxygen of ore-bearing quartz and sulfur sulfide, as well physicochemical conditions for formation of mineralization by studying fluid inclusions in minerals. Objects of researches is igneous rocks and ores of the Alexandrovskoe gold deposit, located in the Eastern Transbaikalia. Methods. To determine the elemental composition of the rocks, we used the X-ray fluorescence method and standard chemical analysis, the concentrations of rare-earth elements were measured using the method of sorption-atomic emission analysis with inductively coupled plasma (GIN SB RAS, Ulan-Ude). The study of isotopic composition of sulfur sulfides, Au and Ag contents, and isotopic age (⁴⁰Ar/³⁹Ar) was carried out at the multicomponent and isotopic research center of the Siberian Branch of the Russian Academy of Sciences (Novosibirsk). Fluid inclusions in quartz of ore veins were studied by traditional methods of thermobarogeochemistry and Raman spectroscopy. Results. The age (⁴⁰Ar/³⁹Ar) of synrudic sericite (162 ±2,3 Ma) was determined, which corresponds to the age of intrusions of the Amujikan-Shakhtaminsky complex. It was established that variations in isotopic composition of oxygen in ore-bearing quartz and isotopes of sulfur sulfides correspond to a fluid of magmatic nature. An analysis of distribution of rare and rare earth elements indicates that the sources of mineralization were deep-seated, differentially differentiated magma chambers that functioned both in the upper and lower continental crust. According to the study of fluid inclusions in quartz of ore veins, crystallization of mineral paragenesis of ore deposits occurred in the temperature range from 150 to 402 °C. A productive mineral association was formed at 402-360 °C. The paper indicated the participation of ore-forming fluids that differ in salt composition in ore formation. This indicates different conditions for generation and nature of ore-forming fluids. A distinctive feature of the deposit is ore formation due to various depth different degrees differentiated ore-bearing magmatic sources.