MINERALOGO-GEOCHEMICAL ZONATION AND PECULIARITIES OF FORMATION OF GOLD-SILVER DEPOSITS IN NORTHEASTERN RUSSIA

The paper generalizes the results of a complex mineralogo-geochemical investigation of the gold-silver (Au-Ag) deposits of the Upper Gizhigi n volcanic field (UGVF) constituting the Even volcanic zone - one of t he largest structures of the Okhotsk-Chukchi volcanogenic belt (Northe astern Russia). The UGVF metallogeny is the result of hydrothermal-act ivity processes which took place at the different stages of the Late C retaceous. The main stage of ore mineralization in this region involve s formation of Au-Ag deposits.;Metallization-enclosing rocks here are Upper Cretaceous effusive bodies of medium and acid compositions. The metallization was preceded by regional (areal) propylitization of rock s. By average chemical composition, the regional propylites scarcely d iffer from the unaltered effusive bodies. They are characterized by si gnificant areas of occurrence, a small degree of metasomatic transform ations, and isochemical compositions. The transformations of the regio nal propylites at the Au-Ag mineralization stage resulted in a complex metasomatic zonation. Three main types of altered wall-rock metasomat ites have been established: quartz-adularian, quartz-sericitic, and qu artz-hydromicaceous. The metasomatites are characterized by increased contents of volatiles, K2O, SiO2, and low contents of other components , and have a distinct zonal structure and the same alteration characte r. Up the ore-bearing zones, the propylites change into quartz-adulari an, quartz-sericitic, and quartz-hydromicaceous metasomatites. The geo chemical fields of concentration (GFC) of the ore elements of the stud ied Au-Ag deposits are characterized by a linear morphology and a rela tively simple component composition. Such elements as Au, Ag, Hg, Sb, As, Mo, W, Pb, and Zn form aureoles. The element distribution is intim ately spatially and genetically related to the metasomatic zonation. U p the ore-bearing zones, the lowly contrasting GFC of Mo, W, Pb, Au, a nd Ag (propylites) successively change into the medium-contrasting GFC of Au, Ag, As, and Mo (quartz-adularian metasomatites) and contrastin g and highly contrasting GFC of Au, Ag, Sb, As, and 11g (quartz-serici tic and quartz-hydromicaceous metasomatites). The main factors determi ning the composition and zonal structure of the GFC are the duration a nd depth of their formation and a change of physicochemical conditions (acidity-alkalinity, oxidation and sulfide potentials). The studied A u-Ag deposits follow the general series of vertical geochemical zonati on (from bottom to top): Zn, Pb-W, Mo-Au-Ag-As-Sb-Hg.