The Ages and Isotope Compositions of the Gold–Silver Deposits and Occurrences of the Evensk Group (Okhotsk–Chukotka Volcanogenic Belt, Northeast Russia)

Abstract

The existing and recently acquired data on the isotope ages and compositions of volcanogenic Au–Ag deposits and occurrences of the Evensk group (Okhotsk–Chukotka volcanogenic belt, Northeast Russia) are summarized. The K–Ar isotope and Rb–Sr isochron ages of Au–Ag ores from the studied deposits and occurrences are 82.7 ± 3–77.5 ± 3 Ma and 84.1 ± 2–79.7 ± 5 Ma, respectively. Au–Ag mineralization was formed within ~5 Ma. Au–Ag mineralization typical of productive ore stages and practically unaffected by later thermal processes is characterized by low initial ⁸⁷Sr/⁸⁶Sr₍₀₎ = 0.7055–0.7059 ratios, which are close to the initial ⁸⁷Sr/⁸⁶Sr₍₀₎ ratios in wall-rock metasomatites (0.7033–0.7082) and unaltered host rocks (0.7045–0.7048), thus indicating a pronounced predominance of mantle Sr in the ores. The composition points of ore Pb on the ²⁰⁷Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb evolution diagram are grouped in close proximity to the Pb isotope composition points in the depleted mantle with a trend towards the subduction-related volcanic rocks typical of the continental margin. A close genetic relationship between the ore and magmatic processes has been confirmed. It is assumed that this relationship is due to the functioning of a single mantle basaltic chamber, which served as a source of metal-bearing fluids and is probably the primary source of Au and Ag.