Abstract
Existing and recently acquired data on the isotope age and composition of volcanogenic Au-Ag deposits and occurrences of the Evensk group (Okhotsk-Chukotka volcanogenic belt, Northeast Russia) are summarized. The K-Ar isotopic and Rb-Sr isochron ages of Au-Ag ores from the deposits and occurrences studied is 82.7 ± 3–77.5 ± 3 Ma and 84.1 ± 2–79.7 ± 5, respectively. Au-Ag mineralization formed during a 5 million-year time interval. Au-Ag mineralization, typical of productive ore stages and practically unaffected by later thermal processes, is characterized by low values of primary 87Sr/86Sr(0) = 0.7055–0.7059 ratios close to primary 87Sr/86Sr(0) ratios in wall-rock metasomatites (0.7033–0.7082) and unaltered host rocks (0.7045–0.7048) indicative of 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 exhausted (depleted) mantle with a trend towards the volcanic rocks formed in subduction zones typical of the continental margin. A close genetic relationship between ore and magmatic processes is confirmed. It is assumed that this relationship is due to the functioning of a single mantle basaltoid chamber, a source of metal-bearing fluids and, probably, the primary source of Au and Ag.
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