TYPES OF EPITHERMAL SILVER DEPOSITS, NORTHEASTERN RUSSIA

Three main epithermal silver provinces occur in northeastern Russia: ( 1) the Cretaceous Okhotsk-Chukotka volcanic plutonic belt (Russia's pa rt of the Circum-Pacific belt), (2) adjacent Mesozoic orogenic belts, and (3) Paleozoic complexes formed of intermediate massifs. Within the most studied Okhotsk-Chukotka province, three types of deposits are d istinguished by structural position and geologic environments: (1) tho se in volcanic intrusive domes, (2) those in siliceous subvolcanic bod ies, and (3) those in contact halos around granodioritic intrusions. G old-silver, silver-lead-zinc, and tin-silver ore deposits are distingu ished, according to the ore composition. The silver deposits of the Du cat ore district are examples of these three types. Genetically, ore m ineralization of the three types differs. Gold-silver mineralization w as formed in near-surface, open hydrothermal systems during mixing of metalliferous solutions with meteoric waters, as indicated by low sali nities (2.5-3.5% NaCl equiv) of the fluid inclusions. Subvolcanic and intrusive bodies were the local heat sources, which caused a field wit h high thermal gradients (avg 200-degrees-370-degrees-C). Boiling of t he fluids was the most effective mechanism of ore deposition with whic h ore shoot formation was connected as evidenced by texture and fluid inclusion data. In contrast, silver-lead-zinc and tin-silver mineraliz ations formed in closed hydrothermal systems in which boiling did not occur. Their relatively low uniform temperatures (avg 240-degrees-320- degrees-C) and low thermal gradients were associated with a more stead y regime of Ag-Pb-Zn-Sn ore deposition compared to the gold-silver dep osits. The relatively high salinity of fluids (up to 7.5% NaCl equiv) resulted from the absence of mixing of metalliferous solutions and met eoric waters.