Types of epithermal silver deposits, northeastern Russia

Abstract

Three main epithermal silver provinces occur in northeastern Russia: (1) the Cretaceous Okhotsk-Chukotka volcanic plutonic belt (Russia's part 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 distinguished by structural position and geologic environments: (1) those in volcanic intrusive domes, (2) those in siliceous subvolcanic bodies, and (3) those in contact halos around granodioritic intrusions. Gold-silver, silver-lead-zinc, and tin-silver ore deposits are distinguished, according to the ore composition. The silver deposits of the Ducat ore district are examples of these three types.Genetically, ore mineralization of the three types differs. Gold-silver mineralization was formed in near-surface, open hydrothermal systems during mixing of metalliferous solutions with meteoric waters, as indicated by low salinities (2.5-3.5% NaCl equiv) of the fluid inclusions. Subvolcanic and intrusive bodies were the local heat sources, which caused a field with high thermal gradients (avg 200 degrees -370 degrees C). Boiling of the fluids was the most effective mechanism of ore deposition with which ore shoot formation was connected as evidenced by texture and fluid inclusion data. In contrast, silver-lead-zinc and tin-silver mineralizations 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 steady regime of Ag-Pb-Zn-Sn ore deposition compared to the gold-silver deposits. The relatively high salinity of fluids (up to 7.5% NaCl equiv) resulted from the absence of mixing of metalliferous solutions and meteoric waters.