Dvoinoe Au–Ag Epithermal Deposit, Chukchi Peninsula, Russia

The Dvoinoe Au-Ag low-sulfide epithermal deposit is located in the Ilirnei ore district (western Chukotka) within the outer zone of the Okhotsk–Chukotka volcanic belt (OChVB). The paper considers the results of geological–structural, mineralogical–geochemical, and thermobarogeochemical studies of the deposit. The ores of the deposit are characterized by dominant colloform-banded (often combined with brecciated) structures; a high Au/Ag ratio (1: 1 to 1: 2); low sulfidity (<0.5%); the presence of hessite, small sulfide segregations (0.005–0.015 mm), and a larger amount of native gold (0.01–0.07 mm); high Au, Ag, and Sb contents; relatively high As, Cu, Pb, and Cd contents; higher Zn, Bi, and Te contents; a low total REE; and negative Eu and positive Ce anomalies. The geochemical features are consistent with the ore mineral composition. The high Rb/Sr, Th/La, Y/Ho, and U/Th ratios of ores are most likely caused by the position of the deposit in the zone of influence of the Ilirnei granitic pluton. A fluid inclusion study showed that the ores were formed from Na–K–Mg low-saline (5.58–0.2 wt % NaCl-equiv.) hydrothermal fluids with a small amount of CO2, H2S, and CH4 with decreasing temperatures from 370 to 130°C and the following fluid parameters: CO2/CH4 = 217–53, Na/K = 5–2, and K/Rb = 2722–202. The results are compared with original and published data on the Kupol, Moroshka, and Sentyabr’sky deposits. Our data are important for regional forecasting, interpreting metallogeny, and the search for and evaluation of Au–Ag epithermal deposits.