Mineral Composition and Physicochemical Conditions of Formation of the Pepenveem Epithermal Au–Ag Deposit (Chukchi Peninsula)

Abstract

—We have studied the mineral composition of ores from the Pepenveem epithermal Au–Ag deposit, which is a promising new object of the Chukchi Peninsula. It has been found that the ore formation process was developed in the following sequence: Pyrite, arsenopyrite, and marcasite were deposited at the early stage, next were Pb, Zn, and Cu sulfides; at the late stage, native gold, pyrargyrite, stephanite, proustite, minerals of the pearceite–polybasite series, acanthite, and other Ag minerals were deposited. The results of fluid inclusion studies indicate that the Au–Ag mineralization formed from low-temperature (236–137 °C) low-concentration chloride hydrotherms (0.18–1.57 wt.% NaCl eq.). The results of calculation of thermodynamic equilibria have shown that in the temperature range from 200 to 100 °C, there were a decrease in the fugacity of sulfur (lg fS2 from –10 to –21) and oxygen (lg fO2 from <–36 to <–48) and a change from near-neutral to acidic solutions. Compared to other Au–Ag deposits on the Chukchi Peninsula (Corrida and Valunistoe), which are characterized by wide distribution of Se- and Te-bearing Au–Ag chalcogenides (naumannite, cervelleite, and hessite), ore formation with gold–silver–sulfosalt mineralization at the Pepenveem deposit took place at lower temperatures and lower selenium, tellurium, and oxygen fugacity. The data obtained permit us to refer the Pepenveem deposit to the group of epithermal low-sulfidation (LS) deposits.