Physicochemical Conditions of Formation of Gold and Silver Parageneses at the Valunistoe Deposit (Chukchi Peninsula)1

Abstract

Abstract —The mineral composition of ores from the Gornyi occurrence of the Valunistoe epithermal Au–Ag deposit (Chukchi Peninsula) has been studied. It has been found that, in addition to native gold, the Au–Ag mineralization comprises chalcogenides (uytenbogaardtite, petrovskaite, acanthite, naumannite, and cervelleite) and minerals of the pearceite–polybasite series, which occur as microinclusions in fine-grained pyrite. The physicochemical conditions of formation of productive mineral assemblages have been estimated based on the chemical composition of Au and Ag minerals and their relationships with other minerals. It is shown that ores were deposited from weakly acid solutions at the late stages, on the background of a temperature decrease from 350 to 100 °C, a decrease in the fugacities of sulfur (logfS2, from −2 to −23), tellurium (logfTe2, from –5 to –27), and selenium (logfSe2, from −16.5 to −28), and changes in the redox conditions (logfO2, from −23 to −48).