Au-Ag-S-Se-Cl-Br Mineralization At The Corrida Deposit (Russia) And Physicochemical Conditions Of Ore Formation

The mineral and chemical compositions of ores from the Corrida epithermal Au-Ag deposit (Chukchi Peninsula, Russia) were studied using the optical and scanning electron microscopy with X-ray energy-dispersion microanalysis. The deposit was formed at the time close to the period when the basic volume of acid magmas had been emplaced within the Okhotsk-Chukotka belt (84 to 80 Ma). The Au-Ag mineralization is distinguished with Au-Ag sulphides and selenides (uytenbogaardtite-fischesserite solid solution, Se-acanthite, S-naumannite) and Ag halides of the chlorargyrite-embolite-bromargyrite series. The ores were formed in two stages. Using microthermometric methods, it has been established that the ore-bearing quartz was formed in the medium-temperature environment (340-160 degrees C) with the participation of low-salt (3.55 to 0.18 wt.% NaCl eq.) hydrotherms, mostly of the NaCl composition with magnesium, iron and low-density CO2. According to our results of thermodynamic modeling at temperatures from 300 to 25 degrees C and data on mineral metasomatic alterations of the host rocks, the Au-Ag-S-Se-Cl-Br mineralization was formed at decreasing temperature and fugacity of sulphur (logfS(2) from -6 to -27), selenium (logfSe(2) from -14 to -35), and oxygen (logfO(2) from -36 to -62), with near-neutral solutions replaced by acid solutions. Analysis of the obtained data shows that the Corrida refers to the group of the LS-type epithermal deposits. This deposit is a new example of epithermal deposits with significant quantities of Au-Ag chalcogenides (acanthite, uytenbogaardtite, fischesserite, naumannite and others).