Genesis and Timing of Low-Sulphide Gold–Quartz Mineralization of the Upryamoye Ore Field, Western Chukotka

The Upryamoye ore field is located in the Chukotka metallogenic belt in Northeast Russia. The orebodies are hosted within Late Jurassic–Early Cretaceous greenschist-facies metamorphosed rocks and structurally controlled by NW-trending fold-and-thrust dislocations. Based on geological exploration, petrographic, mineralogical, and geochronological studies, new data on the geological structure and composition of gold–quartz mineralization of the Upryamoye ore field are presented. Optical and scanning microscopy were used to study the lithological features of the host rocks and determine the ore textures and the morphology and internal structure of native gold, auriferous pyrite, and arsenopyrite. Qualitative and quantitative characterization of the ore minerals was carried out using SEM-EDS and EPMA. To determine the age of the gold mineralization, Re-Os dating of arsenopyrite and U-Th/He dating of pyrite were performed. The results show that the orebodies comprise carbonate–quartz and sulphide–carbonate–quartz saddle reef veins in both the fold hinge and limbs, as well as mineralized shatter zones and mylonite zones that trace thrust faults. The main ore minerals are arsenopyrite and pyrite, associated with minor amounts of galena, sphalerite, chalcopyrite, tetrahedrite, and bournonite. Native gold is distributed extremely unevenly, forming thin and finely dispersed inclusions in pyrite and arsenopyrite. U-Th/He isotopic analyses of auriferous pyrites suggest that gold mineralization in the Upryamoye ore field occurred at 123 ± 4 Ma. The data obtained by Re–Os dating of auriferous arsenopyrite are inconsistent with direct geological observations but indicate that Os in the arsenopyrite was derived from the crustal source. According to a number of characteristic features of mineralization, the Upryamoye ore field is attributed to a metamorphic genetic type of orogenic low-sulphide gold–quartz deposits. The ore-forming process was long and multi-stage, occurring during the final collisional phase and the beginning of the extensional phase of the Chukotka orogen.