Epithermal Ag–Au Mineralization in the Kyplatap Volcanic Field, Central Chukotka

This paper discusses mineralogic and geochemical features of dominantly silver mineralization in secondary quartzites of the Kyplatap volcanic field (KVF) confined to the southeastern flank of the eponymous intrusive dome structure (IDS) that complicates the Palyavaam–Pykarvaam volcano-tectonic depression which is part of the Chaun zone of the Central Chukchi sector of the Okhotsk–Chukchi volcanogenic belt (OChVB). The Kyplatap IDS was formed as a large laccolite-like subvolcanic body intruded into the Alkakvun rhyolites and trachyrhyolites. The KVF contains an abundance of secondary quartzites that compose both linear zones along faults mostly northeast striking and fault-bounded isometric fields. Potential ore bodies are represented by linear zones of crushed and brecciated secondary quartzites accompanied by quartz veinlets and veins. The ore bodies are long up to 200 m and 2–5 m thick. Spotty, veinlet-impregnated, and brecciated structures predominate in the ore bodies. The ore mineralization is mostly impregnated, less frequently in veinlets. The main vein minerals are quartz (30–70%), hydromica (15–20%), sericite, kaolinite, adularia, ferrous carbonate, zircon, chlorite, and alunite. The main ore minerals are pyrite, arsenopyrite, acanthite, polybasite, pearceite, and native silver. The typomorphic feature of the mineralization is thin intergrowths of silver sulfosalts, native silver, and iron hydroxides. We note that dominantly silver mineralization in secondary quartzites within the OChVB was found for the first time ever. The results of surveys classify the Kyplatap ore occurrence as a gold-silver mineral type deposit (Au/Ag = 1 : 450 on average) of the selenium subtype because of considerable admixture of selenium in the ore minerals. The occurrence of Ag mineralization in secondary quartzites and mudstones may provide evidence that it belongs to the high-sulfidation epithermal class. These features of the KVF epithermal mineralization are largely similar to the formation of siliceous and quartz–alunite lithocaps that are formed above degassing intrusions. In such conditions, the HS-type ore-bearing fluids are either not formed in the system or did not reach the epithermal depths. The ore field is poorly or moderately eroded, what is indicated by a practically complete absence of minerals of the polymetallic association and by the dominance of acanthite and silver sulfosalts over native phases.