Tetrahedrite-group minerals record hydrothermal fluid evolution in Southern Kamchatka

Introduction: The Vilyuchinskoe gold–silver (Au-Ag) ore occurrence (Southern Kamchatka) is spatially associated with the Rodnikovoe paleovolcano and hosted by Oligocene andesite–dacite tuffs and lava flows intruded by gabbro–diorite. This study aims to reconstruct the physicochemical evolution and timing of ore formation using tetrahedrite-group minerals, fluid inclusions and potassium-argon (K-Ar) geochronology. Materials and methods: The chemical compositions of ore minerals were analyzed using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS). Fluid inclusion microthermometry was performed on vein quartz using a Linkam THMSG 600 heating–freezing stage. K-Ar dating was carried out on adularia and adularia–quartz fractions. Results: Three new dates obtained from vein adularia yield ages of 4.8 ± 0.3 Ma, 1.49 ± 0.19 Ma and 1.18 ± 0.18 Ma. The tetrahedrite-group minerals were formed in three stages. Stage I consists of fahlore-I (tennantite and tetrahedrite), pyrite, sphalerite (FeS 1.51 mol.%) and minor chalcopyrite. Stage II is represented by fahlore-II (tennantite and tetrahedrite) replacing chalcopyrite, associated with sphalerite (FeS 8.32 mol.%) and galena. Fahlore-II shows elevated Ag (up to 7.84 wt.%) and Bi (up to 5.11 wt.%). Stage III comprises tellurides, selenides, and fahlore-III (tellurium-bearing fahlore and goldfieldites). Fluid inclusion data were as follows: Stages I–II—236–277 °C (1.6–2.6 wt.% NaCl eq.); Stage III—210–249 °C (0.5–1.7 wt.% NaCl eq.). Conclusions: New geochronological data obtained in this study indicate that the Vilyuchinskoe ore occurrence formed during at least two distinct pulses of hydrothermal activity, corresponding to the Pliocene (~5 Ma) and Early Pleistocene (~1.3 Ma). The mineralogical study, together with fluid inclusion data, indicates that fahlores were deposited from hydrothermal fluids at 210–277 °C under decreasing sulfur fugacity and increasing tellurium activity due to fluid cooling f Te 2 . The Vilyuchinskoe ore occurrence is classified as a transitional subepithermal intermediate-sulfidation system, contributing to the understanding of such systems and to prospecting in Southern Kamchatka and other regions.