Gold-Telluride Mineralization in Ore of the Pionerskoe Gold-Quartz Deposit (Eastern Sayan, Russia)

The Pionerskoe gold-quartz deposit is the first primary gold deposit discovered in the southeastern part of the Eastern Sayan. The orebodies of the Pionerskoe deposit are low-sulfidation pyrite-quartz and carbonate-pyrite-quartz veinlets and veins that occur in crush, mylonitization, and shear zones. Pyrite predominates among the ore minerals; chalcopyrite, pyrrhotite, galena, tellurides, native gold, and single fahlore grains are less common. In the quartz veins occurring in listvenite bodies, in addition to the listed ones, are Ni and Co minerals: cobaltite, alloclasite, gersdorffite, and pentlandite. Among the tellurides, the following minerals have been identified: altaite, petzite, hessite, calaverite, melonite, tellurobismuthite, pilsenite, coloradoite, rucklidgeite, volynskite, tsumoite, and tetradymite. Native gold is associated with tellurides; single grains of low-grade native gold are found in association with galena and sphalerite. Native gold disseminations are present in ores as small veinlets and irregularly shaped grains in quartz and pyrite; they often form intergrowths with telluride minerals. The ores contain three mineral assemblages: quartz-pyrite, quartz-polysulfide, and gold-telluride. Studies have shown that during the formation of these assemblages, the sulfur fugacity decreased and the tellurium fugacity increased. Fluid inclusion study and mineral thermometry made it possible to establish the general temperature range of mineral formation from 285 to 225 degrees C, and the formation of the gold-telluride assemblage, from 225 to 227 degrees C. Sulfur isotope compositions in pyrite and equilibrium fluid have values typical of juvenile sulfur. Oxygen isotope composition studies indicate that magmatic fluids were involved in ore formation. The interaction of the primary fluid with the host rocks containing increased concentrations of gold and, possibly, tellurium, led to additional enrichment in these elements and the formation of a small (in terms of reserves) but rich gold deposit with a wide range of telluride minerals.