THE EPITHERMAL GOLD-TELLURIDE KOCHBULAK DEPOSIT (UZBEKISTAN)

The Paleozoic epithermal gold-telluride Kochbulak deposit is situated on the northern slope of the Kurama Range in the central Tien Shan (Uz bekistan). The deposit is located within an andesite-dacite volcanic s equence (C2-3) intruded by subalkaline granodiorite and granosyenite p orphyry dikes (C-3-P-1) The ore-forming system of the deposit originat ed and evolved during the orogenic uplift associated with aerial volca nism and emplacement of porphyritic granitoids. The Kochbulak deposit contains a great diversity of ore-forming sulfides, sulphosalts, tellu rides, and selenides, several of which were first found there. The dep osit has features of epithermal mineralization of both the adularia-se ricite and acid-sulfate genetic types and mesothermal gold mineralizat ion. The deposit consists of high- and low-angle veins and ore-bearing breccia pipes. The pipe-shaped bodies and veins differ in ore, gangue , and metasomatic mineral assemblages. The ore bodies hosted by explos ive-hydrothermal breccias contain goldfieldite, famatinite, luzonite, enargite, diaspore, and pyrophyllite and, therefore, have pronounced f eatures of epithermal mineralization of the acid-sulfate type. The ore veins show evidence of deeper formation and are associated with quart z-carbonate-sericite wall rock alteration. Our study of mineral assemb lages, fluid inclusions, sulfur, oxygen, and hydrogen isotopes, and th ermodynamic calculations show that the Kochbulak deposit was formed du ring a cyclic multistage process with periodic alternation of sealing along with tectonic and explosive opening of the fluid channel ways. T he ore-forming process is subdivided into the preore, I to III ore, an d postore stages related to fracture opening. Three types of solutions took part in the formation of gold-sulfide-sulphosalt-telluride miner alization. They were distinct in temperature, composition, salinity, a nd proportion of meteoric and magmatic water and other volatile compon ents. The temperature generally decreased from 465 degrees C to <100 d egrees C, and slightly increased at the beginning of each stage of the ore-forming process. Pressure, salinity, ion and gas composition of s olutions, and the oxygen isotope composition of water varied as well. The deposition of gold-productive mineral assemblages was caused by ch anges in pH, Eh, activities of S-2, O-2, and Te-2. The recurrence of t hese changes and repeated mineral deposition within the same depth int ervals formed bodies very rich in ore components. The above-mentioned data suggest the Kochbulak deposit represents a particular type of gol d-telluride mineralization, which originated within a wide range of ph ysicochemical conditions at hypabyssal to subsurface depths.