MINERALOGY, GEOCHEMISTRY, AND ORIGIN OF THE NEZHDANINSK GOLD DEPOSIT (SAKHA-YAKUTIA, RUSSIA)

The Nezhdaninsk gold deposit is located at a junction of four regional faults in the South Verkhoyansk synclinorium. The ore bodies are the mineralized cataclitic zones and the veins. They are hosted by metamor phosed terrigenous sandstone-slate sequences intruded by stocks and di kes of silicic and intermediate rocks. The age of the magmatic rocks w as found to vary from 154 to 94 Ma. The deposit was formed during thre e stages. Gold-bearing ore bodies were formed during the second stage. A regeneration of quartz and sulfides, and a crystallization of silve r-bearing mineral assemblages occurred during the third stage. The gol d deposition began at the metasomatic alteration of the host rocks. Th e altered rocks consist of quartz, sericite, carbonates, and dissemina ted pyrite and arsenopyrite, These sulfides contain ''invisible'' chem ically bound gold. Native gold crystallized in quartz veins. The nativ e gold is associated with coarse-grained arsenopyrite and pyrite, and is coeval with sphalerite-galena-sulfosalt aggregates. The fluid inclu sion study showed that the ore formation occurred from two immiscible fluids. One of them was a H2O + CO2-dominated fluid with N-2, CH4, and soluble chlorides (up to 4.5 wt % equiv NaCl). The second fluid was a vapor phase consisting mainly of CO2 and CH4. The stable isotope (S, C, and O) study in the minerals suggests that sulfur, water, and CO2 o f magmatic origin prevailed in fluids. They were mixed with components incorporated from the host rocks. The magmatic and host rocks interac ted with the isotopically light and possibly meteoric waters. The mine ral crystallization occurred at temperatures of 360 to 175 degrees C, and under pressures of 1.2 to 1.7 kbar from mineral-forming fluids wit h a pH equal to 5.5-6.1, The CO2 mole fraction of the fluids was from 0.25 to 0.3, The formation of the economic gold ores was linked with t he postaccretion tectono-magmatic activation of the South Verkhoyansk synclinorium in the Early to Late Cretaceous.