Mineralogy, Geochemistry, and Origin of the Nezhdaninsk Gold Deposit (Sakha-Yakutia, Russia)

Abstract

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 meta-morphosed terrigenous sandstone-slate sequences intruded by stocks and dikes of silicic and intermediate rocks. The age of the magmatic rocks was found to vary from 154 to 94 Ma. The deposit was formed during three stages. Gold-bearing ore bodies were formed during the second stage. A regeneration of quartz and sulfides, and a crystallization of silver-bearing mineral assemblages occurred during the third stage. The gold deposition began at the metasomatic alteration of the host rocks. The altered rocks consist of quartz, sericite, carbonates, and disseminated pyrite and arsenopyrite. These sulfides contain "invisible" chemically bound gold. Native gold crystallized in quartz veins. The native gold is associated with coarse-grained arsenopyrite and pyrite, and is coeval with sphalerite-galena-sulfosalt aggregates. The fluid inclusion study showed that the ore formation occurred from two immiscible fluids. One of them was a H 2O + CO 2-dominated fluid with N 2, CH 4, and soluble chlorides (up to 4.5 wt % equiv NaCl). The second fluid was a vapor phase consisting mainly of CO 2 and CH 4. The stable isotope (S, C, and O) study in the minerals suggests that sulfur, water, and CO 2 of magmatic origin prevailed in fluids. They were mixed with components incorporated from the host rocks. The magmatic and host rocks interacted with the isotopically light and possibly meteoric waters. The mineral crystallization occurred at temperatures of 360 to 175°C, and under pressures of 1.2 to 1.7 kbar from mineral-forming fluids with a pH equal to 5.5-6.1. The CO 2 mole fraction of the fluids was from 0.25 to 0.3. The formation of the economic gold ores was linked with the postaccretion tectono-magmatic activation of the South Verkhoyansk synclinorium in the Early to Late Cretaceous.