Geological setting, mineral composition, and formation conditions of the Kyuchus gold-mercury deposit (Sakha, Russia)

The results of a comprehensive study of the Kyuchus gold-mercury deposit lo cated within the Momsk rift structure are considered. Genetical features of the Au-Hg, Sb, and Hg mineralization are characterized. Ore bodies of the deposit are located in the black shale sequence of Middle Triassic age, whi ch is folded in an anticline. The fold is crossed by a complicated diagonal system of the branching brecciation and shear zones. The carbonate-quartz veins and lenses with a sulfide mineralization are observed in these zones. They are surrounded by a halo of sericite-carbonate metasomatites with dis seminated pyrite and arsenopyrite. Four principal successively formed miner al assemblages, are distinguished in the deposit, namely: (1) pyrite-arseno pyrite with finely dispersed gold, (2) stibnite-quartz with finely disperse d and native gold, (3) cinnabar-metacinnabarite-kaolinite with mercury gold , and (4) carbonate (postproductive). The composition and internal structur e of native gold, ore, and mercury-bearing minerals were investigated in de tail by electron microprobe analysis. Mineralogical, geochemical, and isoto pe data indicates that mineral assemblages at the deposit were precipitated from fluids which contained isotopically identical oxygen and carbon of a juvenile origin and sulfur compositionally close to meteoritic standard. Th e mineral-forming process occurred at the temperature drop from 300 to 100 degrees C and below, pressure decrease, and at an increasing alkalinity and oxidation-reduction potential in an unstable physicochemical environment. Geological, mineralogical, and geochemical features of the Kyuchus deposit are similar to those of deposits hosted by black shale sequences as well to those of the telethermal antimony-mercury occurrences. These facts evidenc e its polygenous and, possibly, polyhronous nature.