Features of the Formation of the Golets Vysoschaishii Gold-Ore Black-Shale Deposit (Bodaibo Ore District)

Abstract

Abstract The gold deposits of the Bodaibo ore district are enclosed in a thick (several kilometers) sequence of Riphean–Vendian carbonaceous carbonate–terrigenous deposits. Gold is concentrated at several stratigraphic levels; the largest deposit in Russia, Sukhoi Log, and its counterpart, Golets Vysochaishii, are confined to terrigenous deposits of the Riphean Khomolkho Formation. The characteristic components of the Khomolkho Formation are lenticular interlayers and layered disseminations of pyrrhotite and pyrite with clearly pronounced lithological–stratigraphic control of their distribution. The Golets Vysochaishii deposit is confined to the largest thickening of sulfide layers in carbonaceous chlorite–quartz–sericite phyllite schists of the lower part of the Khomolkho Formation. The sulfide content in the ore-bearing sequence is 5 wt %; native gold is enclosed in sulfide layers. According to the data of previous researchers, based on the distribution patterns of sulfur isotopes, sulfide ore-bearing deposits of the deposit were formed by hydrothermal–sedimentary pathways. The primary gold content of sulfide sediments is assumed in analogy with the neighboring Sukhoi Log and Verna deposits. The distribution of Au, As, Ag, and carbonaceous matter in the sulfide-bearing strata of the Golets Vysochaishii deposit is substratal, reflecting their accumulation during sedimentation. The rocks were metamorphically altered under the greenschist facies conditions, during which they recrystallized and sulfides and gold were redistributed with the rocks retaining their layered structure. There are no significant differences in the contents of the rock-forming components between the ore deposit and host rocks, which indicates the absence of metasomatic alteration in rocks associated with ore formation. Metamorphic processes at the deposits of the Khomolkho Formation were accompanied by replacement of ilmenite by rutile and pyrrhotite by pyrite, as well as a change in the composition of chlorites and carbonates with increasing iron content; these processes were regulated by the fugacity of oxygen and carbon dioxide. Metamorphic alteration, based fluid inclusion study data, took place in the temperature range 230–510°C with the participation of oxidized carbon dioxide–aqueous fluids (CO 2 /CH 4 up to 17) with a salinity of 7–12\xa0wt % equiv. NaCl and reduced (CO 2 /CH 4 < 0.3) methane–nitrogen fluids with a salinity of 11–13 wt % equiv. NaCl.