Gold deposit in the Butarny granitoid stock, Russian Northeast

Abstract

The Butarny gold deposit is situated in the central part of the Khurchan-Orotukan Zone of tectonomagmatic activation, which is traced for 150 km in the near-meridional direction, and localized in the slightly eroded Late Jurassic granitoid stock of the same name. The explored orebodies are quartz veins and pinnate veinlets with low-grade pocket-disseminated sulfide (mainly arsenopyrite) mineralization containing native gold. The Bi-bearing gold-pyrite-arsenopyrite and the quartz-lollingite-arsenopyrite-stibnitejamesonite stages of the veined low-sulfide ore formation have been distinguished. The main mineral assemblage consists of arsenopyrite, native gold, and native bismuth. The minerals-carriers of gold were deposited during the final stage of ore-bearing quartz crystallization at 334−245°C from low-concentrated pneumatolytic-hydrothermal carbonated fluid containing CO2 and CH4 (5.8−2.2 and 1.6−0.5 mol/kg of solution, respectively). The ore-bearing fluid had variable salinity (5.3−2.2 wt % NaCl equiv). It is quite probable that the gas-saturated fluid participated in transport and precipitation of ore matter. Its density varies from 1.02 to 0.77 g/cm3. The pressure is estimated at 1600 to 780 bar. The fluid regime of ore formation at the Butarny deposit is similar to that of typical intrusion-related gold deposits. The Au tenor of beresitized granodiorite, numerous quartz veinlets, and extensive Au-bearing weathering mantle allow us to suggest stockwork mineralization.