The Ore Mineral Composition of Gold-Bearing Metasomatites of the Kutyn Deposit, Khabarovsk Krai

The paper presents the first results of mineralogical studies of gold-bearing quartz–carbonate–sericite metasomatites of the Kutyn deposit in Khabarovsk krai. The mineral assemblages with native gold and variations in its morphology, size, and chemical composition are studied. It is established that native gold with a fineness of 650–780‰ is associated with Sb-bearing arsenopyrite, pyrite, fahlores, galena, and hessite. Native gold with higher fineness (810–850‰) fills fractures in arsenopyrite. The fineness of native gold from quartz veinlets is 730–770‰. The Te minerals include hessite, altaite, petzite, stützite, telluranthimonite, and coloradoite. Successive pyrite–arsenopyrite, native gold–tetrahedrite–arsenopyrite, native gold–pyrite, and telluride ore assemblages of the hydrothermal–metasomatic stage are distinguished on the basis of the analysis of the composition of minerals and their morphology, interrelations, and trace-element composition of pyrite (up to 6.9 wt % As) and arsenopyrite (up to 2.8 wt % Sb and up to 5.4 wt % Te), which are ubiquitous minerals of this system. The supergene stage led to the formation of a goethite–arsenate assemblage with ultrafine native gold inclusions, which formed during oxidation of gold-bearing pyrite and arsenopyrite. Based on a chlorite geothermometer (Cathelineau and Nieva, 1985), the estimated temperature of early metasomatic process is 229–278°C. According to a stability diagram of the Au–Ag–Te system (Bortnikov et al., 1988), the formation temperature of native gold–tetrahedrite–arsenopyrite assemblage could not be lower than 170°C. The narrow variations in values of S (logS 2 from –16.8 to –15.0) and Te (logTe 2 from –16.5 to –15.2) fugacity at 200°C are estimated for the first time, which correspond to the formation of the studied native gold–tetrahedrite–arsenopyrite assemblage.