Intrusion-related gold-bismuth deposits of North-East Russia: PTX parameters and sources of hydrothermal fluids
Abstract Intrusion-related Au-Bi deposits of North-East Russia are related to Late Mesozoic orogenic S- and I-type granites of ilmenite series. The studied deposits differ in their position relatively to the plutons, alteration, ore body morphology and sulphide content in ores. Based on mineral composition of ores, studied deposits are divided into bismuth-sulphotelluride-quartz, bismuth-arsenide-sulpharsenide and bismuth-siderite-polysulphide types. Bismuth-sulphotelluride-quartz deposits (Levo-Dybinskoe, Kurum, Ergelyakh, Tuguchak, Basugunya) are characterised by low-sulphide (≤3 vol%) mineralisation. Native gold associates with bismuth minerals (bismuthinite, sulphotellurides and tellurides, maldonite, jonassonite, native bismuth). Bismuth-arsenide-sulpharsenide deposits (Myakit, Chepak, Dubach, Chistoe, Kandidatskoe) host As-rich mineralisation. Lollingite and arsenopyrite are main minerals, and their contents vary from 5 to 60 vol% (commonly ∼10%). The intergrowths of native gold with bismuth minerals (sulphotellurides, tellurides, native bismuth) occur mainly in arsenopyrite. Bismuth-siderite-polysulphide (Arkachan) type is characterised by high-sulphide (5–15 vol%) and high-carbonate (up to 35 vol%) ores. Native gold associates with bismuthinite and other sulphobismuthites. Minerals of tellurium are rare. Fluid inclusion and stable isotope study of samples from eighteen Au-Bi deposits constrains the fluid composition, formation temperatures, pressures, and fluid sources. Four types of fluid inclusions (FI) were revealed: (I) two-phase FI, consisting of H2O liquid and CO2 vapour, and three-phase FI with H2O liquid, CO2 vapour and CO2 liquid; (II) vapour-rich CO2 one- or two-phase FI with minor (rarely dominant) CH4 and N2, sometimes with a thin liquid rim; (III) two-phase liquid-vapour aqueous inclusions; (IV) three- or multiphase FI, consisting of H2O liquid, gas bubble and one or more daughter minerals. Au-Bi mineralisation formed at 437–200 °C (mainly from 400 to 250 °C) and 0.1–1.9 kbar from a H2O-CO2-NaCl fluid, which forms an immiscible brine and CO2-bearing vapour at low pressure (≤1.3 kbar) as well as low- to moderate salinity CO2-H2O mixtures without brines at higher pressure (≥1.3 kbar). The studied deposits formed at shallow (Ergelyakh, Tuguchak, Arbatskoe at 1–2 km, Kurum, Levo-Dybinskoe at 2–3 km) and deep (Chuguluk, Shkolnoe, Arkachan at ∼4 km, Dubach at >5 km) depth. Bismuth-sulphotelluride-quartz deposits commonly occur at shallow depth, whereas bismuth-arsenide-sulpharsenide and bismuth-siderite-polysulphide deposits are mainly formed in deeper environment. The stable isotope data suggest predominantly magmatic source of gold-bearing fluids, but magmatic fluid was enriched in light O, C and S isotopes as a result of fluid immiscibility. The magmatic source is consistent with the overlap in lead isotope compositions of ores and related intrusions, as well as synchronism of magmatism and hydrothermal activity according to geochronology data.