ИСТИНА

INTRODUCTION The origin of fluids deposited vein gold deposits is still debuted. To better understand this problem the world-class gold deposit Darasun produced 110 t and having 100 t as proven recources REE distribution nd carbon and oxygen stable isotopes in vein carbonates were studied. This deposit has previously considered as typical mesothermal one. However, some features of its porphyry stile have been recorded. GEOLOGICAL BACKGROUND The Darasun consists of more than 200 steeply dipping gold-mineralized quartz veins spatially related to the K-rich granodiorite porphyry intrusion of the Amudzhikan-Sretensky complex (J2-K1). Explosive breccias and a disseminated mineralisation deposited from a high-temperature brines were found as well (Prokof’ev et al., 2008). The host rocks are the Early Paleozoic gabbroids and Middle to Early Paleozoic granitoids. The ore bodies are located within and around the stock and its dike-like apophyses of granodiorite porphyries derived from a high K, calc-alkaline magma of latitic composition. The intense propylitic alteration predates ore formation. A sericite-chlorite-quartz-pyrite-carbonate halos are temporaly related to formation of auriferous veins. The concentric zoning of mineralisations and their regular distribution relative to the stocks was described (Timofeyevsky, 1972): a quartz–tourmaline assemblage occurs in the center, while pyrite–arsenopyrite assemblage is a distal one. A galena-sphalerite assemblage forms a halo around the stock. The zoning is observed due to zones of geochemical anomalies of B and other ore elements around the stock. Mutual croscutting, textuaral and structural relationships between minerals aggregates of different mineral composition indicate that the Darasun deposit formed during three stages. A tourmaline, pyrite, quartz assemblage, and a pyrite, arsenopyrite, quartz, siderite, chalcopyrite (with sphalerite and anhydrite inclusion), and native gold assemblage were formed at the earliest stage. Economically valuable amount of native gold that associates with pyrite, chalcopyrite galena, sphalerite, quartz, dolomite, pyrrhotite, sulfosalts and tellurides, stibnite, and calcite were deposited during the consecuentive stage. The formation of quartz–calcite has completed the mineral-forming process. METHODS A chemical composition of carbonate minerals was studied with a Camebax SX 100 electrone microprobe analyses The REE analysis in carbonates was carried out using ICP MS and instrumental neutron activation (INAA) techniques. The carbon and oxygen isotopic compositions of carbonates were analyzed on an MI-1201 mass spectrometer. Gas compositions in fluid inclusions were analyzed by gas chromatography. RESULTS Siderite, calcite and dolomite-ankerite and kutnagorite were identified and studied. The dramatic difference in the Σ REE contents in Mg-Fe carbonates and calcite is found. The Mg-Fe carbonates are depleated in ΣREE (from 2.8 to 73 g/t) relative to the late calcite (ΣREE from 18 до 390 g/t). The d13C values of Mg-Fe carbonates and in calcite varies from +1.1 to –6.7 ‰ and from –0.9 to –4.9 ‰, respectively. The d18О values of Mg-Fe carbonates and calcite range from +17.6 to +3.6 ‰, and from +15.7 to –0.5 ‰ , respectively. The zonality of ΣREE and carbon and oxygen isotope compositions in ore vein carbonates relative to granodiorit-porphyry central stock is revealed. The relationship between variations of carbon and oxygen isotope compositions in ore vein carbonates and ΣREE magnitude is found. These data reflects the evolution of ore deposition physicochemical conditions and ore-forming fluid composition. The fluid isotope composition is calculated and its possible sources are considered. The magmatic source of ore-forming fluid and the participation of meteoric waters in the ore-forming process are detected. The geochemical proofs of the interaction of ore-forming fluid of Darasun deposit with host rocks are obtained. The interrelation between ΣREE and δ13С and δ18О values under fluid isotope composition evolution at hydrothermal ore-forming processes is revealed.