G. N. Gamyanin

Fluid Composition and Origin in the Hydrothermal System of the Nezhdaninsky Gold Deposit, Sakha (Yakutia), Russia

AbstractPetrochemical characteristics of igneous, sedimentary, and metasomatic rocks; chemical and isotopic compositions of minerals and fluids; and PT parameters of mineral formation at the Nezhdaninsky deposit are reported. A model of hydrothermal system formation is developed on this basis. In addition to decreasing Ba/Rb and Li/Mg ratios in the course of the hydrothermal process, resulting in the formation of ore-bearing metasomatic rocks, increasing K/Ba and diminishing K/Cs ratios indicate the probable participation of magmatic fluid in the ore deposition. The agreement of the K/Rb and K/Ba ratios with the values typical of the main trend of igneous rocks (MT) implies that the K, Rb, and Ba contents were distributed in the ore-forming hydrothermal fluid according to the ratios in the source magmatic chamber. The K/Rb ratios in metasomatic rocks correspond to the MT and approach the pegmatitic-hydrothermal trend and the composition of orthomagmatic fluid of Mo-W greisen. Similar REE patterns of igneous and terrigenous rocks do not allow the REE source to be constrained unequivocally. The lithological control of lithophile element distribution testifies to the supply of host rock components to the hydrothermal system. All studied rocks and minerals are enriched in LREE. The REE total and the contribution of HREE decrease from preore to synore metasomatic rocks, from preore to regenerated carbonates, and from older to younger scheelite. A similar tendency is noted in granitoids of the Kurum pluton. The δ18O values of quartz range from +10.3 to +12.6‰ in Au-Mo-W zones, from +15.9 to +16.4‰ in metasomatic rocks, from +14.8 to +16.6‰ in gold-ore veins, and from +13.5 to +16.9‰ in silver-base-metal ore mineralization. The estimates of

Ag-Sb Mineralization of the Yana-Kolyma Belt, Northeast Russia

\delta ^{18} O_{H_2 O}

Sulfur isotopic composition of sulfides at the Mangazeya silver deposit, Eastern Sakha-Yakutia, Russia

suggest that water was supplied from a magmatic source (δ18O = +(5.5−9.0‰)) and as a product of sedimentary rock dehydration. High-temperature (up to 390°C) and highly concentrated (up to 31 wt % NaCl equiv) fluids participated in the mineral formation. The phase separation of the fluid into H2O-CO2 liquid and predominantly carbon dioxide gas was combined with mixing of a high-temperature and relatively highly concentrated chloride solution with a low-temperature and poorly mineralized fluid. The redox conditions varied from equilibrium with CH4-bearing fluid at the gold-molybdenum-tungsten stage to equilibrium with CO2-bearing fluid during the gold-ore stage.

Structures, mineralogy, and fluid regime of ore formation in the polygenetic Malo-Taryn gold field, Northeast Russia

A collection of galena from the Nezhdaninsky gold deposit (62 samples), as well as galena from the Menkeche silver-base-metal deposit and the Sentyabr occurrence and K-feldspar from intrusive rocks of the Tyry-Dyby ore cluster have been studied using the high-precision (±0.02%) MC-ICP-MS method. Particular ore zones are characterized by relatively narrow variations of isotope ratios (no wider than σ6/4 = 0.26%). Vertical zoning of Pb isotopic composition is not detected. Variation in Pb isotope ratios mainly depends on the type of mineral assemblage. Galena of the gold-sulfide assemblage dominating at the Nezhdaninsky deposit is characterized by the following average isotope ratios: 206Pb/204Pb = 18.472, 207Pb/204Pb = 15.586, and 208Pb/204Pb = 38.605. Galena from the regenerated silver-base-metal assemblage is distinguished by less radiogenic lead isotope ratios: 18.420, 15.575, and 38.518, respectively. In lead from the Nezhdaninsky deposit, the component, whose source is identified as Permian host terrigenous rocks, is predominant. The data points of isotopic composition of lode lead make up a linear trend within the range of μ2 = 9.5-9.6. K-feldspar of granitic rocks has less radiogenic and widely varying lead isotopic composition compared to that of galena. The isotopic data on Pb and Sr constrain the contribution of Late Cretaceous granitic rocks as a source of gold mineralization at the Nezhdaninsky deposit. The matter from the Early Cretaceous fluid-generating magma chamber participated in the ore-forming system of the Nezhdaninsky deposit. The existence of such a chamber is confirmed by the occurrence of Early Cretaceous granitoid intrusions on the flanks of the Nezhdaninsky ore field. The greatest contribution of magmatic lead (∼30%) is noted in galena from the silver-base-metal mineral assemblage. This component has isotopic marks characteristic of lower crustal lead: the elevated 208Pb/206Pb ratio relative to the mean crustal value and the lower 207Pb/204Pb ratio. Taken together, they determine a high Th/U ∼ 4.0 in the source and μ2 = 9.37–9.50. This conclusion is consistent with the contemporary tectonic model describing evolution of the South Verkhoyansk sector of the Verkhoyansk Foldbelt and the Okhotsk Terrane.

Arkachan: A New Gold–Bismuth–Siderite–Sulfide Type of Deposits in the West Verkhoyansky Tin District, Yakutia

The mineral assemblages of the Ag-Sb deposits from the Yana-Kolyma Foldbelt were studied. The compositions of ore minerals, the isotopic compositions of sulfur in ore minerals, and the carbon and oxygen in carbonates are given. Arsenopyrite of the deposits is significantly enriched in Sb (1–16 wt %), which is related to the primary enrichment of the ore-hosting sequences in Sb and the ore formation at shallow depths. Based on the fluid inclusions study, the deposits were formed at T = 329–149°C and P = 0.30–1.04 kb from low-salinity chloride-sulfate-bicarbonate solutions enriched in Sb and Ag. The sequence of precipitation of Ag-Sb minerals was mainly controlled by the Sb concentration and the sulfur fugacity and potential in the fluid.

Geochronology of Igneous Rocks at and near to the Nezhdaninka Gold Deposit, Yakutia, Russia: U-Pb, Rb-Sr, and Sm-Nd Isotopic Data

The interrelations of gold-quartz and antimony mineralization in long-living tectonic zones of the Maltan deposit (Northeast Russia) have been examined. It has been demonstrated that the orogenic gold-quartz mineralization is confined to interlayered lengthwise thrusts, splay fault zones, and quartz veins and threads. The antimony mineralization, as the gold one, developed within the same structures during secondary reactivation in strike-slip tectonic stress fields. Index microstructure markers that were formed in antimony ores and the host rocks typical for multistage deformation processes have been described. The mineral associations and typomorphic features of the minerals of the gold and antimony ore-forming stages have been studied. The Maltan deposit was found to be similar to the typical large Au-Sb Sarylakh deposit of the Adycha-Taryn zone. The voluminous and extensive effect of the mineral-forming fluid of the antimony oregenerating stage on the early mineral matrix of low-sulphide gold-quartz paragenesis has been demonstrated.

The mangazeya Ag–Pb–Zn vein deposit hosted in sedimentary rocks, Sakha-Yakutia, Russia: Mineral assemblages, fluid inclusions, stable isotopes (C, O, S), and origin

The succession of the formation of ore zones and sulfur isotope ratio of sulfides at the Mangazeya Ag deposit have been studied. The deposit is located in the Nyuektame Fault Zone in the eastern limb of the Endybal Anticline. The ore zones are hosted in the Middle Carboniferous to Middle Jurassic terrigenous sequences of the Verkhoyansk Complex intruded by the Endybal subvolcanic stock and felsic and mafic dikes. Three ore stages are distinguished: (I) gold-rare metal, (II) cassiterite-sulfide, and (III) silver-base-metal. Products of these stages are spatially isolated. The δ34S of sulfides ranges from −6.4 to +8.0‰. In the sulfides of the gold-rare metal assemblage, this value varies from −1.8 to +4.7‰; in the sulfides of the cassiterite-sulfide stage, −6.4 to +6.6‰; and in the sulfides of the silver-base-metal assemblage, -5.6 to +8.0‰. A sulfur isotope thermometer indicates the temperature of mineral deposition at 315–415°C for the first stage and 125–280°C for the third stage. Possible causes of variable sulfur isotopic composition in sulfides are discussed. The data on the sulfur isotope ratio is interpreted in terms of involvement of magmatic fluid (δ34S ∼ 0) in the mineralizing process along with low-temperature fluid taking sulfur from host rocks (δ34S ≫ 0). Boiling and mixing of magmatic fluid with heated meteoric water were important at the last stage of the deposit formation.

Silver-cobalt mineralization in the Upper Seymchan ore cluster, Northeastern Russia

Detailed structural study of ore zones and deformations in host rocks reveals the multistage tectonic evolution of the Malo-Taryn ore field. Gold–quartz mineralization is confined to the interstratal and longitudinal thrust faults, where vein–stringer bodies are systematically oriented relative to the structures of the thrust stage. Antimony mineralization is superposed on gold–quartz mineralization in the same structures, being associated with veins formed during the shear stage of deformations. It is determined that the types of mineralization were formed in the following sequence: gold–bismuth, low-sulfide gold–quartz, berthierite–antimonite, and silver–antimony. Data on mineral assemblages and typomorphic features of minerals are reported. The isotope composition of minerals, temperature conditions, and composition of orebearing fluids are studied. It is established that the same minerals ascribed to different types of mineralization sharply differ in typomorphic features, which is caused by the different composition and genesis of the hydrothermal fluids. The superposition of different types of mineralization on one another within repeatedly activated tectonic structures that were fluid-permeable for a long time is an important indication of large-scale deposits.