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Геология России

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Physicochemical model for the genesis of Cu-Ag-Au-Hg solid solutions and intermetallics in the rodingites of the Zolotaya Gora gold deposit (Urals, Russia)
(2018) Murzin V.V.; Chudnenko K.V.; Palyanova G.A.; Varlamov D.A.; Naumov E.A.; Pirajno F.
In this contribution we examine the compositions of solid solutions and intermetallics of the system Cu-Ag-Au-Hg and the physicochemical conditions of their formation in rodingites from the Zolotaya Gora gold deposit (Southern Urals, Russia). Thermodynamic calculations, modeling the formation of mineral assemblages of rodingite and Cu-Ag-Au-Hg mineralization, were carried out using a “Selektor-C” software package. Two probable models for the genesis of Au-Ag-Cu-Hg solid solutions and Au-Cu intermetallics in rodingites are: 1) hydrothermal; the result of single-stage discharge in open space of deep-sourced gold-bearing fluid with the composition corresponding to rodingite, taking into account its interaction with host serpentinites. 2) metasomatic; deep-seated gold-bearing fluid (W) rising to the surface interacts with early formed rodingite (R) at different ratios (W/R). T and P-conditions of modeling: 450 °C, 3 kbar; 350 °C, 2 kbar; 250 °C, 1 kbar. Results of the calculations on the “hydrothermal” and “metasomatic” models showed different degrees of similarity of natural and theoretical model associations of rodingites. The metasomatic model is better for corresponding to real mineral compositions and mineral paragenesis in the natural Cu-Ag-Au-Hg system observed at the deposit. In this model the chlorite-garnet-pyroxene rodingite is replaced by a chlorite-rich rock with increasing W/R. In this case all gold minerals of Zolotaya Gora deposit (Au-Cu intermetallics and Au-Ag solid solutions) are formed at 250–450 °C. Gold-copper solid solution formed at a temperature of 450 °C (W/R >10). Au-Ag-Hg solid solutions and native copper are formed only at 250 °C. According to the hydrothermal model native copper and AuCu3 were absent phases, but other Au-Cu intermetallics (AuCu, Au3Cu) precipitate if gold concentration in the solution is higher than 0.5 ppm. Thermodynamic calculations proved the possibility of formation of equilibrium assemblages of rodingite minerals and gold-bearing minerals with the participation of water-chloride complexing and low CO2 fluids. At a temperature <350 °C the main status of gold in solution are Au(HS)2− and AuHS0, while at higher temperatures it occurs as AuOH0. Formation of Au-Cu intermetallics occurred under the effect of weak-acid hydrothermal solutions (pH = 3.5 ÷ 5) with low fugacity of O2 and S2: log fO2 = −26 ÷ −47, log fS2 = −8 ÷ −20. Both models (hydrothermal and metasomatic) explain the formation of Au-bearing rodingites and can be used for predicting potential gold-bearing rodingite targets.
Mineralogy, age and genesis of apatite-dolomite ores at the Seligdar apatite deposit (Central Aldan, Russia)
(2017) Prokopyev I.R.; Doroshkevich A.G.; Ponomarchuk A.V.; Sergeev S.A.
The Seligdar apatite deposit is located in the Aldan-Stanovoy shield of the Siberian platform in Russia. This deposit is a typical ore deposit of the Nimnyrskaya approximately N-S apatite zone, which is about 400 km long. The genesis of the apatite-dolomite ores at the Seligdar deposit is a matter of debate. This article presents new evidence of the carbonatitic genesis of the apatite-dolomite rocks at the Seligdar deposit based on modern methods of mineralogical, geochronological, melt and fluid inclusion investigations. According to our data, the age of the apatite-dolomite ores is 1880 ± 13 Ma (U-Pb SHRIMP, zircon). Study of melt inclusions indicates that the ores were formed from a carbonate melt of dolomitic composition with alkali (sulphates, chlorides and fluorides of Na and K) and silica components (1–10 wt.%) at a temperature of > 1100 °C. The dolomite carbonatites have been subsequently exposed to the intense processes of hydrothermal-metasomatic alteration and metamorphism. The evolution of mineral parageneses from the magmatic apatite-magnetite-dolomite carbonatite stage to the hydrothermal stages with quartz, calcite, monazite-Ce, xenotime-Y, haematite, thorite, thorianite, sulphates and sulphides mineralization agrees with the fluid inclusion regime evolution from the carbonate melt to the chloride brines, and the varying concentrations of the chloride solutions are also described in this article. The investigation of the apatite deposits within the Aldan shield not only allows us to take a new look at the question of their origin but also helps us to study the composition of the ancient mantle, as well as the specifics of apatite-dolomite carbonatite and related hydrothermal Fe and Th-REE mineralization in this region.
Ultrafine particles in ground sulfide ores: A comparison of four Cu-Ni ores from Siberia, Russia
(2017) Mikhlin Yu.; Romanchenko A.; Vorobyev S.; Karasev S.; Volochaev M.; Kamenskiy E.; Burdakova E.
Nano-, submicro- and micrometer mineral particles may have an important role in beneficiation of metal ores and environmental impact, but their origin and characteristics are poorly understood. Here, we report data for the yield and the composition of fine fractions, and surfaces of several ground Cu-Ni sulfide ores studied using laser diffraction, scanning electron microscopy and energy dispersive X-ray analysis, X-ray photoelectron spectroscopy. Colloidal particles were characterized using dynamic light scattering, zeta-potential measurement, transmission electron microscopy and electron diffraction. The production of fines by dry milling was found to increase from about 0.01 vol.% to 0.2 vol.% for submicrometer particles and from ~ 0.5 vol.% to about 1.5 vol.% for particulate material less than 5 μm in the following order: Noril'sk disseminated low sulfide ore ≤ Noril'sk Cu-rich sulfide ore < Noril'sk valleriite ore < Kingash ore. For wet milling, the yield may be several times higher. Both surfaces of the milled ores and colloids were enriched in O, Mg, Si (largely as serpentine slimes) and depleted in sulfur, basic metals and iron, but colloidal valleriite, chalcopyrite, and oxidized pyrrhotite were found in the respective supernatants too. Typically, the colloidal particles form aggregates with an average hydrodynamic diameter of about 1 μm and a smaller number of ~ 5 μm species, except for valleriite ore, which exhibits a single size distribution peak at 2.7 μm. Zeta-potential, which characterizes the electric charge of the particles and dispersion stability of colloids, changed from − 25 mV for the low sulfide ore to about 0 mV for valleriite ore, and to + 15 mV for Kingash ore. Poor flotation recovery of metal from Kingash ore and Noril'sk valleriite ore is suggested to be due to both the large quantities and positive charge of hydrophilic ultrafine serpentine and/or magnesium hydroxide minerals. Resistance to oxidation and hence stability against aggregation of copper-bearing sulfide colloids in waste waters is expected to result in a negative impact on the environment.
REE mineralogy and geochemistry of the Western Keivy peralkaline granite massif, Kola Peninsula, Russia
(2017) Mikhailova J.A.; Pakhomovsky Ya.A.; Ivanyuk G.Yu.; Bazai A.V.; Yakovenchuk V.N.; Elizarova I.R.; Kalashnikov A.O.
The authors have studied the geology, geochemistry, petrology and mineralogy of the rare earth elements (REE) occurring in the Western Keivy peralkaline granite massif (Kola Peninsula, NW Russia) aged 2674 ± 6 Ma. The massif hosts Zr- and REE-rich areas with economic potential (e.g. the Yumperuaiv and Large Pedestal Zr-REE deposits), where 25% of ΣREE are represented by heavy REE (HREE). The main REE minerals are: chevkinite-(Ce), britholite-(Y) and products of their metamict decay, bastnäsite-(Ce), allanite-(Ce), fergusonite-(Y), monazite-(Ce), and others. The areas contain also significant quantities of zircon reaching potentially economic levels. We have discovered that behavior of REE and Zr is controlled by alkalinity of melt/solution, which, in turn, is controlled by crystallization of alkaline pyroxenes (predominantly aegirine) and amphiboles (predominantly arfvedsonite) at a late magmatic stage. Crystallization of mafic minerals leads to a sharp increase of K2O content and decrease of SiO2 content that cause a decrease of melt viscosity and REE and Zr solubility in the liquid. Therefore, REE and zirconium immediately precipitate as zircon and REE-minerals. There are numerous pod- and lens-like granitic pegmatites within the massif. Pegmatites in the REE-rich areas are also enriched in REE, but HREE prevails over light REE (LREE), about 88% of REE sum.
Typomorphic features of placer gold of Vagran cluster (the Northern Urals) and search indicators for primary bedrock gold deposits
(2017) Lalomov A.V.; Chefranov R.M.; Naumov V.A.; Naumova O.B.; LeBarge W.; Dilly R.A.
he Vagran placer cluster is located on the eastern slope of Northern Urals. During > 100 years of gold mining history approximately 40 tons of gold have been extracted from the placer deposits. Bedrocks of the region consist of high metamorphic Upper Proterozoic and Paleozoic terrigeneous, terrigeneous-volcanogenic and igneous rocks. Gold placer deposits are mostly alluvial genesis deposits and of Quaternary to Oligocene (?) age. The alluvial deposits consist of gravel with pebbles, boulders, and sandy clay covered by sandy silt and a soil layer. The thickness of the alluvial sequence is usually 5–10 m and reaches 18 m in the main watercourses of the third order. Nearly all of the alluvial sediments are gold bearing but concentrations of economic importance prevail in the bottom part of the sequence above the bedrock. There are four different types of gold particles: (I) rounded and well-rounded particles of high fineness and homogeneous inner structure, (II) rounded to sub-rounded high fineness particles with a pure gold rim developed over a core, (III) crystallomorphic (idiomorphic) high fineness with a homogeneous inner structure, and (IV) irregular angular and subangular particles of medium fineness with a significant content of Ag (10–40 wt.%) and elevated Hg (up to 1.15 wt.%). The first type is prevalent and comprises up to 65% of the total gold particles; it is uniformly distributed throughout the territory. There are features with initially complicated dendritic and laminar shaped particles which were rounded during transportation. The second and third types have a propensity for zones of the inherited erosion–tectonic depressions. Apparently, types I, II and III are related with orogenic mesothermal gold-sulfide-quartz mineralization; the differences of these types depend on the primary zonation of ore bodies and supergenic transformation of the alloys. They were connected with middle-depth ore bodies of an orogenic gold-sulfide-quartz formation. The fourth type is evident of nearby transportation from primary sources and a short duration of supergenic influence. It is controlled by a zone of NW-SE orientation, diagonal to the main structures of Ural Fold Belt. The plot of Au content vs coefficient of heterogeneity (ratio of the Au content in the core and in the rim of the grains) is the distinguishing factor between the four types of gold grains both by primary hypogenetic characteristics and supergenetic features. No corresponding lode occurrence of gold-sulfide-quartz mineralization has been identified to date in this region. Placer gold concentrations are related to the intermediate hosts of the Mesozoic-Cenozoic surfaces of the Ural peneplain uplift in the Oligocene and eroded in Miocene-Quaternary time. This factor determines the widespread distribution of placer gold in the territory of the Vagran cluster. The large, Carlin-type Vorontsovsk gold deposit is located 60 km south-east from the Vagran area. It has a shallow erosional level, small size of native gold, and its distal location from the placer deposits makes it an unlikely primary source for the Vagran placers. However, mineralization of this type of deposit is noted within the cluster. Gold of the fourth type nearly resembles the gold of the Vorontsovsk deposit and, apparently, the source is related to the same hydrothermal mineralization event. ICP MS analyses of the quartz-sulfide lodes in the floor of gold-bearing valleys revealed a gold content of 2.0–6.9 g/t in the zone of type IV distribution. Therefore, gold of the fourth type can be used as an indicator for the exploration of primary bedrock mineralization. The geological setting and typomorphic features of this placer gold shows that the primary gold mineralization is similar to the Vorontsovsk deposit and within the zone of distribution of the placer gold of the fourth type.