Gold–Silver mineralization in porphyry–epithermal systems of the Baimka trend,western Chukchi Peninsula,Russia

Mineralogical, fluid inclusion, and geochemical studies of precious metal mineralization within the Baimka trend in the western Chukchi Peninsula have been preformed. Porphyry copper–molybdenum–gold deposits and prospects of the Baimka trend are spatially related to monzonitic rocks of the Early Cretaceous Egdygkych Complex. Four types of precious metal-bearing assemblages have been identified: (1) chalcopyrite + bornite + quartz with high-fineness native gold enclosed in bornite, (2) low-Mn dolomite + quartz + sulfide (chalcopyrite, sphalerite, galena, tennantite-tetrahedrite) ± tourmaline with low-fineness native gold and hessite, (3) rhodochrosite + high-Mn dolomite + quartz + sulfide (chalcopyrite, sphalerite, galena, tennantite- tetrahedrite) with low-fineness native gold, electrum, acanthite, Ag and Au–Ag tellurides, and Ag sulfosalts, and (4) calcite + quartz + sulfide (chalcopyrite, sphalerite, galena) with low-fineness native gold, Ag sulfides and selenides, and Ag-bearing sulfosalts. Study of fluid inclusions from quartz, sphalerite, and fluorite have revealed that hydrothermal ores within the Baimka trend precipitated from fluids with strongly variable salinity at temperatures and pressures ranging from 594 to 104°C and from 1200 to 170 bar, respectively. An indicator of vertical AgPbZn/CuBiMo geochemical zoning is proposed. The value range of this indicator makes it possible to estimate the erosion level of the porphyry–epithermal system. The erosion level of the Baimka deposits and prospects deepens in the following order: Vesenny deposit → Pryamoi prospect → Nakhodka prospect → Peschanka deposit → III Vesenny prospect.     

Unusual acid melts in the area of the unique Rosia Montana gold deposit, Apuseni Mountains, Romania: Evidence from inclusions in quartz

Crystalline and melt inclusions were studied in large (up to 2 cm across) dipyramidal quartz phenocrysts from Miocene dacites in the area of the Rosia Montana Au-Ag deposit in Romania. Data were obtained on the homogenization of fluid inclusions and the composition of crystalline inclusions and glasses in more than 40 melt inclusions, which were analyzed on a electron microprobe. The minerals identified in the crystalline inclusions are plagioclase (An 51–62), orthoclase, micas (biotite and phengite), zircon, magnetite (TiO₂ = 2.8 wt %), and Fe sulfide. Two types of the melts were distinguished when studying the glasses of the melt inclusions. Type 1 of the melts is unusual in composition. The average composition of 20 inclusions is as follows (wt %): 76.1 SiO₂, 0.39 TiO₂, 6.23 Al₂O₃, 4.61 FeO, 0.09 MnO, 1.64 MgO, 3.04 CaO, 2.79 Na₂O, 3.79 K₂O (Na₂O/K₂O = 0.74), 0.07 P₂O₅, 0.02 Cl. The composition of type 2 of the melts is typical of acid magmas. The average of 23 inclusion analyses is (wt %) 79.3 SiO₂, 0.16 TiO₂, 10.27 Al₂O₃, 0.63 FeO, 0.08 MnO, 0.29 MgO, 1.83 CaO, 3.56 Na₂O, 2.79 K₂O (Na₂O/K₂O = 1.28), 0.08 P₂O₅, 0.05 Cl. The compositions of these melts significantly differ in concentrations of Ti, Al, Fe, Mg, Ca, Na, and K. The high analytical totals of the analyses (close to 100 wt %, more specifically 98.9 and 99.0 wt %, respectively) testify that the melts were generally poor in water. Two inclusions of type 1 and two inclusions of type 2 were analyzed on an ion probe, and their analyses show remarkable differences in the concentrations of certain trace elements. These concentrations (in ppm) are for the melts of types 1 and 2, respectively, as follows: 10.0 and 0.69 for Be, 29.3 and 5.7 for B, 6.4 and 1.4 for Cr, 146 and 6.9 for V, 74 and 18 for Cu, 92 and 29 for Rb, 45 and 15 for Zr, 1.7 and 0.6 for Hf, 10.3 and 2.3 for Pb, and 52 and 1.3 for U. The Th/U ratio of these two melt types are also notably different: 0.04 and 0.19 for type 1 and 2.0 and 2.9 for type 2. These data led us to conclude that the magmatic melts were derived from two different sources. Our data on the melts of type 1 testify that the magmatic chamber was contaminated with compositionally unusual crustal rocks (perhaps, sedimentary, metamorphic, or hydrothermal rocks enriched in Si, Fe, Mg, U, and some other components). This can explain the ore-forming specifics of magmatic chambers in the area.     

Features of Ore Mineralization of Alyarmaut Rise (Western Chukotka)

The Au mineralization of Alyarmaut Rise was formed from the ore-forming fluids of two types strongly differing in pressure. The first type of fluid is similar to fluids of the orogenic Au–quartz deposits in terms of the physicochemical parameters and composition. These fluids were formed at a high pressure from heterogeneous, carbon dioxide–aqueous fluids with a low salinity at a depth of 5–7 km. The second type of fluid, which is also heterogeneous, has a shallower origin with a wide range of salinity variation and a small amount of carbon dioxide. This type of fluid is similar to the fluids of epithermal deposits. The wide range of pressure variation characterizing the mineralization of the rise indicates the vertical displacement of the block of the host rocks followed by erosion. Judging by the presence the secondary halos of Au, Ag, As, Sb, Pb, Zn, and Cu in the Upper Paleozoic carbonate–terrigenous rocks of the Alyarmaut Rise, there are prospects to reveal sites with new epithermal Au–Ag mineralization, which are essential for the area and unusual for the Anyui-Chukotka Fold System.     

Physicochemical parameters and geochemical features of fluids of precamrbian gold deposits

This paper analyzes literature data on physicochemical parameters and chemical composition of fluids of Precambrian endogenous gold deposits. The average values and ranges of temperature, pressure, and salinity of fluids from the Archean and Proterozoic gold deposits are estimated. It is revealed that fluids of Archean deposits are dominated by methane, while those of Proterozoic deposits, by nitrogen. It is proposed that the accumulation of nitrogen in the atmosphere is related to the intense nitrogen degassing from the Earth’s interior. The highest pressures of endogenous fluids in this period could reflect specifics of deep geodynamics of the planet in the Proterozoic. The large gold deposits (>100 tons) are characterized by narrower range of physicochemical parameters as compared to small deposits. The contribution of heated chloride brines in the formation of majority of large Proterozoic deposits is established.     

Genesis of apatite-fluorite rock in the Burpala pluton

Burpala is a unique peralkaline pluton known to the world. Alkaline pegmatites of the pluton contain about 70 rare-metal minerals. A new scheme of rock crystallization is offered: shonkinite → nepheline syenite → alkali syenite → quartz syenite → vein rocks: mariupolite, rare-metal pegmatite, apatite-fluorite, and alkali granite. Investigation of fluid inclusions in fluorite from the apatite-fluorite rocks established the high temperatures (520–560°C) of homogenization of multiphase salt inclusions. Fluids from inclusions are dominated by hydrocarbonates and chlorides as anions and sodium and calcium as cations; microelements include strontium, barium, boron, iron, manganese, lithium, rubidium, and cesium, i.e., components characteristic of magmatogenic fluids. These rocks are analogous to foskorites of carbonatite complexes in the high calcium content, but calcite is replaced with fluorite along with other foskorite minerals such as apatite, magnetite, mica, and pyroxene.     

Fluid inclusion evidence for barbotage and its role in gold deposition at the Darasun goldfield (Eastern Transbaykalia,Russia)

“Barbotage” is a process of gas penetration through a layer of liquid as a flow of dispersed small gas bubbles. Barbotage is widely used in technology particularly due to its high coefficients of interphase energy and material transfer. It promotes the heat- and mass-exchange processes as well as the chemical interactions in gas-liquid systems. Upon investigation of the fluid regime at the magmatic-hydrothermal system of Darasun goldfield (Eastern Transbaikalia, Russia) we have found evidences of barbotage as an effective mechanism resulting in the formation of vein gold deposits. In spite of the fact that early associations of gold ores in the Darasun goldfield deposits were formed from a heterogeneous fluid, their histograms of fluid salinity distributions are quite different. These differences can be explained by including into the ore-forming process the stage at which the vapour phase generated by boiling rose up as a stream of small bubbles. The corresponding scheme of the formation of the Darasun goldfield hydrothermal ore-bearing system is proposed and discussed. The possibility to use gallium fractionation and the fluid salinity distribution in hydrothermal systems as the indicators of barbotage participation in hydrothermal ore-forming processes is evaluated.     

Ore-forming fluids of Xishimen skarn iron deposit,China

Fluid inclusions in calcite and diopside from the Xishimen skarn iron deposit in China were studied. The deposit ores were formed from chloride solutions with salinity ranging from 8.4 to 59.6 wt % NaCl equiv within the temperature range of 210 to 590°C. The model of skarn ore formation involving high-temperature magmatic fluids is discussed.     

Impurity Elements in Quartz from Gold Deposits of the Darasun Ore Field (Eastern Transbaikalia,Russia): Electron Paramagnetic Resonance Data

The distribution of substitutional Al, Ti, and Ge impurities in quartz samples from the Darasun, Teremkinskoe, and Talatui gold deposits, located in the Darasun ore field, were studied by electron paramagnetic resonance. The relationship between the isomorphous substitution and dynamic recrystallization of quartz was studied by optical and scanning electron microscopy. It was found that analysis of the plots of interdependence between the concentrations of various substitutional impurities in quartz (isogens) can detect development trends of isomorphous substitution. Two isomorphous substitution stages were recognized, one associated with quartz crystallization, and the other, with its subsequent dynamic recrystallization. The first stage is characterized by incorporation of Al impurity into the quartz crystal lattice, and the second, by incorporation Ti impurity. A Ge impurity is a catalyst for isomorphous substitution, and its concentrations vary widely. It is noted that the second stage plays a decisive role, because it accounts for the incorporation of the larger part of substitutional impurities. This process is facilitated by the dynamic recrystallization of quartz. Four genetic quartz groups, described by individual isogens, have been recognized in the Darasun ore field. Two of them correspond to quartz crystallized directly from a magmatogenic fluid or redeposited with the melt’s participation, and the other two groups, to quartz crystallized from an altered fluid. It was found that substitutional Al concentrations are retained in quartz after redeposition, whereas substitutional Ti concentrations decrease dramatically Mineral formation processes at each gold deposit are reviewed. Two types of temperature zoning, normal and reverse, have been recognized at the Darasun deposit. Each is characterized by an individual genetic quartz group and the degree of closedness of the mineral formation system. The genetically similar magmatogenic quartz samples found at the Darasun and Talatui deposits indicate the uniformity of the mineralization process in the Darasun ore field. The established trends of isomorphous substitution in quartz are useful in studies of the ore formation histories of gold and other ore deposits.

     

Seismogenic Nature of Fluid-Dynamic Structural Parageneses of the Uryakh Gold Ore Field (Northeastern Transbaikalia)

Geology of Ore Deposits - New data on the geological structure and ore-bearing structural parageneses of the Uryakh gold ore field are presented. The formation of the deposit occurred in dynamic...     

Mineralogy and Formation Conditions of Novoshirokinsky Base Metal–Gold Deposit,Eastern Transbaikal Region,Russia

This article presents the new mineralogical, fluid inclusion, and isotopic data for ores of the Novoshirokinsky base metal–gold deposit. Mineralogical sequence is supplemented and specified. The mineral assemblages containing native gold are studied. Morphology, grain size and chemical composition of native gold are described. Major parameters and composition of mineralizing fluids of the main ore stages at the deposit are estimated: main base metal (mid-temperature conditions, fluid salinity 3.1–13.1 wt % equiv NaCl) and carbonate–base metal (low-temperature conditions, fluid salinity 1.0–12.9 wt % equiv. NaCl). Sulfur isotopic composition of sulfides from commercial mineral assemblages has been studied. The δ³⁴S value (+10.5 ± 1‰) of mineralizing fluid has been calculated. The Novoshirokinsky deposit is similar to epithermal deposits and is spatially related to the Late Jurassic porphyry system. Evidence is provided on carbonate rocks of basement involved in the ore-forming process.