Цифровой репозиторий ГГМ РАН
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listelement.badge.dso-typeЭлемент, Composition and origin of rare-metal (Nb–Ta, REE) and sulfide mineralization in magnesiocarbonatites from the Yenisei Ridge, Central Siberia(2019) Vrublevskii V.V.; Bukharova O.V.; Nebera T.S.; Sveshnikova V.L.The Penchenga Neoproterozoic linear fenite-carbonatite complex in the Yenisei Ridge consists of several magnesiocarbonatite sheet-like bodies which lie nearly concordantly with the metamorphic country rocks and are surrounded with alkaline metasomatic fenite aureoles. Their mineralogy includes primary high-temperature phases (ferrodolomite, calcite, phlogopite, sodic amphibole of the eckermannite – magnesio-arfvedsonite series, fluorapatite, magnetite, and ilmenite), late-magmatic accessories (fluorocalciopyrochlore, pyrrhotite, pyrite, Cu-Pb-Zn-Ag sulfide and hessite, monazite-(Ce) and REE-carbonates of synchysite, cordylite, cebaite, and ancylite), as well as secondary phases (Sr-Ba pyrochlore, ferrocolumbite, fersmite, and Fe and Sr carbonates) of hydrothermal and supergene origin that replace primary magmatic minerals. According to their main features of mineralogy and chemistry, the carbonatites correspond to magnesian derivates of a high-pressure alkali-dolomite melt that forms by melting of carbonated peridotite, as obtained in experiments. The Nd, Sr, and Pb isotope compositions of the Penchenga rocks and minerals (εNd ≈ +6; εSr ≈ −19; ∼0.81 207Pb/206Pb; ∼1.99 208Pb/206Pb) suggest a mantle PREMA + EM 2 source of carbonatite magma and mineralization (including Nb and REE). The presence of carbonates with relatively high δ18O and pyrrhotite enriched in δ34S and 207Pb may be due to continental crust inputs to the source of metals. As estimated approximately from the composition of the pyrrhotite-sphalerite assemblage, sulfide minerals crystallize at a depth of ∼30 km in the lower crust, under a pressure of ∼8 kbar. Degassing of the juvenile alkali-dolomite melt and its interaction with meteoric waters leads to fenitization and formation of late magmatic CO2–H2O fluids. These “carbohydrothermal” fluids form from salt melts, and increase in H2O upon cooling from 600 to 200 °C. At lower temperatures of 480–240 °C, the O–H isotope systems of magmatic minerals undergo inversion and ensuing closure. REE-bearing mineral phases precipitate from REE-carrier fluids.listelement.badge.dso-typeЭлемент, The sources of mineralizing fluids of orogenic gold deposits of the Baikal-Patom and Muya areas, Siberia: Constraints from the C and N stable isotope compositions of fluid inclusions(2019) Prokofiev V.Yu.; Safonov Yu.G.; Lüders V.; Borovikov A.A.; Kotov A.A.; Zlobina T.M.; Murashov K.Yu.; Yudovskaya M.A.; Selektor S.L.The compositions of fluid inclusions in quartz and sphalerite from five gold deposits of the Baikal-Patom (Sukhoi Log, Verninsk and Dogaldyn) and Baikal-Muya (Uryakh and Irokinda) foldbelts in the northern margin of the Central Asian Orogenic Belt, along its boundary with the Siberian craton, indicate that the gold mineralization was formed predominantly from heterogeneous CO2-H2O orogenic fluid. All deposits are structurally controlled and composed of veinlet-disseminated and vein to stockwork types of ores that are hosted by rocks that underwent regional metamorphism to the greenschist facies in Baikal-Patom and up to the granulite facies in Baikal-Muya. The gold-quartz veins of all the Baikal-Patom deposits and the Uryakh were formed by fluids of similar composition with salinity of 1.4–9.5 wt% NaCl equiv. and CO2 content of 1.4–8.6 mol/kg of solution, under comparable physical conditions, at temperatures of 128–385 °C and pressures of 570–3290 bar. Fluids of the granulate-hosted Irokinda deposit are distinguished for the highest temperatures and pressures and involve relics of medium- to high-temperature (179–453 °C) brines with salinity of 4.6–46.3 wt% NaCl equiv. The temperature and pressure of the mineralizing fluid increase roughly from north to south, which correlates with the increase in the metamorphic grade of the host rocks. The increase is accompanied by the simultaneous systematic shift in the C isotope composition of CO2 (released from the quartz by the chrush-leach method) from −1.9 to −5.5‰ δ13CCO2 in the Baikal-Patom and Uryakh from −0.6 to +0.7‰ δ13CCO2 at Irokinda. The C and N isotope compositions of CO2 suggest that Irokinda fluids were partly derived from decarbonated marine limestone, whereas the orogenic gold-bearing fluids of the other deposits are interpreted to have been composed mostly of fluid from a crustal magmatic (granitic?) source.listelement.badge.dso-typeЭлемент, The nature and partitioning of invisible gold in the pyrite-fluid system(2019) Pokrovski G.S.; Kokh M.A.; Proux O.; Hazemann J.-L.; Bazarkina E.F.; Testemale D.; Escoda C.; Boiron M.-C.; Blanchard M.; Aigouy T.; Gouy S.; de Parseval P.; Thibaut M.The most characteristic feature of hydrothermal deposits of gold is its intimate association with pyrite. Microscopically visible gold occurs in pyrite ore as metal particles of >0.1 µm in size, together with so called “invisible” gold, undetectable by conventional microscopic methods. The chemical, redox and structural state of this invisible gold and the mechanisms of its incorporation into pyrite remain both inconsistent and controversial since the dawn of economic geology. To clarify these issues, we performed laboratory experiments to simulate interactions of gold-bearing sulfur-rich hydrothermal fluids with arsenic-free pyrite at temperatures from 350 to 450 °C and pressures from 400 to 700 bar, typical of the formation conditions of many types of gold deposits. Gold solubility was measured in these fluids as a function of sulfur speciation and acidity. Gold redox and structural state in pyrite was characterized by high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS), together with more traditional analytical techniques such as scanning electron microscopy (SEM), x-ray diffraction (XRD), electron probe micro analysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), and inductively coupled plasma atomic emission spectrometry (ICP-AES). Results show that dissolved Au in sulfide-sulfate solutions forms complexes with hydrogen sulfide, and tri- and di-sulfur radical ions whose amounts depend mostly on the fluid pH and total sulfur concentration. Invisible gold in pyrite occurs as Au metal submicron- to nano-sized particles and chemically bound Au(I) in the form of (poly)sulfide clusters composed on S-Au-S linear units, similar to those in aqueous complexes. Our findings contest the common belief that Au(I) substitutes for Fe and/or S in the structure of As-poor pyrite. The partition coefficient of Au(I) between pyrite and the fluid, Dpy/fl, is determined to be 0.15 ± 0.07 at 450 °C in a wide range of Au fluid phase concentrations (10–1000 ppm), but much higher Dpy/fl values, between 10 and 50, are found at 350 °C. These Au partitioning trends coupled with the new data on Au molecular environment in pyrite suggest a control of Au(I) incorporation in the mineral by a chemisorption step. Extrapolated to Au contents of hydrothermal fluids of the Earth’s crust which are typically below 1 ppm, our Dpy/fl values reproduce fairly well the natural Au tenors in As-poor pyrites (∼0.1–1 ppm Au), which are 100–1000 times lower than those typically observed in arsenian pyrites and arsenopyrites (10–1000 ppm Au at As tenors of 0.01–10 wt%). Our results thus indirectly highlight a key role played by arsenic in gold enrichment in As-bearing iron sulfide ore, a role that yet remains to be fully understood and quantified.listelement.badge.dso-typeЭлемент, U-Pb SIMS and Ar-Ar geochronology, petrography, mineralogy and gold mineralization of the late Mesozoic Amga alkaline rocks (Aldan shield, Russia)(2019) Prokopyev I.R.; Doroshkevich A.G.; Ponomarchuk A.V.; Redina A.A.; Yegitova I.V.; Ponomarev J.D.; Sergeev S.A.; Kravchenko A.A.; Ivanov A.I.; Sokolov E.P.; Kardash E.A.; Minakov A.V.Late Mesozoic Upper Amga alkaline rocks are located in the Amga tectonic mélange zone within the Aldan-Stanovoy shield, Russia. The main phases of the Upper Amga complex consist of alkaline syenite intrusions as well as the lamprophyre dikes and sills. The mineralogical features indicate that the lamprophyre may belong to minette. Recent geological works revealed the linear and area-specific geochemical gold anomalies in association with late Mesozoic alkaline magmatism in the Upper Amga gold district. The age of the syenite intrusions is 131.4 ± 1.5 (2.9) Ma, and the lamprophyre sills and dikes were formed at 134.9 ± 1.6, 132.3 ± 1.5 and 117.7 ± 3.4 Ma. The mineralization is represented by the REE-Ti-Th, polymetallic (sulfide) and telluride-gold types in the quartz-calcite-chlorite alteration of the main magmatic phases of the alkaline rocks. The fluid inclusions study shows that the sulfides were formed from hydrothermal fluids of CO2-N2-NaCl-H2O composition at temperatures ranging from 390 to 340 °C, with 44–22.7 wt% NaCl-eq, whereas the epithermal telluride-gold mineralization originated from low-temperature CO2 ± N2-NaCl-H2O fluids, with the homogenization temperature of 230–210 °C and a of 9.2–3.3 wt% NaCl-eq.listelement.badge.dso-typeЭлемент, Neotectonics, remote sensing and erosion cut of ore-controlling structures of the Mnogovershinnoe gold-silver deposit (Khabarovsk Krai, Russian Far East)(2019) Shevyrev S.The Mnogovershinnoe epithermal gold-silver deposit is located on the northern part of the East Sikhote-Alin magmatic belt of Northeast Russia. This deposit is related to the intrusion of a large multiphase Paleogene Bekchiul granitoid pluton. The research area underwent several episodes of intrusion magmatism, volcanism and isostatic uplifting because of its location at the junction point of large tectonic structures of Asia. The integration of in situ observations, geological mapping data, analysis of the Landsat 8 images and digital elevation data provides insights into the Meso-Cenozoic evolution of the Bekchiul magmatic center. Events in the Neogene-Quaternary were sufficiently restored by detecting areas with anomalous differences from the base level surfaces for the second- and third-order streams and numerical modeling. The anomalous values of the base levels difference far exceed the amplitude of sea level regression during the Neo-Pleistocene. The proposed methodological approach considered isostatic uplifting and sustaining activity as guiding factors in the development of the Bekchiul volcano-plutonic center.