Valeria Olegovna Krutikova

—Here we report results of microforms’ studies of native gold and its alloys in igneous rocks, modified to varying degrees by secondary processes. We discuss the composition and occurrence of both the deep-seated magmatic gold-bearing alloys and the products of their transformation under conditions of the upper Earth’s crust. Gold-bearing Kamchatka adakites and ankaramites, Ildeus massif mafic-ultramafic intrusions and adakites from the Stanovoy fold system as well as dacites from the Bolivian Andes were formed during melting of either the suprasubduction mantle wedge or the subducted oceanic crust. In depleted peridotites from the Avachinsky Volcano in Kamchatka as well as suprasubduction ophiolites from Polar Urals, Eastern Sayan and the Western Mediterranean Betic–Rifean belt, the gold-bearing mantle was hybridized by subduction-related melts and high-temperature fluids. Volcanic rocks associated with the Lesser Khingan Fe–Mn deposits and Zolotaya Gora Au deposit in Southern Urals as well as Taragai ultramafic rocks in the South Khingan Range display subduction-related geochemical characteristics. Gold-bearing trachytes in the Virginian Appalachians (USA) represent felsic differentiates of mafic intraplate magmas. We propose that one of the principal forms of gold transport into the upper crustal environments is represented by Cu–Ag–Au alloys, which precipitated from mantle-derived silicate melt enriched in chalcophile and siderophile elements. Such Cu–Ag–Au alloy-rich magmatic rocks can either constitute primary sources of precious metals in the mantle-crust system or serve as geochemical precursors to the formation of native gold assemblages in epithermal and mesothermal ore deposits. Presence of magmatic gold particles in subduction-related igneous rocks and mantle restites hybridized by subduction-derived melts and high-temperature fluids suggest the existence of gold-rich horizons in the Earth’s mantle at depths comparable to typical depths of generation of primary convergent zone and some within-plate magmas.

Mesozoic gabbro from the Stanovoy convergent margin and adakitic dacite lava from the Pliocene–Quaternary Bakening volcano in Kamchatka contain iron–titanium oxide–apatite–sulfide–sulfate (ITOASS) microinclusions along with abundant isolated iron–titanium minerals, sulfides and halides of base and precious metals. Iron–titanium minerals include magnetite, ilmenite and rutile; sulfides include chalcopyrite, pyrite and pyrrhotite; sulfates are represented by barite; and halides are predominantly composed of copper and silver chlorides. Apatite in both gabbro and adakitic dacite frequently contains elevated chlorine concentrations (up to 1.7 wt.%). Mineral thermobarometry suggests that the ITOASS microinclusions and associated Fe-Ti minerals and sulfides crystallized from subduction-related metal-rich melts in mid-crustal magmatic conduits at depths of 10 to 20 km below the surface under almost neutral redox conditions (from the unit below to the unit above the QFM buffer). The ITOASS microinclusions in gabbro and adakite from the Russian Far East provide possible magmatic links to iron oxide–apatite (IOA) and iron oxide–copper–gold (IOCG) deposits and offer valuable insights into the early magmatic (pre-metasomatic) evolution of the IOA and ICOG mineralized systems in paleo-subduction- and collision-related geodynamic environments.

Iron and iron–manganese deposits form three closely spaced clusters within the Lesser Khingan Range of the Russian Far East. Fe-Mn mineralization is hosted in Vendian–Cambrian carbonates and composed of magnetite, hematite, braunite, haussmanite, rhodochrosite and pyrolusite. The iron–manganese ores are closely associated with explosive intermediate–felsic breccias, magnetite-rich lavas, dolerites and mineralized lithocrystalloclastic tuffs. Magmatic rocks display both concordant and discordant relationships with Fe-Mn mineralization and contain abundant xenoliths of host carbonates. Both magmatic rocks (with the exception of Nb-enriched dolerites) and Fe-Mn ores are characterized by variable enrichments in large-ion lithophile and light rare earth elements and strong depletions in high-field strength elements compatible with the broad subduction setting for explosive volcanism and associated hydrothermal Fe-Mn ore mineralization. Nd-Sr isotope systematics suggest contamination by both ancient and juvenile continental crust and the involvement of recycled pelagic sediment in the formation of Fe-Mn deposits in the Lesser Khingan Range of the Russian Far East.

Using the methods of X-ray fluorescence, electron microscopic, sedimentation analysis, and laser diffractometry, the composition of bottom sediments of sur

The Ildeus mafic–ultramafic complex represents plutonic roots of a Triassic magmatic arc tectonically emplaced into the thickened uppermost crust beneath the Mesozoic Stanovoy collided margin. The mafic–ultramafic complex cumulates host Ni-Co-Cu-Pt-Ag-Au sulfide-native metal-alloy mineralization produced through magmatic differentiation of subduction-related primary mafic melt. This melt was sourced in the metal-rich sub-arc mantle wedge hybridized by reduced high-temperature H-S-Cl fluids and slab/sediment-derived siliceous melts carrying significant amounts of Pt, W, Au, Ag, Cu and Zn. Plutonic rocks experienced a pervasive later-stage metasomatic upgrade of the primary sulfide–native metal–alloy assemblage in the presence of oxidized hydrothermal fluid enriched in sulfate and chlorine. The new metasomatic assemblage formed in a shallow epithermal environment in the collided crust includes native gold, Ag-Au, Cu-Ag and Cu-Ag-Au alloys, heazlewoodite, digenite, chalcocite, cassiterite, galena, sphalerite, acanthite, composite Cu-Zn-Pb-Fe sulfides, Sb-As-Se sulfosalts and Pb-Ag tellurides. A two-stage model for magmatic–hydrothermal transport of some siderophile (W, Pt, Au) and chalcophile (Cu, Zn, Ag) metals in subduction–collision environments is proposed.

Igneous rocks from the Russian Far East contain Cu-Ag-Au microspherules with distinct exterior and interior structures, compositions and assemblages of Cu-rich micro-inclusions. Natural microspherules are compared in this study with technogenic Cu-Ag-Au microspherules, which are experimentally produced and extracted from gold scrap jewelry. The following set of diagnostic criteria are considered to distinguish natural from technogenic microspherules on a genetic basis as follows: (1) compacted-related features versus cellular appearance of the exterior; (2) lack of exsolution- or crystallization-related features in natural and domain-type internal structure in technogenic microspherules; (3) absence of spherical copper-oxide inclusions along with meniscus-type textural boundaries in technogenic mcirospherules; (4) pure copper-oxide composition of inclusions versus the common presence of Fe, Ni, Zn, Cu and Na in natural microspherules. The diagnostic characteristics of natural Cu-Ag-Au microspherules suggest extremely fast cooling rates during their formation, which is possible during violent explosive volcanic eruptions or injection of partially molten, pulverized metal alloys into shallow intra-crustal cavities and fault-related tectonic gashes.

The Paleozoic volcano-sedimentary rocks within the structural basement of the horst-and-graben system of the Catalan Coastal Ranges (CCR) are composed of black shale, volcaniclastic sediments, lava flows, sills and lithocrystalloclastic tuffs. Paleozoic sediments are depleted in high-field strength elements (HFSE) such as Nb, Ta, Zr, Hf and Ti, suggesting their accumulation within the Andean-type active continental margin environment. Volcanic rocks within the Paleozoic CCR sequence belong to shoshonitic and high-K volcanic series and are enriched in Cs, Rb and Ba and depleted in HFSE, which is consistent with their derivation from metasomatized (possibly through deep recycling of pelagic sediments) subduction-related mantle source. The presence of sills (sill-sediment complex) suggests that Paleozoic basement of the CCR was formed within the rifted active continental margin or an arc-back-arc basin system akin to the modern Western Pacific subduction configuration. This complex volcanic terroir hosts world-class wines of the Priorat DOQ region. The presence of framboidal pyrite and magnetite, siderite, sphalerite xenotime, (La–Ce–Nd)-monazite, zircon and baddeleyite, as well as cuprite, tenorite and cupriferous and native silver in volcanic-derived black shale (and consequently in the world-famous “llicorella” soil overlying it) may have had dramatic effects on wine quality and sensory characteristics. These mineral features, together with strong enrichment of Priorat shale in Au, Ag and, to a lesser extent, Pt could have pronounced effects on (1) rates and specific types of chemical reactions; (2) plant metabolism; (3) response to nutrient components and (4) determination of grape flavor. Volcanic wine terroirs, such as the Priorat DOQ region, are special geologic environments for wine growth, capable of producing unique wine aromatic and gustatory characteristics.

Introduction. Metal-ceramic coatings based on titanium silicide are promising for protecting stainless steel AISI 304 from wear, corrosion and high-temperature oxidation. Purpose of the work: to investigate the stainless steel AISI 304 surface layer structure after electrospark deposition in a mixture of titanium granules with silicon powder, and to study oxidation resistance, corrosion resistance and tribotechnical properties of the obtained coatings. Research methodology. Fe-Ti-Si coatings on the stainless steel AISI 304 samples were obtained by electrospark machining with a non-localized electrode consisting of titanium granules and 2.6-6 vol.% mixture of titanium and crystalline silicon powders. Results and discussion: it is shown that a stable positive gain of the cathode is observed when the proportion of silicon in the powder mixture does not exceed 32 vol.%. The phase composition of the coatings includes: a solid solution of chromium in iron, titanium silicide Ti5Si3, titanium and silicon, which is confirmed by the energy dispersion analysis data. The microhardness of Fe-Ti-Si coatings ranges from 10.05 to 12.86 GPa, which is 5-6 times higher than that of uncoated steel AISI 304. The coefficient of friction of the coatings is about 20% lower compared to steel AISI 304 and hovers around 0.71-0.73. Wear tests in dry sliding mode show that Fe-Ti-Si coatings can increase the wear resistance of steel AISI 304 up to 6 times. The oxidation resistance of the coatings at a temperature of 900 ̊С is 7-12 times higher as compared to steel AISI 304. The conducted studies have shown that new electrospark Fe-Ti-Si coatings can increase corrosion resistance, oxidation resistance, microhardness, as well as reduce the coefficient of friction and wear rate of the stainless steel AISI 304 surface.

Nb-rich trachytes from Virginia, shoshonite from Colorado and high-K dacite from the Bolivian Andes contain diverse suites of native metals and alloys. Virginia trachytes are differentiates of mafic within-plate magmas derived from oceanic island basalt (OIB)-type mantle sources in the post-orogenic extensional regime of the Appalachian mobile belt. Colorado shoshonite and Bolivian dacite were produced via crystal fractionation of Fe–Mg silicates from trachybasalt and basalt magma, respectively, derived from subduction-related mantle modified by either crustal contamination (both regions are underlain by thick, mature continental-type crust) or recycling of marine sediment. Metal assemblages include refractory metals (Pt with minor Fe and Cr, native W, Co–W and Fe–W alloys), Cu–Ag–Au alloys, chalcophile metals (Cu–Sn alloy, native Cu and Zn) and associated sulfides (galena). Textural evidence (metals occur as inclusions in phenocrysts, high temperatures of their formation, etc.) and compositional features of metallic phases suggest their magmatic origin. Metal-rich magmas (both mafic and differentiated) transport metals in convergent and within-plate tectonic settings and deliver ore metals to the oxidized porphyry and epithermal environments in the upper crust.

Introduction. Austenitic stainless steel AISI 304 is the most widely used type of stainless steel. However, it is subject to wear due to relatively low hardness, and also begins to oxidize intensively in air at a temperature above 800 °C. The use of coatings based on chromium boride can improve its tribotechnical properties and oxidation resistance. The purpose of the work: to study the effect of chromium diboride concentration in the anode mixture on the structure, wear behavior, oxidation resistance and corrosion properties of electric spark coatings on AISI 304 steel. The research methods. Electric spark treatment of AISI 304 steel was carried out in a mixture of iron granules with the addition of CrB2 powder in amount of 5, 10 and 15 vol.%. The structure of the coatings was studied by X-ray analysis, scanning electron microscopy, and electron dispersion spectroscopy analysis. The wear resistance of the coatings was studied under dry friction condition at a load of 10 N. The oxidation resistance test was carried out at a temperature of 900 °C for 100 hours. Results and Discussion. According to X-ray analysis, it is shown that under the conditions of electric spark exposure, CrB2 interacts with iron melt; this has resulted in the formation of chromium and iron borides. Corrosion properties, microhardness, coefficient of friction and wear are investigated in comparison with AISI 304 steel. Samples with coatings showed a lower corrosion potential and corrosion current density compared to the substrate in 3.5% NaCl solution and from 5 to 15 times higher oxidation resistance. The microhardness of the coatings increased from 6.25 to 7.60 GPa with an increase in the addition of chromium diboride in the electrode mixture. The coefficient of friction and the wear rate of all coatings were lower than that of AISI 304 stainless steel, while the coating prepared with the addition of 5 vol.% chromium diboride had the best tribotechnical characteristics.