Silver sulfoselenide from ore of the Valunisty Au-Ag deposit,Chukchi Peninsula

Silver sulfoselenide (Ag,Cu)₉Se₂S₄ from ore of the Valunisty Au-Ag deposit on the Chukchi Peninsula is described for the first time. The mineral occurs in mineralized quartz-adularia veins, where it associates with chalcopyrite, sphalerite, galena, and electrum and replaces arsenpolybasite. It forms anhedral grains up to 0.2 mm in size. The reflectivity of sulfoselenide is moderate, no anisotropy is observed, and the microhardness is very low. The chemical composition of the mineral differs from other known members of the Ag-Se-Se-S system: the S/Se ratio of the mineral is 2/1 and the (Ag + Cu)/(Se + S) ratio is about 1.5.     

Mineralogy and formation conditions of ore from the Biksizak silver-base-metal occurrence in the South Ural,Russia

The Biksizak silver-base-metal occurrence is situated in the Birgil’da-Tomino ore cluster of the East Ural Zone. The mineralization is hosted in the Silurian marble (Eastern site) and limestone interbeds in andesitic dacite (Western site). Four mineral assemblages have been established: the earliest hematite-magnetite, the subsequent pyrite-arsenopyrite and chalcopyrite-sphalerite occurring only in the Eastern site, and fahlore-chalcopyrite known only from the Western site. The closest positive correlation links Cu-Zn-Ag, Zn-Pb, Cu-Ag, and Zn-Au. The correlation between chemical elements varies depending on the localization of the ore. Correlation pairs Au-Ag, Au-Cu, Pb-Ag, and Pb-Cu are characteristic of ore from the Eastern site and are not established in the Western site. In the Eastern site Cu/(Zn + Pb) in ore is < 1, whereas in the Western site this ratio is markedly higher than unity. As follows from fluid inclusion study and mineral geothermometry, ore minerals at the Bilsizak occurrence were formed at a temperature of 300 to 150°C from low- and moderate-saline chloride fluids with 1–9 wt % NaCl equiv. The data obtained show that the Biksizak occurrence was localized at the margin of a porphyry system characterized by hydrothermal and skarn processes.     

Conditions of Formation of Iron–Carbon Melt Inclusions in Garnet and Orthopyroxene under <Emphasis Type="Italic">P-T</Emphasis> Conditions of Lithospheric Mantle

Of great importance in the problem of redox evolution of mantle rocks is the reconstruction of scenarios of alteration of Fe⁰- or Fe₃C-bearing rocks by oxidizing mantle metasomatic agents and the evaluation of stability of these phases under the influence of fluids and melts of different compositions. Original results of high-temperature high-pressure experiments (P = 6.3 GPa, T = 1300–1500°С) in the carbide–oxide–carbonate systems (Fe₃C–SiO₂–(Mg,Ca)CO₃ and Fe₃C–SiO₂–Al₂O₃–(Mg,Ca)CO₃) are reported. Conditions of formation of mantle silicates with metallic or metal–carbon melt inclusions are determined and their stability in the presence of CO₂-fluid representing the potential mantle oxidizing metasomatic agent are estimated. It is established that garnet or orthopyroxene and CO₂-fluid are formed in the carbide–oxide–carbonate system through decarbonation, with subsequent redox interaction between CO₂ and iron carbide. This results in the formation of assemblage of Fe-rich silicates and graphite. Garnet and orthopyroxene contain inclusions of a Fe–C melt, as well as graphite, fayalite, and ferrosilite. It is experimentally demonstrated that the presence of CO₂-fluid in interstices does not affect on the preservation of metallic inclusions, as well as graphite inclusions in silicates. Selective capture of Fe–C melt inclusions by mantle silicates is one of the potential scenarios for the conservation of metallic iron in mantle domains altered by mantle oxidizing metasomatic agents.     

The reflection of geodynamic processes in local geoacoustic emissions

This paper is concerned with the correlation of fast local processes that generate geoacoustic emissions and the regional character of excitation in the geologic medium prior to large earthquakes. It is shown that this comparison is possible based on the unifying process of vertical energy transport due to the earth’s degassing. For the first time here, we examine local geoacoustic characteristics by analyzing Poincare diagrams and developing difference equations that describe dynamic changes in the characteristics of acoustic emission sources. Based on this concept, and incorporating the continuity of geoacoustic emissions, we suggest a decompressional model of seismoacoustic noise and geoacoustic emissions as a component of the noise.     

Hydrothermal systems in peridotites of slow-spreading mid-oceanic ridges. Modeling phase transitions and material balance: Downwelling limb of a hydrothermal circulation cell

A model is development for the kinetic and thermodynamic simulation of the interaction of seawater and its metamorphosed derivatives with crustal rocks in slow-spreading ridges. The thermodynamic modulus of the model is based on the GEOCHEQ complex, which makes it possible to simulate equilibria in systems of aqueous solutions-minerals-gases. The calculating code was modified and adjusted for the thermodynamic-kinetic simulation of the passage of irreversible solution-rock reactions with time. The simulations were carried out for a simplified crustal vertical section of slow-spreading (Hess-type) ridges, which consist only of mantle peridotites (spinel harzburgites). The results of our simulations demonstrate that the degree of peridotite serpentinization under the effect of low-temperature seawater when the rocks are exposed at the seafloor surface remains very low even after 10000 years of interaction. Serpentinization becomes efficient only at temperatures of 130–150°C at crustal depths of 3.5–4.5 km. The results of our simulations allowed us to develop a thermodynamic model for the origin of hydrothermal systems in peridotites in slow-spreading ridges, with regard for the major stages in the material and tectonic evolution of the Hess crust.     

Hydrothermal systems of the hadean ocean and their influence on the matter balance in the crust-hydrosphere-atmosphere system of the early earth

This paper reports the first results of kinetic and thermodynamic modeling of interaction between the komatiitic material of the earliest terrestrial ocean and seawater. The modeling was aimed at reconstructing geochemical effects accompanying the early evolution of the earth’s outer shells, atmosphere, and hydro-sphere. We also explored the character of mineral transformations in the protooceanic crust during its hydro-thermal alteration. Kinetic and thermodynamic simulation by means of the GEOCHEQ program complex was the main tool of our study. The results of modeling allowed us to estimate the character of the environment and the duration of the formation of the oldest banded iron formation (BIF) and volcanogenic massive sulfide (VMS) deposits and reconstruct main trends in the compositional variations in the hydrosphere and atmosphere that coexisted with them.     

Mineralogy of oxidized ores at the Ik-Davlyat gold-base-metal deposit,the southern Urals

Three types of oxidized ores are identified in the Ik-Davlyat gold-base-metal deposit in the southern Urals: (1) carbonate-sericite-chlorite mineralized rock, (2) vein-shaped quartz-goethite-illite clay, and (3) limonitized rock related to veins. Heavy concentrate of the first type of ore is composed of goethite, rutile, native gold Au_0.91Ag_0.08Cu_0.01, and chalcophanite Zn_1.02Mn_2.98O₄ · 3H₂O. The second type of ore contains goethite, rutile, Pb-bearing jarosite, native gold Au_0.90?0.93Ag_0.06?0.08Cu_0?0.01Fe_0?0.01, silver amalgamide (schachnerite) Ag_0.75Hg_0.97Au_0.98-Ag_0.75Hg_0.97Au_0.28, coronadite (Pb_1.72Mn_7.51Fe_0.41Cu_0.36)₈O₁₆, a chalcophanite-hydrohetaerolite mixture, and cerussite. Gold of the highest fineness (Au_0.98Ag_0.01Cu_0.01) is associated with silver amalgamide. The third type of ore is quite similar to the first variety but contains a jarosite impurity. The composition of oxidized ores indicates a difference in composition of primary ores, in particular, the presence of lead minerals in primary veins. The first finding of chalcophanite in Russia is confirmed by chemical, optical, and X-ray data.     

Formation of the Fe,Mg-Silicates,Fe<Superscript>0</Superscript>, and Graphite (Diamond) Assemblage as a Result of Cohenite Oxidation under Lithospheric Mantle Conditions

Experimental studies in the Fe₃C–SiO₂–MgO system (P = 6.3 GPa, T = 1100–1500°C, t = 20–40 h) have been carried out. It has been established that carbide-oxide interaction resulted in the formation of Fe-orthopyroxene, graphite, wustite, and cohenite (1100 and 1200°C), as well as a Fe–C–O melt (1300–1500°C). The main processes occurring in the system at 1100 and 1200°C are the oxidation of cohenite, the extraction of carbon from carbide, and the crystallization of metastable graphite, as well as the formation of ferrosilicates. At T ≥ 1300°C, graphite crystallization and diamond growth occur as a result of the redox interaction of a predominantly metallic melt (Fe–C–O) with oxides and silicates. The carbide–oxide interaction studied can be considered as the basis for modeling a number of carbon-producing processes in the lithospheric mantle at fO₂ values near the iron–wustite buffer.