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.     

Mineralogical and Geochemical Features of Oolitic Ironstones from the Sinara–Techa Deposit,Kurgan District,Russia

The paper discusses the mineralogical and geochemical features of oolitic ironstones from the Sinara–Techa deposit, Transural region, Kurgan district. The ore unit is localized in the lower part of a thick Mesozoic–Cenozoic sequence of sedimentary rocks that fill the West Siberian Basin beneath calcareous clay and overlying beds enriched in glauconite and clinoptilolite. The ironstone consists of goethite ooids in smectite–opal cement. Accessory minerals are pyrite, galena, sphalerite, and monazite. The texture and structure make it possible to suggest the formation of sediments enriched in iron as a result of colloid coagulation. The most probable source of iron is related to inland drift. Deposition of iron took place in the estuaries of subtropical rivers due to mixing of colloidal solution of river water with seawater electrolyte. The chemical features of rocks are controlled by the composition of the adsorbed iron oxi/hydroxide complex.     

Hydrothermal systems in peridotites at slow-spreading ridges. Modeling phase transformations and material balance: Role of gabbroids

This work was aimed on the estimation of role of gabbro in the ore mobilization during hydrothermal transformation of oceanic (gabbro-peridotite) crust at slow-spreading mid-ocean ridges. Kinetic-thermodynamic modeling was used to reconstruct the geochemical and mineralogical trends of evolution of gabbroids during their hydrothermal interaction with marine fluid. The results of our simulation offered a new insight into some problems of material balance and ore formation during hydrothermal process in the slow-spreading mid-ocean ridges. It was shown that the root zones of all known MAR hydrothermal fields related to serpentinites are made up of ultrabasic rocks and located within peridotite protolith near hot and uncooled gabbroic bodies. It was also demonstrated that the observed mineral and geochemical diversity of metagabbros of slow-spreading mid-ocean ridges was provided by the interaction of hydrothermal fluid percolating through the Hess-type oceanic crust with gabbro bodies. It was established that almost cooled gabbroid bodies, being involved in hydrothermal circulation in the shallow root zones, may play an important role in the redistribution of the material within the Hess-type oceanic crust.     

Hydrothermal systems hosted in peridotites at slow-spreading ridges. Modeling phase transformations and material balance: Upwelling limb of the hydrothermal cell

The research was centered on the estimation of geochemical and mineralogical effects related to the transport of hydrothermal fluid to the seafloor surface in the upwelling limb of a hydrothermal system hosted in peridotites at slow-spreading mid-oceanic ridges. The three variants of the location of the root zone of the circulation cell considered in this research were as follows: (1) shallow-depth, with T = 107°C, P = 1.14 kbar; (2) moderate low depths, with T = 151°C, P = 1.4 kbar; and (3) deep, with T = 500°C, P = 4 kbar. The modeling results demonstrate that ore material is accumulated in the discharge zones of serpentinite-related hydrothermal systems only at a high temperature of the fluid in the discharge zone of the upwelling limb of the circulation cell. The root zones at hydrothermal fields that meet this condition should be situated at a significant depth in the crustal section. It was also established that a significant volume of ore material involved in hydrothermal material exchange between the peridotites and fluid is redeposited in the downwelling limb of the hydrothermal system and gives rise to disseminated ore mineralization, which is typical of many serpentinized abyssal peridotites. The activity of moderately low-temperature and low-temperature hydrothermal systems in peridotites does not concentrate ore material in the discharge zone, and no hydrothermal edifices can grow at such systems.     

Diagnosing the precatastrophic state of the environment in a subduction zone prior to very strong mega-seismic events

The barrier-free seismic process in the subduction zone is considered. The Kurils–Kamchatka subduction zones are characterized by two major environments of oscillatory processes expressed in seismic events distributed long-term throughout the zone and concentrated short-term in a local regional area. The first environment is related to the background seismic process. The fast localization of processes confirms the idea of synchronous movement of regional environment components and is indicative of precatastrophic conditions in the zone. Diagnosis of precatastrophic conditions in the zone makes it possible to control the transition from the background seismic process to the short-term regime of the most likely very strong seismic events. A short-term forecast of the epicentral area of the strongest mega-seismic events in subduction zones is implemented.     

The Koklan W–Mo Deposit,Transuralia: Mineralogical–Geochemical Zoning

Geology of Ore Deposits - The Koklan W–Mo deposit in Transuralia is confined to the eponymous Middle Permian leucogranitic pluton, which intrudes a sequence of the Lower Paleozoic crystalline...     

Mineralogy of Precious Metals in Ores of the Biksizak Base–Metal Deposit,South Urals,Russia

Geology of Ore Deposits - Gold and silver mineralogy is studied in ores of the Biksizak base-metal deposit (South Urals, Russia). This is a skarn-related carbonate replacement mineralization...     

Native bismuth,tsumoite, and Pb-bearing tsumoite from the Tarn’er copper-zinc massive sulfide deposit,northern Urals

Bismuth mineralization, including native bismuth, tsumoite (Bi_1.99–2.03Te_2.00), and Pb-bearing tsumoite (Bi_1.56–1.88Pb_0.45–0.14)_2.00–2.03Te_2.00, was identified in the Au-enriched disseminated ore at the Tarn’er massive sulfide deposit formed under the effect of a large diorite intrusion. Native bismuth associated with hessite forms idiomorphic inclusions in chalcopyrite. The assemblage of Pb-bearing tsumoite, hessite, and altaite occurs as angular allotriomorphic-granular inclusions in silicates or at the contact between silicate and sulfide aggregates. Tsumoite in allotriomorphic-granular aggregates with galena, hessite, and sphalerite is devoid of lead. Gold (Au_0.65Ag_0.35) was identified along with bismuth tellurides. The temperature of contact methamorphism (500–800°C) was estimated from the stability of andalusite, sillimanite, and cordierite. The morphology of the bismuth telluride aggregates in silicates and graphic intergrowth of tsumoite with galena suggest possible crystallization from anatectic melt. The positive correlation between Bi, Te, and Au confirms their probable joint transportation in the melt.     

Supergene sulphides and related minerals in the supergene profiles of VHMS deposits from the South Urals

The mineralogy and structure of the supergene profile in recently-exploited volcaniс hosted massive sulphide (VHMS) deposits of Cyprus, Uralian and Kuroko type in the South Urals, Russia, have been studied. Specific subzones enriched in secondary sulphides and associated minerals have been distinguished in residual pyrite and quartz–pyrite sands at the Gayskoye, Zapadno-Ozernoye, Dzhusinskoye and Alexandrinskoye deposits. Besides minerals which are common to the cementation subzones (covellite, chalcocite and acanthite), non-stoichiometric colloform and framboidal pyrite, pyrite–dzharkenite, pyrrhotite-like and jordanite-like minerals, metacinnabar, sphalerite, selenium-enriched tetrahedrite and unidentified As-, Sb sulphosalts of Pb or Hg and Ag, sulphur-bearing clausthalite, naumannite and tiemannite were also found. Secondary sulphide minerals in VHMS deposits of the South Urals region are characterized by light sulphur isotope compositions (− 8.1 to − 17.2‰). Superposition of the advanced oxidation of colloform pyrite, an enrichment in impurities (sphalerite, galena, and tennantite) from the primary ores, stagnant water conditions, an elevation of the water table during oxidation, and bacterial activity led to supergene concentrations of the base metals as sulphide, selenides or sulphosalts.     

Gossan of the Yubileinoe Massive Sulfide Deposit (South Urals): Evidence for Formation on the Seafloor

Lithology and Mineral Resources - Oxidation zone of the Yubileinoe massive sulfide deposit, South Urals, is buried beneath Jurassic sediments containing coalified plant remains. Mineralogy of...