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listelement.badge.dso-typeЭлемент, RB-SR-ИЗОТОПНАЯ СИСТЕМАТИКА ГИДРОТЕРМАЛЬНЫХ МИНЕРАЛОВ, ВОЗРАСТ И ИСТОЧНИКИ ВЕЩЕСТВА ЗОЛОТОРУДНОГО МЕСТОРОЖДЕНИЯ НЕЖДАНИНСКОЕ (ЯКУТИЯ)(2010) Чугаев А.В.; Чернышев И.В.; Гамянин Г.Н.; Бортников Н.С.; Баранова А.Н.Проблема связи орогенных (мезотермальных) золоторудных месторождений с магматическими и метаморфическими процессами наиболее дискуссионна. Типичным и крупным объектом указанного типа является месторождение Нежданинское, поэтому возраст гидротермальной минерализации и источники металлов относятся к числу ключевых вопросов в изучении конкретно этого месторождения и в разработке указанной выше проблемы в целом. Наиболее актуальной сейчас является задача прямого датирования гидротермальных образований месторождения Нежданинское. Такие изотопно-геохронологические данные отсутствуют, что оставляет открытым вопрос о временно&й корреляции рудообразующих и магматических процессов, проявленных на этом месторождении.listelement.badge.dso-typeЭлемент, Mineralogy, Geochemistry, and Origin of the Nezhdaninsk Gold Deposit (Sakha-Yakutia, Russia)(1998) Bortnikov N.S.; Gamyanin G.N.; Alpatov V.A.; Naumov V.B.; Nosik L.P.; Mironova O.F.The Nezhdaninsk gold deposit is located at a junction of four regional faults in the South Verkhoyansk synclinorium. The ore bodies are the mineralized cataclitic zones and the veins. They are hosted by meta-morphosed terrigenous sandstone-slate sequences intruded by stocks and dikes of silicic and intermediate rocks. The age of the magmatic rocks was found to vary from 154 to 94 Ma. The deposit was formed during three stages. Gold-bearing ore bodies were formed during the second stage. A regeneration of quartz and sulfides, and a crystallization of silver-bearing mineral assemblages occurred during the third stage. The gold deposition began at the metasomatic alteration of the host rocks. The altered rocks consist of quartz, sericite, carbonates, and disseminated pyrite and arsenopyrite. These sulfides contain "invisible" chemically bound gold. Native gold crystallized in quartz veins. The native gold is associated with coarse-grained arsenopyrite and pyrite, and is coeval with sphalerite-galena-sulfosalt aggregates. The fluid inclusion study showed that the ore formation occurred from two immiscible fluids. One of them was a H 2O + CO 2-dominated fluid with N 2, CH 4, and soluble chlorides (up to 4.5 wt % equiv NaCl). The second fluid was a vapor phase consisting mainly of CO 2 and CH 4. The stable isotope (S, C, and O) study in the minerals suggests that sulfur, water, and CO 2 of magmatic origin prevailed in fluids. They were mixed with components incorporated from the host rocks. The magmatic and host rocks interacted with the isotopically light and possibly meteoric waters. The mineral crystallization occurred at temperatures of 360 to 175°C, and under pressures of 1.2 to 1.7 kbar from mineral-forming fluids with a pH equal to 5.5-6.1. The CO 2 mole fraction of the fluids was from 0.25 to 0.3. The formation of the economic gold ores was linked with the postaccretion tectono-magmatic activation of the South Verkhoyansk synclinorium in the Early to Late Cretaceous.listelement.badge.dso-typeЭлемент, GARNET-BEARING XENOLITHS FROM SALT LAKE CRATER, OAHU, HAWAII: HIGH-PRESSURE FRACTIONAL CRYSTALLIZATION IN THE OCEANIC MANTLE(2007) Keshav S.; Sen G.; Presnall D.C.The focus of this study is a suite of garnet-bearing mantle xenoliths from Oahu, Hawaii. Clinopyroxene, olivine, and garnet constitute the bulk of the xenoliths, and orthopyroxene is present in small amounts. Clinopyroxene has exsolved orthopyroxene, spinel, and garnet. Many xenoliths also contain spinel-cored garnets. Olivine, clinopyroxene, and garnet are in major element chemical equilibrium with each other; large, discrete orthopyroxene does not appear to be in major-element chemical equilibrium with the other minerals. Multiple compositions of orthopyroxene occur in individual xenoliths. The new data do not support the existing hypothesis that all the xenoliths formed at 1.6–2.2 GPa, and that the spinel-cored garnets formed as a consequence of almost isobaric subsolidus cooling of a spinel-bearing assemblage. The lack of olivine or pyroxenes in the spinel–garnet reaction zones and the embayed outline of spinel grains inside garnet suggest that the spinel-cored garnets grew in the presence of a melt. The origin of these xenoliths is interpreted on the basis of liquidus phase relations in the tholeiitic and slightly silica-poor portion of the CaO–MgO–Al 2 O 3 –SiO 2 (CMAS) system at pressures from 3.0 to 5.0 GPa. The phase relations suggest crystallization from slightly silica-poor melts (or transitional basaltic melts) in the depth range ∼110–150 km beneath Oahu. This depth estimate puts the formation of these xenoliths in the asthenosphere. On the basis of this study it is proposed that the pyroxenite xenoliths are high-pressure cumulates related to polybaric magma fractionation in the asthenosphere, thus making Oahu the only locality among the oceanic regions where such deep magmatic fractional crystallization processes have been recognized.listelement.badge.dso-typeЭлемент, MAGNETO-BIOSTRATIGRAPHY OF THE MIDDLE TO UPPER TRIASSIC TRANSITION, CENTRAL SPITSBERGEN, ARCTIC NORWAY(2007) Hounslow M.W.; Hu M.; Mørk A.; Vigran J.O.; Weitschat W.; Orchard M.J.Palaeomagnetic and biostratigraphic data were obtained for the latest Ladinian and most of the Carnian (Botneheia Formation to basal Kapp Toscana Group), from two sections in central Spitsbergen (Svalbard archipelago). Thermal and alternating field (AF) demagnetization reveals a magnetization of both normal and reversed polarities. The mean directions pass reversal and fold tests and are similar to other European Late Triassic palaeopoles. One of the sections displays 15° of vertical-axis, clockwise, tectonic rotation on a décollement in the underlying Botneheia Formation. The magnetostratigraphy is dominated by normal polarity in the uppermost Botneheia Formation and into the Tschermakfjellet and basal De Geerdalen formations. A substantial hiatus characterizes the Ladinian–Carnian boundary in central Spitsbergen, so reverse magnetozones, identified in Tethyan sections near this boundary, are absent. Magnetostratigraphic correlation, along with palynostratigraphic constraints, indicates that most of the De Geerdalen Formation is Lower Carnian. The magnetostratigraphy and palynology indicates that the Isfjorden Member (upper unit of the De Geerdalen Formation) is probably mid-Carnian in age. Change in the lithological architecture in the Isfjorden Member, compared with the underlying parts of the De Geerdalen Fm, suggests a hiatus near the base of the member, which may represent a mid-Carnian unconformity, not previously recognized on Spitsbergen.listelement.badge.dso-typeЭлемент, NATIVE Hg liq IN THE METALLIFEROUS SEDIMENTS OF THE EAST PACIFIC RISE (21°S)(2007) Dekov V.M.Native Hg droplets, documented here for the first time in mid-ocean ridge metalliferous sediments, were found in the surface sediment of a magmatically robust and ultra-fast spreading mid-ocean ridge, the East Pacific Rise. The surface sediment and Fe–Mn micronodules in it are enriched in Hg. Two different processes can produce Hg0liq in this environment: hydrothermal and biogenic. Bio-transfer of ionic Hg dissolved in the hydrothermal fluids into Hg0 in the sediment can account for the eventual native Hg occurrences in the close vicinity of the hydrothermal vents. The Hg0liq occurrences in the distal metalliferous sediments most probably have originated directly from the hydrothermal fluids and been transported by hydrothermal plumes away from the vents. Condensation of Hg0liq from Hg0vap in the vent fluids subjected to near critical phase separation seems the most plausible mode of origin of the native Hg occurrences. The high-temperature vents at the global spreading ridge chain introduce an assessed amount of 1.62–4.86 × 103 t/yr Hg0liq into the ocean. With time, the deposited Hg0liq will likely be dissolved in the overlaying seawater and will not survive in the geologic record.