Au–Ag–Te Mineralization of the Low‐Sulfidation Epithermal Aginskoe Deposit,Central Kamchatka,Russia

Mineralogic studies of major ore minerals and fluid inclusion analysis in gangue quartz were carried out for the for the two largest veins, the Aginskoe and Surprise, in the Late Miocene Aginskoe Au–Ag–Te deposit in central Kamchatka, Russia. The veins consist of quartz–adularia–calcite gangue, which are hosted by Late Miocene andesitic and basaltic rocks of the Alnei Formation. The major ore minerals in these veins are native gold, altaite, petzite, hessite, calaverite, sphalerite, and chalcopyrite. Minor and trace minerals are pyrite, galena, and acanthine. Primary gold occurs as free grains, inclusions in sulfides, and constituent in tellurides. Secondary gold is present in form of native mustard gold that usually occur in Fe‐hydroxides and accumulates on the decomposed primary Au‐bearing tellurides such as calaverite, krennerite, and sylvanite. K–Ar dating on vein adularia yielded age of mineralization 7.1–6.9 Ma. Mineralization of the deposit is divided into barren massive quartz (stage I), Au–Ag–Te mineralization occurring in quartz‐adularia‐clays banded ore (Stage II), intensive brecciation (Stage III), post‐ore coarse amethyst (Stage IV), carbonate (Stage V), and supergene stages (Stage VI). In the supergene stage various secondary minerals, including rare bilibinskite, bogdanovite, bessmertnovite metallic alloys, secondary gold, and various oxides, formed under intensely oxidized conditions. Despite heavy oxidation of the ores in the deposit, Te and S fugacities are estimated as Stage II tellurides precipitated at the log f Te₂ values ?9 and at log fS₂ ?13 based on the chemical compositions of hypogene tellurides and sphalerite. Homogenization temperature of fluid inclusions in quartz broadly ranges from 200 to 300°C. Ore texture, fluid inclusions, gangue, and vein mineral assemblages indicate that the Aginskoe deposit is a low‐sulfidation (quartz–adularia–sericite) vein system.     

Fluid Inclusion and Stable Isotope Studies at Don Sixto,a Precious Metal Low Sulfidation Deposit in Mendoza Province,Argentina

The Don Sixto mining area in Mendoza province, central‐western Argentina, contains an epithermal low sulfidation Au–Ag deposit. It is a small deposit (～4 km²), with a gold resource of 36 t. In Don Sixto, ore minerals are disseminated in the hydrothermal quartz veins and hydrothermally altered volcanic‐pyroclastic rock units of Permian–Triassic age. On the basis of the texture, ore mineral paragenesis and cross cutting relationship of gangue minerals, seven stages of mineralization were recognized and described. The first six stages are characterized by quartz veins with minor amounts of base metal minerals and the last stage is represented by fluorite veins with minimal quantities of base metal minerals; the precious metal mineralization is mainly related to the fourth stage. The hydrothermal veins exhibit mainly massive, crustiform and comb infilling textures; the presence of bladed quartz replacement textures and quartz veins with adularia crystals are indicative of boiling processes in the system. Fluid inclusion and complementary stable isotope studies were performed in quartz, fluorite, and pyrite samples from the vein systems. The microthermometric data were obtained from primary, biphasic (liquid‐vapor) fluid inclusion assemblages in quartz and fluorite. The maximum values for salinity and homogenization temperature (T_h) came from the stage IV where quartz with petrographic evidence of boiling has average values of 4.96 wt% NaCl_equiv. and 286.9°C respectively. The lower values are related to the last stage of mineralization, where the fluid inclusions in fluorite have average salinities of 1.05 wt% NaCl_equiv. and average homogenization temperatures of 173.1°C. The oxygen and sulfur isotopic fractionation was analyzed in quartz and pyrite. The calculated isotopic fractionation for oxygen in the hydrothermal fluid is in the range of δ¹⁸O_H2O = ?6.92 up to ?3.08‰, which indicates dominance of a meteoric source for the water, while sulfur reaches δ³⁴S_H2S = 1.09‰, which could be reflecting a possible magmatic, or even a mixed source.     

安徽庐枞火山盆地铀矿床类型研究

在收集整理前人成果基础上,着重分析了庐枞盆地铀成矿地质特征及地球化学特征。盆地内铀矿化与石英正长岩具密切时空关联,石英正长岩是与火山岩同源并有时空联系的潜火山岩;潜火山岩外带砂岩层间破碎带和贯通潜火山岩与砂岩的断裂及其旁侧裂隙密集带构造是主要控矿构造,隐爆角砾岩构造亦控矿;与铀矿物伴生的金属矿物为中低温热液矿床标型矿物,铀矿石伴生元素及矿床内普遍而强烈发育的硅化,与潜火山岩型金属矿床特征相似。认为火山盆地内铀矿床类型应归属于潜火山岩型矿床,并进而对今后的铀矿勘查工作提出了建议。     

A 300-kyr record of aridity and wind strength in southwestern Africa: inferences from grain-size distributions of sediments on Walvis Ridge, SE Atlantic

The terrigenous fraction of sediments recovered from Walvis Ridge, SE Atlantic Ocean, reveals a history of southwestern African climate of the last 300 kyr. End-member modelling of a data set of grain-size distributions (n=428) results in three end members. The two coarsest end members are interpreted as eolian dust, the third end member as hemipelagic mud. The ratio of the two eolian end members reflects the eolian grain size and is attributed to the intensity of the SE trade winds. Trade winds were intensified during glacials compared to interglacials. Changes in the ratio of the two eolian end members over the hemipelagic one are interpreted as variations in southwestern African aridity. Late Quaternary southwestern African climate was relatively arid during the interglacial stages and relatively humid during the glacial stages, owing to meridional shifts in the atmospheric circulation system. During glacials the polar front shifted equatorward, resulting in a northward displacement of the zone of westerlies, causing increased rainfall in southwestern Africa. The equatorward shift of the polar front is coupled with an increase of the meridional pressure gradient, leading to enhanced atmospheric circulation and increased trade-wind intensity.     

Pre-accretionary and parent body histories of a cometary aggregate particle

Chondrite aggregate interplanetary dust particle IDP L2011K7, collected in the Earth’s lower stratosphere, is an agglomerate of diopside, Mg,Fe-olivine, rare Fe-sulfide and abundant amorphous Mg,Fe-silicates. The overwhelming majority of amorphous silicates have a serpentine-dehydroxylate [(Mg,Fe)₃Si₂O₇] composition; a few have a smectite-dehydroxylate [(Mg,Fe)₆Si₈O₂₂] composition. The cation ratios of the amorphous silicates are notably identical to those of serpentine and smectite phyllosilicates. This paper follows the chronological changes in the amorphous silicates that include (1) formation of nanometer scale crystalline silicates (Mg,Fe-olivine and pyroxene), (2) partial hydration and formation of antigorite-serpentine proto-phyllosilicates, (3) partial dehydration of these proto-phyllosilicates, and finally oxidation and Fe-oxide formation by flash heating during atmospheric entry. Environmental conditions capable of driving these changes in the diffuse interstellar medium or solar nebula, in a comet nucleus, or in circumsolar orbit as a cometary meteoroid were marginal at best. These changes could only proceed because of the unique amorphous silicate compositions. While this study cannot make a firm statement about an interstellar or solar nebula origin for its amorphous silicates that are irradiation-induced olivine, this study does find that amorphous silicates with serpentine and (rare) smectite compositions are an important fraction of the amorphous silicates in comets in addition to amorphous olivine and pyroxene. It is noted that an ice and water-free, millimeter-scale, structurally coherent crumb would be an ample ‘microenvironment’ to evolve micrometer-scale dust. After all IDP L2011K7 only measures 22 × 17 μm.     

Micrometer-sized ice particles for planetary-science experiments - I. Preparation, critical rolling friction force, and specific surface energy

Coagulation models assume a higher sticking threshold for micrometer-sized ice particles than for micrometer-sized silicate particles. However, in contrast to silicates, laboratory investigations of the collision properties of micrometer-sized ice particles (in particular, of the most abundant H₂O-ice) have not been conducted yet. Thus, we used two different experimental methods to produce micrometer-sized H₂O-ice particles, i.e. by spraying H₂O droplets into liquid nitrogen and by spraying H₂O droplets into a cold nitrogen atmosphere. The mean particle radii of the ice particles produced with these experimental methods are (1.49 ± 0.79) μm and (1.45 ± 0.65) μm. Ice aggregates composed of the micrometer-sized ice particles are highly porous (volume filling factor: ? = 0.11 ± 0.01) or rather compact (volume filling factor: ? = 0.72 ± 0.04), depending on the method of production. Furthermore, the critical rolling friction force of F_Roll,ice = (114.8 ± 23.8) × 10⁻¹⁰ N was measured for micrometer-sized ice particles, which exceeds the critical rolling friction force of micrometer-sized SiO₂ particles . This result implies that the adhesive bonding between micrometer-sized ice particles is stronger than the bonding strength between SiO₂ particles. An estimation of the specific surface energy of micrometer-sized ice particles, derived from the measured critical rolling friction forces and the surface energy of micrometer-sized SiO₂ particles, results in γ_ice = 0.190 J m⁻².     

The extinction curve in the visible and the value of RV: Addendum to AN 333, 160

This paper corrects and completes a previous study of the shape of the extinction curve in the visible and the value of R_V. A continuous visible/infrared extinction law proportional to 1/λ^p with p close to 1 (± 0.4) is indistinguishable from a perfectly linear law (p = 1) in the visible within observational precision, but the shape of the curve in the infrared can be substantially modified. Values of p slightly larger than 1 would account for the increase of extinction (compared to the p = 1 law) reported for λ > 1 μ m and deeply affect the value of R_V. In the absence of gray extinction R_V must be 4.04 if p = 1. It becomes 3.14 for p = 1.25, 3.00 for p = 1.30, and 2.76 for p = 1.40. Values of p near 1.3 are also attributed to extinction by atmospheric aerosols, which indicates that both phenomena may be governed by similar particle size distributions. A power extinction law may harmonize visible and infrared data into a single, continuous, and universal interstellar extinction law (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

INFLUENCE OF SUBARCTIC VEGETATION COVER ON SNOWMELT

An examination of the three major components of the energy balance has shown that subarctic vegetation cover markedly alters the relative importance of these components for the snowmelt. It has been found that the contribution of net radiation decreases as forest cover decreases while sensible and latent heat contributions to the snowmelt increase with decreasing forest cover. The decrease in the importance of net radiation with decreasing forest cover is due to the effect of the forest cover on albedo and the effects of the forest cover on the long-wave radiation component of the radiation balance. This situation seems to be typical for open canopy forests. The increase in importance of sensible and latent heat in the open is due to the increase in wind speed from the forest to open sites.