Evolution of the Vagran Gold-Bearing Placer Cluster (Northern Urals) and Prospects for Revealing Bedrock Mineralization

Geology of Ore Deposits - Abstract—More than 10 t of gold have been mined within the Vagran placer cluster (Northern Urals), and the identified primary sources are limited to single...     

A new generation of Pulkovo observatory meridian instruments and the first results of observations

The results of investigations and observations with classical meridian circle MK-200 and photographic vertical circle (PVC) after its modernization and automation are discussed. The data concerning the stability of the instrumental system and the flexure of the Pulkovo Horizontal meridian circle (HMC) are given. New work on the establishment of in axial meridian circle in Nikolaev, Ecker meridian circle and reflecting one in Pulkovo is mentioned.     

Volatiles in basaltic magmas of ocean islands and their mantle sources: I. Melt compositions deduced from melt inclusions and glasses in the rocks

Statistical analysis of a data bank of the compositions of glasses and melt inclusions in minerals from ocean-island basalts. The initial database contains more than 45 000 published analyses of ocean-island igneous rocks from around the world. Much attention was given to the contents of volatiles (H₂O, Cl, F, and S) and their ratios to one another and to nonvolatile components of close incompatibility (Ti, P, K, and Ce). The average compositions of melt inclusions are similar to those of glasses of the rocks, including volatiles, with consideration for a somewhat higher degree (by approximately 20%) of the differentiation of glasses. The average compositions of ocean-island melts differ from those of mid-ocean basalts in having wider variations and elevated contents of some of the most incompatible elements (Sr, Nb, Ta, Ba, U, Th, and others), as well as H₂O, F, and Cl. Based on the correlation of volatiles to one another and to incompatible elements, three groups of ocean-island basalts are distinguished: (I) low-K, P, Ti magma compositions approximating mid-ocean ridge magmas, (II) high-K, Ce, P, and Ti magmas that resemble continental rift magmas but differ from them in low H₂O content, and (III) high-K, H₂O, Ce, P, and Ti magmas close to continental rift magma. All three types of the melts were found only in the Hawaiian Archipelago, whereas other ocean islands are dominated by any one of these types. The distinguished melt types presumably reflect the differences (heterogeneity) in the compositions of the sources.     

Average compositions of magmas and mantle sources of mid-ocean ridges and intraplate oceanic and continental settings estimated from the data on melt inclusions and quenched glasses of basalts

Based on the generalization of the compositions of melt inclusions and quenched glasses from basaltic rocks, the average compositions of magmas were estimated for mid-ocean ridges (MOR), intraplate continental environments (CR), and ocean islands and plateaus (OI). These compositions were used to constrain the average contents of trace and volatile elements in mantle sources. A procedure was developed for the estimation of the average contents of incompatible elements, including volatiles (H₂O, Cl, F, and S), in the mantle. A comparison of the obtained average contents for the depleted mantle (DM) with the available published estimates showed that the contents of most incompatible trace elements (H₂O, Cl, F, Be, B, Rb, Sr, Zr, Ba, La, Ce, Nd, Sm, Eu, Hf, Ta, Th, and U) can be reliably estimated from the ratio of K to the desired trace element in the MOR magmas and the average content of K in the DM. For Nb, Ti, P, S, Li, Y, and heavy REE, we used the ratios of their contents to an element with a similar degree of incompatibility in MOR magmas (U for Nb and Dy for the other elements). This approach was used to determine the average contents of incompatible elements in oceanic plume mantle (OPM) and the subcontinental mantle of intraplate settings or continental plume mantle (CPM). It was shown that the average composition of both suboceanic and subcontinental mantle plumes is moderately enriched compared with the DM in the most incompatible elements (K, U, Ba, and La) and volatile components (H₂O, Cl, and F). The extent of volatile component enrichment in the plume mantle (500–1500 ppm H₂O) is insufficient for a significant depression of the mantle solidus. Therefore, mantle plumes must be hotter than the ambient depleted mantle. The average contents of incompatible trace elements in the OPM are similar to those of the primitive mantle, which could be related either to the retention of primitive mantle material in the regions of plume generation or to DM fertilization at the expense of the deep mantle recycling of crustal materials. In the latter case, the negative anomaly of water in the trace-element distribution patterns of the OPM is explained by the participation of dehydrated crust in its formation. Variations in the compositions of magmas and their sources were considered for various geodynamic settings, and it was shown that the sources are heterogeneous with respect to trace and volatile components. The chemical heterogeneity of the magma sources and gradual transitions between them suggest that the mantle reservoirs interact with each other. Chemical variations in continental and oceanic plume magmas can be attributed to the existence of several interacting sources, including one depleted and at least two enriched reservoirs with different contents of volatiles. These variations are in agreement with the zoned structure of mantle plumes, which consist of a hot and relatively dry core, a colder outer shell with high contents of volatile components, and a zone of interaction between the plume and depleted mantle.     

Paleomagnetism and petrophysics of the Jänisjärvi impact structure,Russian Karelia

Abstract— Paleomagnetic, rock magnetic, and petrophysical results are presented for impactites and target rocks from the Lake Jänisjärvi impact structure, Russian Karelia. The impactites (tagamites, suevites, and lithic breccias) are characterized by increased porosity and magnetization, which is in agreement with observations performed at other impact structures. Thermomagnetic, hysteresis, and scanning electron microscope (SEM) analysis document the presence of primary multidomain titanomagnetite with additional secondary titanomaghemite and ilmenohematite. The characteristic impact‐related remanent magnetization (ChRM) direction (D = 101.5°, I = 73.1°, α₉₅ = 6.2°) yields a pole (Lat. = 45.0°N, Long. = 76.9°E, dp = 9.9°, dm = 11.0°). Additionally, the same component is observed as an overprint on some rocks located in the vicinity of the structure, which provides proofs of its primary origin. An attempt was made to determine the ancient geomagnetic field intensity. Seven reliable results were obtained, yielding an ancient intensity of 68.7 ± 7.6 μT (corresponding to VDM of 10.3 ± 1.1 times 10²² Am²). The intensity, however, appears to be biased toward high values mainly because of the concave shape of the Arai diagrams. The new paleomagnetic data and published isotopic ages for the structure are in disagreement. According to well‐defined paleomagnetic data, two possible ages for magnetization of Jänisjärvi rocks exist: 1) Late Sveconorwegian age (900–850 Myr) or 2) Late Cambrian age (?500 Myr). However, published isotopic ages are 718 ± 5 Myr (K‐Ar) and 698 ± 22 Myr (³⁹Ar‐⁴⁰Ar), but such isotopic dating methods are often ambiguous for the impactites.     

俄罗斯Kurile-Kamchatka地区安山质熔体的成分、挥发分组成和微量元素

Melt inclusions in minerals from some volcanoes of the Kurile-Kamchatka region were examined.The studied basaltic andesites and andesites were sampled from volcanoes of the Central Kamchatka depression(Shiveluch and Bezymyannyi),Eastern Kamchatka volcanic belt(Avachinskii and Karymskii),and Iturup Island,Southern Kuriles(Kudryavyi).Basalts of the 1996 eruption of the Karymskii volcanic center and dacites of Dikii Greben'volcano,Southern Kamchatka were also studied.More than 260 melt inclusions from 31 rock samples were homogenized,and quenched glasses were analyzed using electron and ion microprobes.The compositions of melt inclusions in andesitic phenoerysts vary in silica contents from 56 to 80wt%.Al_2 O_3 ,FeO,MgO,CaO decrease and Na_2O and K_2O increase with increasing SiO_2.Many inclusions(about 80% )are dacitic or rhyolitic.However,the compositions of silicic glasses(>65wt% SiO_2)in andesites significantly differ in TiO2,FeO,MgO,CaO,and K_2O contents from those in dacites and rhyolites.High-potassium melts(K_2O 3.8～6.8wt% )with various SiO_2 from 51.4 to 77.2wt% were found in minerals of all volcanoes studied.This indicates a contribution of a component selectively enriched in potassium to magmas of the whole region.A great compositional diversity of melt inclusions in plagioelase phenocrysts from the Bezymyannyi andesites suggests a complex history of plagioclase crystallization and magma evolution in the andesite formation.Melts from different volcanoes strongly vary in volatile contents.The highest H_2O contents are found in the melts from Shiveluch(3.0～7.2wt%,4.7wt% on average)and Avachinskii (4.7～4.8wt%);while those are lower in melts of Kudryavyi(0.1～2.6wt% ),Dikii Greben'(0.4～1.8wt%),and Bezymyannyi (<1wt%).Chlorine contents are also variable.The lowest values are found in the Bezymyannyi melts(0.09wt% on average),the highest Cl contents are typical of melt inclusions in minerals from the Karymskii andesites(0.26wt% on average).The melts from Avachinskii,Dikii Greben',Kudryavyi,and Shiveluch show intermediate Cl contents(0.13～0.20wt% ).The pressure of 350～1600 bar determined by CO_2 fluid inclusions in plagioclase from the Shiveluch andesites suggests a magma chamber at a depth of 1.5～6 km. Concentrations of 17 elements were determined in glasses of melt inclusions in plagioclases from five volcanoes(Avachinskii, Bezymyannyi,Dikii Greben',Kudryavyi,and Shiveluch).The studied melts show similar trace-element patterns with Nb and Ti minima and B,K,Be,and Li maxima.The melts are close to typical island arc magmas by Sr/Y,La/Yb,K/Ti,and Ca/St ratios, and have some specific regional geochemical features.REE patterns sensitive to degree of magma differentiation indicate that Kudryavyi magmas are most primitive,while Shiveluch magmas are most evolved.     

Age of the Raygorodok Au-bearing Gabbro—Monzodiorite Massif (Northern Kazakhstan)

Accessory zircon from the polyphase Raygorodok stock (Northern Kazakhstan) has been dated by local U–Pb analysis (SIMS and LA-ICP-MS). This Au-bearing intrusion has been dated to 442–447 Ma, suggesting its emplacement at the very end of the Ordovician and Early Silurian, allowing the stock’s correlation with the Stepnyak Complex of small intrusions. Thus, control of small intrusions of the Stepnyak type over gold mineralization has been corroborated permitting their use as a regional prospecting characteristic.     

Mercuriferous gold and amalgams in Mesozoic-Cenozoic rocks of the Vyatka-Kama depression

Mercuriferous gold and amalgams of two types have been detected in the Lower Triassic, Middle Jurassic, and Holocene alluvial deposits in the sedimentary cover of the Vyatka-Kama depression. Electron microscopic studies revealed that some of these aggregates represent intergrowths of individual segregations (globules) ranging in size from 1 to 300 μm. At high magnifications (up to 100 000 times or more), electron photomicrographs show that the smallest globules include tiny (up to nanoscale) gold particles. Microprobe analysis made it possible to detect significant chemical variations in the globules mainly due to alterations in the Hg content (from 0 to 58.25 wt %). The major part of such aggregates belongs to solid Au-Hg solid solutions. Amalgams with Hg >19.8 at % are represented by several phases: (Au,Ag)₃Hg and (Au,Ag)₂Hg prevail; (Au,Ag)₃Hg₂, (Au,Ag)₅Hg₄, and (Au,Ag)Hg are subordinate. In terms of composition, some globules correspond to argentiferous gold (without Hg), mercuriferous electrum, and Au-Pb intermetallides with Hg, Ag and Cu. Another, more widespread type is represented by lumpy massive grains (with mercuriferous gold shell) and porous amalgam. Their formation is related to natural amalgamation initiated by the Early Mesozoic tectonomagmatic activation of the platform and adjacent deep faults. The results obtained can be used in prospecting for Au-Hg mineralization in the sedimentary cover of the Vyatka-Kama depression.     

Rendering technique of multi-layered domain boundaries and its application to fluid flow in porous media visualizations

Current visualization techniques for computational fluid dynamics applications are sophisticated and work well in simple geometries. For complex geometries such as pore spaces, multiple domain boundaries obstruct the view and make the studying of fluid flow fields difficult. To overcome these deficiencies, we use two-sided materials to render the domain boundaries. Using this technique, it is possible to place the camera inside the domain and have a non-obstructed view of the surrounding flow field without losing spatial reference to the domain boundaries. As a result, a larger part of fluid flow visualization is visible. Two-sided material rendering was successfully applied to display still images with Blender Cycles renderer, in a virtual reality environment, and several implementation techniques were explored for using the Visualization Toolkit.