Age boundaries of formation of the Tomtor alkaline-ultramafic pluton: U-Pb and 40Ar/39Ar geochronological studies

This paper presents the results of geochronological studies of the Tomtor alkaline-ultramafic pluton, one of the largest Nb, Y, Sc, and TR deposits. A new scheme of its magmatism is given. The current K-Ar and Rb-Sr ages of different igneous rocks of the Tomtor pluton range from 800 to 250 Ma [Zaitsev et al., 1992; Frolov et al., 2003]. Such dispersion is probably related to the intense carbonatization of the rocks. The U-Pb zircon and ⁴⁰Ar/³⁹Ar mica ages indicate two stages of the formation of the pluton (700 and 400 Ma), which agrees well with the age of cycles of rift-related tectonogenesis of the Siberian platform.     

Cobalt mineralization in the Altai–Sayan orogen: age and correlation with magmatism

The paper discusses the spatiotemporal and genetic relationships of hydrothermal Co mineralization in the Altai–Sayan orogen with mafic, alkaline mafic, and granitoid magmatism on the basis of isotopic, geochemical, and geochronological investigations. Four stages of Co mineralization have been distinguished for this region: Early Devonian (D₁), Late Devonian–Early Carboniferous (D₃–C₁), Permo-Triassic (P₂–T), and Early Cretaceous (K₁). They correspond to periods of large-scale mafic magmatism. Isotopic (Pb, Sr, He) and geochemical studies have shown that Co mineralization is genetically related to mafic and granitoid magmatism. Also, these studies have confirmed that Co deposits are formed with the participation of mantle fluids and are related to chambers of mafic and alkaline mafic melts. Besides, it has been found that ore originated both from magmatic sources and host rocks. A pulsed facies endogenic zonation has been established for Co deposits, Co-bearing ore clusters, and zones with high-temperature Co–As and low-temperature Ni–Co–As mineralization. It has been first established that ores at hydrothermal Co deposits are rich in Pt and Pd.     

The stages and duration of formation of gold mineralization at copper-skarn deposits (Altai–Sayan folded area)

Gold mineralization at copper-skarn deposits (Tardanskoe, Murzinskoe, Sinyukhinskoe, Choiskoe) in the Altai–Sayan folded area is related to different hydrothermal-metasomatic formations. It was produced at 400–150 ºC in several stages spanning 5–6 Myr, which determined the diversity of its mineral assemblages. Gold mineralization associated with magnetite bodies is spatially correlated with magnesian and calcareous skarns, whereas gold mineralization in crushing zones and along fault sutures in moderate- and low-temperature hydrothermal-metasomatic rocks (propylites, beresites, serpentinites, and argillizites) is of postskarn formation. Different stages were manifested with different intensities at gold deposits. For example, the Sinyukhinskoe deposit abounds in early high-temperature mineral assemblages; the Choiskoe deposit, in low-temperature ones; and the Tardanskoe and Murzinskoe deposits are rich in both early and late gold minerals. Formation of commercial gold mineralization at different copper-skarn deposits is due to the combination of gold mineralization produced at different stages as a result of formation of intricate igneous complexes (Tannu-Ola, Ust’-Belaya, and Yugala) composed of differentiated rocks from gabbros to granites.     

Photometric studies of comet C/2009 P1 (Garradd) before the perihelion

The results of the photometric observations of comet C/2009 P1 (Garradd) performed at the 60-cm Zeiss-600 telescope of the Terskol observatory have been analyzed. During the observations, the comet was at the heliocentric and geocentric distances of 1.7 and 2.0 AU, respectively. The CCD images of the comet were obtained in the standard narrowband interference filters suggested by the International research program for comet Hale-Bopp and correspondingly designated the “Hale-Bopp (HB) set.” These filters were designed to isolate the BC (λ4450/67 Å), GC (λ5260/56 Å) and RC (λ7128/58 Å) continua and the emission bands of C₂ (λ5141/118 Å), CN (λ3870/62 Å), and C₃ (λ4062/62 Å). From the photometric data, the dust production rate of the comet and its color index and color excess were determined. The concentration of C₂, CN, and C₃ molecules and their production rates along the line of sight were estimated. The obtained results show that the physical parameters of the comet are close to the mean characteristics typical of the dynamically new comets.     

Jonassonite (AuBi<Subscript>5</Subscript>S<Subscript>4</Subscript>) from the Dakripen gold deposit in Vietnam

For the first time in ore deposits of Vietnam, a mineral phase containing Au, Bi, and S as major elements was found in the gold ore of the Dakripen deposit. Pb is also present as a minor isomorphic impurity. Rare irregular or ellipsoid grains of that mineral up to 35 μm in size were identified in polished sections together with pyrite; galena; sphalerite; bismuthinite; ikunolite; native bismuth; and, occasionally, gold. All these species are related to the third stage of the ore formation. In reflected light, the mineral is bluish white, the reflectance is comparable with ikunolite and slightly higher than that of bismuthinite, and weak pleochroism and visible anisotropy are established. The mineral is opaque and brittle without internal reflections. According to 18 microprobe analyses, its average chemical composition and quantitative variations for the major elements are as follows (wt %): 14.02 (13.11–14.58) Au, 76.37 (74.93–76.91) Bi, 0.49 (0.10–1.00) Pb, 9.80 (8.87–10.07) S, and 100.68 in total. The empirical formula calculated for the average element contents—Au_0.96(Bi_4.91Pb_0.03)_4.94S_4.10—is similar to the idealized formula of jonassonite from the Nagybörzsöny deposit (Hungary)—Au(Bi,Pb)₅S₄—approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The typical mineral assemblage, optical properties, and chemical composition of this mineral allow us to regard it as a low-Pb variety of jonassonite. It may be assumed that the real formula of jonassonite without sporadic impurities of Pb and other elements must be AuBi₅S₄, as was stated before in the first communication of the Commission on New Minerals and Mineral Names of the International Mineralogical Association and is typical of minerals from Kazakhstan, Russia, Japan, Germany, the Czech Republic, the United States, and Australia.     

Physical characteristics of the plasma tail of comet C/2009 R1 (McNaught)

We determined brightness distribution in the plasma tail of comet C/2009 R1 (McNaught) using observations with a small Newtonian reflector (200/1000) on June 9?C12, 2010. Images of the comet were detected using short exposures with a Canon CMOS APS-C camera. The brightness distribution is simulated and the parameters of the cometary plasma tail are obtained within the diffusion model. The magnetic field induction in the cometary tail, lifetime of light particles, and the lengthwise and transverse ion diffusion coefficients are estimated.     

Geochronology and Ore Mineralization of the Dzheltula Alkaline Massif (Aldan Shield,South Yakutia)

The Dzheltula alkaline massif is located in the Tyrkanda ore region of the Chara–Aldan metallogenic zone of the Aldan–Stanovy Shield (South Yakutia). The region contains separate placer gold objects, which are being explored at the present time, and ore-bearing Mesozoic alkaline intrusions, which are weakly studied due to their poor accessibility. The Dzheltula massif (DM) is the largest exposed multiple-ring intrusion within the Tyrkanda ore region; therefore, it is considered as a typical object for geological, petrological, geochronological, and metallogenic studies. The DM consists of five magmatic phases of syenite composition. ⁴⁰Ar–³⁹Ar dating has established that the crystallization age of the oldest phase, the leucocratic syenite porphyry (pulaskite), is 121.1 ± 1.3 Ma. The crystallization age of the cross-cutting phases represented by syenite–porphyry dikes (laurvikites and pulaskites) ranges from 120.1 ± 2 to 118.3 ± 2.1 Ma. The youngest phase of the massif, trachyte, crystallized at 115.5 ± 1.6 Ma. According to the mineralogical and geochemical studies, two types of ore mineralization, namely gold and uranium–thorium–rare-earth (U–Th–REE), are established within the DM. The gold mineralization was found in the quartz–chlorite–pyritized metasomatites. It is confined to the NNE- and NNW-trending fault zones and coincides with the strike of the syenite porphyry dike belt. Uranium–thorium–rare-earth mineralization has been established in the quartz–feldspathic metasomatites localized in the outer contact of the massif. The juxtaposition of mineralization of different types in some zones of the Dzheltula syenite massif significantly increases the ore potential of the studied object within the Tyrkanda ore region.     

Age constraints and tectonic settings of metallogenic and magmatic events in the Verkhoyansk-Kolyma folded area

The paper presents new isotope geochronological data for several mineral deposits, ore occurrences, and related igneous bodies (plutons and dikes) in the Verkhoyansk-Kolyma folded area, eastern Yakutia. Twenty-one ⁴⁰Ar/³⁹Ar mica and four U-Pb zircon dates provide the first age constraints on key metallogenic units in the area. The dating results allow correlation between tectonic, magmatic, and metallogenic events. The sampled mineral deposits within the Adycha-Taryn fault zone in the southeastern Verkhoyansk-Chersky orogen apparently formed at the Jurassic-Cretaceous boundary during the final phase of the collision between the Siberian (North Asian) craton and the Kolyma-Omolon microcontinent (Kupol’noe deposit and the early metallogenic pulse of the Malotarynskoe deposit, ~ 143-144 Ma) and in the latest Early Cretaceous, in the beginning of the orogen collapse (Tallalakh and Dora-Pil’ deposits and the Malotarynskoe late metallogenic pulse, ~ 126 Ma). According to the suggested new classification of metallogenic units, these deposits belong to the Late Jurassic-Early Cretaceous Yana-Kolyma metallogenic belt. The Kyuchus deposit (~ 106 Ma), the Deputatsky ore cluster (~ 106-113 Ma), and the Khotoidokh deposit (~ 116 Ma) in the northern Verkhoyansk-Kolyma folded area belong to the North Verkhoyansk metallogenic belt. Their origin was associated with accretional and collisional processes that produced the Novosibirsk-Chukotka orogen in the middle Cretaceous. The Mangazeya ore cluster (~ 100 Ma, Early-Late Cretaceous boundary) in the southwestern end of the North Tirekhtyakh magmatic transverse belt belongs to the West Verkhoyansk metallogenic belt. The Nezhdaninskoe, Zaderzhnoe, Kurum, and Kuta deposits of the South Verkhoyansk area (~ 125-120 and ~ 100-95 Ma) can be joined into a single Verkhoyansk-Okhotsk metallogenic belt. The belt resulted from accretion and collision along the East Asian active continental margin and the related formation of the South Verkhoyansk orogen in the Early Cretaceous.     

Mineralogical,geochemical, and age characteristics of the rocks of the Inagli dunite-clinopyroxenite-shonkinite massif with platinum-chromite and Cr-diopside mineralization (Aldan Shield)

We consider the mineralogical and geochemical features of the rocks of the Inagli dunite-clinopyroxenite-shonkinite massif with platinum-chromite and unique jewelry Cr-diopside mineralization, which is a reference object of concentric zonal complexes. The massif rocks, from dunites to pulaskites, including peridotites, clinopyroxenites, shonkinites, and melanocratic alkali syenites, form a single continuous comagmatic series. This is confirmed by a clear dependence of the compositions of olivine, pyroxene, phlogopites, and Cr-spinels on the MgO content of the rocks and on the behavior of trace elements in them. The similar compositions of pyroxenes and trace-element patterns of clinopyroxenite rocks and Cr-diopsidite veins indicate a genetic similarity of these rocks. The age and mineralogical and geochemical compositions of the rocks and the geologic and morphological features of the intrusion prove that the Inagli massif formed from high-K picritoid melts, which underwent gradual decompression solidification during the ascent and formed a cylindrical diapir-like body at the subsurface level in the Early Cretaceous. The new portions of differentiates supplied from the lower horizons of the magma column determined the complex composition of the massif: It has a concentric zonal structure cut by numerous radial-circular vein bodies of pegmatites and pure anchimonomineral rocks (Cr-diopsidites), in places, of jewelry quality.