Home – Home – Jornals – Russian Geology and Geophysics 2012 number 5

We consider the structure, composition, and age of Early Paleozoic intrusive associations of the Bumbat-Hairhan area located in the central part of the island-arc system of the Lake Zone, western Mongolia. There are large polychronous plutons (Bumbat-Hairhan and Gundguzin) and small intrusions in this area. We have established that they are composed of granitoid and gabbroid associations of different lithology, ages, and geodynamic nature. The results of U-Pb dating show that granitoid and basic magmatism took place within 551-465 Ma. Two main stages of intrusive magmatism are recognized, which reflect two stages of the geodynamic evolution of the region: island-arc (551-524 Ma) and accretion-collision (511-465 Ma). At all stages, predominant rocks in the region were diorite-tonalite-plagiogranite and plagiogranite associations of calc-alkaline series.

The Sin Quyen Cu-Fe-Au-REE deposit is localized in the Proterozoic deposits of the Phan Xi Pang zone, northern Vietnam. The mineralization is formed by lenticular and sheet-like bodies occurring concordantly with the host rocks. Seventeen orebodies have been recognized in the deposit, which form an ore horizon up to 140 m in total thickness, about 2 km in strike, and up to 350 m in dip. The ores are of simple mineral composition: Au-rich copper and iron sulfides (chalcopyrite, pyrite, pyrrhotite) and iron oxides (magnetite, hematite). Gold and silver are distributed unevenly in the ores: Their contents vary from hundredths and tenths of ppm to 1.8 ppm. Copper sulfide ores are the main concentrator of gold and silver. All ores are characterized by high REE contents, tens and hundreds of times exceeding the element clarkes. The highest contents have been revealed for Ce and La. Orthite is the main carrier of REE. No correlation between REE and ore elements of sulfide-oxide ores has been revealed, which points to the independent formation of the mineralization.
Orebodies together with the host rocks underwent metamorphism at 500-600 to 630-685?C and 3-7 kbar.
The spatial association of the mineralization with amphibolites (metamorphosed basites) and the mineral composition of ores suggest that the Sin Quyen deposit is of Cyprus volcanogenic type.

Gold and silver minerals from the Kupol epithermal deposit, Chukchi Peninsula, were studied. A schematic sequence of mineral formation has been compiled. Specific mineral assemblages have been revealed in jarosite breccias: with native gold, yutenbogaardtite, fischesserite, acanthite, and native sulfur. We considered a possible mechanism of formation of gold and silver sulfides and selenides in volcanogenic deposits: They might have formed during solfatara and postsolfatara processes with the participation of volcanic gases or their condensates and sublimates as well as liquid sulfur (and selenium) and sulfuric hydrotherms produced under the interaction of volcanic gases with meteoric waters. The specific features of the Kupol deposit confirm the solfatara genesis of Au-Ag sulfides and selenides.

The Verkhoyansk-Chukchi orogenic area formed on the vast land mass that had appeared as a result of collision in the Early Cretaceous and that had consolidated the Mesozoic protoorogenic structures in northeastern Asia. This heterogeneous land mass was the basis on which spatially and structurally coupled orogens developed in the late Mesozoic and Cenozoic. The protoorogenic stages in the evolution of the Verkhoyansk-Chukchi area were followed by several large orogenic stages: Albian-Cenomanian, Eocene, and Pliocene-Quaternary. The Okhotsk-Chuckchi volcanic-block and the Yana-Kolyma fold-block orogenic systems formed at stage 1. The Taui-Anadyr' continental-margin rift system formed at stage 2. This shaped the structure of the Verkhoyansk-Chukchi area. The internal structure of the orogenic systems formed through the Pliocene-Quaternary (neotectonic) stage, which was dominated by uplifting throughout the Verkhoyansk-Chukchi area. The neotectonic structure was mostly inherited from the previous orogenic stages.

The paper concerns with mathematical modeling of two-velocity media with reference to data on Stoneley wave propagation at the fluid-porous solid interface from experiments by K.W. Winkler, H.L. Liu, and D.J. Johnson. Models obtained within the limits of two theories, that of Biot (Biot-Johnson) and continuum filtration, are investigated in comparison. On this basis, it is discussed whether it is reasonable to include pore tortuosity and frequency-dependent permeability and tortuosity when modeling the mechanics of deforming fluid-saturated porous solids.

The accuracy (detail, resolution) of controlled-source electrical prospecting is a combination of several disparate elements. They include the model framework used in the interpretation. A real geologic medium has a complex structure. Sedimentary rocks have a layered structure with fractal properties: The layers split into smaller ones. Large-scale geoelectrical studies (for example, electrical prospecting) require a proper geoelectrical model. By necessity, the 1D geoelectrical model in electrical prospecting is horizontally layered, with thick (hundreds of meters) homogeneous layers, whereas a fine structure is neglected. To study some aspects of this problem, we performed a set of numerical experiments. They were aimed at studying the TEM response from a formation consisting of many thin layers with random geoelectric parameters, mainly resistivity.

Growth lines in the mineralized tissues of living and fossil organisms often exhibit regular patterns that record daily, monthly, or annual cycles. Growth laminations in fossil corals and other marine invertebrates indicate long-term deceleration of the Earth's rotation, probably largely due to tidal friction, resulting in a decline in the number of days per year over the Earth's history. Fossils suggest the rate of decline has not been uniform, with the trend between the late Carboniferous and Cretaceous in particular departing from preceding and subsequent periods. However, insufficient data have obscured the nature and cause of the apparent halt in despinning within this time interval. Here we present new fossil geochronometer data that reveal a sustained acceleration in the Earth's rotation in the early Mesozoic Era, lasting about 90 Myr and having led to a decrease in the length of day (LOD) at an average rate of about 3 ms/kyr. The coincidence of this acceleration with certain geophysical events, including the final assembly of Pangaea and a change in the intensity and stability of the geomagnetic field, strongly suggests that its cause is rooted in the deep interior of the Earth. A similar explanation has been proposed for observed decadal variations in the Earth's rotation. Our results suggest large-scale linkage of rotational variation, tectonics, and the geomagnetic field to core-mantle boundary (CMB) dynamics. Furthermore, the newly identified acceleration in the Earth's rotation which began at the end of the Paleozoic and the geophysical factors that are associated with it can ultimately clarify the causal mechanisms of the Permo-Triassic mass extinction.