Formation stages and ore matter sources of the Devdoraki copper deposit,Kazbek volcanic center,the Greater Caucasus

Comprehensive petrological–mineralogical, geochronological, and isotope-geochemical studies have been carried out at the Devdoraki copper deposit situated in the Kazbek neovolcanic center, the frontier territory between Georgia and Russia. The formation history of this deposit has been deciphered on the basis of K–Ar isotopic geochronological data, and the multistage evolution of ore–magmatic system has been established. The subeconomic disseminated and less abundant stringer pyrite mineralization formed at the first stage in the Early Cretaceous back to 130–120 Ma at the retrograde stage of regional metamorphism. The second productive stage was related to intense Quaternary volcanism of the Kazbek center. The late stringer base-metal mineralization formed about 400 ka ago in connection with the activity of minor volcanoes in the eastern part of deposit. In its western part adjoining the Kazbek volcanic cone, ore formation apparently continued over the entire period of recent magmatic activity from 400 to 100 ka ago. It is quite probable that this process is currently proceeding at deep levels of the Devdoraki deposit. Pb–Pb isotope-geochemical data show that Jurassic metasedimentary rocks that host sulfide mineralization could have been a main source of matter for early pyrite. At the second stage of base-metal mineralization formation, the source of ore matter was earlier metamorphic pyrite combined with hydrothermal solutions related to Quaternary endogenic activity within the Kazbek volcanic center. Gangue mineral matter (quartz, carbonates) was supplied simultaneously from the postmagmatic hydrothermal solution and host shale.     

The “Upper Basalts” of the East Chukotka Segment of Okhotsk–Chukotka Belt: the Along Strike Migration of Volcanic Activity or the Overprint by Later Magmatic Event?

Doklady Earth Sciences - New U–Pb and 40Ar/39Ar isotopic dates on volcanic rocks of the East Chukotka segment of the Okhotsk–Chukotka Belt correspond to 76–71 Ma span, which is...     

Neogene–Quaternary Magmatism of the Çaldıran Plain and its Vicinity (Eastern Turkey): an Example of Post-Collisional Transition from Subduction to Intraplate Type

This paper is aimed at studying the chronological evolution of the Neogene–Quaternary volcanic activity within the Çald?ran plain and its mountainous framing (Eastern Turkey). It is shown that the last pulse of continental-margin magmatism related to the subduction and closure of Neotethys oceanic basin occurred in the Middle Miocene (13.5–12.5 Ma). The post-collision volcanism proceeding simultaneously with large-scale regional tectonic rearrangement and initiation of the long-term Çald?ran fault began in the Late Miocene (7–6 Ma), and reached maximum activity in the Middle Pliocene (4.7–3.6 Ma). The Quaternary period in the region evolution was marked by the abundant within-plate magmatic activity restricted to the regional SW–NE trending zone, and the formation of Eastern Turkey’s largest Tendürek shield volcano (Late Pleistocene–Holocene). Petrological-geochemical data indicate that magmas during the overall evolution of young volcanism of the Çald?ran plain was generated from a single mantle reservoir, whose composition gently one-way evolved with time. Calculations show that melting occurred in the upper part of the asthenosphere (immediately near the boundary with thinned lithospheric mantle), which was metasomatized by pre-existing long-continued subduction. The chemical variations of mantle source with time (from the Middle Miocene to Quaternary) were mainly determined by a decrease of subduction component and the presence of aqueous phases, with a general trend from E-MORB to OIB-type for generated magmas. The composition of Late Quaternary basic lavas of Tendürek Volcano in terms of most petrological-geochemical characteristics corresponds to within-plate alkaline basalts. The main trend of geochemical evolution of mantle source is correlated with a systematic change of the predominant serial affinity of igneous rocks from calcalkaline through moderately alkaline to Na-alkaline varieties. Discrete character of young magmatism within the Çald?ran plain, and its subsequent evolution (sulrasubduction → post-collision → within-plate) were mainly determined by periodical large-scale changes in geotectonic setting within the Eurasian–Arabian collision zone: (1) cessation of subduction, (2) break-up and deepening of oceanic slab with its subsequent break off, (3) inferred emergence of incipient rift setting under conditions of intense submeridional compression.     

Modeling changes in the permeability in a gypsified fractured-porous rock massif

A method is proposed for evaluating rock hydraulic conductivity (k _x;y , LT^?1) in grid model blocks to be used in finite-difference simulation of mass exchange in gypcified fractured-porous rocks containing groundwater. The values of k _x and k _y were determined taking into account the dominating form of transport (advection or transverse dispersion), contributing most to gypsum dissolution. The method was used to simulate groundwater flow in a vertical section of gypsified rocks in the base of the dam at the Lower-Kafirniganskii Hydropower System. The changes in rock permeability in the base of the dam were used to evaluate whether there is a need to take seepage-control measures (cement-grout curtain, blanket, and a gypsum fill over the blanket). A leaching zone was found to form near the curtain and blanket during the simulation period (100 years) under background seepage conditions. In this period, the total water flow in the lower pool can increase by no more than 10–15%. Under such conditions, the “seepage-control” measures can enhance the reliability of the dam at the Lower-Kafirniganskii Hydropower System.     

Probability of radial anisotropy in the deep mantle

It is well established that the Earth's uppermost mantle is anisotropic, but observations of anisotropy in the deeper mantle have been more ambiguous. Radial anisotropy, the discrepancy between Love and Rayleigh waves, was included in the top 220 km of PREM, but there is no consensus whether anisotropy is present below that depth. Fundamental mode surface waves, for commonly used periods up to 200 s, are sensitive to structure in the first few hundred kilometers and therefore do not provide information on anisotropy below. Higher mode surface waves, however, have sensitivities that extend to and below the transition zone and should thus give insight about anisotropy at greater depths, but they are very difficult to measure. We previously developed a new technique to measure higher mode surface wave phase velocities with consistent uncertainties. These data are used here to construct probability density functions of a radially anisotropic Earth model down to approximately 1500 km. In the uppermost mantle, we obtain a high probability of faster horizontally polarized shear wave speed, likely to be related to plate motion. In the asthenosphere and transition zone, however, we find a high probability of faster vertically polarized shear wave speed. To a depth of 1500 km in the lower mantle, we see no significant shear wave anisotropy. This is consistent with results from laboratory measurements which show that lower mantle minerals are anisotropic but LPO is unlikely to develop in the pressure–temperature conditions present in the mid-mantle.     

Influence of fluid composition on the elastic properties of sandstone and quartzite at high temperature and pressure in application to the problem of crustal waveguides

Recent geophysical studies have revealed zones with anomalously low seismic velocities and increased electric conductivity in the middle part of the Earth’s crust. It is suggested that these zones can probably be related with the variations in the porosity and permeability of the rocks and the presence of fluids, and that they can explain the existence of crustal waveguides. In order to verify this model, we carried out an experimental study. Among the studied metamorphic processes, the acid metasomatose most completely explains the silicification of rocks. The present work considers the physico-chemical causes of the redeposition of quartz from the solutions, the influence of the pH of the solutions on the mass transfer and deposition of quartz during silicification, and alteration of the physical properties of the rocks. Silicification leads to the increase in density, change in microtexture, and cementation of rocks. It also causes the change in seismic velocities in the Earth’s crust.     

Forced imbibition into a limestone: measuring P‐wave velocity and water saturation dependence on injection rate

Quantitative interpretation of time‐lapse seismic data requires knowledge of the relationship between elastic wave velocities and fluid saturation. This relationship is not unique but depends on the spatial distribution of the fluid in the pore‐space of the rock. In turn, the fluid distribution depends on the injection rate. To study this dependency, forced imbibition experiments with variable injection rates have been performed on an air‐dry limestone sample. Water was injected into a cylindrical sample and was monitored by X‐Ray Computed Tomography and ultrasonic time‐of‐flight measurements across the sample. The measurements show that the P‐wave velocity decreases well before the saturation front approaches the ultrasonic raypath. This decrease is followed by an increase as the saturation front crosses the raypath. The observed patterns of the acoustic response and water saturation as functions of the injection rate are consistent with previous observations on sandstone. The results confirm that the injection rate has significant influence on fluid distribution and the corresponding acoustic response. The complexity of the acoustic response —‐ that is not monotonic with changes in saturation, and which at the same saturation varies between hydrostatic conditions and states of dynamic fluid flow – may have implications for the interpretation of time‐lapse seismic responses.     

Research note: the effect of strain amplitude produced by ultrasonic waves on its velocity

The effect of the amplitude of ultrasonic waves propagating through a sample is not often taken into account in laboratory experiments. However, ultrasonic waves can produce relatively large strain inside the sample, and thus change the properties of the sample. To investigate the effect of strain amplitude on the P-wave velocity, a series of ultrasonic wave propagation experiments were carried out on three different media. All measurements were performed at 1 MHz central frequency and at the strain levels inside propagating waves of  ∼3.0 × 10⁻⁶ to 6.0 × 10⁻⁵ without applying confining pressure to the sample. Strains in the waves were measured by a laser Doppler interferometer upon wave arrival on a free surface of the sample. The ultrasonic velocities were measured by a pair of P-wave transducers located at the same measuring point as the laser beam of the LDI. The effect of strain on P-wave velocity varied for different material. The P-wave velocity was calculated using both a first arrival and a first maximum peak at different applied voltage. The P-wave velocity remained unchanged for a pure elastic medium (aluminium); however, the velocity increased continuously with the increasing of the strain for polymethylmethacrylate and Gosford sandstone. For Gosford sandstone, velocity increases up to 0.8% with strain increase from 7.0 × 10⁻⁶ to 2.0 × 10⁻⁵. This effect of velocity increase with the strain induced by an ultrasonic wave can be explained by the in-elasticity of both the polymethylmethacrylate and Gosford sandstone samples.