Experimental study of gold-hydrosulphide complexing in aqueous solutions at 350-500°C, 500 and 1000 bars using mineral buffers

The solubility of gold was measured in KCl solutions (0.001-0.1 m) at near-neutral to weakly acidic pH in the presence of the K-feldspar-muscovite-quartz, andalusite-muscovite-quartz, and pyrite-pyrrhotite-magnetite buffers at temperatures 350 to 500°C and pressures 0.5 and 1 kbar. These mineral buffers were used to simultaneously constrain pH, f(S₂), and f(H₂). The experiments were performed using a CORETEST flexible Ti-cell rocking hydrothermal reactor enabling solution sampling at experimental conditions. Measured log m(Au) (mol/kg H₂O) ranges from −7.5 at weakly acid pH to −5.9 in near-neutral solutions, and increases slightly with temperature. Gold solubility in weakly basic and near-neutral solutions decreases with decreasing pH at all temperatures, which implies that Au(HS)₂ is the dominant Au species in solution. In more acidic solutions, solubility is independent of pH. Comparison of the experimentally measured solubilities with literature values for Au hydrolysis constants demonstrates that at 350°C dominates Au aqueous speciation at the weakly acidic pH and f(S₂)/f(H₂) conditions imposed by the pyrite-pyrrhotite-magnetite buffer. In contrast, at temperatures >400°C becomes less important and predominates in weakly acid solutions. Solubility data collected in this study were used to calculate the following equilibrium reaction constants:

     

Age of tin ore from the Solnechnoe quartz-tourmaline-cassiterite deposit, the Khabarovsk krai, Russia from the results of Rb-Sr dating of quartz and adularia

The age of the main productive phase of ore formation at the large Solnechnoe tin deposit has been estimated for the first time based on the study of the Rb-Sr isotopic system of hydrothermal quartz and adularia from ore veins and metasomatic rocks. The Rb-Sr isochron age (84 ± 1 Ma) of mineralization coincides with the age of intrusive rocks pertaining to the third phase of the Silinka Complex, which control tin mineralization. The ⁸⁷Sr/⁸⁶Sr ratios of ore-forming solution and granitic rocks of the final intrusive phase are close to each other, indicating that the granitic melt was most likely one of the main sources of metals. The long and multistage formation history of the deposit could have been caused by complex geodynamic evolution of the Sikhote-Alin accretionary fold region in the Cretaceous.     

Structure of the lithosphere below the southern margin of the East European Craton (Ukraine and Russia) from gravity and seismic data

The present study was undertaken with the objective of deriving constraints from available geological and geophysical data for understanding the tectonic setting and processes controlling the evolution of the southern margin of the East European Craton (EEC). The study area includes the inverted southernmost part of the intracratonic Dnieper-Donets Basin (DDB)–Donbas Foldbelt (DF), its southeastern prolongation along the margin of the EEC–the sedimentary succession of the Karpinsky Swell (KS), the southwestern part of the Peri-Caspian Basin (PCB), and the Scythian Plate (SP). These structures are adjacent to a zone, along which the crust was reworked and/or accreted to the EEC since the late Palaeozoic. In the Bouguer gravity field, the southern margin of the EEC is marked by an arc of gravity highs, correlating with uplifted Palaeozoic rocks covered by thin Mesozoic and younger sediments. A three-dimensional (3D) gravity analysis has been carried out to investigate further the crustal structure of this area. The sedimentary succession has been modelled as two heterogeneous layers—Mesozoic–Cenozoic and Palaeozoic—in the analysis. The base of the sedimentary succession (top of the crystalline Precambrian basement) lies at a depth up to 22 km in the PCB and DF–KS areas. The residual gravity field, obtained by subtracting the gravitational effect of the sedimentary succession from the observed gravity field, reveals a distinct elongate zone of positive anomalies along the axis of the DF–KS with amplitudes of 100–140 mGal and an anomaly of 180 mGal in the PCB. These anomalies are interpreted to reflect a heterogeneous lithosphere structure below the supracrustal, sedimentary layers: i.e., Moho topography and/or the existence of high-density material in the crystalline crust and uppermost mantle. Previously published data support the existence of a high-density body in the crystalline crust along the DDB axis, including the DF, caused by an intrusion of mafic and ultramafic rocks during Late Palaeozoic rifting. A reinterpretation of existing Deep Seismic Sounding (DSS) data on a profile crossing the central KS suggests that the nature of a high-velocity/density layer in the lower crust (crust–mantle transition zone) is not the same as that of below the DF. Rather than being a prolongation of the DDB–DF intracratonic rift zone, the present analysis suggests that the KS comprises, at least in part, an accretionary zone between the EEC and the SP formed after the Palaeozoic.     

Deep Structure and Geodynamics of the Black Sea–Caspian Region

Geotectonics - New data on the crust structure of the Black Sea?Caspian region, including the Scythian and Anatolian plate margins, the Caucasus, Black Sea and Southern Caspian structures are...     

Crustal Structure of the Crimean Mountains along the Sevastopol–Kerch Profile from the Results of DSS and Local Seismic Tomography

The paper discusses the velocity structure of the crust beneath the Crimean Mountains from the results of active and passive seismic experiments. Based on a new interpretation of seismic data from the old Sevastopol–Kerch DSS profile by modern full-wave seismic modeling methods, a velocity model of the crust beneath the Crimean Mountains has been constructed for the first time. This model shows the significant differences in the structure of two crustal blocks: (1) one characterized by higher velocities and located in the western and central Crimean Mountains, and (2) the other characterized by lower velocities and located in the east, in the Feodosiya–Kerch zone, which are subdivided by a basement uplift (Starokrymskoe Uplift). The former block is characterized by a more complex structure, with the Moho traced at depths of 43 and 55 km, forming two Moho discontinuities: the upper one corresponds to the platform stage, and the lower one, formed presumably at the Alpine stage of tectogenesis as a result of underthrusting of the East Black Sea microplate beneath the southern margin of the Scythian Plate in Crimea. At depths of 7–11 km, velocity inversion zone has been identified, indicating horizontal layering of the crust. Local seismic tomography using the data on weak earthquakes (m_b ≤ 3) recorded by the Crimean seismological network allowed us to obtain data on the crustal structure beneath the Crimean Mountains at depths of 10–30 km. The crustal structure at these depths is characterized by the presence of several high-velocity crustal bodies in the vicinity of cities Yalta, Alushta, and Sudak, with earthquake hypocenters clustered within these bodies. Comparison of this velocity model of the Crimean Mountains with the seismicity distribution and with the results from reconstruction of paleo- and present-day stress fields from field tectonophysical study and earthquake focal mechanisms allowed the conclusion that the Crimean Mountains were formed as a result of on mature crust at the southern margin of the East European Platform and Scythian Plate, resulting from processes during various phases of Cimmerian and Alpine tectogenesis in the compressional and transpressional geodynamic settings. The collisional process is ongoing at the present-day stage, as supported by high seismicity and uplift of the Crimean Mountains.     

U-Pb systems and U isotopic composition of the sandstone-hosted paleovalley Dybryn uranium deposit, Vitim uranium district, Russia

The isotopic (U-Pb, ²³⁸U-²³⁵U, ²³⁴U-²³⁸U) and chemical study of whole-rock samples and finegrained fractions of rocks in a vertical section of the terrigenous sequence at the Dybryn uranium deposit in the Khiagda ore field shows that a wide U-Pb isotopic age range (26.9-6.5 Ma) is caused by oxidation and disturbance of the U-Pb isotopic system in combination with protracted uranium ore deposition. The oxidation of rocks resulted in the loss of uranium relative to lead and eventually to an overestimated ²⁰⁶Pb/²³⁸U age at sites with a low U content. The ²³⁸U/²³⁵U ratios in the studied samples are within the range of 137.74–137.88. Samples with a high uranium content are characterized by a decreasing ²³⁸U/²³⁵U ratio with a decrease in ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²³⁸U ages. A nonequilibrium ²³⁴U/²³⁸U ratio in most studied samples furnishes evidence for young (<1.5 Ma) transformation of the Miocene uranium ore, which is responsible for uranium migration and its redeposition.     

Age of a Cross-Layered Sand Sequence of the Yar Sredny Reference Section (Central Kamchatka): Data on Combined 230Th/238U and 14С Dating of Peat

Doklady Earth Sciences - The first quantitative data on the radio-isotope age are presented for a sequence of cross-layered sands abundant in the Neopleistocene reference sections of loose...