Home – Home – Jornals – Russian Geology and Geophysics 2013 number 5

Holocene shallow (0.5–4.5 m, rarely more) and Pliocene–Pleistocene deep (>25 m) placers occur within the China tectonic depression. The shallow placers are associated with the formation of the present-day drainage valleys of the China River under permafrost conditions, and the deep ones are localized within the preglacial paleovalleys of the river basin. An integrated geological and geochemical study was carried out at ten shallow commercial placers, eight of which are classified as poorly studied and “unconventional.” Placers are considered “unconventional” based on their technological characteristics (commercial gold is small (–0.25 to +0.1 mm), thin (–0.1 mm), and micron-sized or “bound” (invisible)), geomorphologic conditions of formation, confinement to the oxidized zone of active permafrost, significant portion of fine hydrogenic gold, and several other minor features. The formation of shallow “unconventional” placers is controlled by the conditions of active permafrost. Under aerobic conditions, suprapermafrost waters form an oxidized zone, in which iron hydroxides impart a yellowish reddish color to water-bearing rocks. Long-lived geochemical barriers (biogenic, reduction, electrochemical, sorption, and others), including gravitational differentiation, play an important role in the concentration of small and thin gold. Alluvial deposits in Meso-Cenozoic tectonic depressions, such as the China basin, are the most promising in terms of “unconventional” placers. The main factors favoring the formation of these localities and the criteria for their assessment are large feeding sources of gold (mainly carbonaceous and sulfide) mineralization, endogenic and exogenic dispersion aureoles with thin and invisible gold; increased thickness of the suprapermafrost active layer and its temporal and spatial stability, contributing to the formation and functioning of oxidized horizons with the accumulation of ferric hydroxide and hydrogenic gold; specific morphologic varieties of hydrogenic gold, which are the fundamental criterion for primary gold mineralization with migratable metal; fine-clastic clay-rich composition of recent alluvial or alluvial-talus sediments, produced by water reworking of ancient gold-bearing weathering crusts; and development of broad floodplains filled with Holocene sediments and their junction with talus-solifluction erosional slopes.

The chemical, gas, and isotopic compositions of nitric thermal waters in the Baikal Rift Zone are considered. It is shown that the behavior of sulfate and carbonate ions in hydrothermal systems is different, which indicates that they are of two different origins. The studied thermal waters are of five chemical types formed in different geologic conditions. Special attention is given to the genesis of hydrotherms, the geologic and geomorphologic conditions of their recharge, and their equilibrium with rocks. It has been established that most of chemical elements of the waters migrated from rocks, but a significant portion of them is bound by secondary minerals, which results in their deep differentiation, accumulation, or precipitation. Thus, the so-called redundant elements appear, which were earlier considered to be of mantle origin.

Model experiments were performed on the solubility of sulfide ores fr om the Kyzyl-Tashtyg deposit in distilled water and fulvic-acid (FA) solutions of different concentrations. It has been established that the oxidation of sulfide minerals in ores under atmospheric conditions might produce acid drainage waters with toxic heavy metals (Cu, Zn, Cd). The influence of natural organic acids depends on the capability of ore and host rock to neutralize the acidity of solutions. If carbonate content is enough at the first stage, a considerable increase in the pH value of FA-free solutions ensures Fe and Cu removal into the solid phase, wh ereas the formation of metal fulvate complexes hinders this process. However, the rocks exhaust their neutralizing potential, all the solutions remain acidic for >100 days of leaching. In this case, FAs, on the contrary, inhibit the oxidation of the surface of sulfide minerals and reduce the removal of heavy metals into the solution.

We present data on the location, chemical composition, and contents of trace elements in thin ferromanganese crusts at two sites of the submarine Vityaz’ Ridge: Diana and Bussol’ test grounds. The crusts abound in inclusions of grains of nonferrous (Cu, Zn, Pb, Sn, Ni, W) and noble (Au, Ag, Pd, Pt) metals in the form of native elements, sulfides, sulfates, oxides, or intermetallic compounds. The crusts at the Diana test ground contain mainly grains of nonferrous-metal minerals, and those at the Bussol’ test ground, mainly noble-metal minerals. There are also sites with Ni-rich (up to 3.5%) manganese crust. A detailed study of the ore crusts from the Vityaz’ Ridge showed that they are probably at the initial stage of formation.

The three-mode distribution of trace-element (TE) concentrations is observed in accordance with three main forms of TE occurrence in mineral: structural, surficial, and phase (native TE phases). Minerals of hydrothermally synthesized pyrite–pyrrhotite–magnetite–greenockite assemblage in the presence of Au and Mn are studied. Discrimination of modes is made, using the method of statistical sample of analytical data for single crystals, which is based on the postulate that individual crystals can contain predominantly one of possible forms. This is supported by study of Cd modes of occurrence by element thermorelease atomic-absorption spectrometry. X-ray photoelectron spectroscopy and atomic-force microscopy were used to examine the surficial TE forms. It has been confirmed that the dependence of the TE content on the crystal size in the sample is mainly due to surficial nonautonomous phases absorbing TEs. The effect of correspondence of chemical-component forms on the surfaces of coexisting minerals is also corroborated. This phenomenon is not related to the mutual contamination of phases but is due to the induction of the corresponding states of chemical forms in coexisting nonautonomous phases. It is possible to obtain true coefficients of interface distribution, characterizing structural TE impurities. These coefficients differ strongly fr om apparent distribution coefficients calculated from the bulk contents of impurities, except for Mn in pyrrhotite and magnetite, wh ere its structural mode is predominant. The results obtained show that TEs can be used as quantitative geochemical indicators, which help to obtain correct information about the parameters of mineral formation and TE contents in the fluid phase.

We present a thermophysical model for interaction between the conduit of a thermochemical plume and free convection flows in the mantle. The mantle flow incident on the plume conduit melts at the conduit boundary (front part) and crystallizes at its back. Geological data on the intensity of plume magmatism over the last 150 Myr are used to estimate the total thermal power of mantle plumes. A possible scenario for plume-related mantle recrystallization by plumes is proposed. Over the lifespan of a thermochemical plume, mantle melts and recrystallizes owing to the motion of the plume source and interaction between the plume conduit and free convection flows. The plume conduits can melt and recrystallize the entire mantle over a certain period of time. The model for the interaction of drifting plume conduits with mantle flows and the estimated total thermal power of mantle plumes are used to estimate the duration of plume-related melting and recrystallization of the entire mantle. The influence of mantle plumes on the convective structure of the mantle through melting is judged from the model for plume interaction with horizontal mantle flows.

Geological and geophysical interpretation of TEM data has revealed changes to the subsurface from the «Kraton-3» peaceful underground nuclear explosion (PUNE). The explosion was conducted on 24 August 1978 at a depth of 577 m in Middle Cambrian limestone on the eastern periphery of the Tunguska basin (Western Yakutia). The site is located in an area of 100 to 300 m thick permafrost and pressurized aquifers with Na-Ca-Cl brines (up to 400 g/l TDS) and cryopegs. The «Kraton-3» epicenter is only 160 m away from a fault emerging along the Markha River. TEM responses collected at 22 stations along three profiles image a layered-earth background resistivity pattern. The highly resistive uppermost layer, ~ 150–200 m thick, consists of perennially frozen ice-rich rocks. Dry permafrost on watersheds of the Markha right side reaches 1200 Ohm · m, while the hypsometrically lower frozen ground along the fault is 10 to 40 times less resistive. That is exactly the place of the PUNE epicenter, and the resistivity lows may record permafrost degradation and taliks (unfrozen layers). The layers below are conductive and correspond to Upper Cambrian and Middle Cambrian (I) aquifers with brines. The top of the Upper Cambrian aquifer along the central profile is highly variable in depth, especially along the fault on the river left bank. The data indicate a local groundwater anomaly above the explosion: the Middle Cambrian I brines, which show up as a conductor in the resistivity pattern, become ~300 m shallower, most likely rising along the rubble chimney above the UNE containment cavity; the lateral extent of the anomaly reaches 400 m. There may exist paths for mass and heat transport maintained by pressurized brines in the system «containment cavity–rubble chimney–fault zone–ground surface».