Home – Home – Jornals – Russian Geology and Geophysics 2015 number 10

New data on the mineral composition and the first data on the geochemical composition of ores of the Rogovik gold-silver deposit (Omsukchan ore district, northeastern Russia) have been obtained. Study of the regularities of the spatial distribution of ore mineralization shows that the deposit ores formed in two stages. Epithermal Au-Ag ores of typical poor mineral and elemental compositions were generated at the early volcanic stage. The major minerals are low-fineness native gold, electrum, acanthite, silver sulfosalts, kustelite, and pyrite. The typomorphic elemental composition of ores is as follows: Au, Ag, Sb, As, Se, and Hg. The content of S is low, mostly ≤1%. Silver ores of more complex mineral and elemental compositions were produced under the impact of granitoid intrusion at the late volcanoplutonic stage. The major minerals are high-Hg kustelite and native silver, silver sulfosalts and selenides, fahlore, pyrite, chalcopyrite, galena, and sphalerite. The typomorphic elemental composition of ores is as follows: Ag, As, Sb, Se, Hg, Pb, Zn, Cu, and B. The content of S is much higher than 1%. The ores also have elevated contents of Mo, Ge, F, and LREE (La, Ce, and Nd). At the volcanoplutonic stage, polychronous Au-Ag ores formed at the sites of the coexistence of silver and epithermal gold-silver mineralization. Their specific feature is a multicomponent composition and a strong variability in chemical composition (both qualitative and quantitative). Along with the above minerals, the ores contain high-Hg gold, hessite, argyrodite, canfieldite, orthite, fluorapatite, and arsenopyrite. At the sites with strongly rejuvenated rocks, the ores are strongly enriched in Au, Ag, Hg, Cu, Pb, Zn, Ge, Se, La, Ce, Nd, S, and F and also contain Te and Bi. The hypothesis is put forward that the late silver ores belong to the Ag-complex-metal association widespread in the Omsukchan ore district. A close relationship between the ores of different types and their zonal spatial distribution have been established. In the central part of the Rogovik deposit, epithermal Au-Ag ores are widespread in the upper horizons, Ag ores are localized in the middle horizons, and rejuvenated polyassociation Au-Ag ores occur at the sites (mostly deep-seated) with ore-bearing structures of different ages.

We studied the mineralogic and geochemical features of metasomatic rocks and ores from the Pogromnoe gold deposit, which is unconventional for Transbaikalia. The deposit, which formed in the Early Cretaceous, at the rifting stage of the regional evolution, is localized in the dynamoclastic strata of the Mongol-Okhotsk suture, along which the Siberian continent joined the Mongolia-China continent in the Early-Middle Jurassic. Gold mineralization occurs as two morphologic types of ores: stockwork quartz-carbonate-arsenopyrite-pyrite ores in altered volcanics (orebody 1) and veinlet-vein quartz ones (with disseminated sulfides) in altered carbonaceous shales (orebody 10). The host rocks of the deposit are the highly altered volcanosedimentary rocks of the Butorovskii Formation (Shadoron Group, J _2-3), which transformed into metasomatic (by composition) and dynamoclastic (by texture and structure) rocks. It has been found that the formation of the metasomatic rocks and mineralization proceeded in several stages. Propylites formed at the preore stage (J ₃); tectonic schists and albitophyres, at the late preore stage; and sericitolites and albite-carbonate-sericite-quartz metasomatic rocks (quartzites), at the synore stage. The ⁴⁰Ar/ ³⁹Ar age of the stockwork system of ore-bearing fractures and metasomatic rocks which formed at the late preore stage is estimated as 139.5 ± 1.8 Ma. The gold-bearing rocks at the deposit are the late preore and synore metasomatic rocks formed after volcanics with sulfide mineralization (gold concentrators are pyrite II and III and arsenopyrite I and II) and after altered carbonaceous shales (gold concentrators are vein quartz and arsenopyrite II). Gold grade is completely consistent with silicification, saturation with quartz-sulfide and sulfide microveinlets, and fine sulfide dissemination. By genesis, the Pogromnoe deposit belongs to objects which formed in shear zones with the contribution of gold-bearing mantle fluids. The authors presume that the sources of mineralization are the ore-producing granitoids of the Amudzhikan-Sretensk intrusive assemblage within the Aprelkovo ore-magmatic system (OMS) (Os’kina and Urguchan plutons). This is confirmed by Pb isotope compositions ( ²⁰⁷Pb/ ²⁰⁴Pb and ²⁰⁶Pb/ ²⁰⁴Pb) for the pyrite and arsenopyrite of the Pogromnoe gold-bearing ores, which testify to the widespread occurrence of “mantle” Pb isotope signatures. The ⁴⁰Ar/ ³⁹Ar age of the ore-producing granitoids of the Aprelkovo OMS is 131.0 ± 1.2 Ma. Gold in the orebodies occurs in native form and is fine and very fine. By gold grade, the Pogromnoe deposit deserves very close attention as a new commercial type of gold mineralization in Transbaikalia.

The typomorphic features of pyrite of the Sukhoi Log deposit were studied by a set of volumetric and surface methods: electron probe microanalysis, scanning electron and probe microscopy, powder X-ray diffraction, X-ray photoelectron and Auger electron spectroscopy, atomic-absorption spectrometry in the SSADSC (method of statistical sample of analytical data for single crystals) version, and atomic-emission spectrometry. Pyrite from the Sukhoi Log deposit has the following distinctive features: permanent presence of sulfite ion, which often dominates over other surface sulfur anions; weakly determined size dependence of the content of uniformly distributed Au owing to the presence of an internal concentrator of gold - dispersed carbonaceous material - in pyrite from ore zones; cell sculptures of the crystal faces, which appeared owing to the nanofragmentation of the growth surface; micro- and nanoinclusions of carbonaceous phases within crystals, associated with defects in their structure; and thin films enriched in O and C on the surface of and within the crystals. It has been shown that gold-sulfide mineralization at the Sukhoi Log deposit formed in a single ore-generating hydrothermal system, in which gold, sulfur, and carbon belonged to a microparagenesis. Some features (composition of surface, characteristics of submicroscopic structure, and elemental composition) evidence that the conditions of crystallization of pyrite in inter-ore space were different from the conditions of its genesis in the ore zones, which suggests the presence of at least two genetic types of pyrite. Carbonaceous micro- and nanoparticles and O- and C-containing films can favor an increase in the adsorption of gold from cyanide solutions on pyrite. To reduce this effect during gold recovery, a technique for surface modification should be elaborated. The ways for solving the most complicated problems dealt with the source of noble metals (NM) and the ore specialization of the deposit have been outlined. For this purpose, a detailed analysis of the main ore minerals for trace-element speciation is required. In the case of the magmatic source of NM, correlation between the contents of Au and PGE structural forms should exist. On the other hand, there is no correlation between the structural forms of Au or Pt and elements whose contents in fluid are determined by the host rock rather than the magmatic source.

The Ta Nang gold deposit is localized in Middle Jurassic black shales. The ore zone is a series of layer-by-layer crush zones and zones of hydrothermal rock alteration, ≤10 m in thickness and >2 km in length. It consists of quartz-sulfide veins, sulfidized black shales, and their hydrothermally altered varieties. Sulfide mineralization occurs as two assemblages: early pyrite-arsenopyrite and late chalcopyrite-sphalerite-galena. The pyrite-arsenopyrite assemblage is composed of different morphogenetic varieties. Coarse-crystalline arsenopyrite and pyrite aggregates and metacrystals of different orientations, 0.1 to 10 mm in size, are the most widespread. The chalcopyrite-sphalerite-galena assemblage is scarcer. Along with the main ore minerals, it includes more rare minerals: pyrrhotite, lead sulfosalts (tsugaruite), and gold, which form a spatial assemblage with the main minerals or small inclusions in them. Gold occurs mainly as fine dissemination in cracks in pyrite, arsenopyrite, chalcopyrite, and quartz. Gold content in sulfidized carbonaceous shales is no more than tenths of ppm, averaging 0.38 ppm. This content in the quartz veins is considerably higher, averaging 3.92 ppm. Silver contents in the shales and quartz veins are similar and equal to 2.68 and 5.30 ppm, respectively. Also, the sulfidized rocks and veins have elevated contents of Fe, As, Pb, Zn, Cu, Cd, Ni, and Co; most of these elements (Fe, As, Pb, Zn, and Cu) make up their own sulfide minerals, and the others are trace elements. According to ³⁹Ar/ ⁴⁰Ar dating of sericite from the quartz-sulfide veins, their age is 129.3 ± 5.6 Ma, which is close to the age of the Cretaceous granite intrusions of the Ðeo Ca complex. These veins formed from moderately strong solutions (11.7-6.4 wt.% NaCl equiv.) with the CH ₄ + N ₂ + CO ₂ gas phase at 340-130 °C. Judging from the S isotope composition (δ ³⁴S = 1.6-4.3‰), predominantly deep-seated endogenic sulfur participated in the formation of ore sulfide associations. Analysis of the distribution of gold shows that it was deposited together with sulfide minerals (galena, sphalerite, and chalcopyrite) at a later stage.

The protocol for analysis of rock-forming mineral compositions by X-ray electron probe microanalysis used at the Institute of Geology and Mineralogy, Novosibirsk, Russia, is described. The analysis is conducted with a JXA-8100 electron probe microanalyzer capable to support a highly stable beam at relatively high probe currents for a long time. Elements that can be assayed range fr om sodium (atomic number Z = 11) to zinc ( Z = 30). The operation conditions for routine analyses are substantiated: accelerating voltage 20 kV, probe current 50-100 nA, and signal accumulation time 10 s at both the peak and the background. The method of analytical-problem formulation for measurements is presented. It is proven that the proprietary software is insufficient with the presence of the binary matrix effect and better correction methods are required. Metrological characteristics of the protocol have been studied. The variation coefficient, describing the reproducibility of results, averages 0.9 % for major components ( C > 10 %), 2.5% for minor components (1 < C < 10 %), and 6.8 % for accessory components (0.3 < C < 1 %). With still lower contents (0.05 < C < 0.3 %), the standard deviation of reproducibility is 0.02 %. The values of the variation coefficient and standard deviation for measurement repeatability are approximately two times lower. The relative trueness of the method is within 1%. The detection lim it (3Г criterion) is generally within 0.01-0.03 %. It can be improved by an order of magnitude by increasing the accumulation time and probe current.

We studied recent sedimentation in small saline and brackish lakes located in the Ol’khon region (western Baikal area) with arid and semiarid climate. The lakes belong to the Tazheran system; it is a series of compactly located closed shallow lakes, with a limited catchment area and different mineralization, under the same landscape, climatic, geologic, and geochemical conditions. Two complementary approaches are applied in the research: (1) a detailed study of individual lake and (2) a comparison of the entire series of lakes, which can be considered a natural model for studying the relationship between endogenic mineral formation and the geochemistry of lake waters. The lake waters and bottom sediments were studied by a set of modern methods of geochemistry, mineralogy, and crystal chemistry. The mineral component of the bottom sediments was analyzed by powder X-ray diffraction (XRD), IR spectroscopy, and electron microscopy. The lakes are characterized by predominart carbonate sedimentation; authigenic pyrite, smectite, chlorite, and illite are detected in assemblage with carbonate minerals in the bottom sediments. Carbonate phases have been identified, and their proportions have been determined in the samples by decomposition of the complex XRD profiles of carbonate minerals into peaks using the Pearson VII function. Mathematical modeling of the XRD profiles of carbonates has revealed that predominantly Mg-calcite with variable Mg content and excess-Ca dolomite accumulate in the lake bottom sediments influenced by biogenic processes. Aragonite, monohydrocalcite, and rhodochrosite form in some lakes along with carbonates of the calcite-dolomite series. We show a close relationship between the composition of the assemblages of the newly formed endogenic carbonate minerals and their crystallochemical characteristics and the chemical composition of the lake waters.

The biostratigraphic study of the Low Cretaceous sediments of the Middle Messoyakha swell (Malaya Kheta region) distinguished 12 biostratons based on biofossil data and thus confirmed Boreal Berriassian, Valanginian, and Lower Hauterivian strata. The genetic types of the sediments were determined by lithofacies and biofacies analyses. In the deposit, the Bazhenovo and Kulomzino horizons (Berriassian to basal Valanginian) consist of relatively deep-water marine, shallow-water marine, and coast-continental facies that displace one another along the strike. The Tara and Ust’-Balyk horizons (middle Lower Valanginian to Lower Hauterivian) are composed of shallow-water marine facies. Paleontological data suggest a low-hilled island covered with conifer-gingko forests with fern understory in the place of the Middle Messoyakha swell in the Berriassian Age.

A new type of jet from continental Devonian sediments is described. The substrate for gagatization was fragments of Archaeopteris plants, whose burials in sandstone strata are characterized by a kind of spheroidal weathering of organic matter; the material is completely fusainized. This type of jet combines the properties of classical jet and inertinite. The new type of jet was named tsilmanite, after the place of its discovery.

Moraines studied in the Chon-Kyzylsuu River valley (southeastern Issyk-Kul region, Tien Shan) were mobilized during historic and prehistoric large earthquakes. Seismic triggers of moraine mobilization included the M > 8 Kebin earthquake of 1911 and prehistoric events that produced rockslides, landslides, and multiple fault scarps. Rockslides in the Chon-Kyzylsuu basin are located in the hanging wall of the Terskey border thrust fault. The observed deformation results from at least four prehistoric earthquakes in the second half of the Holocene (early 20th century BC, early 11th century BC, middle 8th century BC, and early 2nd century BC), with local shaking intensity I ≥ 7.

The Bolnay (Hangayn) fault is an active shear system which generated the M = 8.2-8.5 Bolnay earthquake of 23 July 1905, one of world’s largest recorded intracontinental event. The fault follows the Mesozoic suture formed during the closure of the Mongolia-Okhotsk ocean. The Late Cenozoic faulting in the region was induced by propagation of strain from the India-Eurasia collision that had reached Mongolia at about 5 ± 3 Ma. The left-lateral strike slip almost all over the fault length is compensated in its western end by Late Quaternary reverse motion. We estimated coseismic slip associated with the event of 1905 and the previous earthquakes in the eastern fault end and checked whether vertical offset compensates the strike slip in this part as well. The 1905 coseismic slip measured from a displaced dry stream bed and pebble bars in the Hasany-Gol river valley was 6.5-7.5 m. The 13 ± 1 m left-lateral displacement of pebble bars in the same valley represents a cumulative slip of two events. Paleoseismological studies across the strike of surface ruptures reveal at least two generations of rupture in two events that postdated the deposition of sediments with a ¹⁴C age of 4689 ± 94 yr. Hypsometry of the alluvial surface in the zone of deformation shows gradual elevation increase toward the mountains, but without abrupt change across the fault. This means the absence of vertical offset and reactivation of the fault as a left-lateral strike slip. The horizontal slip in the eastern extension of the Bolnay fault is compensated rather by parallel fault-bounded pull-apart basins trending northeastward oblique to the principal fault strike. The age of their sedimentary fill suggests no older than middle Pleistocene normal faulting that compensated the Bolnay strike slip.

Seismotectonic deformations of crustal volumes related to geographical coordinates were calculated from data on earthquake focal mechanisms. The crust of the western part of Asia, including the Tien Shan, Tarim massif, Tibet, Pamir-Karakorum, and Kun Lun, undergoes predominantly longitudinal shortening and latitudinal extension. In the eastern part, longitudinal extension and latitudinal shortening are observed. The notional boundary separating these parts is determined over a fairly wide range between longitudes 95º and 103º E and is apparently related to the northward compression from the Indian plate in the south and the westward compression from the Okhotsk and Philippine plates in the east. At the same time, this boundary may indicate the maximum zone of influence of the Indian plate. The boundaries of the Amurian plate are inferred from changes in seismotectonic deformations in the eastern part of Asia. Differences in the seismotectonic deformation of the Earth’s crust are found within the northern part of the Okhotsk plate and the surrounding area.

Data of magnetovariational sounding (MVS) under the IMAGE (International Monitor for Auroral Geomagnetic Effects) project and data of magnetotelluric sounding (MTS) under the BEAR (Baltic Electromagnetic Array Research) project are processed with consideration of the lateral inhomogeneity of the electromagnetic field in order to fully determine the components of the electrical conductivity tensor of Baltic Shield rocks. In the reconstruction of the anisotropy characteristics from the MVS and MTS data, an approximate domain description of the bounded polygon area with an effective electrical conductivity tensor is used. Maps of the lateral distributions of its eigenvalues and the strike azimuth of maximum electrical conductivity are constructed. According to available experimental data, an azimuth angle of 40-60 ° best describes the direction of the principal axis of the tensor with maximum electrical conductivity on the Baltic Shield. Comparison of the results obtained from seismic and electromagnetic data indicates a possible correlation between the strike azimuths of maximum electrical conductivity and low seismic velocities, as well as similarity of the boundaries of the corresponding domain structures of the Baltic Shield.