Home – Home – Jornals – Russian Geology and Geophysics 2020 number 2

We present results of geochemical and isotope (Rb-Sr, Sm-Nd, and Lu-Hf) studies of the early Paleozoic plagiogranitoid associations in the south of the Lake Zone in Western Mongolia, which formed at the island-arc and accretion-collision stages of the regional evolution. According to the petrogeochemical composition, the early Paleozoic plagiogranitoid associations of the island-arc (Tugrug, Hatan-Hunga, Udzur-Hunga, and Bayasgalant plutons, 531-517 Ma) and accretion-collision (Tugrug, Mandalt, and Dut Uul plutons, 504-481 Ma) stages are high- and low-alumina rocks. The recognized types of plagiogranitoids, with regard to their trace-element composition, indicate that their parental melts were generated from MORB-type metabasites at ≥10-12 kbar, in equilibrium with garnet-containing restite, and at ≤8 kbar, in equilibrium with plagioclase-containing restite. The Sr-Nd isotope data on the rocks and the Lu-Hf isotope parameters of their magmatic zircons show two groups of plagiogranitoids, with different sources of melts. The first group includes plagiogranitoid associations of most plutons (Tugrug, Udzur-Hunga, Hatan-Hunga, Bayasgalant, and Dut Uul) with isotope parameters (ε_Nd = 8.5-4.6, (⁸⁷Sr/⁸⁶Sr)₀ = 0.7034-0.7036, and ε_Hf = 14.7-11.9) indicating the juvenile nature of their sources. The second group includes plagiogranitoids of the Mandalt pluton; their isotope parameters (ε_Nd = 1.4-0.2, (⁸⁷Sr/⁸⁶Sr)₀ = 0.7053, and ε_Hf = 7.2-5.4) indicate that the parental melts were generated mostly from enriched-mantle metabasites. The Hf isotope data on inherited and xenogenic zircons (664-519 Ma) from the early Paleozoic plagiogranitoid associations of the southern Lake Zone permit us to separate these rocks into three groups according to their ε_Hf values (14.5-12.8, 2.9, and 10.6-6.7). The Hf isotope parameters of magmatic and inherited zircons, with regard to their age, indicate that the source of the parental melts lacked rocks with a long crustal history, such as the early Precambrian associations of the Dzavhan microcontinent.

The Kyzyl-Tashtyg deposit and the Kyzyl-Tashtyg ore field are included in the Kyzyl-Tashtyg ore cluster, which is located in the southeastern part of the Ulugoi structure-metallogenic zone. The geologic section of the ore field is composed of lower Cambrian terrigenous and volcanic deposits of the Tumat-Taiga and Tapsa Formations that include a homodromous sequence of early basaltic andesites transitioning to felsic dacite-rhyolite rocks with widespread subvolcanic intrusions. The pyrite-polymetallic mineralization of the ore field spatially associates with a central-type volcanic structure and is localized within the «motley» unit consisting of three lithologic/stratigraphic horizons. The Kyzyl-Tashtyg pyrite-polymetallic deposit is the main ore occurrence of the ore field, which contains the main reserves of pyrite-polymetallic ores. The mineralization outcrops are 650 m in length and 60-65 m in width. The reserves of commercial ore are 12,920 mln tons, with the following average contents of ore metals: Pb-2.8%, Zn-10.3%, Cu-0.65%, Au-1.03 ppm, and Ag-48.71 ppm. The ores have typically simple mineral compositions: pyrite, chalcopyrite, sphalerite, galena, and baryte. Secondary minerals include enargite, hessite, sylvanite, proustsite, and native silver. Based on the dominant mineral or groups of minerals, the ores are classified into sulfur-pyrite (pyrite), copper-zinc (pyrite-chalcopyrite-sphalerite), polymetallic (sphalerite-galena), and baryte-polymetallic (baryte-sphalerite-galena) varieties. The main commercial type is copper-zinc mineralization. The widest range of impurity elements and their highest contents have been identified in copper-zinc and polymetallic ores, in which the main mineral is sphalerite, the carrier of the largest amount of impurity elements. In these ores, commercial contents of gold and silver have been detected: Au-0.8 and 2.3 ppm and Ag-26 and 78 ppm, respectively. Formation temperatures of different types of ores of the deposit range from 400-305 to 270-150 °C. The formation temperatures regularly decrease from sulfur-pyrite to copper-zinc and to baryte-polymetallic ores. This temperature pattern forms a vertical mineralization zoning from bottom to top of the deposit. The formation of the deposit was related to early Cambrian volcanism and included different mechanisms of ore deposition. The hill-like shape of the main ore lode with intense hydrothermal alteration of rocks in its base indicates a hydrothermal and hydrothermal-sedimentary genesis on the floor of a sea basin. The occurrence of tube-like orebodies in the root area of the mineralization zone with veinlet-disseminated and massive ore structures indicates a hydrothermal-metasomatic origin.

We studied the mineralogical and geochemical features and formation conditions of productive mineral assemblages of the Tardan gold-sulfide-quartz deposit located in the endo- and exocontact zone of the Kopto-Bai-Syut gabbro-diorite-plagiogranite massif of the Ordovician Early Tannu-Ola complex (O₁tn). Postskarn mineralization of vein-dissemination type in skarns, quartz diorites, and carbonate rocks is limited by tectonic crushing zones and conjugated with beresitization and listwaenitization of the ore-bearing rocks. Mineralogical and geochemical research has shown the formation of ultrahigh-fineness (986-952‰) and high-fineness (947-918‰) gold at the first productive gold-quartz-calcite substage, of high-fineness gold (918-904‰) → medium-fineness gold (896-809‰) → low-fineness gold (798-756‰) ± hessite Ag₂Te ± volynskite AgBiTe₂ at the second productive gold-telluride-sulfide-quartz-carbonate substage, and of medium-fineness gold (897-802‰) → low-fineness gold (799-717‰) → electrum (691-612‰) → mercurian electrum (471-451 ‰) ± hessite Ag₂Te ± acanthite Ag₂S ± matildite AgBiS₂ at the third productive gold-sulfosalt-sulfide-quartz substage. High- and medium-fineness gold prevails in the ores, ultrahigh- and low-fineness gold is subordinate, and electrum and mercurian electrum are scarce. The fineness of native gold in the ores varies from 451 to 986‰, averaging 858‰. The productive mineral assemblages of the Tardan deposit formed from aqueous fluids containing Mg, Na, and K chlorides (salinity is 6.1-12.9 wt.% NaCl eq.), with a decrease in the mineral formation temperature from 380 to 150 ºC and variations in f _O2, f _S2, f _Se2, and f _Te2.

The Lugokanskoe deposit is located in southeastern Transbaikalia and has been studied for a long time by many researchers. However, the type of its formation is still debatable. In this paper we study the mineral composition of ores by modern methods, recognize and describe the main gold mineral assemblages, and present detailed data on the chemical composition of native gold and sulfide minerals and their isotope composition. We have established that gold-pyrite-chalcopyrite-arsenopyrite and gold-bismuth parageneses localized in skarn deposits are the main productive assemblages. Study of the sulfur isotope composition of sulfide minerals has shown an endogenous source of sulfur of the ore minerals. The carbon and oxygen isotope compositions of carbonates of ore-bearing veins indicate the participation of a magmatic fluid. The established age of the gold mineralization and igneous rocks of the Shakhtama complex, together with direct geological observations, points to their spatial, temporal, and genetic relationships. According to their petrochemical and geochemical characteristics, the igneous rocks of the Shakhtama complex are I -type ilmenite (reduced) granitoids. Study of fluid inclusions by heating and cooling and Raman spectroscopy has shown that the mineral formation was accompanied by a gradual decrease in the content of salts in the ore-forming fluids and by a decrease in their homogenization temperatures. Optical observations demonstrate that the fluid was heterogeneous at the early stages of the mineral formation. The evolution of the ore system was accompanied by a change in the gas phase composition of fluid inclusions from predominantly nitrogen-carbon dioxide to essentially aqueous, with carbon dioxide impurity (H₂O + CO₂ ± ± N₂ → H₂O ± CO₂). The research data testify to the magmatic nature of fluids and the participation of meteoric waters at the late stages of the ore-forming process. The data obtained have led to the conclusion that the Lugokan gold deposit is related to reduced intrusions formed at a shallow depth.

The chemical and isotope compositions of nitric and carbon dioxide thermal waters in Jiangxi Province (China) are considered. The nitric thermal waters are ultrafresh (TDS = 0.26-0.42 g/L) and highly alkaline (pH = 8.73-8.87), with excess of SiO₂, F-, Na, etc. but ultralow concentrations of Ca2+, Mg2+, and Cl-. The carbon dioxide thermal waters are more saline (TDS = 0.3-3.9 g/L) but have lower pH values (6.7-7.8). Major anions in both types of waters are HCO₃^- and Na⁺, but SO₄^2-, F^-, CO₂, and H₂S also play a crucial role. The equilibria of the thermal waters with a complex of secondary minerals (carbonate, fluoride, clay, zeolite, etc.) have been calculated. The thermal-water-rock system is shown to be in the equilibrium-nonequilibrium state. During the transfer into deep horizons and back to the surface, the hydrotherms continuously dissolve all minerals that are in nonequilibrium with them (K-feldspar, anorthite, etc.) and form new minerals, which are in equilibrium with these waters (calcite, albite, etc.). The composition of the solution and the type of secondary minerals change with time because of the change in the proportion of chemical elements: Some elements are removed from the solution, while others continue to accumulate. A dynamic equilibrium between the elements entering and leaving the nitric thermal waters is established very early, when the waters are still ultrafresh, which is due to the high pH and low p _CO2. This equilibrium inhibits an increase in the salinity of the nitric hydrotherms, and they remain lowly mineralized. Owing to the higher p _CO2 and, correspondingly, lower pH values, the carbon dioxide thermal waters reach a dynamic equilibrium at a later stage, when their salinity is higher than 3 g/L; therefore, they are more mineralized.

We study the morphology and sedimentary facies of trace fossils formerly identified as Arctichnus found at the base of the Jurassic section near Cape Airkat (northern Siberia). They are most often found in shoreface silty sand and are similar to Rosselia socialis ichnospecies in morphology, taphonomy, and depositional environments. On the basis of this similarity, the Airkat trace fossils should be identified as Rosselia socialis Dahmer, 1937. Analysis of the type collection of Arctichnus arcticus has revealed new morphological features of the taxon.

Spectral characteristics of the medium around 23 digital strong-motion seismic stations of the Kamchatka region have been studied from local earthquake data relative to a reference bedrock station (Petropavlovsk, PET). Spectra are determined by multiband filtering. In each band peak velocity amplitudes, levels of Fourier S-spectra and mean-square coda amplitudes were compared. Average Fourier spectra were obtained from S-wave energy using Parseval’s equation. The difference in hypocentral distances for pairs of stations was compensated by empirical S -wave attenuation functions. Records of more than 300 events were processed, with M = 5-6 and hypocentral distances mainly 100-600 km. The spectral ratios estimated by the three methods show behavior diversity. Some nonrock stations show expected spectral characteristics at high frequencies. The conditions at other stations can be considered similar to those at PET. Some stations show amplifications of up to 10 times in the 20-30 Hz frequency range. In general, the obtained spectral characteristics within 3-5 Hz are consistent with the expected trends corresponding to known local geology around strong-motion stations.

Magnetotelluric sounding (MTS) usually records variations in five components of the electromagnetic field. Interpretation of distorted MT responses of an inhomogeneous earth may be problematic. The problem can be solved by recording additionally the vertical component E_z of the electric field in order to improve the quality of inversion and to gain more information. Currently E_z variations are estimated by measuring voltage on a vertical line immersed in water. The land measurements of this kind require special drilling or using the existing boreholes, which is not always feasible. Otherwise, a circular electric dipole (CED) can be used as a receiver to record the electric field behavior along the vertical component. Its applicability to MT soundings is analyzed in this paper.