Alexander Kravchenko

Determining the metallogenic specialization of igneous rocks in ore districts remains a pressing and often controversial challenge. Addressing this issue requires a comprehensive approach, that integrates petrographic and petrogeochemical data with geochronological studies of individual formations. Such integrated analysis, when contextualized within specific formation conditions and metallogenic assessments, provides a robust framework for interpretation. The Evotinsky massif, located within the eponymous Evotinsky placer ore region in the central Aldan-Stanovoy shield, represents one such critical formation. This study investigates its petrogenesis based on rock samples collected during fieldwork from 2018 to 2020, focusing on the massif’s main phases. Subsequent processing and major element analysis were performed using silicate and multielement techniques at the Department of Physicochemical Analysis Methods of the Diamond and Precious Metals Geology Institute SB RAS (Yakutsk). Trace element concentrations were determined by mass spectrometry at the САС Plasma LLC laboratory (Tomsk). Finally, geochronological U-Pb dating of zircons was conducted using SIMS (SHRIMP-II) at the Center for Isotope Research of the Karpinsky Russian Geological Research Institute (VSEGEI). The Evotinsky massif displays an antidromous structure and comprises two intrusive phases: an earlier phase of amphibole quartz syenites and a later phase of amphibole-pyroxene quartz monzonites. The rocks are classified within the high potassium calc-alkaline series, corresponding to derivatives of the shoshonite-latite series. Geochemical data indicate a mixed mantle and crustal source for the magmas. Petrochemical parameters and trace element ratios (Rb, Y, Yb, Ta) show compositions analogous to volcanic rocks from continental margins and island arcs. U-Pb zircon geochronology (SHRIMP II) constrains the emplacement of the amphibole quartz syenites to 119 ± 1 Ma and the amphibole-pyroxene quartz monzonites to 117 ± 1 Ma (Aptian, Early Cretaceous). In conclusion, the Evotinsky massif formed under back-arc extensional conditions during the final stages of the Mongol-Okhotsk Ocean closure. Furthermore, these findings demonstrate the massif’s significant potential for gold mineralization.

The article considers the features of the petrographic and geochemical compositions of the Mesozoic igneous rocks of the Polovinkinskaya depression from earth bores K1, K2, K3, located within the central part of the Aldan-Stanovoy shield, on the boundary of the Tynda and Nimnyr terranes within the Amginskaya zone of tectonic mélange. Based on the relationship of rocks, as well as on the features of the petrographic composition, the following sequence of introduction and formation is assumed: 1) gabbro; 2) hornblende-augite andesites. This is confirmed by the values and variations of the petrochemical composition. According to the petrochemical and geochemical characteristics, gabbros are close to the tholeiitic series. Andesites belong to the calc-alkaline petrochemical series, latites of the shoshonite type. The presence of such rocks as gabbro and andesites is typical for mature island arcs and active continental margins, which does not contradict the existing ideas of predecessors. Similar in geological structure objects are gold-producing gabbro-tonalite-granodiorite-granite (GTGG) massifs of the Urals, which are supra-subduction formations on the active continental margin of the Andean type. The presence of rocks of the GTGG formation within the Polovinkinskaya depression serves as a favorable factor for noble-metal ore-generating processes.

The Evotinskii Ore and Placer Region (EOR) is located in the central part of the Aldano-Stanovoy Shield and is characterized by a significant presence of gold-bearing placers, many of which have undergone extensive mining activities. Despite geological investigations that have been conducted since the 1930s, the main sources of numerous placers remain largely unidentified. This study employed traditional mineralogical and geochemical methods, along with LA-ICP-MS analysis, to investigate placer gold within the Evotinskii Ore and Placer Region. The findings indicate that native gold within the placers of the EOR is predominantly represented by particles measuring between 0.1 and 1.2 mm, which primarily exhibit forms of cementation, fracture, and interstitial characteristics. The degree of roundness varies, and high-fineness shells and intergranular high-fineness veins are not uncommon. Structural etching revealed a multi-phase, granular, concentric, and vaguely zonal internal structure of the native gold. Furthermore, granulation and twinning structures were also identified. The LA-ICP-MS analysis of the native gold from the placers indicated a diverse array of trace element impurities within the particles. These extensive investigations suggest that the primary sources of placer gold are likely to be ore occurrences similar to the P. Pinigin deposit, as well as sulfide and epithermal occurrences associated with alkaline Mesozoic magmatism, which are prevalent in the Central-Aldan ore region. The possibility of a certain amount of gold originating from mineralizations associated with porphyry systems and iron ore deposits cannot be excluded. The discovery of ore-like particles may signify the proximity of bedrock sources and enhances the potential of the region for the discovery of previously unidentified ore occurrences.

ГЕОДИНАМИКА И РУДОНОСНОСТЬ ПРОТЕРОЗОЙСКИХ МАФИТОВ ЦЕНТРАЛЬНОЙ ЧАСТИ АЛДАНО-СТАНОВОГО ЩИТА (ЮГ СЕВЕРО-АЗИАТСКОГО КРАТОНА)

Описание и интерпретация вещественного состава магматических серий раннего докембрия способствует реконструкции геодинамических обстановок формирования фундамента древних платформ. Центральная часть Алдано-Станового щита Сибирской платформы характеризуется разнообразием магматических комплексов со сложной и длительной историей формирования. В связи с наложенными процессами метаморфизма, изменившими первоначальный облик пород, проблема расчленения и корреляции комплексов, геодинамического районирования на вещественной основе далека от решения. В этой статье описаны вещественные характеристики унгринского магматического комплекса близкого по возрасту образования к палеопротерозойской надсубдукционной обстановке. Рассмотрены его вероятные аналоги среди других магматических комплексов и нерасчленённых ортопород, уточнена площадь распространения. Сделан вывод о внедрении комплекса после аккреционных событий.

Natural chemical systems are an excellent object for studying the properties of various elements. The most diverse and informative geological complexes are crystalline rocks of the Precambrian. These rocks are exposed near the northern and southern margins of the Siberian craton. The chemical composition of rocks, the contents of impurity elements, and metals were studied by us using chemical and spectral analysis methods. Microprobe studies were performed. Using regression and multivariate statistical methods of analysis, the regularities of the distribution of chemical elements were found. It is shown that the distribution of precious metals and carbon dioxide in rocks is attributed to their chemical properties and comparable with close in-chemical properties’ rock-forming elements. It is found that the factor analysis reflects the uniform regularities of the distribution of elements in different regions and rocks. These regularities are similar on macro and micro levels. Comparison of the distribution patterns with the results of geochemical and petrological studies of other authors shows the leading role of the redox potential and acidity of the environment in the formation of rocks and minerals. The role of mathematical statistics for solving problems of chemical petrology and chemical systems analysis is underlined.

Placers of precious metals and other mineral resources are associated with the uplifted basement of the North-Asian craton and metamorphic terrains near its borders. The complex geological structure of these territories does not allow to make obvious conclusions about the sources of the placers. One of the areas of the southern outcrop of the craton basement, the Evota gold-bearing region of the Aldan-Stanovoy shield was studied. Analysis of the morphology of native gold selected from creeks of the region and eluvium from one of the primary deposits show that grains with similar morphological features occur. Probably, not far from the points of sampling, primary sources are located. Possible erosion and sediment removal should occur in positive forms of relief. Therefore, an attempt to determine the location of primary sources by modeling positive forms of relief, where erosion occurs and removal could be manifested, was made. The methodology for modeling these forms using modern geoinformation systems was considered as an attempt to analyze the obtained results. Digitized relief maps (topographic base) of the area and sample points with discovered gold were a basis for the calculations. Further steps were: building a surface of relief as a regular network of highs, calculating directions and slope angle of the surface, building a map of removal vectors, constructing a path with a maximum slope upward from the point with gold particles discover. Lines of positive forms of relief confined to gold discover points were constructed as a result. Field works for identifying signs of gold content in these lines and point sampling of rock debris were carried out. Microprobe analyses of rock debris were performed. Minerals of gold and silver were discovered in the debris. The obtained data shows that this method of analysis can be useful for deciphering ore-bearing structures and searching for primary sources.

The study of the structure of the Precambrian crystalline complexes raised and exposed along the margins of the North Asian craton allows us to characterize the geological features of various stages of the evolution of the lithosphere. Based on these data identification of the promising metallogenic zones and local potentially ore-bearing objects can also be achieved. The goal of the performed research was to create objective models of the areas characterized by the presence of mineral deposits to identify characteristics and promising structures. Using modern geoinformation systems, the maps of the high gradient zones of the local components of geophysical fields have been created. These maps reflect the boundaries of geological complexes with contrasting physical properties. For clarity, the geophysical background was removed from the maps, and only the zones of high gradients of various geophysical fields were left. A combination of maps was performed and it was found that high-gradient zones of the various geophysical fields complement each other along the strike. Based on the obtained geophysical schemes, geological maps and field observations of geological structures, already known and new folds and faults have been deciphered. As a result, models of folds and faults that control the location of mineral resources were created. Models allow finding the common features and evaluating the minerals prospects of areas. In the framing of the Upper-Timpton dome of the Central-Aldan superterrane and in the Shamanikhin fault zone of the Prikolyma terrane, there are many high-gradient zones of the various geophysical fields observed. New ore objects can be expected to be discovered in these areas.

The study of the composition of the Precambrian crystalline complexes raised and exposed along the margins of the North Asian craton allows us characterizing the conditions of formation and clarifying metallogenic typification of the geological structures of various stages of evolution of the lithosphere. Based on these data identification of the promising metallogenic zones and local potentially ore-bearing objects can also be achieved. The goal of the research was to characterize the chemical composition features of geological complexes in the Tyrkanda tectonic zone between the granulite terrains of the Aldan-Stanovoy shield in the south of the North Asian craton. The object is interesting due to the presence of a large number of different types of metamorphic and magmatic rocks. Complex geological structures with numerous stages of intensive ductile and brittle deformations of various kinematics are observed. As a part of the research for the rocks of the zone, the petrographic observation was done and chemical composition was studied. Characteristics and typification of rocks were carried out. Reconstruction of the primary (premetamorphic) types of rocks was performed using the methods of Predovsky and Neelov. A cluster analysis of geochemical data was performed. The excess of background concentrations of chemical elements in clusters was calculated. According to the calculation results, metamorphosed ultramafic and mafic rocks of tholeiitic magmatic series form a single cluster. These rocks are richer with indicator elements for diamond and precious metals. A short description of the probable types of mineralization in these rocks was provided. The most promising for precious metals surrounding areas was determined.

Researched magmatic rocks, which undergone the process of polymetamorphism in granulite, amphibolite and greenschist facies, earlier by some geologist was defined as paragneisses and (or) intrusive rocks. In this paper based on geological and geochemical data argue the magmatic nature of rocks and identity to the calc-alkaline series. The series consists of gabbros, diorites, quartz diorites, granodiorites, tonalites and trondhjemites. Contents of the rock-forming oxides and rare earth elements in these rocks are close to the widely known early Proterozoic gabbro-diorite-tonalite-trondhjemite series of the South-West Finland. These features indicate the probability of formation of the researched rocks with the participation of the subduction. As a results of studies established what in M complex possible inclusion of rocks and bodyes from other sites such as Ust-Timpton, Idzhek-Seym and Upper-Gonam pluton. The features of the MS include the linkage with the long-lived faults and associated dynamometamorphic changes of the bodies, polymetamorphism, superimposed chemical changes in the composition of primary rocks, as well as rarely preserved migmatite veinlets in the host rocks. Geochemical similarity of trondhjemite-, tonalites, granodiorites Mariin series and associated rocks with well-known early Proterozoic differentiated series of southwest Finland typical for the Proterozoic and Paleozoic continental margins suggests that the plutons of the Mariin series formed in an active continental margin setting.