Anatolii Zhuravlev

This study presents an inaugural analysis of rock-forming mineral chemistry within andesites from the Polovinkinskaya Depression, situated at the junction of the Tynda and Nimnyr terranes in the central Aldan- Stanovoy Shield. Utilizing microprobe data processed through MagMin PT, WinPyrox, and COMAGMAT, we evaluated the systematics, thermodynamic crystallization conditions, and potential metallogenic significance of these rocks. The mineral assemblage includes plagioclase (oligoclase and two generations of andesine), groundmass andesine microlites, pyroxenes (augite and diopside), Ca-amphiboles (edenite and pargasite), and Mg-biotite. Our findings indicate that plagioclase crystallization occurred under conditions analogous to those found in island arcs and active continental margins, characterized by high oxygen fugacity (logfO ₂ =-5.04 to -6.86), temperatures ranging from 1090°C to 1124°C, and pressures decreasing from 12 to 4,5 kbar. These data support the hypothesis that subducted oceanic crust from the Mongol-Okhotsk Basin contributed to the Mesozoic Aldan magmatic sources. Furthermore, the identified P-T depth intervals (36–24, 24–12, 12–6, and 6–2 km) suggest the presence of intermediate Mesozoic magma chambers. Finally, an assessment of amphibole and biotite typomorphism, combined with the geological evolution of the andesites, indicates a high potential for epithermal gold mineralization in the region.

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 search for kimberlites in covered territories remains highly relevant today, as there are no longer any easily accessible kimberlite objects within the Yakut diamond-bearing province that are exposed at the surface and clearly visible in geophysical fields. Therefore, planning search operations for diamond-bearing kimberlites requires a novel, integrative approach that combines geological, mineralogical, structural, and other criteria. We found that in the dolerites of the Vilyui-Markhinsky belt—located near kimberlites in the Nakynskoye, Malo-Botuobinskoye, and Syuldyukarskoye kimberlite fields—the content of titanium oxide and several high field strength and rare earth elements (Th, Ta, Hf, Nd, Tb, Gd) is approximately twice as high compared to their typical content in dolerites of the kimberlite fields overall. We propose using this feature of the above-mentioned elements as one of the criteria for kimberlite searching. Considering the presence of dolerites with unusually high concentrations of titanium oxide and several high field strengths (Th, Hf, Ta, Zr, Y) and rare earth (Nd, Tb, Gd) elements within the Charo-Sinsk dike swarm, a comparative study of dolerites from dikes near these bodies was conducted. A gradual decrease in the concentration of these reference elements was observed with increasing distance from the dike exhibiting the maximum element content. We suggest considering such areas as promising targets for kimberlite discovery. Specifically, areas within the CharoLensky interfluve and the right bank of the Lena River opposite the village of Sinsk are classified as prospective. The results obtained in this study and the identified target areas are recommended to geological and mining companies for conducting focused exploration of kimberlites in these specific local areas.

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.

The petrographic and geochemical characteristics of the Mesozoic igneous rocks of the Mikhailovsky massif, including the sills and dikes of the Evotinsky ore district located in the central part of the Nimnyr terrane of the Aldan-Stanovoy shield, were examined. This study revealed for the first time that the Mikhailovsky Massif is composed of quartz monzonites, while the sills and dikes are composed of quartz syenites. These rocks are geochemically close to the shoshonite-latite series of island arc rocks and are enriched in LREE and depleted in HREE, characterized by aweak negative Eu-anomaly. The sequence of magmatism development in the Mesozoic has been established: the formation of rocks of the Mikhailovsky massif and most small bodies occurred in the post-collision period, followed by the intrusion of quartzsyenite dike bodies in theanorogenicstages of territorial development.The study suggeststhat all the studied igneous rocks of the northeastern part of the Evotinsky region are characterized by a duality of geochemical composition,which indicates either intense mantle-crustal interactionor the existence of two sources of both mantle and crustal originat that time.

The Tomtor massif is a polychronous ring zonal complex of alkaline ultramafic and carbonatite rocks containing unique Nb and REE deposits. Mineralogical and geochemical studies of minerals from different types of silicate rocks and carbonatites of the Tomtor massif were performed. For excluding traces of the interaction between silicate and carbonatite melts, we limited ourselves to the study of independent small secant bodies located in the immediate vicinity of the massif itself. The presence of through mineral series in various silicate igneous rocks and carbonatite ores of high-titanium chromium spinels, rare-metal, ore and other exotic phases with similar compositional trends was defined. Such studies will help reveal the mineralogical criteria for the genetic relationship between silicate melts and associated carbonatite derivatives, which can form rich rare elements mineralization. Also, such studies help to improve the petrochemical and mineralogical criteria for dividing potentially diamond-bearing magmatites (typical kimberlites) from non-diamond-bearing kimberlites, alpicrites and other non-diamond-bearing rocks convergent to kimberlites, which are formed under different physicochemical conditions. The existence of polychronous complex ore–magmatic ring complexes, such as the Tomtor massif, indicates the existence of large deep intraplate magma-generating chambers in the lithospheric mantle.

В работе рассматриваются петрографический и петрохимический составы мезозойских магматических пород массива Михайловский и малых тел. Изученные магматические образования пространственно расположены в северо-восточной части Эвотинского рудного района центральной части Нимнырского террейна. Несмотря на то, что в пределах Центрально-Алданского рудного района основная часть золоторудных месторождений связана с многофазовым мезозойским магматизмом (Рябиновое, Самолазовское и др.), на смежной территории Верхне-Амгинского рудного района поля рудоконтролирующими в образовании золотосульфидного оруденения являются группы мезозойских однофазовых массивов сиенитов и монцонитов Лебединского монцонит-сиенит-гранитового комплекса. Нами при полевых исследованиях было установлено, что Михайловский массив имеет однофазовое строение, изучению, вещественного состава которого до последнего времени не уделялось значительного внимания, равно как и малым телам рассматриваемой территории. По результатам проведенных исследований, сделаны основные выводы: по различным классификационным данным породы Михайловского массива сложены кварцевыми монцонитами и близки к породам как высококалиевой известково-щелочной, так и большей мере к шошонитовой петрохимической сериям. Образования силла относятся к кварцевым монцонитам высококалиевой известково-щелочной петрохимической серии. Малые тела даек сложены кварцевыми сиенитами и в большей мере отвечают породам шошонитовой петрохимической серии магматических пород. Отношение к шошонитовой серии пород массива Михайловский, а также тел даек сиенитов предполагает мантийную природу образующих их расплавов. По проведенным исследованиям выдвинута последовательность внедрения магматических пород, от образований массива Михайловский до внедрения тел даек сиенитов. На основе петрохимического анализа, а также по схожести с массивами сиенитов и монцонитов Лебединского монцонит-сиенит-гранитового комплекса Верхне-Амгинского рудного района выдвинуто предположение о потенциальной рудоносности последнего.

The paper presents the results of the petrographic and geochemical studies of igneous rocks of the Medvedev and Taezhniy massifs, including their first absolute dating. The massifs are located in central Nimnyr block of the n shield within the Leglier ore cluster of the Evotinskiy ore district (Southeast Russia, Aldan Shield). For the first time, the three-phase structure of the Medvedev massif has been defined, as observed in our expedition and petrographic studies. Rocks from the three phases of the Medvedev massif include quartz syenites, syenites, and monzonites, and rocks from the two phases of the Taezhniy massif include quartz monzonites and syenites. Geochemically, the rocks are close to volcanic island arcs, the formation of which was related by subducted oceanic crust of the Mongol–Okhotsk Ocean. The defined duality of the geochemical compositions of the igneous rocks of the massifs may be due to the presence of both mantle and crustal sources; however, it is most likely that these rocks resulted from the melting of a mixed mantle source or the latter was contaminated by the crust with further differentiation of melts in intermediate crust chambers. Additionally, geochemical characteristics suggest that the analyzed rocks are close to latite and shoshonite derivatives and can be considered as part of the monzonite–syenite formation type. The first identified periods of formation of igneous rocks in the Medvedev massif are 122.0–118.0 Ma and Taezhniy 117.5–114.5 Ma, which correspond to the Early Cretaceous (Aptian).