We consider the mineralogical and geochemical features of the rocks of the Inagli dunite-clinopyroxenite-shonkinite massif with platinum-chromite and unique jewelry Cr-diopside mineralization, which is a reference object of concentric zonal complexes. The massif rocks, from dunites to pulaskites, including peridotites, clinopyroxenites, shonkinites, and melanocratic alkali syenites, form a single continuous comagmatic series. This is confirmed by a clear dependence of the compositions of olivine, pyroxene, phlogopites, and Cr-spinels on the MgO content of the rocks and on the behavior of trace elements in them. The similar compositions of pyroxenes and trace-element patterns of clinopyroxenite rocks and Cr-diopsidite veins indicate a genetic similarity of these rocks. The age and mineralogical and geochemical compositions of the rocks and the geologic and morphological features of the intrusion prove that the Inagli massif formed from high-K picritoid melts, which underwent gradual decompression solidification during the ascent and formed a cylindrical diapir-like body at the subsurface level in the Early Cretaceous. The new portions of differentiates supplied from the lower horizons of the magma column determined the complex composition of the massif: It has a concentric zonal structure cut by numerous radial-circular vein bodies of pegmatites and pure anchimonomineral rocks (Cr-diopsidites), in places, of jewelry quality. (C) 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

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).

The article discusses petrography and features of the chemical composition of the Mesozoic alkaline intrusions of the Khokhoi ore field located within the Upper Amga gold-mining district in the Aldan Shield. This gold-mining district is located in the Aldan-Stanovoy Province rich in gold. Tectonic position and magmatic events of this Province are close to those positions and events of Shandong Gold Province of Sino-Korean Craton. The investigation of the genesis of the magmatic rocks and comparison of the Provinces can shed light on the genesis and regularity of location of the gold deposits in them. Intrusions of the Upper Amga gold-mining district passed through the metamorphic rocks of the Precambrian and intruded into the Cambrian carbonate rocks and the Jurassic sandstones. Petrographic observations found that large intrusive massifs of the ore field consist mainly of potassic feldspar and plagioclase. Mineral composition, structure and texture of the rocks correspond to syenite and monzonite. Blocks of melanocratic rocks – presumably tephrites are rare on the surface. Several generations of feldspar and dark-colored minerals are observed within all these rocks, indicating melt fractionation. High Sr and K contents were identified in the rocks. Contents decrease in the series of tephrite-syenite-monzonite. This feature, judging by other petrochemical data, can be associated not only with magmatic differentiation, but also with the melting and interaction of the lithosphere. In sienites, ovoids of quartz and the first-generation potassic feldspar melting margin were found. It could be possibly resulted from the capture of surrounding rock material and changes in the thermodynamic conditions of crystallization. It is found out that potassium and silicon contents in rocks depend inversely on calcium content. Interaction of melts and the host carbonate rocks with removal of silicon and potassium and formation of gold-bearing sericite-quartz and feldspar-quartz metasomatites is assumed. The loss of potassium probably has led to potassium metasomatism at the contacts of intrusions. This metasomatism produced the gold ores.

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 article considers features of geochemical composition of the Mesozoic igneous rocks of the Khokhoi ore field, spatially located in the Upper-Amga gold-mining district of the north-western part of the Aldan-Stanovoy shield, at the junction of two large structural units - the Olekma granite-greenstone region and the Aldan granulite-gneiss area, in the zone of meridional deep Amga fault. Magmatic formations are represented by the Northern Boskho, Western Boskho and Upper Khokhoi massifs of the Lebedinsky monzonite-syenite-granite complex, intruding rocks of the crystalline basement and terrigenous-carbonate complex of the Siberian platform cover, and by rare tephrite dikes. Due to the accessibility of the study area, little attention was focused on the mentioned igneous rocks until recently. The study showed that magmatic formations of the ore field are geochemically specialized in lithophile and siderophile elements, and they belong to derivatives of trachy-andesite and andesi-basalt magma. All studied rocks have sufficiently close spectra of rare-earth element distribution, in particular, the rocks are poor in heavy rare-earth elements relative to light rare-earth elements. Presence of low negative Eu-anomaly for the Northern Boskho and Upper Boskho massifs may indicate the fact that they were formed from residual differentiates during fraction crystallization of rock-forming minerals with the involvement of plagioclases. Low Eu/Eu* values also indicate the fact that these rocks were formed during the late stage of the area evolution. Negative niobium anomaly, identified for all igneous rocks of the ore field, indicates magma contamination with continental crust. Considering interpretation of rare-earth elements and their ratio, it was supposed that igneous rocks of the ore field were formed in intracontinental conditions. It was generally concluded that there was an intensive mantle-crust interaction throughout the magmatism evolution of the Khokhoi ore field, beginning from intrusion of tephrite dikes bodies to intrusion of syenite rocks. The presence of long-lived mantle source, as well as contamination with crust material, was responsible for heat and fluid inflow that supplied circulation of thermal spring in the system. Heat and fluids caused additional mobilization of ore material from surrounding rocks, with further formation of gold-sulfide mineralization close to tephrite dikes and the Northern Boskho massif.

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.