The development of a refined conceptual model for the generation of volcanic-related uranium deposits includes studies in the character of magmatic, hydrothermal-metasomatic, and filtration-transport processes, as well as of the physicochemical conditions favoring the transport and deposition of uranium. We consider these issues using the examples of the Streltsovka caldera and the eponymous ore field in eastern Transbaikalia, the Xiangshan volcanic structure in South China, and the McDermitt caldera in the western United States (Oregon and Nevada). According to the IAEA classification (Geological Classification …, 2018), these ore fields and deposits are of the volcanic-related type, while the Streltsovka and the Xiangshan ore field show a combination of the volcanic-related type in the volcanic-sedimentary cover and the granite-related type at the basement. Most industrially viable uranium deposits of the volcanic-related type were formed in the regions listed above during Mesozoic and Cenozoic times (although we know of older, Paleozoic objects of the type). Although the time spans in which ore-bearing volcanic-related edifices were formed are different, many features in the occurrence of magmatic, hydrothermal, and filtration transport processes in these edifices are rather similar. It is commonly supposed that these features are due to a common effect of intraplate tectonic regimes or to the evolution of outer parts in the ocean-continent zones where magmatic activity produced volcanism of the bimodal series in the dominant basites–acid volcanics–basites sequence, while the migration of uranium-transporting fluids was controlled by a joint action of seismogeodynamic and thermal convective processes.

The causes of metasomatic albitization of host rocks and the formation of the accompanying thorium mineralization in uranium deposits of the Streltsovka ore field located in the Streltsovka volcanic caldera are considered. Data provided by the authors and published data allow us to conclude that the process of hydrothermal albitization does not require additional Na input by the magmatogenic fluids. It could have originated from the host rocks due to pre-ore acid leaching; accessory minerals in the same rocks were a source of Th. The increase in the pH value necessary for the deposition of albite under low-T conditions is ensured by a sharp change in the physicochemical conditions due to seismotectonic impulses. The latter were accompanied by opening of the fault and fracture systems, a decrease in pressure (decompression), and degassing. The alkaline character of the fluids that caused albitization is also emphasized by the behavior of trace and rare-earth elements. It is likely that thorium migrated in a sodium alkaline environment probably in the form of mobile tetra- or pentacarbonates: Na4[Th(CO3)4] or Na6[Th(CO3)5].

— This article provides a comparative analysis of data on the petrology of ore-bearing felsic volcanics and low-temperature near-ore metasomatites of the Streltsovka volcanic structure in Eastern Transbaikalia, the Xiangshan structure in Southern China, and the McDermitt structure in the Western United States. The ore-bearing structures are represented by relatively large resurgent (revived) calderas (Streltsovka and McDermitt) and the Xiangshan volcanic dome with several small calderas in its apical part. The leading geodynamic mechanism of the development and functioning of the magmatic ore systems of these volcanic structures is a crustal extension setting expressed by rifting, which took place during the Late Jurassic–Early Cretaceous in Eastern Transbaikalia; the Late Cretaceous and Early Paleocene in Southern China; and the Miocene, in the McDermitt caldera within the Yellowstone hotspot. Magmatic activity produced bimodal volcanic series of basites–felsic volcanics–basites, and the host rocks of uranium mineralization, as a rule, consist of metaluminous or moderately peraluminous high-K effusive and/or subvolcanic rock types corresponding to A2-type “anorogenic granites.” Rhyolites, rhyodacites, trachyrhyolites, extrusive syenites, quartz syenites, rhyolite dikes and domes of all three volcanic structures are enriched in fluorine and display a fairly high degree of fractionation. The leading types of near-ore metasomatic alteration are preore illitization and argillization, which are succeeded by synore albitization, carbonatization, chloritization, and fluoritization, followed by postore argillization. Ore field structures are defined by the presence of delimiting (ring) faults and the proportions of intracaldera fluid conduits; and ore deposit and orebody structure, by the combination of intrastratal steeply dipping and gently dipping fault. It is demonstrated that, despite different time frames and evolutionary profiles of the structure-forming processes, these volcanic structures display numerous similarities in the evolution of magmatic and hydrothermal processes, and this accounts for their definition as “typical” ore-bearing structures in the current IAEA classification of volcanic-related uranium deposits.

The article discusses problems of migration, sorption, and redistribution of uranium in felsic volcanic rocks (ignimbrites) and volcanic glasses of different compositions from the Tulukuev and Novogodnee deposits, located at the upper structural level (the cover of volcano-sedimentary rocks) of the Streltsovka caldera, which hosts Russia’s largest Streltsovka uranium ore field (SOF). The research covers the entire sorption series of rocks and minerals: from abnormally high uranium contents in felsic volcanic rocks and volcanic glasses of the Novogodnee deposit, located in a reducing geochemical environment, to complete uranium removal from mineral concentrators in an oxidizing environment in the Tulukuevsky open pit deposit. The uranium distribution and variations in its content were studied using f-radiography in different zones of metasomatic aureoles, minerals, rock fragments, the matrix and fiamme of ignimbrites, elements of deformational alterations, including mineralized and open fractures of different morphology, as well as in cataclasis, microbrecciation, and veinlet zones, etc. Integrated geological-structural, mineralogical-geochemical, and petrophysical studies and hydrogeochemical and isotope-geochemical monitoring studies of fracture-vein and atmospheric waters have been conducted since 2000 and continue at present. It is shown that the Tulukuev and Novogodnee deposits are unique objects, which can be used for studying the conditions, migration paths, migration mechanisms, and accumulation of uranium in different structural settings under varying redox conditions. It was established that the most important mechanism of uranium retardation is sorption processes on permeable reaction barriers under reducing conditions, formed currently within hydraulically active faults, crosscutting blocks of oxidized rocks. At these natural physicochemical barriers, U(VI) is effectively retained and transformed into insoluble U(IV) form due to the reactivity of Fe–Mn oxyhydroxides, impregnated carbonaceous matter, and vital activity products of microorganisms (ferrihydrides). Comparing the sorption capacity of minerals with respect to uranium allowed us to develop a comparative series of minerals and mineral aggregates in descending order from amorphous Fe and Ti oxides to feldspar and quartz. The above studies can be used when substantiating the search, exploration, and mining of uranium ores at uranium-ore deposits and when considering possible sources of ore matter. The radiogeoecological aspect of surveys involved with substantiating the long-term isolation of radioactive materials and remediation of radionuclide-polluted areas and groundwater horizons is also crucial.

The Valunistoe Au–Ag deposit is the third largest among epithermal deposits in Chukotka after the Kupol and Dvoinoe. It is located at the western closing of the East Chukotka flank zone of the Okhotsk-Chukotka volcanic belt. Volcanic domes (Pravogornenskaya, Zhil’ninskaya, Shakhskaya, Valunistaya, Shalaya, and Oranzhevaya, each is 3–6 km in diameter) have the main ore-controlling significance in the area; they form a chain elongated to the northeast, along the Kanchalan fault zone. Near the deposit, Upper Cretaceous volcanics are widespread: ignimbrites, lavas and tuffs ranging from rhyolite to basaltic composition, and lenses and interbeds of sedimentary rocks, subvolcanic bodies and dikes of andesites, basalts, and dacites. The structure of the deposit is caused by its localization within the limits of the eponymous (Valunistaya) volcanic dome. Twelve ore-bearing vein zones with thicknesses ranging from several to several tens of meters have been revealed at the deposit. The Glavnaya (Main) and Novaya (New) vein zones have been studied in detail; they are traced along their strikes to a distance of more than 1500 m and consist of en echelon veins 1.0 m thick on average, with lengths varying from 100 to 400 m. Based on the sampling data, Au and Ag contents in ores are 0–474.3 and 0–3794.23 g/t, respectively. Colloform-banded structures are frequently encountered, often combined with breccia structures. The main vein minerals are quartz and adularia; calcite, chlorite, fluorite, sericite, pyrophyllite, kaolinite, montmorillonite, gypsum, and epidote are less frequent. The main ore minerals are pyrite, acanthite, chalcopyrite, galena, sphalerite; secondary ore minerals are native Au and Ag and polybasite; rare ore minerals are pearceite, magnetite, hematite, marcasite, freibergite, tetrahedrite, bournonite, hessite, matildite, and others. Ores are characterized by an Au/Ag ratio from 1 : 5 to 1 : 10 and sulfidity (0.5–5%). Ores are enriched in many elements (Au, Ag, Sb, Cd, Pb, Cu, Zn, As, Se, Mo, Te, and Cr), with enrichment factors ranging from several times (Se, Mo, Te, and Cr), to tenfold (Cd, Pb, Cu, and Zn) and hundredfold (Sb) levels, reaching an excess of tens and hundreds thousand times for Au and Ag (Fig. 7). Ores are characterized by a low total REE and demonstrate positive Eu anomalies. Geochemical features are consistent with the mineral composition of ores. Full homogenization of fluid inclusions in quartz occurs at temperatures of 203–284°C and 174–237°C in calcite, while the salt concentration in both cases is from 0.2 to 0.7 wt % NaCl equiv. Fluid density changes from 0.87 to 0.56 g/cm 3 . The results give grounds to attribute the Valunistoe deposit to the low-sulfidized epithermal class. The data provided in the article are of practical value for regional forecast–metallogenic maps and can be used in searching for and appraising epithermal Au–Ag deposits.

The characteristic features of the manifestations of acid magmatism and metasomatic hydrothermal alteration processes in Southeastern Transbaikalia, an important mining region in Russia, are discussed. The zoned or beltlike distribution of the ores of various metals, uranium in particular, and other mineral deposits is due to the general evolutionary trends of the geodynamic regimes of the respective blocks of the territory and the subsequent magmatic differentiation trends. The highly differentiated and fluorine-rich granitoids of Southeastern Transbaikalia are located only in the zones of the consolidated crust, which are characterized by low gravity and negative quiet magnetic field values. In other zones, i.e., volcanic belts, volcanogenic troughs, and the regions dominated by the intrusive massifs of the Shakhtama complex, rare metal manifestations are absent. It was demonstrated that there is no correlation between the localization of large uranium deposits and leucocratic rare metal granite domains and that these deposits are concentrated in large volcanic calderas or volcanic troughs, controlled by long-lived fault systems. In addition to uranium, these volcanogenic structures control base metal, molybdenum, gold, fluorite, and some other mineral deposits. The results of a comparative analysis of the geochemical features of the acid volcanics that host the uranium mineralization and are classified as the last differentiation products of the contrasting latite volcanism, on the one hand, and the subvolcanic and hypabyssal derivatives of the rare metal granites, on the other, are presented. The analysis revealed the signatures of their fundamental difference from each other and that they belong to different branches of magmatic differentiation. A comparative analysis of the metasomatic aureoles of the altered wallrocks in the Late Mesozoic volcanics and the Shakhtama granitoid domains, on the one hand, and the leucocratic rare metal granite domain, on the other, is given. The data presented attest to the existence of a mixed geodynamic regime within the study area of the Central Asian mobile belt during the Jurassic and the earliest Cretaceous time: residual collisions with strong deformations in a compressional setting, accompanied by the development of late acid differentiation products of the magmatic latite series, and the incipient rifting associated with the intrusion of the leucogranites of the Kukul’bei complex in a more quiet extensional setting.

The evolution of the geological structure in the Southern Argun’ Region is studied in terms of changing geodynamic conditions of the Proterozoic, Caledonian, and Variscan Tectonomagmatic Cycles, which also under Mesozoic tectonomagmatic activation led to the formation of latite igneous rocks enriched in Au, Cu–Mo, Pb–Zn–Ag, volcanic and plutonic complexes of the caldera structures with Mo–U, Pb–Zn, and fluorite ores, and rare-metal granites with a Sn–W–Li–Ta spectrum.

The unique combination of several exogenic processes augmenting uranium mineralization followed one another in time at deposits of the Khiagda ore field and gave rise to the formation of uranium resources exceptional for the paleovalley geologic and economic type. The specific geological evolution, volcanic activity, and regional climatic conditions taken together became the main cause of local occurrence of these deposits.

The development of vertical zoning of wall-rock metasomatic alteration is considered with the Mesozoic Strel’tsovka caldera as an example. This caldera hosts Russia’s largest uranium ore field. Metasomatic rocks with the participation of various phyllosilicates, carbonates, albite, and zeolites are widespread in the ore field. In the eastern block of the caldera, where the main uranium reserves are accommodated, hydromica metasomatic alteration gives way to beresitization with depth. Argillic alteration, which is typical of the western block, is replaced with hydromica and beresite alteration only at a significant depth. Postore argillic alteration is superposed on beresitized rocks in the lower part of the section. Two styles of vertical metasomatic zoning are caused by different modes of deformation in the western and eastern parts of the caldera. Variations of the most important petrophysical properties of host rocks—density, apparent porosity, velocities of P- and S-waves, dynamic Young’s modulus, and Poisson coefficient—have been determined by sonic testing of samples taken from different depths. It is suggested that downward migration of the brittle-ductile transition zone could have been a factor controlling facies diversity of metasomatic rocks. Such a migration was caused by a new phase of tectonothermal impact accompanied by an increase in the strain rate or by emplacement of a new portion of heated fluid. Transient subsidence of the brittle-ductile boundary increases the depth of the hydrodynamically open zone related to the Earth’s surface and accelerates percolation of cold meteoric water to a greater depth. As a result, the temperature of the hydrothermal solution falls down, increasing the vertical extent of argillic alteration. High-grade uranium mineralization is also localized more deeply than elsewhere.

Mineral transformation of host rocks and localization of orebodies at the unconformity-type uranium deposits are considered for the Karku deposit in the northern Ladoga region. It is shown that the great depth of uranium mineral formation and the peculiar composition of host rocks, along with temperature and chemistry of fluids, played a critical role in variation of lithostatic and fluid pressure, porosity, and permeability. The compaction of quartz sandstone and gravelstone, which are typical host rocks at unconformity-type deposits, the development of microstylolithic sutures, conformal structures, pressure solution and deposition of quartz in free pores gave rise to the closure or constraint of pore space and to increase in pore pressure of fluids in the deep part of the Riphean troughs with approaching lithostatic loading. A transitional zone between hydrostatic and lithostatic pressure controlled localization of orebodies and was decisive for uranium mineral formation. This zone coincided with the Riphean-Paleoproterozoic unconformity and sank somewhat into the crystalline basement. Below this transitional zone, the intergranular fluid was under a pressure that was close to the pressure on solid phases, i.e., P tot ≈ P fl. The reliability of this phenomenon is confirmed by cessation of pressure solution-redeposition of quartz and distinct deceleration of dehydration of hydrous minerals. As is shown for the Karku deposit, the highly hydrated clay minerals of the illite-smectite series are widespread in its subore portion and lacking at the supraore levels along with termination of quartz regeneration. It is suggested that a zone of superhigh fluid pressure in deep parts of sedimentary basins constrains localization of uranium orebodies by structural and stratigraphic unconformity between Riphean and Paleoproterozoic rocks. It is stated that altered wall rocks at the unconformity-type uranium deposits cannot be identified with products of hydrothermal phyllic and argillic alteration of host rocks at the medium- and low-temperature endogenic deposits. The main distinctions consist in lack of wall-rock metasomatic reaction zoning and acid-alkaline evolution of solutions. All transformations of host rocks should be classified as diverse manifestations of deep catagenesis of sedimentary sequences and buried regolith. The carbon and oxygen isotopic compositions of calcite from host rocks at the Karku deposit are far from those of magmatic and hydrothermal carbonates. They are characterized by a high δ18O = +17 to +25‰ and a high dispersion of δ13C = −1.5 to almost −15‰. No granitoid magmatism is known in the regions, where the unconformity-type uranium deposits occur. Therefore, the rocks of the crystalline basement are the most probable source of uranium, which precipitated on the reductive barrier as a product of interaction with bitumen contained in the Riphean basal beds.

Бугдаинское золотоносное W-Mo-месторождение (Восточное Забайкалье) приурочено к центральной части вулкано-купольной структуры в пределах крупного плутона варисских гранитоидов. По своим характеристикам оно соответствует месторождениям типа Клаймакс или риолитовому субклассу Mo-порфировых месторождений. Его повышенная золотоносность обусловлена относительно широким распространением жильного и жильно-прожилкового золото-полиметаллического оруденения. В штокверковых и жильных рудах сейчас диагностировано более 70 минералов, включая самородные элементы, сульфиды, сульфосоли, теллуриды, оксиды, молибдаты, вольфраматы, карбонаты и сульфаты, в том числе впервые установленные здесь джалиндит, гринокит, молибденсодержащий штольцит, амальгамы серебра и золота, штромейерит, сервеллеит, берриит и др. Выделено четыре стадии минералообразующего процесса. Наиболее ранняя дорудная стадия проявилась после внедрения субвулканического штока риолит(гранит)-порфиров в виде калишпатизации и интенсивного окварцевания. Вслед за ними сформировалась штокверковая и жильно-прожилковая W-Mo-минерализация кварц-молибденитовой стадии. Серицитизация, пиритизация и последующее образование кварц-сульфидных жил и прожилков с самородным золотом, сульфидами полиметаллов и разнообразными Ag-Cu-Pb-Bi-Sb-сульфосолями золото-полиметаллической стадии происходили после изменения регионального плана тектонических деформаций. Завершался гидротермальный процесс отложением аргиллизитовой (каолинит-смектитовой) ассоциации послерудной стадии.

The Bugdaya Au-bearing W-Mo porphyry deposit, Eastern Transbaikal Region, Russia, is located in the central part of volcanic dome and hosted in the large Variscan granitic pluton. In its characteristics, this is a Climax-type deposit, or an Mo porphyry deposit of rhyolitic subclass. The enrichment in gold is related to the relatively widespread vein and veinlet gold-base-metal mineralization. More than 70 minerals (native metals, sulfides, sulfosalts, tellurides, oxides, molybdates, wolframates, carbonates, and sulfates) have been identified in stockwork and vein ores, including dzhalindite, greenockite, Mo-bearing stolzite, Ag and Au amalgams, stromeyerite, cervelleite, and berryite identified here for the first time. Four stages of mineral formation are recognized. The earliest preore stage in form of potassic alteration and intense silicification developed after emplacement of subvolcanic rhyolite (granite) porphyry stock. The stockwork and vein W-Mo mineralization of the quartz-molybdenite stage was the next. Sericite alteration, pyritization, and the subsequent quartz-sulfide veins and veinlets with native gold, base-metal sulfides, and various Ag-Cu-Pb-Bi-Sb sulfosalts of the gold-base-metal stage were formed after the rearrangement of regional pattern of tectonic deformation. The hydrothermal process was completed by argillic (kaolinite-smectite) assemblage of the postore stage. The fluid inclusion study (microthermometry and Raman spectroscopy) allowed us to establish that the stockwork W-Mo mineralization was formed at 550–380°C from both the highly concentrated Mg-Na chloride solution (brine) and the low-density gas with significant N2 and H2S contents. The Pb-Zn vein ore of the gold-base-metal stage enriched in Au, Ag, Bi, and other rare metals was deposited at 360–140°C from a homogeneous Na-K chloride (hydrocarbonate, sulfate) hydrothermal solution of medium salinity.

Alteration of rocks and localization of uranium mineralization in the northwestern Strel'tsovka caldera are exemplified in the Dal'nee deposit. In the main parameters of hydrothermal mineralization (temperature, pH, pressure, and composition of solution), the Dal'nee deposit differs from the deposits of the Strel'tsovka ore field located in the central part of the caldera. The localization of high-grade stratiform ore-bodies are interpreted in light of kinematic relations between steeply and gently dipping faults that formed in the tectonic setting of the NE-SW-trending, long-living, right-lateral, strike-slip faulting. The wide halos of argillic alteration and the structural control of uranium mineralization are caused by the fact that the deposit is located at the margin of the geological block, which has developed since the Late Triassic in a regime of extension (pull-apart) to form a depression, which is arranged en echelon relative to the main caldera and comparable to it in area. Currently, this depression is overlapped by sediments of the Sukhoi Urulyungui Basin. Such a structure markedly increases the probability of finding hidden uranium ores associated with low-temperature argillic alteration in the volcanosedimantary rocks and granitoid basement of the northwest-ern Strel'tsovka caldera.