During cruise 41 of the R/V Professor Logachev in 2019, two new ore fields were discovered within the Russian exploration area of the Mid-Atlantic Ridge. The Korallovoe ore field is located in the middle part of the rift valley ridge and lies on a gentle slope in the depth range of 2800–2850 m. The Molodezhnoe ore field is located approximately 5 km northeast of the Korallovoe field and lies on a less steeper section of the slope at depths of 3500–3550 m. The Korallovoe and Molodezhnoe fields are confined to an outcrop of a gabbro–peridotite massif on the western side of the Mid-Atlantic Rift valley. The total number of discovered ore fields in the Russian exploration area has increased to 14.

During cruise 41 of the R/V Professor Logachev in 2019, two new ore fields were discovered within the Russian exploration area of the Mid-Atlantic Ridge. The Korallovoe ore field is located in the middle part of the rift valley ridge and lies on a gentle slope in the depth range of 2800-2850 m. The Molodezhnoe ore field is located approximately 5 km northeast of the Korallovoe field and lies on a less steeper section of the slope at depths of 3500-3550 m. The Korallovoe and Molodezhnoe fields are confined to an outcrop of a gabbro-peridotite massif on the western side of the Mid-Atlantic Rift valley. The total number of discovered ore fields in the Russian exploration area has increased to 14.

The work is devoted to the study of ore minerals from the surface horizon of ore-bearing sediments in the Pobeda hydrothermal cluster using the following methods: optical microscopy, scanning electron microscopy, and X-ray spectral microanalysis. It was found that ore minerals are represented by fragments of Cu–Fe sulfides (isocubanite, chalcopyrite, and pyrite), newly formed iron hydroxides, and atacamite. In addition, barite and edaphogenic material as talcified silicate clasts, sometimes with sulfide inclusions, are present. Structural-morphological types of iron hydroxides are distinguished. Based on the hydrophysical data, the location of assumed active hydrothermal vent in the Pobeda-3 ore occurrence area was updated. Distribution of the studied minerals depending on the location relative to active hydrothermal vents is described. Decrease in the size and amount of hydrothermal mineral clasts and edaphogenic material, as well as increase in the degree of sulfide replacement by iron hydroxides, were observed when moving away from the sources. Moreover, decrease in the Cu/Fe ratio in the chemical composition of Cu–Fe sulfides is also noted. An unidentified phase of Cu3.57‒4.22Fe1.71‒2.19S4.99‒5.31 with the chalcopyrite lamellas was established in the surface horizon of the column at station 37L245g.

Research subject. Carbonate formations raised from depths up 1986 to 2973 m in the off-axis zone of the rift valley of the North Atlantic Ocean in areas of active young volcanism. The ocean floor here is composed of basaltoids and serpentinized gabbro-peridotites fragmentarily overlain by carbonate pelagic sediments.Aim. To confirm the organogenic nature of these carbonate formations and to reveal new features of deep-water carbonate structures of this type.Materials and methods. The research objects comprised 100 samples of branched and cone-shaped/crater-like carbonate formations, the primary studies of which were carried out directly on the research vessel. Analytical methods included optical microscopy, electron microscopy, X-ray fluorescence spectroscopy, X-ray diffractometry, infrared spectroscopy, inductively coupled plasma mass spectrometry, and isotope mass spectrometry.Results. Among the most important features of the studied formations were found to be a concentric-zonal structure, which forms around the axial channel, and a thin dark brown crust of carbonate-ferromanganese composition. The abundance of planktonic fauna fossils and the distribution of mineralized biofilms with bacteriomorphic structures and glycocalyx were found in the body of crusts of the studied formations. More than 50 trace elements were found, including 11 essential (vital), 18 physiogenicallly-active and 22 antibiotic elements. The ratios of group contents of essential and antibiotic elements vary from 0.67 in the upper part of the structures to 0.001 in their lower part and up to 0.0006 in the volcanogenic substrate of the carbonate buildups. The ratio of the concentrations of essential zinc to physiogenically-active copper behaves similarly. In calcite, the isotopic composition of carbon, δ13СPDB = = –0.16 ± 1.03‰, corresponds to marine sedimentary carbonates; conversely, while oxygen exhibits anomalously isotopically heavy values, δ18OSMOW = 34.44 ± 3.21‰. In ferromanganese carbonates, the corresponding values are –3…1 and 32– 35‰.Conclusions. The studied carbonate formations are solid solutions based on calcite in their body and based on siderite-rhodochrosite binary series in the composition of brown crusts. Specific features of the chemism and minal compatibility of carbonate solid solutions reflect the conditions of microbially-stimulated mineral formation. The conducted isotopic studies discovered the phenomenon of a combination of carbon and oxygen, fundamentally different in genetic nature, in the studied formations. For the explanation of this fact, a scheme for isotopic exchange of oxygen between marine bicarbonate and sulfate with the active participation of sulfate-reducing bacteria was proposed.

The paper presents the mineral and chemical compositions of carbonate, metalliferous, and ore-bearing sediments developed within the Pobeda ore cluster based on the materials of stations 37L244g, 37L245g, and 37L184k by the chemical, optical, electron microscopic, electron probe, and X-ray methods. The compositions are compared with those of background biogenic carbonate sediments developed beyond the hydrothermal cluster. The results revealed that the hydrothermal activity zone is marked by various types of mineral and geochemical zoning: (1) increase in the ore element concentration downward the sediment section leading to a downsection change of carbonate sediments by the ore-bearing variety (St. 37L184k); (2) layer-by-layer enrichment of sediments with ore components (St. 37L244g); and (3) confinement of concentration zones to the roof, middle part, and bottom of sedimentary deposits (St. 37L245g). The relationship of various types of mineral and geochemical zoning with the physical and chemical parameters of interstitial waters in the metalliferous and ore-bearing sediments is considered.

The results of mineralogical and geochemical studies of deepsea carbonate buildups dredged from areas of modern volcanism in the rift valley of the northern Atlantic Ocean were obtained for the first time. Morphology, anatomy, chemical and microelement compositions of buildups are characterized. Mineralized biofilms with bacteriomorphic structures were revealed. Using the methods of X-ray diffraction and RSMZ analysis, the main carbonate minerals of the buildups were diagnosed and studied, and the phenomenon of the anomalous nature of the isotopic composition of carbon and oxygen in them was revealed. It has been suggested that the cause of the abnormally heavy oxygen isotopic composition in carbonate buildups is the borrowing of oxygen by carbonate-forming bacteria directly from sulfates, often distinguished by isotopeheavy oxygen.

The study of samples of volcanic rocks, sulfide ores, and near-ore metasomatites from the bottom of the Atlantic Ocean resulted in the discovery of vital forms of organisms: foraminiferal shells, mineral pseudomorphs after the bodies of polychaetes, deposit feeders, and other fauna. This allows us to understand not only modern geological processes in the depths of the oceans, but also to reconstruct events of the distant geological past based on the principle of actualism.

Based on materials obtained in Cruises 33 and 34 of the R/V Professor Logachev, the paper addresses formation conditions, morphology, structures, mineral composition of the present-day oceanic sulfide ores, and their relationships with the host (biogenic carbonate) bottom sediments in the 19°–20° N MAR (Zenith-Victoria and Petersburg hydrothermal fields) region. The grain size distribution, mineral composition of the carbonate (background) and ore-hosting sediments, as well as physicochemical parameters of their interstitial waters, are examined. The results suggest a significant role of hydrothermal-metasomatic processes in the formation of ores and ore-bearing sediments. A model is proposed for the formation of sulfide mineralization in oceanic sediments at the geochemical barrier in the zone of their interaction with the acid hydrothermal (diffuse-type) ore-bearing solutions delivered from rocks of the ocean floor.

The optimal set of prospecting methods, including geophysical (geoelectrical), geological (mineralogical-geochemical), and hydrological observations, was developed during the long-term investigations of the sulfide mineralization in the northern equatorial zone of the Mid-Atlantic Ridge. The application of these methods made it possible to discover six massive sulfide deposits and considerably extend the boundaries of another two ore objects. The ores associated with ultramafic rocks are characterized by elevated Cu, Au, and Co concentrations. It is established that the ore formation was a multistage process that resulted in the accumulation of large deposits (over 10 million tons).

На оригинальном материале восьми колонок, полученных в ходе 32-й экспедиции на НИС “Профессор Логачев” в 2009 г., выполнены литолого-фациальные, биостратиграфические, минералогические и геохимические исследования осадков, расположенных в пределах Северо-Западного (активного), и Восточного (не активного) гидротермальных полей рудного узла “Семенов”. Выделяются минеральные типы осадков, и предложена принципиальная схема вертикального строения гидротермально-осадочного разреза, перекрывающего массивные сульфидные руды. Установлено, что рудоносные осадки характеризуются вертикальной зональностью из последовательно сменяющих друг друга минеральных ассоциаций, которые в свою очередь контролируются активностью кислорода. Рассмотрены механизмы формирования атакамита – CuCl2 · 3Cu(OH)2, широко развитого в оксидно-железистых красноцветных апосульфидных отложениях (sulfide gossans).

New material from eight columns recovered during Cruise 32 of the R/V Professor Logachev in 2009 was used to explore the lithological facies, biostratigraphy, mineralogy, and geochemistry of sediments from the northwestern (active) and eastern (inactive) hydrothermal vent fields of the Semenov cluster. Mineral types of sediments were distinguished, and a general scheme was proposed for the vertical structure of the hydrothermal-sedimentary sequence overlying massive sulfide ores. It was found that the ore-bearing sediments exhibit a vertical zoning in the distribution of mineral assemblages, which are controlled by oxygen activity. The mechanisms of the formation of atacamite, CuCl2 · 3Cu(OH)2, which is a widespread mineral in red iron-oxide bodies replacing sulfides (gossans), were evaluated.

Внутренниеокеанические комплексы и высокоамплитудные сдвиги растяжения, характерные для медленно-спредингового Срединно-Атлантического хребта, имеют принципиальное значение для структурного контроля крупных гидротермальных систем, в том числе формирующих придонное сульфидно-полиметаллическое рудоотложение. Рассмотрены структурно-геологические, петрографические и минералогические данные по внутреннему океаническому комплексу (ВОК), вмещающему серию недавно открытых неактивных гидротермальных сульфидных полей на 13°31 с.ш. Срединно-Атлантического хребта: Семенов-1, -2, -3, -4 и -5. Внутренний океанический комплекс представлен серпентинизированными и оталькованными перидотитами и редкими габброидами, однако все гидротермальные поля обнаруживают вещественные признаки базальтового субстрата. Наложенные на внутренний океанический комплекс вулканические структуры выделяются по ряду выходов пиллоу-лав со свежими закалочными стеклами. Долериты рассматриваются как подводящие каналы этих структур, вероятно – разрозненные дайковые рои. Наложенные вулканические структуры преимущественно развиты в субширотной крутопадающей тектонической зоне на линии гидротермальных сульфидных полей Семенов-1, -2, -5 и -3. Гидротермально-метасоматические преобразования пород можно отнести к нескольким принципиально разным проявлениям. Широко распространенные по массиву ВОК апосерпентинитовые талькиты с пирротин-пиритовой минерализацией, а также находка апобазальтовых тальк-хлоритовых метасоматитов с пиритовой минерализацией могут быть интерпретированы как продукты гидротермальной активности в проницаемой зоне высокоамплитудного сдвига. Хлоритизация и брекчирование базальтов с наложением кварцевой (или опаловой), баритовой и пиритовой (или халькопиритовой) минерализаций связано непосредственно с придонным сульфидоотложением. Для самородно-медной минерализации в практически неизмененных базальтах в поле Семенов-4 предполагается отложение из рудообразующих флюидов перед их поступлением в зону придонного сульфидоотложения. Высокотемпературные апобазитовые амфиболиты с плагиогранитными прожилками интерпретируются как тектонические фрагменты наиболее высокотемпературных частей гидротермальных систем, где могло происходить частичное плавление базитового материала в присутствии водного флюида с образованием плагиогранитных расплавов. Кремнекислые породы (плагиограниты, диориты и тоналиты), обнаруженные в тектонической зоне контроля придонных гидротермальных полей Семенов-1, -2, -5 и -3, относятся как к плутоническим, так и субвулканическим телам и рассматриваются как вероятные продукты частичного плавления базитового материала в глубинных частях гидротермальных систем. Структурное положение гидротермальных полей различно. Гигантское поле Семенов-4 приурочено к области выклинивания базальтов висячего бока и выходу зоны высокоамплитудного сдвига к поверхности дна. Серия сравнительно мелких полей (Семенов-1, -2, -3 и -5) развита на массиве внутреннего океанического комплекса, в наложенных вулканических структурах, в пределах субширотной крутопадающей тектонической зоны. Структурный контроль гидротермальных систем при формировании гидротермальных полей на 13°31 с.ш. также интерпретируется по-разному. В случае поля Семенов-4 восходящий поток флюидов мог быть приурочен к проницаемой зоне высокоамплитудного сдвига, а корневая часть гидротермальной системы с магматическим нагревателем могла находиться на значительном удалении, под осевой зоной спрединга. Для серии остальных четырех сравнительно мелких полей предполагается связь восходящих флюидных потоков и корневых частей гидротермальных систем с наложенными на внутренний океанический комплекс вулканическими структурами в крутопадающей тектонической зоне.

Based on studies conducted on the 33rd cruise of the R/V Professor Logachev in 2010, a new type of sulfide mineralization of the mid-oceanic ridges has been established. It was formed in the present-day organic sediments due to diffuse penetration of hydrothermal fluids that emanated along the fractures in basalts on the slopes of the Mid-Atlantic Ridge.

The oceanic core complexes and large-offset detachment faults characteristic of the slow-spreading Mid-Atlantic Ridge are crucial for the structural control of large hydrothermal systems, including those forming sub-seafloor polymetallic sulfide mineralization. The structural-geological, petrographic, and mineralogical data are considered for the oceanic core complex enclosing the Semenov-1, -2, -3, -4, and -5 inactive hydrothermal sulfide fields recently discovered on the Mid-Oceanic Ridge at 13°31′ N. The oceanic core complex is composed of serpentinized and talc-replaced peridotites and sporadic gabbroic rocks, however, all hydrothermal fields reveal compositional indications of basaltic substrate. The volcanic structures superposed on the oceanic core complex are marked by outcrops of pillow lavas with fresh quenched glass. Dolerites regarded as volcanic conduits seem to represent separate dike swarms. The superposed volcanic structures develop largely along the near-latitudinal high-angle tectonic zone controlling the Semenov-1, -2, -5, and -3 hydrothermal sulfide fields. The manifestations of hydrothermal metasomatic alteration are diverse. The widespread talcose rocks with pyrrhotite-pyrite mineralization after serpentinite, as well as finding of talc-chlorite metabasalt are interpreted as products of hydrothermal activity in the permeable zone of detachment fault. Chloritization and brecciation of basalts with superposed quartz or opal, barite, and pyrite or chalcopyrite mineralization directly related to the sub-seafloor sulfide deposition. The native copper mineralization in almost unaltered basalts at the Semenov-4 field is suggested to precipitate from ore-forming fluids before they reach the level of sub-seafloor sulfide deposition. Amphibolites with plagiogranite veinlets are interpreted as tectonic fragments of the highest-temperature portions of hydrothermal systems, where partial melting of basic rocks in the presence of aqueous fluid with formation of plagiogranitic melt is possible. Silicic rocks (plagiogranite, tonalite and diorite) revealed in the tectonic zone controlling the Semenov-1, -2, -5, and -3 hydrothermal sulfide fields are related to both plutonic and subvolcanic bodies and considered to be products of partial melting of basic rocks at deep levels of the hydrothermal systems. The hydrothermal fields differ in their structural position. The giant Semenov-4 field is located at the area where the hanging-wall basalt wedges out and the detachment fault zone reaches the oceanic floor. The range of relatively small Semenov-1, -2, -3, and 5 fields develops on the oceanic core complex massif, being localized in the superposed volcanic structures within the near-latitudinal steeply dipping tectonic zone. The structural control of the hydrothermal fields at 13°31′ N is also interpreted in different ways. For the Semenov-4 field, the ascending fluid flow can be related to the permeable detachment fault zone. The root zone of the hydrothermal system with a magmatic heater could have been localized at a significant distance beneath the axial spreading zone. For the other four relatively small fields, it is suggested that the ascending fluid flows and roots of the hydrothermal systems are controlled by the volcanic structures superposed on the oceanic ore complex within the steeply dipping tectonic zone.