Abstract—Proposed intermediate phases and their thermal decomposition are discussed on the basis of the diagram of low-temperature phase relations and iron sulfide transformations. The formation of pyrite during pyrrhotite oxidation is refined using chemical reactions involving iron and sulfur ions in the thermal decomposition of greigite and iron trisulfide. The further oxidation reactions of disulfide ions of pyrite with the formation of polysufide ions are presented. A relationship between the composition of the nonequilibrium nonstoichiometric layer on the pyrrhotite surface during oxidation and intermediate phases is found. Detailed mineralogical–geochemical data on the hydrothermal sulfide aggregates of “black smokers” are given. Early pyrrhotite is found to form pyrite fringes on the surface at first, and then the pyrrhotite is almost completely substituted by pyrite and/or marcasite.

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.

This paper, devoted to the mineral composition of ore-bearing sediments in the Pobeda hydrothermal cluster in the Mid-Atlantic Ridge (MAR), is a continuation of previous studies (Gablina et al., 2021). Rare zinc minerals (zinc phosphates, zincite, willemite, gahnite(?), and Zn-forsterite) are described. Most of these minerals were first established in bottom sediments of the MAR. The studies were carried out by several methods: scanning electron microscopy, X-ray spectral microanalysis, X-ray diffractometry, Raman spectroscopy, and electron backscatter diffractometry.

The results of long-term studies of the localization conditions, morphology, structure, mineral and chemical compositions of modern ocean sulfide ores, carbonate sediments, as well as ore- and metal-bearing sediments, at 13°–20° N Mid-Atlantic Ridge are summarized. This paper considers the features of sulfide ores formed in various geological and physicochemical environments: (1) on the bottom surface during the hydrothermal solution–seawater (“black smokers”) interaction; (2) under the bottom surface during the hydrothermal solution–biogenic carbonate sediment (hydrothermal-metasomatic ore) interaction. These ore types have some common features, and they are not always distinguished by researchers. A set of distinctive features is proposed to identify the hydrothermal-metasomatic ores that can be used in the search and prediction of sulfide ores in the ocean.

Nonstoichiometric minerals form during the leaching of chalcocite; they form the following sequential row between chalcocite and covellite: djurleite (Cu1.93–1.96S), roxbyite (Cu1.72–1.82S), digenite (Cu1.75–1.78S), anilite (Cu1.75S), geerite (Cu1.5–1.6S), spionkopite (Cu1.4S), and yarrowite (Cu1.1S). A hypothesis that the oxidation of chalcocite (Cu2S) ends in villamaninite (CuS2) rather than covellite (CuS) has been put forward and substantiated. The formation of various copper sulfides during the oxidation of chalcocite is related to the fact that copper has a variable valence and sulfur can be in them in both mono- and disulfide form. A scientific substantiation and chemical reactions are proposed for the formation of elemental orthorhombic sulfur at the last stage of chalcocite leaching, and the possibility of appearance of its other allotropes is shown. When chalcocite and other sulfides are leached below the melting temperature of sulfur (low-temperature regime), cyclic sulfur molecules, which are essentially liquid (molten) sulfur, form. During leaching, sulfate ions and elemental sulfur do not precipitate in two independent processes: they form in one way through the formation of polysulfides, which are transitional phases to molecular sulfur.

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.

In this study, to better understand the influence of hydrothermal processes on ore metal accumulation in bottom sediments, we examined distribution of Fe, Mn, Cu, Zn, As, and Pb in core of metalliferous sediments from the Pobeda hydrothermal cluster, and in core of non-mineralized (background) carbonate sediments (located 69 km northwards). Mechanisms of Fe, Mn, Cu, and Zn accumulation in sediments (12 samples) were evaluated based on sequential extraction of geochemical fractions, including a conditional mobile (F-1, exchangeable complex; F-2, authigenic Fe-Mn oxyhydroxides and associated metals; F-3, metals bound to organic matter/sulfides), and residual (F-4), fixed in crystalline lattices ones. The element contents were determined by the XRF and AAS methods, total carbon (TC) and total organic carbon (TOC) were determined using a Shimadzu TOC-L-CPN. Mineral composition and maps of element distribution in sediment components were obtained using the XRD and SEM-micro-X-ray spectrometry methods, respectively. In metalliferous sediments, according to our data, the major Fe mineral phase was goethite FeOOH (37–44% on a carbonate-free basis, cfb). In the metalliferous core, average contents (cfb), of Fe and Mn were 32.1% and 0.29%, whereas those of Cu, Zn, Pb, and As, were 0.74%, 0.27%, 0.03%, and 0.02%, respectively. Metalliferous sediments are enriched in Fe, Cu, Zn, Pb, and As, relatively to background ones. The exception was Mn, for which no increased accumulation in metalliferous core was recorded. Essential mass of Fe (up to 70% of total content) was represented by the residual fraction composed of crystallized goethite, aluminosilicates, the minerals derived from bedrock destruction processes mineral debris. Among geochemically mobile fractions, to 80% Fe of the (F-1 + F-2 + F-3) sum was determined in the form of F-2, authigenic oxyhydroxides. The same fraction was a predominant host for Mn in both metalliferous and background sediments (to 85%). With these Fe and Mn fractions, a major portion of Cu, Zn, and Pb was associated, while a less their amount was found in sulfide/organic fraction. In the metalliferous sediment core, maximal concentrations of metals and their geochemically mobile fractions were recorded in the deeper core sediment layers, an observation that might be attributed to influence of hydrothermal diffused fluids. Our data suggested that ore metals are mostly accumulated in sediment cores in their contact zone with the underlying serpentinized peridotites.

In this study, to better understand the influence of hydrothermal processes on ore metal accumulation in bottom sediments, we examined distribution of Fe, Mn, Cu, Zn, As, and Pb in core of metalliferous sediments from the Pobeda hydrothermal cluster, and in core of non-mineralized (background) carbonate sediments (located 69 km northwards). Mechanisms of Fe, Mn, Cu, and Zn accumulation in sediments (12 samples) were evaluated based on sequential extraction of geochemical fractions, including a conditional mobile (F-1, exchangeable complex; F-2, authigenic Fe-Mn oxyhydroxides and associated metals; F-3, metals bound to organic matter/sulfides), and residual (F-4), fixed in crystalline lattices ones. The element contents were determined by the XRF and AAS methods, total carbon (TC) and total organic carbon (TOC) were determined using a Shimadzu TOC-L-CPN. Mineral composition and maps of element distribution in sediment components were obtained using the XRD and SEM-micro-X-ray spectrometry methods, respectively. In metalliferous sediments, according to our data, the major Fe mineral phase was goethite FeOOH (37-44% on a carbonate-free basis, cfb). In the metalliferous core, average contents (cfb), of Fe and Mn were 32.1% and 0.29%, whereas those of Cu, Zn, Pb, and As, were 0.74%, 0.27%, 0.03%, and 0.02%, respectively. Metalliferous sediments are enriched in Fe, Cu, Zn, Pb, and As, relatively to background ones. The exception was Mn, for which no increased accumulation in metalliferous core was recorded. Essential mass of Fe (up to 70% of total content) was represented by the residual fraction composed of crystallized goethite, aluminosilicates, the minerals derived from bedrock destruction processes mineral debris. Among geochemically mobile fractions, to 80% Fe of the (F-1 + F-2 + F-3) sum was determined in the form of F-2, authigenic oxyhydroxides. The same fraction was a predominant host for Mn in both metalliferous and background sediments (to 85%). With these Fe and Mn fractions, a major portion of Cu, Zn, and Pb was associated, while a less their amount was found in sulfide/organic fraction. In the metalliferous sediment core, maximal concentrations of metals and their geochemically mobile fractions were recorded in the deeper core sediment layers, an observation that might be attributed to influence of hydrothermal diffused fluids. Our data suggested that ore metals are mostly accumulated in sediment cores in their contact zone with the underlying serpentinized peridotites.

По материалам, полученным в рейсе № 37 нис “Профессор Логачев” в Российском разведочном районе, изучен видовой состав, распространение и сохранность карбонатных раковин нано- и микрофоссилий биогенных донных осадков в пределах гидротермального узла “Победа” (17°07.45´–17°08.7´ с. ш. Срединно-Атлантического хребта). Измерены физико-химические параметры и общая карбонатность осадков, выявлены их изменения по разрезам колонок. По комплексам кокколитов изученные осадки отнесены к верхней части (acme) биостратиграфической зоны Emiliania huxleyi. Установлено, что в пределах зоны гидротермальной активности (рудный узел “Победа”) снижается общая численность микроорганизмов и уменьшается их видовое разнообразие, как правило, в нижней части колонок. Вниз по разрезу колонок снижаются также Eh, pH и карбонатность осадков. Эти изменения объясняются влиянием диффузных гидротермальных флюидов.

Based on materials obtained during Cruise 37 of the R/V Professor Logatchev in the Russian exploration area, the species composition, distribution, and preservation of nanno- and microfossil carbonate shells in biogenic bottom sediments within the Pobeda hydrothermal cluster (17°07.45′–17°08.7′ N MAR) were studied. Physicochemical parameters and total carbonate content were measured and their changes along column sections were revealed. Based on the coccolith assemblages, the studied sediments are assigned to the upper part (acme) of the biostratigraphic zone Emiliania huxleyi. It has been established that the hydrothermal activity zone (Pobeda hydrothermal cluster) is marked usually by a decrease of both total population and species diversity of microrganisms in the lower part of columns. The column is also marked by downsection decrease of Eh, pH, and carbonate content. These changes are attributed to the influence of diffuse hydrothermal fluids.

Comparison of benthic foraminiferal assemblages from the core obtained within the Peterburgskoe ore field (Mid-Atlantic Ridge) and from the core taken five kilometers away from the ore field revealed evident differences in their composition, in the appearance of their shells, and also in the benthic–plankton species ratio. It was noted that the foraminiferal assemblage from the ore-bearing sediments of the Petersburg field was characterized by a higher relative content of benthic species and a large number of chemically altered and broken shells. The first occurrence of the species Osangularia umbonifera, which is able to exist in lowoxygen and CaCO3-undersaturated bottom waters at the boundary of biogenic sediments surrounding the ore field and in the ore-bearing sediments, was established. In the core section sampled beyond the ore field, the composition of foraminiferal assemblages differs insignificantly from typical oceanic ones.

Based on materials obtained in Cruise 37 of the R/V Professor Logatchev (2014‒2015), the paper discusses formation conditions, morphology, isotope age, and mineral and chemical composition of sulfide ores in the Pobeda-1 (17°08.7′ N MAR) and Pobeda-2 (17°07.45′ N MAR) hydrothermal ore fields (Pobeda ore cluster) discovered in blocks 37 and 39 confined to the Russian exploration area. Morphogenetic types of ores are defined and their formation conditions are examined.

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.

Sulfide ores were investigated along with ore-bearing and metalliferous sediments of the hydrothermal fields in the northern near-equatorial Mid-Atlantic Ridge (MAR) zone: Semenov (13°30–31′N), Ashadze-1 (12°58′N), Zenit-Victoria (20°08′N), and Peterburgskoe fields (19°52′N), discovered during legs 26, 32, and 33 of the R/V Professor Logachev FSUE PMGE. Biogenic carbonate and background sediments of this region were also examined. Lithological, biostratigraphic, and geochemical physical-chemical investigations methods were used. Mineragraphic and precision structural and chemical research of typomorphic minerals were carried out at various stages of lithogenesis. It was found out that most sulfide constructions in the Zenit-Victoria and Peterburgskoe fields, as well as the eastern field of the Semenov cluster, are located in biogenic carbonate sediments of the Holocene and Late Pleistocene ages and represent a new type of sulfide mineralization, unknown earlier in the MAR zone. This mineralization was formed by metasomatic replacement of biogenic carbonate sediments by ore minerals, simultaneously with diffuse percolating of hydrothermal solutions through the sediments.