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).

Comparative studies of the composition and distribution of the dispersed organic matter (DOM) were performed for the bottom sediments from active areas of the Ashadze hydrothermal field (the Mid-Atlantic Ridge, 13° N) and for background sediments from the area treated (sampled during the cruises of R/V Professor Logachev in 2003 and 2007). The content of hydrocarbons (HCs) in the hydrothermal samples is about 20 times as high as the background values. The HCs composition includes both low-transformed (isoprenoids, hopenes, ββ-hopanes, and ααα27R-steranes) and geologically mature chemofossils (C16-C35 n-alkanes, geohopanes, moretanes, and polyaromatic hydrocarbons). The polyaromatic hydrocarbons (PAHs) are mainly represented by phenanthrene and its alkyl homologues, i.e., by possible products of the diagenetic transformation of biogenic precursors. The results obtained point to the mixed genesis of the hydrothermal DOM, which is caused first by the diversity of the biota composition in the considered region, as well as by the specificity of the processes of the DOM’s maturing under the extreme environmental conditions.

The ultramafic-rock-hosted and active Ashadze submarine hydrothermal field, located at a depth of 4100-4200 m at 13 degrees N along the Mid-Atlantic Ridge, was discovered and sampled by dredge and grab sampler at four stations in 2003, on the 22(nd) cruise of RV "Professor Logatchev". Recovered material included fragments of sulfide chimneys, massive and veinlet-disseminated ore assemblages, sulfide breccia and mineralized peridotite. Preliminary investigations of the field suggested a zonal structure, which was confirmed by further laboratory investigations. A mineralogical Study was carried out, using EMPA and SEM/ EMD (280 analyses). Data on the morphology and composition of the principal sulfides indicate non-equilibrium conditions of submarine formation of the minerals. The following main characteristics of the massive sulfides in the Ashadze field were established: 1) varied morphologies of the main sulfide minerals (pyrrhotite, sphalerite, chalcopyrite, isocubanite, pyrite and chalcocite). 2) Variations occur in the composition of the main sulfide minerals. Concentrations of minor elements are much higher than the amounts estimated experimentally in corresponding systems under equilibrium conditions, but may also depend on mineral associations. 3) Temporal and spatial transformations of minerals were observed and are reflected by pseudomorphs (most commonly after pyrrhotite grains) and exsolution phenomena in various solid-solutions. For the first time, an exsolution texture was found in phase Y (Cu2FeS5) along with that in isocubanite and bornite. The latter transformations occur as a result of microfacies zoning of chimney walls.

Sulfide ore samples recovered by corers from and beyond Orebody 1 in the Logatchev-1 hydrothermal field (Mid-Atlantic Ridge, 14°45′ N) are studied by optical, electron microscopic, X-ray microspectral, and X-ray diffraction methods. The major and ore minerals are identified. Sulfides of the Cu-S system, the major ore-forming minerals in sediments, are investigated in detail. Specific features of their composition, structure, secondary alterations, and distribution in sediments of the Logatchev-1 field are considered. It has been established that sulfide concretions in modern sediments primarily consist of nonstoichiometric minerals of the chalcocite-digenite series, i.e., djurleite (Cu 1.96 S) and roxbyite (Cu 1.75–1.86 S). It is assumed that copper sulfides primarily precipitated from hydrothermal solutions as high-temperature hexagonal chalcocite that was replaced after the hydrothermal activity by djurleite, roxbyite, and other nonstoichiometric minerals of the Cu-S system. Based on the comparison of their paragenetic associations with those of copper sulfides in hydrothermal chimneys, the paper discusses constraints of the diagenetic transformation of sulfides in ore-bearing sediments and the halmyrolysis of modern hydrothermal edifices located in contact with seawater. Roxbyite recently discovered in oceanic sediments plays a specific role in these processes.

The composition of ore minerals in MAR sulfide occurrences related to ultramafic rocks was studied using methods of mineragraphy, electron microscopy, microprobe analysis, and X-ray analysis. The objects are located at various levels of the maturity of sulfide mounds owing to differences in age, duration, and degree of activity of the following hydrothermal systems: generally inactive Logatchev-1 field ( up to 66.5 ka old), inactive Logatchev-2 field (3.9 ka), and generally active Rainbow field ( up to 23 ka). Relative to MAR submarine ore occurrences in the basalt substrate, mineralization in the hydrothermal fields mentioned above is characterized by high contents of Au, Cd, Co, and Ni, along with the presence of accessory minerals of Co and Ni. The studied mounds differ in quantitative ratios of major minerals and structural - textural features of ores that suggest their transformation. Ores in the Logatchev-1 field are characterized by the highest Cu content and the development of a wide range of multistage contrast exsolution structures of isocubanite and bornite. In the Logatchev-2 field, sphalerite - chalcopyrite and gold - arsenic exsolution structures are present, but isocubanite exsolution structures are less diverse and contrast. The Rainbow field is marked by the presence of homogenous isocubanite and the subordinate development of exsolution structures. We have identified four new phases in the Cu - Fe - S system. Phases X and Y ( close to chalcopyrite and isocubanite, respectively) make up lamellae among isocubanite exsolution products in Logatchev-1 and Logatchev-2. Phase Y includes homogenous zones in zonal chimneys of the Rainbow field. Phases A and B are formed in the orange bornite domain at low-temperature alteration of chalcopyrite in the Logatchev-1 field. Mineral assemblages of the Cu - S system are most abundant and diverse in the Logatchev-1 field, but their development is minimal in the Logatchev-2 field where mainly Cu-poor sulfides of the geerite - covellite series have been identified. Specific features of mineral assemblages mentioned above reflect the maturity grade of sulfide mounds and can serve as indicators of maturity.

Дается сравнительный анализ минералого-геохимических особенностей медно-цинковых сульфидных руд полей Логачев-2 и Рейнбоу Срединно-Атлантического хребта (САХ), приуроченных к протрузии серпентинитов. Установлено, что Zn(Fe)- и CuFe(Zn)-сульфиды отлагались в трубах черных курильщиков практически одновременно из пульсирующих разнотемпературных неравновесных растворов, обедненных H 2S. Изотопный состав свинца подтверждает, что источником свинца являются глубинные участки океанской коры. В рудах поля Рейнбоу содержится в 20 раз больше Со, чем в рудах, приуроченных к базальтам, и наблюдается высокое отношение Со/Ni = 46. Руды поля Рейнбоу обогащены тяжелым изотопом 34S (δ 34S средн. = 10‰) за счет постоянного подтока холодной морской воды в близповерхностную зону гидротермальной системы. Руды поля Логачев-2 обогащены золотом в 8 раз по сравнению с другими районами САХ. По обогащенности полезными компонентами (Zn, Cd, Co и Au) сульфидные руды полей Рейнбоу и Логачев-2 не имеют аналогов среди рудопроявлений САХ.

Mineralogical and geochemical characteristics of Cu-Zn sulfide ores from the Logachev-2 and Rainbow fields in the Mid-Atlantic Ridge (MAR) are compared. Both fields are related to serpentinite protrusions. It was established that Zn(Fe)- and Cu,Fe(Zn)-sulfides were deposited in black smoker pipes virtually contemporaneously from pulsating nonequilibrium H2S-depleted solutions of varying temperature. According to lead isotope data, the deep oceanic crust was a source of lead. The ore from the Rainbow field contains 20 times more Co than the basalt-related ore and shows a high Co/Ni ratio of 46. The ore from the Rainbow field is enriched in S-14 (delta(14)S = 10%) owing to the permanent flow of cold seawater into the shallow zone of the hydrothermal system. The ore from the Logachev-2 field is enriched in gold by a factor of 8 relative to other MAR localities. The sulfide ores from the Rainbow and Logachev-2 fields have no analogs among MAR ore occurrences as regards their enrichment in Zn, Cd, Co, and Au.

The aim of this paper is to analyze and compare the mineralogy and geochemistry of copper–zinc sulfide ores from the Logachev-2 and Rainbow hydrothermal fields of the Mid-Atlantic Ridge (MAR), confined to serpentinite protrusions. It was found that the Zn(Fe) and Cu, Fe(Zn) sulfides had been deposited in black smokers pipes almost simultaneously from intermittently flowing, nonequilibrium H2S-low solutions of different temperatures. The Pb isotope composition confirmed that the source of lead had been the deep oceanic crust. The ores of the Rainbow Field contain 20-fold more Co than the ores restricted to the basalts and show a high ratio of Co/Ni = 46. The Rainbow ore is enriched in heavy S isotope (δSav = 10/oo) because of the constant flow of cold sea water into the subsurface zone of the hydrothermal system. The ore of the Logachev-2 field is 8 times higher in gold compared to the other MAR regions. The sulfide ores of the Rainbow and Logachev-2 fields have no analogues among the MAR ore occurrences in terms of the enrichment in useful components (Zn, Cd, Co, and Au).

Primary sulfides from cores of holes 957M, 957C, and 957H drilled during the ODP Leg 158 on the active hydrothermal TAG mound (Mid-Atlantic Ridge, 26°08′ N) were examined for the concentration of several elements. Based on 262 microprobe analyses, it has been established that the sulfides are characterized by an extremely heterogeneous distribution of noble metals (Au, Ag, Pt, and Pd) and several associated elements (Hg, Co, and Se). Noble metals are arranged in the following order in terms of decreasing abundance, i.e., concentration level above the detection limit (the number of analyses containing the specific element is given in parentheses): Au (65), Ag (46), Pt (21), and Pd (traces). The associated trace elements make up the following series: Co (202), Hg (132), and Se (49). Main carriers of the “invisible” portion of noble metals are represented by pyrite (Au, Hg), marcasite and pyrite (Ag, Co), sphalerite and chalcopyrite (Pt, Pd), and chalcopyrite (Se). The noble metal distribution in sulfides reveals a lateral zonality: the maximal concentration and abundance of Au in chalcopyrite (or Pt and Ag in chalcopyrite and pyrite) increase from the periphery (Hole 957H) to the center (holes 957C and 957M) of the hydrothermal mound, while the Au distribution in pyrite displays a reversed pattern. The Co distribution increases with depth. This work discusses the vertical zonality in the distribution of elements mentioned above and their response to the evolution of ore genesis.

We report the first occurrence of cobalt pentlandite in oceanic deposits at 14DG459N, Mid-Atlantic Ridge (MAR). The mineral occurs as small (less than 20 mu m), round to irregular grains and aggregates associated with chalcopyrite, bornite, isocubanite, chalcocite, digenite, with minor pyrite and marcasite, and with rare covellite and Au-bearing zincian copper. On the basis of twenty-two analyses, the compositional range (in wt.%) is: Co 42.9-53.9, Ni 7.2-12.4, Fe 3.4-5.5, Cu 0.7-9.4 and S 31.6-32.9. Seventeen of the twenty-two compositions plot between Me 9 S 8 and Me 15 S 13 . The narrow range of sulfur contents corresponds closely with those of synthetic pentlandite (44.2-47.9 at.%). The cobalt content of a pentlandite-group mineral depends on the mineral assemblage with which it is associated.

Sidescan sonar and TV/photo surveys, TV-controlled sulphide grab sampling and sediment sampling have been carried out at the TAG and MARK hydrothermal fields of the Mid-Atlantic Ridge during research cruises of the Sevmorgeologija Association, St Petersburg. Detailed mapping of the MARK field (23°N) allowed contouring of the massive sulphide bodies. The largest of them are Cu-enriched and the smallest zincenriched. Sulphides of the TAG field (26°N) active mound appear to be Cu-rich in its central and Zn-rich in its marginal parts. Sampling of the inactive Mir hydrothermal mound of the TAG field revealed an enrichment in Cu in its northern part and an abundance of silica in the south. This mound and especially the north hydrothermal zone, both presently inactive, determined the metal distribution in TAG sediments during the last 10 000–13 000 years. Extensive Cu-rich sulphide deposits were also discovered and sampled near 14°45′N. The dominance of Cu over Zn in mature Mid-Atlantic Ridge hydrothermal deposits, arising at late stages of their formation, can probably be explained by dominant leaching of Cu from basalts at advanced stages of development of the hydrothermal systems and/or by increasing the efficiency of Cu sulphide deposition in the course of formation of the ore body.