Chemical and isotopic compositions of rocks, ores, and hydrothermal minerals from the Rainbow and Logachev-2 hydrothermal fields

The aim of this paper is to analyze and compare 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 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. Pb isotope composition confirmed that the deep oceanic crust had been a source of lead. The ores from the Rainbow field are 20-fold higher in Co than ores restricted to basalts and show a high ratio of Co/Ni=46. The ores from the Rainbow field are enriched in 34S isotope (aver. d34S=10 per mil) because of constant flow of cold sea water into the subsurface zone of the hydrothermal system. Ores from the Logachev-2 field are 8 times higher in gold compared to other MAR regions. Sulfide ores from the Rainbow and Logachev-2 fields have no analogues among MAR ore occurrences in terms of enrichment in valuable components (Zn, Cd, Co, and Au).

Mineral and chemical compositions of hydrothermal ores and sediments from the Menez Gwen Hydrothermal Field, Mid-Atlantic Ridge

Mineralogy and geochemistry of sulfide-bearing rocks and ores discovered within the Menez Gwen Hydrothermal Field are studied. Samples were taken during Cruise 49 of R/V Akademik Mstislav Keldysh of the p.p. Shirshov Institute of Oceanology. Mineral composition of rocks and ores were studied by traditional methods of optical microscopy, scanning electron microscopy (CAMSCAN), and microprobe analysis (EPMA SX-50). Contents of trace elements were determined by laser ablation inductively coupled plasma - mass spectrometry (LA-ICP-MS). Zn-Cu ore comprises zonal sulfide chimney intergrowths. Numerous Se-rich copper ore fragments occur in volcanomictic layered gritstones and/or barite slabs. Mineral composition, zonality and association of trace elements in ore are typical of black smokers formed at the basalt base near the Azores Triple Junction in the MAR. Obtained results make it possible to reconstruct formation history of the Menez Gwen Hydrothermal Field into the high-temperature (Cu-Se association in ore clasts), medium-temperature (Zn-Cu-As association in ore), and recent (Ba-SiO2 association) stages.

Chemical composition of ocean Fe-Mn ores

The monograph includes the study of chemical and mineral compositions of terrestrial and marine manganese ores, methods of their analysis, dependence of manganese crust geochemistry on tectonic position of their formation, problems of manganese genesis and sources of manganese in ocean ores in connection with geohistorical aspects of ocean formation and development. A hypothesis is offered that formation of giant manganese mineral basins on continental margins resulted from a large asteroid fall to the ocean.

Composition and age of ores and bottom sediments accumulated within and near the Rainbow hydrothermal field

Results of direct geological and geochemical observations of the modern Rainbow hydrothermal field (Mid-Atlantic Ridge, 36°14'N; 33°54'W) carried out from the deep-sea manned Mir submersibles during Cruises 41 and 42 of the R/V Akademik Mstislav Keldysh in 1998-1999 and data of laboratory studies of collected samples are under consideration in the paper. The field lacks neovolcanic rocks and the axial part of the rift is filled in with a serpentinite protrusion. In this field there occur metalliferous sediments, as well as active and relict sulfide edifices composed of sulfide minerals; pyrrhotite, chalcopyrite, isocubanite, sphalerite, marcasite, pyrite, bornite, chalcosine, digenite, magnetite, anhydrite, rare troilite, wurtzite, millerite, and pentlandite have been determined. Sulfide ores are characterized by concentric-zoned textures. During in situ measurements during 35 minutes temperature of hydrothermal fluids was varying within a range from 250 to 350°C. Calculated chemical and isotopic composition of hydrothermal fluid shows elevated concentrations of Cl, Ni, Co, CH4, and H2. Values of d34S of H2S range from +2.4 to +3.1 per mil, of d13C of CH4 from -15.2 to -11.2 per mil, and d13C of CO2 from +1.0 to -4.0 per mil. Fluid inclusions are homogenized at temperatures from 140 to 360°C, whereas salinity of the fluid varies from 4.2 to 8.5 wt %. d34S values of sulfides range from +1.3 to +12.5 per mil. 3He/4He ratio in mineral-forming fluid contained in the fluid inclusions from sulfides of the Rainbow field varies from 0.00000374 to 0.0000101. It is shown that hydrothermal activity in the area continues approximately during 100 ka. It is assumed that the fluid and sulfide edifices contain components from the upper mantle. A hypothesis of phase separation of a supercritical fluid that results in formation of brines is proposed. Hydrothermal activity is related to the tectonic, not volcanic, phase of the Mid-Atlantic Ridge evolution.

Composition of bottom sediments, rocks, and ores from the Tropical Atlantic

Geological features of some areas of the Tropical Atlantic (stratigraphy, tectonic structure, lithology, distribution of ore components in bottom sediments, petrography of bedrocks, etc.) are under consideration in the book. Regularities of concentration of trace elements in iron-manganese nodules, features of these nodules in bottom sediments, distribution of phosphorite nodules and other phosphorites have been studied. Much attention is paid to rocks of the ocean crust. A wide range of mineralization represented by magnetite, chromite, chalcopyrite, pyrite, pentlandite, and other minerals has been found.

Geochemistry of hydrothermal solutions, ores, and biota from hydrothermal fields at the East Pacific Rise axis near 9°50'N

Hydrothermal solutions were examined in a circulation system that started to develop after the 1991 volcanic eruption in the axial segment of the EPR between 9°45'N and 9°52'N. Within twelve years after this eruption, diffusion outflow of hot fluid from fractures in basaltic lavas gave way to focused seeps of hot solutions through channels of hydrothermal sulfide edifices. An example of the field Q demonstrates that from 1991 to 2003 H2S concentrations decreased from 86 to 1 mM/kg, and the Fe/H2S ratio simultaneously increased by factor 1.7. This fact can explain disappearance of microbial mats that were widespread within the fields before 1991. S isotopic composition of H2S does not depend on H2S concentration. This fact testifies rapid evolution of the hydrothermal system in the early years of its evolution. Carbon in CH4 from hot fluid sampled in 2003 is richer in 12C isotope than carbon in fluid from the hydrothermal field at 21°N EPR. It suggests that methane comes to the Q field from more than one source. Composition of particulate matter in hydrothermal solutions indicates that it was contributed by biological material. Experimental solutions with labeled substrates (t<70°C) show evidence of active processes of methane oxidation and sulfate reduction. Our results indicate that, during 12-year evolution of the hydrothermal system, composition of its solutions evolved and approached compositions of solutions in mature hydrothermal systems of the EPR.

Chemical composition of Fe-Mn ores from the North Pacific seamounts

Ore crusts from the Mid-Pacific Seamounts were studied by scanning electron microscopy and by atomic-absorption and chemical analysis. Characteristic ultramicroscopic structures of ore material of these crusts are globular, fibrous, conchoidal and cellular. Non-ore components are represented by fragments of bedrocks, zeolites, biogenic carbonates, and apatite. Contents of ore elements are: Fe 5.53-15.82%; Mn 14.92-23.45%; Co 0.32-0.82%; Ni 0.22-0.70%; Cu 0.02-0.12%, Mn/Fe ratio varies from 1.02 to 3.39. In general elevated contents of Co (>0.55%) in Fe-Mn crusts correspond to elevated (>1) Mn/Fe ratios.

Annotated record and chemical composition of iron-manganese nodules from the Baltic Sea

Iron-manganese concretions, closely related to lacustrine ores and deep sea manganese nodules, are presently forming in different parts of Gulfs of Bothnia and Finland. They can be divided according to physical form into three distinct groups: (1) round pea-shaped concretions, (2) ring-shaped concrections, and (3) flat sheets and crusts of concretionary material. A definite correlation was found to exist between the form i.e. type of concretions and their chemical composition (Mn/Fe ratio). Trace element concentrations were generally rather high, although not as high as in deep sea manganese nodules. X-ray and DTA was used to study the mineralogy and crystal structure of the concretions. Surface concentrations and geographical distribution of the concretions were estimated on the basis of samples, diving observations and echo-grams.