Several hydrothermal deposits associated with ultramafic rocks have recently been found along slow spreading ridges with a low magmatic budget. Three preferential settings are identified: (1) rift valley walls near the amagmatic ends of ridge segments; (2) nontransform offsets; and (3) ultramafic domes at inside corners of ridge transform-fault intersections. The exposed mantle at these sites is often interpreted to be a detachment fault. Hydrothermal cells in ultramafic rocks may be driven by regional heat flow, cooling gabbroic intrusions, and exothermic heat produced during serpentinization. Along the Mid-Atlantic Ridge (MAR), hydrothermal deposits in ultramafic rocks include the following: (1) sulfide mounds related to high-temperature low-pH fluids (Logatchev, Rainbow, and Ashadze); (2) carbonate chimneys related to low-temperature, high-pH fluids (Lost City); (3) low-temperature diffuse venting and high-methane discharge associated with silica, minor sulfides, manganese oxides, and pervasive alteration (Saldanha); and (4) stockwork quartz veins with sulfides at the base of detachment faults (15°05′N). These settings are closely linked to preferential circulation of fluid along permeable detachment faults. Compared to mineralization in basaltic environments, sulfide deposits associated with ultramafic rocks are enriched in Cu, Zn, Co, Au, and Ni. Gold has a bimodal distribution in low-temperature Zn-rich and in hightemperature Cu-rich mineral assemblages. The Cu-Zn-Co-Au deposits along the MAR seem to be more abundant than in ophiolites on land. This may be because ultramafic-hosted volcanogenic massive sulfide deposits on slow spreading ridges are usually not accreted to continental margins during obduction and may constitute a specific marine type of mineralization.

In addition to the two new hydrothermal fields Krasnov (l6° 38' N) and Ashadze (13° N) discovered in 2003-2004, some later discoveries were made in the Northern Equatorial segment of the Mid-Atlantic Ridge (MAR). Two new major seafloor massive sulfide (SMS) deposits: Semyenov (13° 31' N) and Zenith-Victory (20° 08' N), as well as the now extended Puy des Folles hydrothermal field (20° 30.5' N), provide enough data to make conclusions about the general characteristics of SMS deposits in the northern equatorial MAR environment. Parameters of SMS deposits, such as their distribution, geological setting, morphology, composition and age, are discussed in this paper. A majority of studied SMS deposits (Ashadze, Logatchev, Semyenov) are associated with uplifted lower crust and mantle rocks (oceanic core complex, or OCC) at the MAR segments with asymmetric mode of accretion. OCC is tectonically uplifted along with detachment faults, which exhume deep-seated rocks onto the seafloor. Detachment faults, in turn, could be the conduits for the circulation of seawater and discharge of hydrothermal fluids. Gabbro-peridotite hosted deposits are characterized by high concentrations of copper, gold and cobalt. The relationship between the size of the deposits and their age is documented.

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.

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