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.

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Two new major hydrothermal fields have been discovered in the rift valley of the MAR at 13 N (Ashadze) and l6 38 0 N (Krasnov). The Ashadze field consists of a cluster of active hydrothermal sites associated with ultramafic rocks and located at the greatest depth in the ocean (4,200 m). By contrast, the Krasnov field consists of inactive sulfide mounds hosted in basalts. The Krasnov is the largest hydrothermal deposit on the MAR (17.4 Mt) so far discovered with iron sulfide as the principal mineral type. By contrast, Cu-Zn sulfides are the major minerals in the Ashadze deposits, which are also enriched in gold and several other metals.

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A radiochemical study was carried out on massive sulfides from Semyenov hydrothermal district at the Mid-Atlantic Ridge. New and published results provide evidence that 230 Th/U ages obtained for massive sulfides are reliable. The sulfide deposits from the West, North-West, North-East, and East hydrothermal sites at the Semyenov hydrothermal district were formed between ~124 ka and ~37 ka ago. The hydrothermal activity might have started in the eastern part of the district and moved to the west by episodic ore formation.

A geochronological and geochemical study on 10 samples of seafloor massive sulfides (SMS) from the inactive Peterburgskoye hydrothermal field at the Mid-Atlantic Ridge (MAR) was carried out. The 230 Th/U ages of the SMS are the oldest for the Quaternary hydrothermal ores ever found at the ocean floor. According to them the hydrothermal activity at Peterburgskoye field started at least 170 ka and continued down to 63 ka. The oldest hydrothermal ores from this field consist mainly of pyrite and chalcopyrite and have geochemical properties typical for SMS associated with basalts.

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