The basalt-hosted Semenov-2 hydrothermal field on the Mid-Atlantic Ridge is host to a rather unique Cu-Zn–\ud rich massive sulfide deposit, which is characterized by high Au (up to 188 ppm, average 61 ppm, median\ud 45 ppm) and Ag (up to 1,878 ppm, average 490 ppm, median 250 ppm) contents. The largest proportion of\ud visible gold is associated with abundant opal-A, which precipitated after a first generation of Cu, Fe, and Zn\ud sulfides and before a second generation of Fe and Cu sulfides. Only rare native gold grains were found in earlier\ud sulfides. Fluid inclusions in opal-A associated with native gold indicate precipitation at 300° ± 40°C from\ud fluids of salinity higher than that of seawater (3.5–6.8 wt % NaCl equiv). According to laser ablation-inductively\ud coupled plasma-mass spectrometry analyses, invisible gold is concentrated in secondary covellite (23–227 ppm)\ud rather than in the primary sulfides (<1 ppm). Silver minerals (native silver, stutzite, and naumannite) rarely\ud occur in the sulfides and in aragonite associated with opal-A; invisible silver was detected in all sulfides, but,\ud again, covellite contains more Ag (>1,000 ppm) than all other sulfides (<250 ppm). Covellite replacing Zn\ud sulfides (covellite-A) is enriched in all analyzed trace elements relative to covellite replacing Cu-Fe sulfides\ud (covellite-B). The enrichment of covellite-A in trace elements may be related to the dissolution of inclusions\ud of various minerals hosted in former sphalerite, which were the source for Au and Ag (native gold), Pb and Tl\ud (galena), Se (chalcopyrite, Se-bearing galena, naumannite), Te and Bi (Bi tellurides), As (tennantite, chalcopyrite),\ud and Sb (tennantite). The formation of covellite-A was favored by hydrothermal fluid/seawater mixing or\ud direct oxidation of sulfides by seawater, as suggested by the relatively high contents of typical “seawater” elements\ud (U and V). The degree of seawater involvement was apparently lower for covellite-B.\ud Although the Semenov-2 field is basalt hosted, several geochemical features of the massive sulfides studied\ud are similar to those of the Mid-Atlantic Ridge ultramafic-hosted Cu-Zn–rich massive sulfides, such as Fe:Cu:Zn\ud ratios close to 1:1:1, high Sn, Se, Au ,and Ag contents, and high Au/Ag ratios. However, the strong enrichment\ud in SiO2, the moderate Mn and Co contents, very low Ni contents, and the Co/Ni ratio >1 are more consistent\ud with a mafic signature. Thermodynamic modeling of hydrothermal fluids produced by reactions between various\ud proportions of seawater and basalt or peridotite at 350°C shows that mineral assemblages broadly similar to\ud those of the Semenov-2 deposit can precipitate from fluids produced in a mafic environment, but that Au and\ud Ag minerals are not predicted to precipitate from such fluids over a wide temperature range. These results suggest\ud that an additional contribution to the hydrothermal system is required in order to achieve saturation in precious\ud metals. A magmatic input is suggested ...

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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