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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Studies of sulfide cores from Site 856, Middle Valley, northern Juan de Fuca Ridge, established the vertical zonation of a large inactive oceanic massive sulfide deposit. Zone 1 consists mainly of pyritic massive sulfide with minor sphalerite and magnetite (0 to 28 mbsf in Hole 856H and 0 to 40 mbsf in Hole 856G). Compared with the rest of the deposit, this zone is strongly enriched in Zn and a large number of trace metals (in particular Cd, Mn, Sn, and Sb), and in elements contained in gangue minerals (Si, Al, Mg, and Ba). Most of these elements reach their maximum concentrations in the upper part of the zone; the lower part is enriched in Cu, Se, and Ca. Zone 2 consists dominantly of pyrrhotite-rich massive sulfide with minor pyrite and magnetite (28 to 48 mbsf in Hole 856H) and is depleted in minor elements. Zone 3 is almost pure pyritic massive sulfide (48 to 75 mbsf in Hole 856H and 40 to 65 mbsf in Hole 856G) and is relatively enriched in Ge, As, Sb, Pb, and Tl. Zone 4 consists of dominant pyrrhotite with minor chalcopyrite (75 to 95 mbsf in Hole 856H) and shows a maximum enrichment in Cu, Co, and Bi. Correlation analysis shows that elements enriched in zone 1 are associated with different ore-forming and gangue minerals: sphalerite (Cd and Pb), talc and/or chlorite (Sc, Th, and REE), silica (Hg and Ta), dolomite (V), and barite (Sr, Zr, and Hf). Sb, As, Sn, Au, Ag, Mn, Mo, and Tl form a separate association, presumably related to sulfosalt minerals. Enrichment of Zn and minor elements in the upper part of the deposit, which is typical of ancient massive sulfide deposits, is attributed to the zone-refining process of progressive upward replacement of lower temperature minerals by higher temperature minerals during growth of the sulfide body, with the concomitant hydrothermal leaching of trace metals. Later, the deposit was hydrothermally altered, probably due to lateral flow of evolved seawater along turbidite layers. The alteration produced replacement of primary pyrrhotite by pyrite in zone 3 and by pyrite + magnetite assemblages in the upper zones. In the upper levels of the deposit, sulfate-rich water acted as a strong oxidant. At lower levels a hotter and more evolved sulfur-enriched pyritizing solution also introduced additional Pb, Sb, As, and other sediment-derived elements into zone 3. The absence of light rare earth element enrichment and the existence of a positive Eu anomaly in the massive sulfides of this zone is connected with the alteration of plagioclase in the associated turbidites. The secondary convection system that caused alteration of the massive sulfides could have been driven by one of the sills emplaced into Middle Valley sediments subsequent to sulfide formation.

Geological features which control massive sulphide formation on the fast-spreading East Pacific Rise (EPR) and slow-spreading Mid-Atlantic Ridge (MAR) can be specified based on data from the Sevmorgeologija Association (St Petersburg) research cruises and results of other studies. Wide crestal surfaces of undisturbed axial volcanoes and the presence of axial grabens that indicate voluminous subsurface magma chambers represent the sites most favourable for sulphide formation on the EPR. The elevation of rift segments and distance from major ridge-axis discontinuities are less important for sulphide formation. Sites of localized magma delivery from subcrustal zones, as indicated by Mg anomalies in basalts, may be favourable. However, at least one site near 21°30′S on the EPR shows evidence of along-axial magma penetration from the central part of the rift segment to its tip, resulting in a lateral shift of hydrothermal activity with time. Higher crustal permeability for magma is required for the formation of subsurface chambers which initiate hydrothermal convection on the magmatically less active MAR. Rift valley marginal faults, and especially their intersections with minor transverse dislocations, locally control hydrothermal activity where magma laterally penetrates from the extrusive zones of the adjacent rift segments.

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