Polymetallic massive sulfides at the modern seafloor a review

Polymetallic massive sulfides on the modern seafloor have been found in diverse volcanic and tectonic settings at water depths ranging from about 3700 to 1500 m. These deposits are located at fast-, intermediate-and slow-spreading mid-ocean ridges, on axial and off-axis volcanoes and seamounts, in sedimented rifts adjacent to continental margins and in subduction-related back-arc environments. High-temperature hydrothermal activity and large accumulations of polymetallic sulfides, however, are known at fewer than 25 different sites. Several individual deposits contain between 1 and 5 million tonnes of massive sulfide (e.g., Southern Explorer Ridge, East Pacific Rise 13°N, TAG Hydrothermal Field) and only two deposits (Middle Valley and Atlantis II Deep, Red Sea) are known to contain considerably higher amounts of sulfides ranging between 50 and 100 million tonnes. This range (1–100 million tonnes) is similar to the size of many volcanic-associated massive sulfide deposits found on land. However, the vast majority of known sulfide occurrences on the modern seafloor amount to less than a few thousand tonnes and consist largely of scattered hydrothermal vents, mounds and individual chimney structures.Recovered samples from about 25 deposits world-wide represent no more than a few hundred tonnes of material. The mineralogy of these samples includes both high (> 300°–350°C) and lower-temperature (< 300°C) assemblages consisting of varying proportions of pyrrhotite, pyrite/marcasite, sphalerite/wurtzite, chalcopyrite, bornite, isocubanite, barite, anhydrite and amorphous silica. Massive sulfide deposits in back-arc environments additionally may contain abundant galena, PbAsSb sulfosalts (including jordanite, tennatite and tetrahedrite), realgar, orpiment and locally native gold. Close to 1300 chemical analyses of these samples indicate that the seafloor deposits contain important concentrations of Cu and Zn comparable to those of massive sulfide deposits on land. The sediment-hosted deposits, while being somewhat larger than deposits on the sediment-starved mid-ocean ridges, appear to have lower concentrations and different proportions of the base metals due to fluid-sediment interaction. Initial sampling of sulfides in the back-arc spreading centers of the West and Southwest Pacific suggests that these deposits have higher average concentrations of Zn, Pb, As, Sb and Ba than deposits at the sediment-starved mid-ocean ridges. Gold and silver concentrations are locally high in samples from a number of mid-ocean ridge deposits (up to 6.7 ppm Au and 1000 ppm Ag) and may reach concentrations of more than 50 ppm Au and 1.1 wt% Ag in massive sulfides from immature back-arc rifts, that are dominated by felsic volcanic rocks. Precious metal contents of seafloor sulfides thus are well within the range of those found in land-based deposits.Although massive sulfide deposits have been found at water depths as shallow as 1500 m, boiling of the hydrothermal fluids may prevent the formation of typical polymetallic massive sulfides at shallower depths (i.e., less than a few hundred meters), where the hydrostatic pressure is too low to prevent phase separation. In this case, mineralization with distinct epithermal characteristics and significant amounts of precious metals can be expected