ABSTRACT
Polymetallic massive sulfide deposits on the modern ocean floor have been found on fast- and slow-spreading ridges, axial and off-axis volcanoes, in sedimented rifts adjacent to continental margins, and in subduction-related arc and back-arc settings. The mineralogy of these deposits includes both high (>300 to 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 sulfides in back-arc spreading centers additionally may contain abundant galena, Pb-As-Sb sulfosalts (including jordanite, tennantite and tetrahedrite), realgar, orpiment, and locally native gold. Initial sampling of sulfides from subduction-related back-arc rifts 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 concentrations are locally high in samples from a number of mid-ocean ridge deposits (up to 7 ppm) but may reach concentrations of more than 50 ppm Au in massive sulfides from immature back-arc rifts. Geochemical studies indicate that factors such as source rock compositions and rock-buffering of the hydrothermal fluids are important controls on the formation of gold-rich massive sulfides. Recent exploration of shallow-marine island arc environments of the western Pacific have also led to the discovery of several new types of seafloor hydrothermal activity with distinctive epithermal characteristics and notable enrichments of gold.
