1995 年 104 巻 3 号 p. 438-448
A compilation of 17 sulfide-bearing, sea-floor hydrothermal sites in western Pacific area was made to compare them with volcanogenic massive sulfide deposits (VMSDs) which were likely to have formed in analogous setting of ancient arc-backarc systems. Critical examination of these modes of occurrence shows that all the hydrothermal activity is limited at or above magmatic centers which occur either on backarc spreading center, in backarc rift, or on volcanic front. About half of the known localities (8 sites) are related to bimodal volcanism and are considered to be the analog of Kuroko type deposit. The other half (9 sites) are accompanied with andesitic to basaltic volcanism and are categolized into Besshi-and Cyprus-type deposits. This coincides well with the statistics that 56% of the known VMSDs belong to Kuroko type (Rona, 1988). Therefore, the present-day sea-floor mineralization well represents ancient mineralization of VMSDs now found in strata of arc-backarc affiliation.
The close spatial relationship between volcanism and hydrothermal activity suggests that both present-day and ancient deposits are of volcanogenic origin. Several lines of evidence such as isotope ratio of constituent elements (Ishibashi and Urabe, 1995) also support this conclusion. The ultimate source of these volatile elements such as oxygen, hydrogen, carbon, sulfur, nitrogen, and probably chlorine is believed to be dehydration of sediments which were subducted with oceanic plate beneath the arc-backarc systems.