Most of the world's economically
significant volcanogenic massive sulphide (VSM) deposits are considered to have
formed in submarine arc and backarc settings. While the Kermadec-Tonga arc is an
excellent example of an intra-oceanic arc setting, it remains largely unexplored
in terms of submarine mineral resources. More than half of this arc (~1400
km of the
>2500-km-long feature) lies within the EEZ of New Zealand, where it is
defined by at least one sub-aerial and 33 submarine volcanoes. The NZAPLUME (New
Zealand American PLUme Mapping Expedition) cruise of March 1999 provided a
systematic assessment ofhydrothermal activity on the southernmost 13 submarine
volcanoes along the southern Kermadec arc. Using prospecting techniques based on
the physical and chemical detection of hydrothermal plumes, we discovered that
more than half (7 of 13) of the southern Kermadec volcanoes host one or more
active hydrothermal systems at depths ranging between <200 to 1800 m.>
Chemical results show that the fluids discharged are compositionally diverse
among volcanoes and between sites at a single volcano. Plumes over more than
half of the volcanoes are gas-rich (He, CO2, H2S) relative
to mid-ocean-ridge (MOR) plumes.In
addition, most sites are highly enriched in Fe, yielding Fe/Mn values that far
exceed those for MORs. One interpretation consistent with these observations is
compositional overprinting by addition of magmatic fluids, which are typically
highly enriched in gaseous and ionic components. Regardless of the source, it is
clear that the ligands (HS-, Cl-) required to efficiently
transport base (e.g. Cu, Zn, Pb) and precious (e.g. Au, Ag) metals to the
seafloor are abundant in Kermadec arc fluids. We conclude that modem analogs of
VMS deposits are likely forming at multiple sites along the Kermadec arc and
within the EEZ of New Zealand.