SHALLOW SUBMARINE HYDROTHERMAL SYSTEMS ASSOCIATED WITH ARC VOLCANOES

Convergent plate boundaries are dominant in the Western Pacific region with ~20,000 km (worldwide 21,700 km) of combined intraoceanic and island arcs. About 30% of the almost 700 volcanoes known to exist along volcanic arcs are submarine. About 2,550 km of intraoceanic arc has been surveyed in detail for submarine hydrothermal venting with another 1,200 km to a lesser degree. About 30% of the 137 volcanoes surveyed were found to be hydrothermally active, expelling hydrothermal liquids and gases into the ocean at water depths between 50 and 1650 m. The vent sites are usually located near the summit of volcanic cones although some occur in deeper depths, near the floor of caldera volcanoes.

We define ‘shallow’ as being less than ~500 m water depth, or where a fluid of seawater composition will boil at temperatures less than ~260 °C. Addition of significant amounts of dissolved gases like CO₂ and H₂S means boiling will occur at greater depths than those for a pure seawater solution, for any given temperature. Of the 43 actively venting sites known along intraoceanic arcs, 19 (44%) are shallow. If we project this over the remaining ~3,150 km of unsurveyed intraoceanic arc, and use a frequency of venting of one vent site every 87 km of arc length, then a further 16 shallow vent sites have yet to be discovered. Nine of the 12 known active vent sites associated with the submarine portions of island arcs are also shallow. Combined, 50% (n=28) of all known active submarine vent sites associated with volcanic arcs occur in water depths of <500 m. A number of shallow vent sites occur along the Kermadec arc, offshore New Zealand, and include; Rumble V, Rumble III, Vulkanolog, Macauley cone, Giggenbach, Ngatoroirangi and Monowai. Depths to the vent sites range from 440 m at Rumble V to 130 m at Vulkanolog. Hydrothermal plumes associated with these volcanoes can be highly enriched in dissolved ionic species such as Fe, and ³He, CO₂ and sulfur gases. Evidence for a magmatic component in these arc vent systems is given by the nature and concentrations of various gases and Fe, and from samples recovered from these sites that commonly contain the mineral assemblage; quartz, opal-A, opal-C, native sulfur, pyrite, marcasite and natroalunite. These sites are ideal candidates for Au-rich mineralization similar to that seen in subaerial epithermal environments.