Submarine Hydrothermal Venting Related to Volcanic Arcs
Cornel E.J. de Ronde, Gary J. Massoth, Edward T. Baker, John E. Lupton, 2005. "Submarine Hydrothermal Venting Related to Volcanic Arcs", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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Volcanic arcs that have a submarine component (n = 21) include both intra-oceanic and island arcs. Combined, they have a total length of almost 22,000 km with ~93 percent in the Pacific region. We estimate that 696 volcanoes occur along these arcs, with at least 209 (30%) being submarine. The 13 best studied arcs total 14,260 km and include 526 volcanoes with a spacing of 22 to 32 km (mean = 27 km). Less than 3 percent of arc length has been systematically surveyed for sea-floor hydrothermal emissions. Submarine hydrothermal venting along these arcs therefore remains overwhelmingly undetected.
The southern Kermadec arc northeast of New Zealand represents ~260 km of intra-oceanic arc that has been systematically surveyed for hydrothermal plumes. Here, seven of the 13 (55%) volcanoes surveyed are hydrothermally active. Depths to the vents range between 250 and 1,660 m below sea level. Venting is characterized by hydrothermal plumes that are chemically heterogeneous when compared to midocean ridge sites, i.e., they range from being highly enriched in dissolved ionic species (e.g., Fe) and 3He, CO2, and sulfur gases, to 3He rich but with very low concentrations of ionic species. By contrast, surveys of the Tabar-Lihir-Tanga-Feni island arc offshore Papua New Guinea and the Ghizo Ridge fore-arc offshore the Solomon Islands show no detectable submarine hydrothermal activity. Instead, hydrothermal and volcanic activity is confined to the summits of subaerial islands, as illustrated by the Luise volcano on Lihir Island, which is host to the giant Ladolam gold deposit.
The Izu-Bonin intra-oceanic arc extends for 1,200 km south from Japan and is host to 26 submarine volcanoes. Myojin Knoll is a caldera volcano that has a number of hydrothermal vents (180°–330°C) in association with the large (~9 × 106 t) gold-rich Sunrise massive sulfide deposit. Suiyo seamount is also host to numerous hydrothermal vents (up to 317°C) associated with sulfide chimneys and mounds. These are the only two submarine arc-related hydrothermal systems for which a complete suite of vent fluid analyses has been obtained.
Thirty-one active vent sites (for the period 1984–2002) have been documented worldwide on submarine arc volcanoes; circumstantial evidence suggests hydrothermal activity at an additional 11 sites. Twothirds of these sites occur in water depths of <1,000 m. The injection of hydrothermal emissions into the mid and upper levels of the oceans has implications for tracer studies of mid-depth geostrophic flow. The shallowest vent sites can deliver Fe to the euphotic zone (100- to 200-m water depth) thus giving arc-related vent sites an environmental influence generally lacking at midocean ridges.
Magma bodies underlying vent sites in arc settings may play a significant role in the supply of magmatic volatiles and metals to their overlying hydrothermal systems. The shallower hydrothermal circulation cells expected on arc volcanoes, relative to midocean ridges, implies that any magmatic fluid component will be more evident in the expelled vent fluids. The venting of hydrothermal systems at relatively shallow depths and with high gas contents will assist phase separation and should promote the formation of economic massive sulfide (Cu-Zn ± Pb ± Au) deposits, making them attractive for exploration and possible exploitation.
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To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic.
The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.