The resource potential of slow- and ultraslow-spreading ridges

Increasing commercial interest in mining of seafloor massive sulfides and the political will to secure metal supply for global industries has led to an ongoing debate about their possible resource potential. The need for such assessments is now more urgent, as a number of countries and international consortia have begun to invest in intensive exploration campaigns. A growing database of global SMS occurrences is providing clues to the likely distribution, size and grade of the deposits. More than 330 sites of seafloor mineralization are now known on the ocean floor. The available data is now sufficient to allow an initial assessment of the global seafloor sulfide potential and the factors controlling metal enrichment in various tectonic settings. Analyses of over 4,950 sulfide samples collected to date from vent sites in different tectonic settings and at various spreading rates has revealed highly variable concentrations not only for base and precious metals, but also for certain rare metals. It has also shown high variability for sites located at the global mid-ocean ridges that are currently being selected as exploration targets in the high seas and approved by the International Seabed Authority. Currently, the 57,000 km of mid-ocean ridges are host to 205 known seafloor sulfide occurrences. Approximately 41 % of these sites are hosted by fast- and ultrafast-spreading mid-ocean ridges (spreading rate of >60 mm/yr) while 22 % of the sulfide occurrences are associated with intermediate-rate (40-60 mm/yr) spreading centers. Slow-spreading (20-40 mm/yr) and ultra-slow spreading ridges (<20 mm/yr) host 23 % and 14 %, respectively. Most of the sulfide occurrences are hosted by mid-ocean ridge basalts, although an increasing number of sites have been found on a substrate of ultramafic rocks, commonly associated with core complex formation. It is this substrate that seems to be particularly favourable for Cu and Au co-enrichment when compared to normal basalt-hosted sites. Recent surveys of slow- and ultraslow spreading ridges have also indicated a larger surface area and possibly tonnage at these sites. Slow- and ultraslow spreading ridges could be more favorable targets for future exploration of base- and precious-metal-rich massive sulfide.