Sub-seafloor massive sulfide deposits: a resource for the near future?

Europe, and the world in general, need new sources of base and strategic metals, given a growing global population, with growing effluence, which will reach ten billion human beings before (slowly) decreasing. One of the new, global scale sources of mineral resources is the deep seafloor, and there are 25 exploration concessions already granted by ISA (International Seabed Authority) in the Area, mostly for polymetallic nodules and sms deposits, and still others in waters under national jurisdiction.

Petersen et al (2015; see also the InterRidge vents database, Beaulieu 2013) have recently updated the inventory of sms deposits and the metal tonnage contained in them, not only for the 218 deposits with significant mineralization already discovered, in neovolcanic zones, but also for similar deposits yet to be discovered, to a total (including arcs) of 750 to 1,500 deposits, with total base metal content (Cu+Zn) of about 30 million tonnes (Mt), roughly 8 Mt Cu and 22 Mt Zn. With the exception of the Red Sea Atlantis II Deep, the larger sms deposits are of the order of 10 Mt of ore, median size 50,000 to 100,000 tonnes of ore. These conclusions stem from 35 years or so of study of ridge hydrothermal systems and their importance cannot be overemphasized. 

The world annual mine production for Cu and Zn was (2014 figures) 18,700 Mt for Cu and 13,300 for Zn (USGS, 2015), meaning that the total resource contained in sms deposits, discovered and yet to be discovered, in neovolcanic zones, would, at current rates, be equivalent to less than 6 months (Cu) and 20 months (Zn) of world consumption. Even the most optimistic estimates suggest that sms deposits cannot make a substantial contribution to global metal production (Singer, 2014). Comparison with volcanogenic massive sulfide (VMS) deposits, on land, shows a striking contrast between the size of seafloor massive sulfide deposits (much smaller) and VMS deposits, for which the “small” division is usually taken as 5-10 Mt. In fact, as pointed out by Galley et al. (2007), much of the VMS ore, on land, is in large to supergiant VMS deposits. 

The main reason for the above discrepancy may result from the fact that seafloor exploration has been focused on outcropping, active deposits, whereas a large number of deposits in the geological record formed under the seafloor, protected from dispersion and oxidation (see Barriga & Fyfe, 1988; Doyle & Allen, 2003; Marques et al, 2007; Dias et al, 2011; Relvas et al, 2014). Another line of reasoning (Cathles, 2011; Hannington, 2013), based on the balance of seafloor hydrothermal activity, suggests that almost all of the base metals leached from seafloor rocks and contributed by magmatic fluids will reprecipitate as sulfides within the crust, as ssms orebodies, or sub-seafloor disseminations, and not in sms deposits. 

Most deep sea exploration has not really focused on sub-seafloor deposits. Current estimates suggest that the potential is very large, but finding these sub-seafloor massive sulfide (ssms) deposits will require exploration strategies very different from those aimed at active dispersion at vents. It will be necessary to detect large masses of sulfides covered by a few meters of sediments or volcanic rocks, in very large exploration areas. Fleets of AUV’s will be a necessity.

In conclusion, we must find the large, ssms deposits, and learn to exploit them, because mining sms deposits alone may not produce a significant contribution to the issue of global metal supply. 

Acknowledgement. This is a contribution to project Blue Mining, EU FP7, EC Contract 604500


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