Base and precious metal resources in seafloor massive sulfide deposits

Abstract

Interest in seafloor massive sulfide deposits as a possible source of metals has been strong since the discovery of submarine hydrothermal vents in 1977. To aid decision-makers, an assessment of undiscovered seafloor massive sulfide deposits is made here. Estimates of number of undiscovered deposits and their metal contents are made and presented in probabilistic form to represent the inherent uncertainties in such estimates. The three-part form of assessment is used as a framework for this assessment because it can provide unbiased and reasonable estimates provided that consistent mineral deposit models are used. Four kilometer wide zones centered on a 256,000km2 tract associated with spreading centers and a tract of 100,000km2 associated with volcanic arcs and back-arc basins are delineated as permissive for undiscovered deposits. Well-explored volcanogenic massive sulfide deposits on land serve both as models of contained metals and as density models of deposit numbers. Density models provide probabilistic estimates of number of deposits in each tract. Frequency distributions of Zn, Cu, Pb, Ag, and Au contents in land deposits are used to represent metal contents in undiscovered seafloor deposits. Metal distributions were grouped into a mafic-related group to apply to the spreading center tract and a felsic and bimodal group to apply to the back-arc and volcanic arc tract. Tests of dependency of resource estimates demonstrate for the first time that independence can be assumed to combine such estimates using Monte Carlo simulation.Simulation is used to combine probabilistic number of deposits estimates for the two tracts and each of the five linked metal content distributions. Median estimates of undiscovered Zn, Cu, Pb, Ag, and Au are 6.4million, 4.6million, 1.2million, 10,000, and 390t respectively. Although there is considerable uncertainty in estimated amounts of Zn, Cu, Pb, Ag, and Au in undiscovered seafloor massive sulfide deposits located in the 4km wide zones, even the most optimistic estimates suggest that these deposits cannot make a substantial contribution to global metal production.