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Corresponding author: e-mail, Mark.Hannington@uottawa.ca
© 2010 Society of Economic Geologists, Inc. Special Publication 15, pp. 317–338

Abstract

Despite recent commercial interest in possible mining of sea-floor massive sulfide (SMS) deposits, there has been a great reluctance to attempt any estimate of their global abundance, owing to the limited exploration of the oceans and the general lack of knowledge of the deposits. The need for such an assessment is now more urgent, as a number of leading companies and international consortia have begun to invest in intensive exploration campaigns for SMS, and governments and other organizations have begun to establish the legal framework for sea-floor exploration and exploitation of mineral resources in territorial and international waters. A growing database of global SMS occurrences is beginning to provide clues to the likely distribution, size, and grade of the deposits. More than 300 sites of sea-floor hydrothermal activity and associated mineralization are now known on the ocean floor; about 200 of these are sites of confirmed high-temperature venting (black smokers) and associated polymetallic sulfide deposits. They occur primarily at mid-ocean ridges (65%) but also in back-arc basins (22%) and on submarine volcanic arcs (12%). More than 3,800 samples have been collected from 95 of the best studied deposits, and preliminary estimates of the sizes of the deposits have been made at 62 sites. The total amount of massive sulfide contained in the known deposits is estimated to be ˜50 million tons (Mt); the top 10 percent of deposits (≥2 Mt) contain about 35 Mt of massive sulfide or about 70 percent of the total. The largest deposits, excluding the Atlantis II Deep in the Red Sea, are on the order of 10 Mt in size. However, the median deposit size is only about 70,000 t. The average concentrations of metals based on analyses of surface samples are 3.6 wt percent Cu, 7.9 wt percent Zn, 0.4 wt percent Pb, 1.7 g/t Au, and 115 g/t Ag, although comparisons with drill core indicate that grades can be significantly lower below the sea floor in many, but not all, deposits.

A number of independent datasets, including global heat flow, circulation models for high-temperature vent fluids, geochemical budgets of the oceans, and the incidence of hydrothermal plumes, all arrive at similar estimates of ˜1,000 active vent sites along the mid-ocean ridges. As many as 500 additional vent sites may be located on submarine volcanic arcs and in back-arc basins, for a total of ˜1,500 sites. However, an analysis of the spatial distribution of known deposits, both on the mid-ocean ridges and in subduction-related environments, suggests that this is likely a maximum and that the total number of significant SMS occurrences along the neovolcanic zones of the world’s oceans is closer to ˜900. If the size distribution of the known deposits is representative of what remains to be discovered, then the total tonnage of SMS, excluding the Red Sea, is expected to be on the order of 600 Mt (˜1,000 deposits with a minimum size of 100 t and a maximum size of 10 Mt). The total contained metal would be about 30 Mt, based on a grade of 5 wt percent combined Zn + Cu + Pb. This estimate is similar to the total discovered metal in Cenozoic VMS deposits on land. However, it does not include long extinct deposits that may be located far off-axis. If present-day rates of massive sulfide formation on the mid-ocean ridges and back-arc spreading centers are extrapolated to older crust, then significant tonnages of massive sulfide may be expected beneath off-axis sediments.

In contrast to land-based exploration, where larger deposits are commonly discovered early in the exploration history of a VMS district, exploration of the modern sea floor has discovered a high proportion of small, widely spaced SMS deposits. Large, inactive deposits are more difficult to identify by current exploration methods but may exist in isolated areas that have yet to be fully explored, such as in heavily sedimented back-arc rifts. This raises the possibility of a dramatically different resource future for SMS if one or more large deposits or “districts” are discovered that contain a high proportion of the total metal.

Introduction

Volcanic-associated and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world’s total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth’s surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific “high-grade“ area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area. Recent developments suggest that the first deep-sea mining of base metals will be from the much smaller, higher grade massive sulfide deposits found at mid-ocean ridges, along active volcanic arcs, and in back-arc basin environments. For a number of reasons, both legal and technical, commercial exploration for these deposits has been restricted to exclusive economic zones, although the majority of high-temperature hydrothermal activity is located on the mid-ocean ridges in international water. One company has conducted sufficient drilling to estimate a NI 43-101 compliant resource at a location in the eastern Manus basin of Papua New Guinea (Solwara 1 deposit of Nautilus Minerals Inc.). However, the United Nations, through its International Seabed Authority, is also currently drafting regulations for prospecting and exploration of those deposits located beyond national jurisdictions.

Traditionally referred to as volcanogenic massive sulfide (VMS) or volcanic-hosted massive sulfide (VHMS) deposits, recent explorers of the oceans have used the acronym SMS for “sea-floor massive sulfide” deposits forming at submarine hydrothermal vents. At present, more than 300 sites of sea-floor hydrothermal activity and associated mineralization have been found. About 200 of these are confirmed SMS occurrences (Fig. 1); 95 of these deposits have now been sampled and at least partly mapped (Table 1), providing the first indications of the likely grades, tonnages, and distribution of deposits that remain to be discovered. Comparisons between modern SMS and ancient VMS deposits also have become increasingly sophisticated, highlighting important similarities and differences in their distribution, sizes, and bulk compositions (Franklin et al., 2005; Hannington et al., 2005).

Fig.1.

Locations of sea-floor massive sulfide (SMS) deposits and related hydrothermal vents (modified from Hannington et al., 2005). Numbers refer to high-temperature hydrothermal vents and related SMS deposits (filled circles). Other low-temperature hydrothermal vents and related mineralization are indicated by open circles. Major spreading ridges and subduction zones are indicated. Note that the current database contains more than 200 discrete deposits; a number of clusters of deposits are indicated here by just one symbol (e.g., Bent Hill, ODP Mound, Area of Active Venting at Middle Valley, no. 89). Ninety-five of these sites have been sampled and at least partly mapped; these are listed in Table 1 and used in the analysis presented in this paper.

Fig.1.

Locations of sea-floor massive sulfide (SMS) deposits and related hydrothermal vents (modified from Hannington et al., 2005). Numbers refer to high-temperature hydrothermal vents and related SMS deposits (filled circles). Other low-temperature hydrothermal vents and related mineralization are indicated by open circles. Major spreading ridges and subduction zones are indicated. Note that the current database contains more than 200 discrete deposits; a number of clusters of deposits are indicated here by just one symbol (e.g., Bent Hill, ODP Mound, Area of Active Venting at Middle Valley, no. 89). Ninety-five of these sites have been sampled and at least partly mapped; these are listed in Table 1 and used in the analysis presented in this paper.

Table 1.

Estimated Sizes (area) and Average Bulk Chemical Compositions of 95 Sea-Floor Massive Sulfide Deposits

LocationEstimated Area (m2)(n)Cu (wt %)ZnPbAu (ppm)Ag
    Mid-ocean ridges 
Juan de Fuca and Explorer 
 S. Explorer Ridge 5,000 51 3.2 5.4 0.1 0.72 125 
 Endeavour Segment 
  High Rise 3,000 49 1.9 9.6 0.2 0.28 63 
  Main field 5,000 97 2.6 6.7 0.5 0.14 260 
  Clam bed <100 0.3 2.8 1.1 0.12 145 
  Mothra 5,000 20 3.3 14.9 0.5 0.05 105 
 Co-axial <100 23.2 0.98 59 
 Northern cleft <100 26 1.7 26.6 0.5 0.25 225 
 Southern cleft <100 16 1.1 30.9 0.1 0.11 230 
East Pacific Rise 
 21°N, EPR <100 46 5.4 12.2 0.1 0.12 83 
 21°N, Green Seamount 300 2.6 0.2 <0.1 0.16 <1 
 14°N, EPR <100 2.8 4.7 <0.1 0.51 48 
 13°N, EPR 5,000 71 6.4 11.1 0.1 0.45 61 
 13°N Seamount 30,000 13 2.4 4.3 0.03 0.80 32 
 11°30'N, EPR <100 23 3.2 1.8 <0.1 0.25 23 
 11°32'N, EPR <100 32 3.4 0.1 <0.1 0.13 
 11°N, EPR <100 18 1.6 26.6 0.1 0.15 37 
 09°50'N, EPR <100 29 7.9 6.3 <0.1 
 07°24'S, EPR <100 14 11.1 2.1 <0.1 0.05 23 
 16°43'S, EPR <100 19 10.2 8.5 <0.1 0.32 55 
 17°26'S, EPR <100 1.3 5.6 <0.1 0.10 31 
 18°25'S, EPR <100 35 4.7 18.6 0.1 0.58 170 
 21°30'S, EPR <100 28 9.8 4.3 0.1 0.41 62 
 21°50'S, EPR <100 2.4 21.7 0.1 0.36 120 
 37°40'S, SEPR <100 24 3.8 3.1 <0.1 0.82 40 
Galapagos rift, 86°W 30,000 128 4.5 3.5 <0.1 0.29 35 
Indian Ocean 
 Sonne field, CIR 50,000 35 13.1 3.0 <0.1 1.02 46 
 Kairei field, CIR 3,000 20.4 10.3 <0.1 7.99 101 
 Edmond field, CIR 3,000 17 6.5 10.7 <0.1 4.01 188 
 Mt. Jourdanne, SWIR <100 1.9 16.7 1.2 4.29 630 
Mid-Atlantic Ridge 
 Broken Spur 5,000 76 4.9 3.7 <0.1 1.64 30 
 24°30'N, MAR 16.2 4.1 <0.1 5.30  
 TAG, surface and core 30,000 310 4.9 6.5 <0.1 1.80 92 
 Mir zone 50,000 137 5.0 8.7 <0.1 4.20 115 
 Alvin zone 100,000 2.0 0.3 <0.1 0.85 17 
 Snakepit 15,000 93 6.5 4.6 0.1 1.66 70 
 Krasnov field 150,000 137 5.6 0.4 <0.1 1.11 22 
 Semyenov, 13°30'N 300,000 
 Turtle pits, 5°S 5,000 43 7.0 2.3 <0.1 0.26 17 
 Nibelungen, 8°S <100 17.3 16.2 0.1 8.95 37 
Ridge-Hot Spot Intersections 
 Lucky Strike 3,000 36 4.5 2.0 <0.1 0.08 35 
 Axial volcano, CASM <100 31 0.3 16.3 0.4 4.12 250 
 Axial volcano, ASHES <100 90 2.3 17.3 0.2 1.56 295 
Uultramafic-hosted 
Mid-Atlantic Ridge 
 Rainbow 30,000 62 11.0 18.0 <0.1 2.53 189 
 Ashadze-1 50,000 104 10.5 16.3 <0.1 2.60 90 
 Ashadze-2 50,000 23 10.4 0.5 <0.1 8.50 
 Logatchev-1 5,000 78 23.4 3.9 <0.1 8.95 37 
 Logatchev-2 1,000 14.7 25.4 <0.1 23.80 92 
Sedimented ridges and rifts: 
 Middle Valley 50,000 100 1.1 2.8 0.03 0.16 
 Escanaba trough 1,000 51 1.8 5.6 1.2 1.29 163 
 Guaymas basin 15,000 22 0.4 2.2 0.4 0.15 82 
Intraoceanic back-arc basins 
Mariana trough 
 Alice Springs, 18°N 1,000 16 1.2 10.0 7.4 0.78 185 
 Forecast, 13°N 300 13 0.8 6.7 0.5 22.90 355 
Manus basin 
 Western Manus, Solwara 11  17 1.8 9.4 1.3 1.07 213 
 Central Manus, Solwara 3  31 1.1 21.3 1.4 15.15 642 
 Central Manus, Solwara 2  44 1.2 22.9 0.2 6.73 234 
 Central Manus, Solwara 10  13 7.1 7.5 0.1 2.31 152 
North Fiji basin 5,000 93 7.8 9.8 0.1 2.55 200 
Lau basin 
 Northeast Lau, NELSC  17 4.5 4.1 <0.1 4.64 62 
 Fonualei rift  29 2.1 14.5 0.3 
 MTJ caldera  29 6.7 9.2 <0.1 20.47 183 
 Kings Triple Junction  78 5.6 28.2 0.1 17 
 Kilo Moana, ELSC  16 3.7 22.9 <0.1 4.16 68 
 Towcam, ELSC  4.9 35.4 0.3 4.44 121 
 ABE, ELSC  23 3.1 29.3 0.3 2.85 107 
 White Church, NVFR  66 3.6 12.0 0.4 2.60 86 
 Tui Malila, NVFR  22 1.4 18.7 0.6 4.87 94 
 Vai Lili, CVFR  96 5.4 26.6 0.2 3.02 113 
 Mariner, CVFR  14 10.8 17.8 0.2 3.11 172 
 Misiteli, SVFR  0.4 8.0 1.9 13.44 656 
 Hine Hina, SVFR  37 4.5 13.8 0.5 3.22 337 
Volcanic arcs 
Izu-Bonin arc 
 Sunrise, Myojin Knoll 150,000 69 6.0 16.8 3.7 16.17 814 
 Kita Bayonnaise  4.7 17.3 0.4 9.13 162 
 Hakurei  24 1.0 37.2 3.1 6.00 692 
 Myojinsho caldera  1.5 25.5 4.6 1.71 205 
 Suiyo Seamount 3,000 32 13.0 13.5 0.5 24.28 229 
Kermadec arc 
 Brothers Seamount 5,000 37 0.9 13.2 1.9 0.5 340 
 Rumble II  9.8 0.1 <0.1 
Transitional arcs and back-arc basins 
Manus basin 
 Solwara 1 (Surface only) 90,000 250 9.7 5.4 1.1 14.96 174 
 Solwara 4 (Pacmanus) 15,000 257 7.3 22.6 1.2 13.38 257 
 Solwara 5  30,000 30 5.4 8.3 1.6 16.06 277 
 Solwara 6 15,000 14 14.0 13.9 0.6 18.84 5702 
 Solwara 7 15,000 27 4.3 19.4 1.3 13.35 317 
 Solwara 8  43 9.0 23.3 1.6 14.37 240 
 Solwara 9  17 6.3 10.6 1.5 19.91 296 
 Solwara 12  10 7.0 22.6 3.5 13.69 425 
 Solwara 13  9.1 30.7 3.3 4.72 546 
Franklin Seamount  34 <0.1 0.4 0.2 13.12 294 
Tyrrhenian Sea 
 Panarea Seamount  <0.1 6.2 2.8 11 
 Palinuro Seamount 3,000 15 0.4 17.9 8.6 3.40 270 
Bransfield, Hook Ridge  2.6 11.8 1.3 0.35 191 
Piip volcano, Aleutians  10 1.2 0.1 
Intracontinental back-arc rift 
Okinawa trough        
 Izena cauldron 5,000 40 3.3 20.2 11.8 3.13 2305 
 North Iheya Ridge  3.0 31.4 13.5 1.20 465 
 Minami Ensei knolls  1.5 12.8 5.0 2.43 90 
 Yonaguni knoll  4.7 17.0 9.8 
LocationEstimated Area (m2)(n)Cu (wt %)ZnPbAu (ppm)Ag
    Mid-ocean ridges 
Juan de Fuca and Explorer 
 S. Explorer Ridge 5,000 51 3.2 5.4 0.1 0.72 125 
 Endeavour Segment 
  High Rise 3,000 49 1.9 9.6 0.2 0.28 63 
  Main field 5,000 97 2.6 6.7 0.5 0.14 260 
  Clam bed <100 0.3 2.8 1.1 0.12 145 
  Mothra 5,000 20 3.3 14.9 0.5 0.05 105 
 Co-axial <100 23.2 0.98 59 
 Northern cleft <100 26 1.7 26.6 0.5 0.25 225 
 Southern cleft <100 16 1.1 30.9 0.1 0.11 230 
East Pacific Rise 
 21°N, EPR <100 46 5.4 12.2 0.1 0.12 83 
 21°N, Green Seamount 300 2.6 0.2 <0.1 0.16 <1 
 14°N, EPR <100 2.8 4.7 <0.1 0.51 48 
 13°N, EPR 5,000 71 6.4 11.1 0.1 0.45 61 
 13°N Seamount 30,000 13 2.4 4.3 0.03 0.80 32 
 11°30'N, EPR <100 23 3.2 1.8 <0.1 0.25 23 
 11°32'N, EPR <100 32 3.4 0.1 <0.1 0.13 
 11°N, EPR <100 18 1.6 26.6 0.1 0.15 37 
 09°50'N, EPR <100 29 7.9 6.3 <0.1 
 07°24'S, EPR <100 14 11.1 2.1 <0.1 0.05 23 
 16°43'S, EPR <100 19 10.2 8.5 <0.1 0.32 55 
 17°26'S, EPR <100 1.3 5.6 <0.1 0.10 31 
 18°25'S, EPR <100 35 4.7 18.6 0.1 0.58 170 
 21°30'S, EPR <100 28 9.8 4.3 0.1 0.41 62 
 21°50'S, EPR <100 2.4 21.7 0.1 0.36 120 
 37°40'S, SEPR <100 24 3.8 3.1 <0.1 0.82 40 
Galapagos rift, 86°W 30,000 128 4.5 3.5 <0.1 0.29 35 
Indian Ocean 
 Sonne field, CIR 50,000 35 13.1 3.0 <0.1 1.02 46 
 Kairei field, CIR 3,000 20.4 10.3 <0.1 7.99 101 
 Edmond field, CIR 3,000 17 6.5 10.7 <0.1 4.01 188 
 Mt. Jourdanne, SWIR <100 1.9 16.7 1.2 4.29 630 
Mid-Atlantic Ridge 
 Broken Spur 5,000 76 4.9 3.7 <0.1 1.64 30 
 24°30'N, MAR 16.2 4.1 <0.1 5.30  
 TAG, surface and core 30,000 310 4.9 6.5 <0.1 1.80 92 
 Mir zone 50,000 137 5.0 8.7 <0.1 4.20 115 
 Alvin zone 100,000 2.0 0.3 <0.1 0.85 17 
 Snakepit 15,000 93 6.5 4.6 0.1 1.66 70 
 Krasnov field 150,000 137 5.6 0.4 <0.1 1.11 22 
 Semyenov, 13°30'N 300,000 
 Turtle pits, 5°S 5,000 43 7.0 2.3 <0.1 0.26 17 
 Nibelungen, 8°S <100 17.3 16.2 0.1 8.95 37 
Ridge-Hot Spot Intersections 
 Lucky Strike 3,000 36 4.5 2.0 <0.1 0.08 35 
 Axial volcano, CASM <100 31 0.3 16.3 0.4 4.12 250 
 Axial volcano, ASHES <100 90 2.3 17.3 0.2 1.56 295 
Uultramafic-hosted 
Mid-Atlantic Ridge 
 Rainbow 30,000 62 11.0 18.0 <0.1 2.53 189 
 Ashadze-1 50,000 104 10.5 16.3 <0.1 2.60 90 
 Ashadze-2 50,000 23 10.4 0.5 <0.1 8.50 
 Logatchev-1 5,000 78 23.4 3.9 <0.1 8.95 37 
 Logatchev-2 1,000 14.7 25.4 <0.1 23.80 92 
Sedimented ridges and rifts: 
 Middle Valley 50,000 100 1.1 2.8 0.03 0.16 
 Escanaba trough 1,000 51 1.8 5.6 1.2 1.29 163 
 Guaymas basin 15,000 22 0.4 2.2 0.4 0.15 82 
Intraoceanic back-arc basins 
Mariana trough 
 Alice Springs, 18°N 1,000 16 1.2 10.0 7.4 0.78 185 
 Forecast, 13°N 300 13 0.8 6.7 0.5 22.90 355 
Manus basin 
 Western Manus, Solwara 11  17 1.8 9.4 1.3 1.07 213 
 Central Manus, Solwara 3  31 1.1 21.3 1.4 15.15 642 
 Central Manus, Solwara 2  44 1.2 22.9 0.2 6.73 234 
 Central Manus, Solwara 10  13 7.1 7.5 0.1 2.31 152 
North Fiji basin 5,000 93 7.8 9.8 0.1 2.55 200 
Lau basin 
 Northeast Lau, NELSC  17 4.5 4.1 <0.1 4.64 62 
 Fonualei rift  29 2.1 14.5 0.3 
 MTJ caldera  29 6.7 9.2 <0.1 20.47 183 
 Kings Triple Junction  78 5.6 28.2 0.1 17 
 Kilo Moana, ELSC  16 3.7 22.9 <0.1 4.16 68 
 Towcam, ELSC  4.9 35.4 0.3 4.44 121 
 ABE, ELSC  23 3.1 29.3 0.3 2.85 107 
 White Church, NVFR  66 3.6 12.0 0.4 2.60 86 
 Tui Malila, NVFR  22 1.4 18.7 0.6 4.87 94 
 Vai Lili, CVFR  96 5.4 26.6 0.2 3.02 113 
 Mariner, CVFR  14 10.8 17.8 0.2 3.11 172 
 Misiteli, SVFR  0.4 8.0 1.9 13.44 656 
 Hine Hina, SVFR  37 4.5 13.8 0.5 3.22 337 
Volcanic arcs 
Izu-Bonin arc 
 Sunrise, Myojin Knoll 150,000 69 6.0 16.8 3.7 16.17 814 
 Kita Bayonnaise  4.7 17.3 0.4 9.13 162 
 Hakurei  24 1.0 37.2 3.1 6.00 692 
 Myojinsho caldera  1.5 25.5 4.6 1.71 205 
 Suiyo Seamount 3,000 32 13.0 13.5 0.5 24.28 229 
Kermadec arc 
 Brothers Seamount 5,000 37 0.9 13.2 1.9 0.5 340 
 Rumble II  9.8 0.1 <0.1 
Transitional arcs and back-arc basins 
Manus basin 
 Solwara 1 (Surface only) 90,000 250 9.7 5.4 1.1 14.96 174 
 Solwara 4 (Pacmanus) 15,000 257 7.3 22.6 1.2 13.38 257 
 Solwara 5  30,000 30 5.4 8.3 1.6 16.06 277 
 Solwara 6 15,000 14 14.0 13.9 0.6 18.84 5702 
 Solwara 7 15,000 27 4.3 19.4 1.3 13.35 317 
 Solwara 8  43 9.0 23.3 1.6 14.37 240 
 Solwara 9  17 6.3 10.6 1.5 19.91 296 
 Solwara 12  10 7.0 22.6 3.5 13.69 425 
 Solwara 13  9.1 30.7 3.3 4.72 546 
Franklin Seamount  34 <0.1 0.4 0.2 13.12 294 
Tyrrhenian Sea 
 Panarea Seamount  <0.1 6.2 2.8 11 
 Palinuro Seamount 3,000 15 0.4 17.9 8.6 3.40 270 
Bransfield, Hook Ridge  2.6 11.8 1.3 0.35 191 
Piip volcano, Aleutians  10 1.2 0.1 
Intracontinental back-arc rift 
Okinawa trough        
 Izena cauldron 5,000 40 3.3 20.2 11.8 3.13 2305 
 North Iheya Ridge  3.0 31.4 13.5 1.20 465 
 Minami Ensei knolls  1.5 12.8 5.0 2.43 90 
 Yonaguni knoll  4.7 17.0 9.8 

Sources: Data compiled from Hannington et al. (2004 and 2005) and references therein; area estimates are indicated only for those deposits for which published maps or other detailed descriptions are available in the literature; numbers represent cumulative area of exposed sulfide where multiple vent complexes or mounds are included as part of one deposit; all values are approximate only; bulk compositions are for data reported in the literature (averages of n samples) and represent surface material only, except for the TAG mound

In this paper, we examine the current database of SMS deposits to gain a better understanding of their diversity and possible future as a source of base metals. Methods for conducting such an assessment have been developed over the past 30 yr, principally within the USGS (Singer, 2010; Singer and Menzie, 2010). The motivation for these assessments also has been clearly articulated: before decisions can be made about exploring the oceans for base metals, decisions must be made about what to explore for and whether it is worth the risk. Using the general principle that the tonnages and grades of well-explored deposits and deposit densities serve as models for the size, grade, and distribution of undiscovered deposits, we have made the first estimates of the total amount of SMS in the volcanically active parts of the present-day oceans. This analysis is based on detailed information from 95 of the best studied deposits, including some data from deep-sea drilling, and analyses of more than 3,800 samples. Comparisons with grade-tonnage models for land-based mineral deposits provide a measure of confidence in the estimated numbers.

References made in this paper to specific deposits or locations on the sea floor are strictly for illustration purposes and are in no way meant to indicate that resources suitable for commercial exploitation are actually present in any given area. Information such as area x thickness of contiguous sulfide bodies, bulk density, grade of metals, or other mineralogical and metallurgical characteristics cannot be used to infer an economic resource in any of the examples cited, and no such resources are implied.

Volcanogenic Massive Sufide Base Metal Resources— Past, Present, and Future

The most recent compilations of VMS deposits on land include about 1,100 deposits in more than 50 countries and 150 different mining camps or districts (Franklin et al., 2005; Mosier et al., 2009). A variety of approaches to the classification of these deposits have been advanced, most recently focusing on lithostratigraphic associations and inferred tectonic settings (Barrie and Hannington, 1999; Franklin et al., 2005). The majority of deposits are recognized as having formed in subduction-related arc and back-arc systems, either in oceanic environments or at the transition from oceanic to continental crust. Uncertainty in assigning specific tectonic settings to individual deposits commonly arises. However, with improved geochronology, lithogeochemistry, and volcanic reconstructions, it has been possible to classify most VMS deposits in terms of tectonic environments, allowing more precise comparisons with modern settings. At the same time, mapping of the ocean floor has resulted in more precise geodynamic models of the plate margins (e.g., Bird, 2003). Whereas the majority of SMS occurrences are still known mainly from the mid-ocean ridges, many deposits have now been located along intraoceanic arcs and back-arc spreading centers as well as at rifted continental margins (Hannington et al., 2005). These are also recognized as important settings for ancient VMS deposits.

In early databases, well-preserved type examples of VMS deposits (e.g., Kuroko-, Cyprus-, and Besshi-type) were the basis for establishing grade and tonnage models (Mosier et al., 1983; Singer, 1986a, b, c; Singer and Mosier, 1986a, b). These models have been refined in recent years with more precise classifications of the deposits (e.g., Mosier et al., 2007, 2009). The grade and tonnage models show expected trends; deposits associated with mafic volcanic rocks tend to be more copper rich and those associated with felsic rocks tend to have higher grades of zinc, lead, silver, and gold (Table 2). Metal contents and deposit sizes can be approximated by lognormal distributions. The largest deposits are on the order of several hundreds of millions of tonnes in size, but they are rare. Only about 10 percent of VMS deposits contain between 25 and 50 Mt of ore, fewer than 5 percent contain between 50 and 100 Mt, and only 3 percent are giant deposits of more than 100 Mt. The median sizes are 0.7, 1.9, and 3 Mt, respectively, in mafic-dominated, bimodal-mafic, and felsic-dominated successions (Mosier et al., 2009). Almost all of the metal is contained in the largest size classes and, in a typical VMS district, the single largest deposit commonly accounts for 60 to 70 percent of the total metal; the second largest deposit may contain only 10 to 20 percent of the metal (Sangster, 1980). Grade and tonnage data also highlight important secular variations in the formation of VMS deposits (Table 3), which correlate broadly with changes in tectonic styles and in the composition of the Earth’s crust and atmosphere through time (Franklin et al., 2005; Huston et al., 2010).

Table 2.

Grade and Tonnage Data for Volcanogenic Massive Sulfide and Sea-Floor Massive Sulfide Deposits Based on Reported Nonzero Grades (data are from Mosier et al., 2009, and this study)

VMS classification (host rock)(n)75th percentile50th percentile25th percentile
Felsic-dominated 
Million tons 421 0.7 3.0 11 
ΣZn + Cu + Pb   6.3 wt %  
Cu (wt %) 412 0.7 1.3 2.2 
Zn (wt %) 350 2.1 4.0 6.6 
Pb (wt %) 273 0.5 1.0 2.2 
Au (ppm) 280 0.4 1.0 2.0 
Ag (ppm) 301 22 42 94 
Bimodal-mafic 
Million tons 272 0.5 1.9 
ΣZn + Cu + Pb   4.0 wt %  
Cu (wt %) 267 0.9 1.4 2.1 
Zn (wt %) 217 1.2 2.3 4.6 
Pb (wt %) 77 0.1 0.3 1.0 
Au (ppm) 158 0.4 1.0 2.0 
Ag (ppm) 172 10 22 49 
Mafic-dominated 
Million tons 174 0.1 0.7 
ΣZn + Cu + Pb   2.8 wt %  
Cu (wt %) 174 1.1 1.7 2.8 
Zn (wt %) 58 0.4 1.1 2.5 
Pb (wt %) 13 <0.1 0.1 1.4 
Au (ppm) 72 0.3 0.7 1.8 
Ag (ppm) 70 12 31 
Sea-floor massive sulfide
Million tons 62 <0.01 0.07 0.7 
ΣZn + Cu + Pb   15.0 wt %  
Cu (wt %) 93 1.7 4.3 7.0 
Zn (wt %) 93 4.3 10.6 18.6 
Pb (wt %) 63 <0.1 0.1 1.2 
Au (ppm) 87 0.3 1.7 5.3 
Ag (ppm) 88 37 107 234 
VMS classification (host rock)(n)75th percentile50th percentile25th percentile
Felsic-dominated 
Million tons 421 0.7 3.0 11 
ΣZn + Cu + Pb   6.3 wt %  
Cu (wt %) 412 0.7 1.3 2.2 
Zn (wt %) 350 2.1 4.0 6.6 
Pb (wt %) 273 0.5 1.0 2.2 
Au (ppm) 280 0.4 1.0 2.0 
Ag (ppm) 301 22 42 94 
Bimodal-mafic 
Million tons 272 0.5 1.9 
ΣZn + Cu + Pb   4.0 wt %  
Cu (wt %) 267 0.9 1.4 2.1 
Zn (wt %) 217 1.2 2.3 4.6 
Pb (wt %) 77 0.1 0.3 1.0 
Au (ppm) 158 0.4 1.0 2.0 
Ag (ppm) 172 10 22 49 
Mafic-dominated 
Million tons 174 0.1 0.7 
ΣZn + Cu + Pb   2.8 wt %  
Cu (wt %) 174 1.1 1.7 2.8 
Zn (wt %) 58 0.4 1.1 2.5 
Pb (wt %) 13 <0.1 0.1 1.4 
Au (ppm) 72 0.3 0.7 1.8 
Ag (ppm) 70 12 31 
Sea-floor massive sulfide
Million tons 62 <0.01 0.07 0.7 
ΣZn + Cu + Pb   15.0 wt %  
Cu (wt %) 93 1.7 4.3 7.0 
Zn (wt %) 93 4.3 10.6 18.6 
Pb (wt %) 63 <0.1 0.1 1.2 
Au (ppm) 87 0.3 1.7 5.3 
Ag (ppm) 88 37 107 234 

1 Estimates from this study; tonnages are based on 62 deposits listed in Table 1; median grades are based on surface samples from 95 deposits listed in Table 1; see text for details

Table 3.

Distribution of Discovered Metals in Volcanogenic Massive Sulfide Deposits by Area and Age (modified from Franklin et al., 2005)

Area of exposed volcanic rocks (km2)Metal content (ΣZn + Cu + Pb) (t)Discovered metal (t/km2)
Archean 621,000 50,301,000 81 
Paleoproterozoic 990,000 43,427,000 44 
Meso- and Neoproterozoic 673,000 8,473,000 13 
Paleozoic 1,189,000 203,104,000 168 
Mesozoic 4,279,000 64,965,000 15 
Cenozoic 4,948,000 18,971,000 3.8 
Modern seafloor1 8,900,000 30,000,000 3.4 
Area of exposed volcanic rocks (km2)Metal content (ΣZn + Cu + Pb) (t)Discovered metal (t/km2)
Archean 621,000 50,301,000 81 
Paleoproterozoic 990,000 43,427,000 44 
Meso- and Neoproterozoic 673,000 8,473,000 13 
Paleozoic 1,189,000 203,104,000 168 
Mesozoic 4,279,000 64,965,000 15 
Cenozoic 4,948,000 18,971,000 3.8 
Modern seafloor1 8,900,000 30,000,000 3.4 

1 Estimated from this study; based on a permissive area 100 km wide encompassing 89,000 km of global mid-ocean ridges, volcanic arcs, and back-arc basins; total metal content is based on 600 Mt of massive sulfide with an average grade of 5% combined Zn + Cu + Pb; see text for details

These statistics are now routinely used, together with deposit density models, to assess the undiscovered mineral potential of different deposit types. The first step is a review of the geologic settings in which the deposits may occur. This is the basis for selection of permissive areas for the occurrence of deposits in unexplored regions and the first part of a three-part resource assessment of the type used by the USGS (Singer, 1993, 2008).

Environments of Sea-floor Massive Sulfide Formation

Almost all sea-floor hydrothermal activity occurs at plate margins, where a strong spatial and temporal correlation exists between magmatism, seismicity, and high-temperature venting. The neovolcanic zones of the plate boundaries have a total strike length in the oceans of 89,000 km. This includes both oceanic spreading centers (mid-ocean ridges) and intracontinental rifts, such as the Red Sea, with a combined length of 64,000 km, and submarine volcanic arcs and back-arc basins with a combined length of 25,000 km (Bird, 2003; de Ronde et al., 2003). The global incidence of sea-floor hydrothermal activity is closely linked to the magmatic budgets of each of these different settings, and the abundance of vents is roughly proportional to the lengths of the ridge and arc segments. Most high-temperature vents (65%) are located at mid-ocean ridges, with the remainder in back-arc basins (22%), along submarine volcanic arcs (12%), and on intraplate volcanoes (1%; Baker and German, 2004; Hannington et al., 2005).

Different types of spreading centers, both at mid-ocean ridges and in back-arc basins, are discriminated on the basis of spreading rate and ridge morphology, which vary in response to regional tectonic stresses and rates of magma supply (e.g., Baker et al., 1996). Fast-spreading ridges, with full spreading rates of 6 to 10 cm/yr, occur in relatively thin oceanic crust and are characterized by abundant volcanic eruptions; intermediate-rate (4−6 cm/yr) and slow-spreading (1−4 cm/yr) ridges occur in relatively thick crust and are characterized by only intermittent volcanism between long periods of essentially amagmatic, tectonic extension, and/or intrusive activity. Fast-spreading ridges account for about 25 percent of the total length of the mid-ocean ridges, whereas 15 percent are classified as intermediate rate, and 60 percent are slow spreading. Superfast-spreading centers (as much as 17 cm/yr), such as the southern East Pacific Rise (EPR), and ultraslow-spreading centers (<1 cm/yr), such as the Arctic and Southwest Indian ridges, are also recognized. The rate of magma supply, the depth of the subaxial magma, and the extent of magmatic versus tectonic extension influence the size and vigor of hydrothermal convection on the ridges. As a result, there is a general correlation between the spreading rate and the incidence of hydrothermal activity (Baker et al., 1995, 1996; Baker and German, 2004); however, the largest sulfide occurrences are commonly found where volcanic eruptions are episodic and alternate with long periods of intense tectonic activity and few eruptions (Hannington et al., 2005). Although most occurrences of SMS in the neovolcanic zones are associated with ongoing hydrothermal venting, about 20 to 25 percent of the deposits are no longer active. This does not include the older inactive deposits that may be buried by off-axis sediments (see below).

On fast-spreading ridges, such as the southern EPR, lavas are extruded onto the sea floor faster than the rate of extension, so the flows accumulate as local volcanic highs as much as 100 m above the surrounding sea floor. The eruptive fissures typically occupy a narrow axial graben (˜1 km wide), and this is the most common location for hydrothermal vents. However, because the eruption rates are so high, lavas can disrupt the flow of hydrothermal fluids and bury sulfide occurrences that are localized along the fissures. Also, because the upflow zones are relatively narrow (e.g., Coumou et al., 2008), they are subject to perturbation by intrusive activity that affects permeability within the neovolcanic zone. As a result, SMS deposits at fast-spreading ridges tend to be abundant but small, and they are rapidly displaced from their heat source by the high-spreading rates.

Slow- and intermediate-rate spreading centers, such as the Mid-Atlantic Ridge and Central Indian Ridge, are characterized by lower rates of magma supply and greater structural control on hydrothermal upflow than at fast-spreading ridges. Slow-spreading ridges, in particular, have wide (up to 15 km) and deep (up to 2 km), fault-bounded axial valleys. Here, eruptions occur only very rarely or at intervals of thousands of years; at the slowest spreading rates, the time between eruptions may be tens of thousands of years. Until 1984, it was generally accepted that hydrothermal activity on slow-spreading ridges would be limited because of the lack of near-sea floor magmatic heat. Following the discovery of the TAG Hydrothermal Field on the Mid-Atlantic Ridge, it became apparent that slow-spreading ridges may host some of the largest hydrothermal systems on the sea floor. Deeply penetrating faults allow circulation of hydrothermal seawater to much greater depths than on fast-spreading ridges, exposing a larger volume of hot rock to the hydrothermal fluids, and this important difference in hydrology contributes to the longevity of the hydrothermal systems and the sizes of the deposits. Recent seismic data from the TAG Field suggest that hydrothermal upflow occupies a large-scale ridge detachment fault and may be sourced at considerable depth (˜7 km) and tens of kilometers from the site of hydrothermal discharge (de Martin et al., 2007). Because deposits on the slow-spreading ridges are commonly located well off-axis, the substrate is stable enough to support hydrothermal upflow for many thousands of years and thus the growth of large sulfide mounds. Large off-axis volcanoes, which may be located 5 to 10 km from the ridges, similarly host important sulfide occurrences (e.g., at 13°N EPR).

The lack of known vents in other parts of the oceans (e.g., polar regions and the Southern Ocean; Fig. 1) mainly reflects the difficulties of marine research at these latitudes. Recent discoveries of hydrothermal plumes and massive sulfide deposits in the high Arctic (e.g., Gakkel Ridge: Baker et al., 2004) and in Antarctica (e.g., Bransfield Strait: Petersen et al., 2004) confirm that sea-floor hydrothermal activity along the neovolcanic zones of remote parts of the oceans is little different from that observed elsewhere. In contrast, almost no high-temperature hydrothermal activity is known to be associated with large intraplate hot-spot volcanoes, except in proximity to ridges (Hannington et al., 2005).

Some heavily sedimented environments, including sediment-covered ridges and rifted margins, are particularly important sites of SMS formation. One of the largest occurrences, Middle Valley on the Juan de Fuca Ridge, is in an area of 100 percent sediment cover and was discovered in the absence of high-temperature venting. High heat flow and hydrothermal manifestations in the sediment overlying the deposit indicate that it formed mainly by near subsea-floor mineralization (Davis et al., 1992). It is noteworthy that many of the largest VMS deposits in the geologic record also formed at least partly or entirely by subsea-floor deposition (discussed further below).

About one-third of the known high-temperature sea-floor hydrothermal systems and SMS deposits occur at submarine volcanic arcs or in back-arc basins (Hannington et al., 2005). Hydrothermal systems in subduction-related environments are broadly similar to those at the mid-ocean ridges, and many aspects of sea-floor hydrothermal activity first recognized on the mid-ocean ridges (e.g., hydrothermal plumes, chimneys, and mounds) are also observed in arc and back-arc settings. In contrast to the mid-ocean ridges, the convergent margin settings are characterized by a range of different crustal thicknesses, heat flow regimes, and magma compositions, depending on the nature of the converging plates (e.g., intraoceanic, continental, or transitional). The compositions of the volcanic rocks, in particular, vary from mid-ocean ridge basalt (MORB) to more felsic lavas (andesite, dacite, and rhyolite), which leads to major differences in the composition of the hydrothermal fluids and the mineralogy and bulk composition of the SMS deposits.

The numbers of vents at the volcanic fronts of the arcs remain poorly known, as these volcanoes are still being discovered and explored. On the Mariana and Kermadec arcs, which are the most completely surveyed, hydrothermal plumes have been found in the summit calderas of one in three arc-front volcanoes, which occur at intervals of ˜70 km along the lengths of the arcs (de Ronde et al., 2003, 2007). However, the majority of the high-temperature vents and the largest sulfide deposits, so far, in subduction-related environments are restricted to the back-arc spreading centers (e.g., Lau basin, North Fiji basin, Mariana trough). Other arc-related rifts, especially in epicontinental or continental margin arc environments (e.g., Okinawa trough, Bransfield Strait), are underexplored, in part because of the sediment cover. However, these may be the most important sites of massive sulfide accumulation in the oceans based on the record of land-based VMS deposits (Franklin et al., 2005; Hannington et al., 2005). The sediment load from adjacent continental margins has an insulating effect, preserving the high heat flow associated with rifting and also increasing the likelihood of preservation of contained sulfide deposits. The combination of continental basement, sediment input, and shallow-water depths also contributes to the complex mineralogy and chemistry of the deposits in epicontinental settings.

Although submarine hydrothermal vents have been found at depths ranging from 4,000 to <100 m, the majority of SMS occurrences occur at water depths of between 2,000 and 3,000 m, which is the depth range of most of the world’s mid-ocean ridges. The deepest vents occur on slow- or ultraslow-spreading centers that lack the crustal buoyancy associated with large volumes of subaxial magma. Although only about 10 percent of known sites are in >3,000 m of water, that number mainly reflects the lack of exploration at greater depths. Restricting exploration to shallow water (e.g., <2,000 m), for as yet unspecified technological reasons, would exclude many areas that are highly prospective for SMS. About one-third of known sea-floor hydrothermal vents occur at depths of less than 1,500 m. At these shallow depths, hydrothermal fluids at 350°C, which is the temperature of typical black smoker vents, will begin to boil, and this has a strong influence on the composition of the deposits forming on the sea floor. Cooling of hydrothermal fluids as a result of boiling causes deposition of metals below the sea floor, so large deposits are less likely to form in very shallow water.

These general characteristics of sea-floor hydrothermal systems can be used to identify permissive areas for SMS exploration, which is the basis for estimating deposit densities (i.e., numbers of deposits per unit area of permissive geology), as discussed further below.

Metal Concentrations in Sea-Floor Massive Sulfide Deposits

Samples collected from SMS deposits worldwide indicate important concentrations of Cu, Zn, and precious metals. Chemical data, originally compiled for the International Seabed Authority in 2002 and again in 2004 (Hannington et al., 2004), have recently been updated and now include analyses of more than 3,800 samples from 95 deposits. Summary data listed in Table 4 reveal systematic differences in the bulk compositions of the deposits in different volcanic and tectonic settings. This reflects the different substrates, including midocean ridge basalt, ultramafic intrusive rocks, more evolved lavas associated with volcanic arcs, and sediments, in both oceanic and continental crust. Superimposed on this source-rock control is the strong temperature dependence of metal concentrations in the vent fluids, as well as contributions of metals directly from the magma. Consistent trends are also observed in the trace element concentrations of the sulfides in different settings (e.g., high concentrations of Co, Se, and Ni in mafic rock-dominated mid-ocean ridge environments and high concentrations of As, Sb, Hg, Ba, and Au in volcanic arcs and young back-arc rifts: Hannington et al., 2005).

Table 4.

Average Bulk Compositions of 95 Sea-Floor Massive Sulfide Deposits by Tectonic Setting (source data in Table 1)

Location(n)Cu (wt %)ZnPbAu (ppm)Ag
Mid-ocean ridges1 2071 5.9 6.1 <0.1 1.6 89 
Sedimented ridges 173 1.1 3.6 0.5 0.5 84 
Intraoceanic back-arc basins 668 3.9 16.4 0.9 6.6 210 
Intraoceanic arcs 169 5.3 17.7 2.4 9.6 407 
Transitional arcs 728 6.4 14.8 2.0 12.2 692 
Continental margin arcs 60 3.1 20.3 10.0 2.3 953 
Location(n)Cu (wt %)ZnPbAu (ppm)Ag
Mid-ocean ridges1 2071 5.9 6.1 <0.1 1.6 89 
Sedimented ridges 173 1.1 3.6 0.5 0.5 84 
Intraoceanic back-arc basins 668 3.9 16.4 0.9 6.6 210 
Intraoceanic arcs 169 5.3 17.7 2.4 9.6 407 
Transitional arcs 728 6.4 14.8 2.0 12.2 692 
Continental margin arcs 60 3.1 20.3 10.0 2.3 953 

Notes: n = numbers of samples

1 Data for the mid-ocean ridges are based on deposits for which there are representative suites of 50 samples or more

Mid-ocean ridge deposits for which there are representative suites of 50 samples or more have average metal concentrations of 5.9 wt percent Cu, 6.1 wt percent Zn, <0.1 wt percent Pb, 1.6 ppm Au, and 89 ppm Ag (Table 4). Calcium, Ba, and SiO2 (in anhydrite, barite, and amorphous silica, respectively) are major constituents of the deposits but on average account for <20 wt percent of most samples. SMS deposits on sedimented ridges have average metal concentrations of 1.1 wt percent Cu, 3.6 wt percent Zn, 0.5 wt percent Pb, 0.5 ppm Au, and 84 ppm Ag. These data reflect the influence of hydrothermal fluids ascending through thick sequences of turbidites, as well as the tendency for precipitation of metals beneath the sediment-seawater interface. Calcite, anhydrite, barite, and amorphous silica are major components of the sediment-hosted deposits and commonly dilute the base metals. Unusually high metal concentrations have been encountered in deposits hosted by ultramafic rocks at several locations on the slow-spreading Mid-Atlantic Ridge (Hannington et al., 2005). Reported metal concentrations in samples collected from these deposits average 14.0 wt percent Cu, 12.8 wt percent Zn, <0.1 wt percent Pb, 9.3 ppm Au, and 83 ppm Ag, although in some cases this may reflect sea-floor weathering and secondary enrichment.

SMS deposits hosted by more evolved volcanic suites, typical of subduction-related arc and back-arc settings, have much higher concentrations of Zn, Pb, Au, and Ag compared to typical mid-ocean ridge deposits. Samples from deposits at intraoceanic back-arc spreading centers have average metal concentrations of 3.9 wt percent Cu, 16.4 wt percent Zn, 0.9 wt percent Pb, 6.6 ppm Au, and 210 ppm Ag. Samples from deposits in the summit calderas of arc volcanoes have average concentrations of 5.3 wt percent Cu, 17.7 wt percent Zn, 2.4 wt percent Pb, 9.6 ppm Au, and 407 ppm Ag. At arc-continent transitions and in intracontinental back-arc rifts, average metal concentrations are 6.4 wt percent Cu, 14.8 wt percent Zn, 2.0 wt percent Pb, 12.2 ppm Au, and 692 ppm Ag, and 3.1 wt percent Cu, 20.3 wt percent Zn, 10.0 wt percent Pb, 2.3 ppm Au, and 953 ppm Ag, respectively. The average metal contents of surface samples collected from all 95 SMS deposits listed in Table 1 are summarized in cumulative frequency plots in Figure 2. The data define general grade curves for Cu, Zn, Pb, Ag, and Au and can be approximated by typical lognormal distributions.

Fig.2.

Cumulative frequency plots of Cu, Zn, Pb, Ag, and Au grades for SMS deposits. The plotted data are based on averages of surface samples collected from 95 SMS deposits listed in Table 1. The data define general grade curves (lognormal distribution), and intercepts for the 90th, 50th, and 10th percentiles of the lognormal distribution are indicated. The summary data are also listed in Table 2.

Fig.2.

Cumulative frequency plots of Cu, Zn, Pb, Ag, and Au grades for SMS deposits. The plotted data are based on averages of surface samples collected from 95 SMS deposits listed in Table 1. The data define general grade curves (lognormal distribution), and intercepts for the 90th, 50th, and 10th percentiles of the lognormal distribution are indicated. The summary data are also listed in Table 2.

In almost all cases, the average metal concentrations of surface samples from SMS deposits are higher than reported grades of VMS deposits on land. This is because of a strong bias in sampling toward high-temperature sulfide chimneys, which are easiest to recover and are often the focus of detailed studies. Much less is known about the interiors of larger sulfide mounds, but it is clear that sulfide chimneys alone are not representative of the bulk compositions of the deposits as a whole. Evidence from a few well-documented examples indicates that most deposits are compositionally zoned; Cu- and Fe-rich sulfide minerals formed at higher temperatures (300°−350°C) are most abundant in the interiors of the mounds, whereas Zn sulfide minerals formed at lower temperatures (250°−300°C) are deposited on the surfaces of the mounds (e.g., Hannington et al., 1995, 1998). Systematic sampling of both high- and low-temperature assemblages across the surfaces of large deposits can provide a more representative picture of their bulk compositions. However, accurate information about the continuity of base and precious metal grades in the interiors of the deposits can only be obtained by drilling. A comparison of data from surface samples and drill core in deposits that have been drilled shows that the grades can be significantly lower below the sea floor in many, but not all, cases (Table 5).

Table 5.

Comparison of Average Bulk Compositions of Surface Samples with Drill Core Samples from Selected Sea-Floor Massive Sulfide Deposits

Location(n)Cu (wt %)ZnPbAu (ppm)Ag
TAG, Mid-Atlantic Ridge 
 Surface samples 176 5.30 8.35 0.03 2.66 171 
 Core samples 311 2.52 0.47 <0.01 0.35 
Snakepit, Mid-Atlantic Ridge 
 Surface samples 93 9.03 4.59 0.03 1.77 62 
 Core samples 14 10.16 4.65 0.03 0.60 16 
Middle Valley, Bent Hill 
 Surface samples 166 0.73 2.77 0.03 0.42 17 
 Core samples 87 1.19 1.51 0.02 0.08 
Middle Valley, ODP mound 
 Surface samples — — — — — — 
 Core samples 17 3.05 14.16 <0.01 — 
Solwara 1, Manus basin 
 Surface samples 250 9.73 5.36 1.08 14.96 174 
 Core samples 984 5.22 0.51 0.10 4.31 24 
Location(n)Cu (wt %)ZnPbAu (ppm)Ag
TAG, Mid-Atlantic Ridge 
 Surface samples 176 5.30 8.35 0.03 2.66 171 
 Core samples 311 2.52 0.47 <0.01 0.35 
Snakepit, Mid-Atlantic Ridge 
 Surface samples 93 9.03 4.59 0.03 1.77 62 
 Core samples 14 10.16 4.65 0.03 0.60 16 
Middle Valley, Bent Hill 
 Surface samples 166 0.73 2.77 0.03 0.42 17 
 Core samples 87 1.19 1.51 0.02 0.08 
Middle Valley, ODP mound 
 Surface samples — — — — — — 
 Core samples 17 3.05 14.16 <0.01 — 
Solwara 1, Manus basin 
 Surface samples 250 9.73 5.36 1.08 14.96 174 
 Core samples 984 5.22 0.51 0.10 4.31 24 

Sources: Data compiled from Hannington et al. (2004) and updated from Hannington et al. (2005); drill core data for TAG, Snakepit, and Middle Valley are from ODP and IODP drilling expeditions; data for Solwara 1 are compiled from publically released data from Nautilus Mineral Inc.

Sizes of Sea-Floor Massive Sulfide Deposits

Significant deposits, with explored dimensions on the order of 5,000 m2 or more, have now been found in most of the settings considered in this study (Table 1), including mid-ocean ridges (Galapagos, TAG, MIR, Ashadze 2, Krasnov, Logatchev, Semyenov), sedimented ridges (Middle Valley), intraoceanic back-arc basins (North Fiji basin), volcanic arcs (Myojin Knoll), rifted arcs in transitional or epicontinental environments (JADE, Solwara 1), and intracontinental rifts (Atlantis II Deep). However, of the more than 200 sites of high-temperature hydrothermal venting and associated mineralization, less than 10 percent are considered to be this large (e.g., Fouquet, 1997; Hannington et al., 1995, 2005). Most consist only of scattered hydrothermal vents and chimneys amounting to less than a few thousand tons.

The deposits listed in Table 1 are a test group considered to have the most reliable information about deposit size, but the true sizes are often incompletely known. Reported dimensions commonly include large areas of discontinuous sulfide outcrop or barren substrate between chimneys and mounds, thereby overestimating the sizes of the deposits. Because of the widespread dusting of metalliferous sediment and debris from collapsed chimneys, the continuity of sulfide bodies is difficult to assess, even with the most detailed surveys. A number of examples illustrate the challenges of reliably determining deposit sizes. On the Endeavour segment of the Juan de Fuca Ridge, 30 different sulfide complexes are distributed among eight vent fields along a 10-km segment of the axial valley. Collectively the hydrothermal activity covers a large area, but if all of the vent complexes were part of a single deposit, it would not be more than 100 m in diameter. When polymetallic sulfides were first discovered at Southern Explorer Ridge, the largest mound was estimated to be 250 × 200 m in area, based on submersible observations of the time. However, recent high-resolution surveys have shown that the mound comprises mainly lava covered by Fe-stained sediment, with only four clusters of chimneys covering a fraction of the area originally considered to be massive sulfide. In a submersible survey of the Sunrise occurrence, on the Myojin Knoll submarine volcano, an area of sulfide mineralization measuring 400 × 400 m was reported (Iizasa et al., 1999). Based on a relief of 30 m and a bulk density of 1.9 gm/cm3, a total accumulation of 9 Mt of massive sulfide was calculated. Three important assumptions were implicit in this calculation: (1) the sulfide outcrop was considered to cover 100 percent of the outlined area, including areas between sulfide ridges and mounds that are concealed by sediment; (2) the observed relief is due entirely to the accumulation of massive sulfide on a flat sea floor and not due to faults or buried volcanic features (e.g., lava domes); and (3) the bulk density is uniform and represents the entire volume used in the calculation. However, dive tracks indicate that submersible surveys cover only about 25 percent of the total area, and the visually identifiable sulfide outcrop cannot be reliably extrapolated to the limits of the mapped area. Thus, the estimate of 9 Mt is considered to be a maximum and possibly overestimated by a considerable margin.

Recent developments in marine geophysical methods, such as deep-towed magnetics and electromagnetic (EM) surveys, are providing tools for reliably determining the surface extent and continuity of massive sulfides on the sea floor (Tivey and Johnson, 2002; Kowalczyk and Jackson, 2007; Kinsey et al., 2008). Deep-towed EM was successfully employed in surveys of the Solwara 1 deposit in the eastern Manus basin to delineate areas of near-surface massive sulfides (Lipton, 2008). However, drilling is required to judge the continuity and thickness of sulfide outcrops in most cases and sometimes just to identify the deposits. For example, at Middle Valley on the Juan de Fuca Ridge, the sea floor is punctuated by numerous large mounds as much as several hundred meters in diameter and 50 m high. Drilling and other detailed surveys showed that most of these mounds are only uplifted blocks of sediment above buried volcanic sills. However, in one of the mounds, Bent Hill, 95 m of massive sulfide was intersected below the sea floor (Davis et al., 1992). A second smaller mound 300 m away (ODP mound) also was shown to be mostly massive sulfide (Zierenberg and Miller, 2000).

Reliable estimates of the total sulfide accumulation have been possible in only a few cases where drilling information is available. The four deep drill holes that penetrated the Bent Hill and ODP mounds at Middle Valley (ODP Legs 139 and 169) indicate a combined tonnage of between 10 and 15 Mt (Fouquet et al., 1998; Zierenberg et al., 1998). Seventeen holes drilled to a maximum depth of 125 m in the large TAG mound on the Mid-Atlantic Ridge, indicated 2.7 Mt of massive sulfide, averaging 2 wt percent Cu and 1.2 Mt of stockwork mineralization at 1 wt percent Cu (ODP Leg 158: Hannington et al., 1998). Data from extensive drilling of the Solwara 1 deposit in the eastern Manus basin (146 holes to a maximum depth of 20 m) have been used to estimate the first NI 43-101 compliant resource for a SMS deposit of 2.1 Mt (Lipton, 2008). Calculated grades for the indicated resource of 870,000 t are 6.8 percent Cu, 4.8 g/t Au, 23 g/t Ag, and 0.4 percent Zn, and the inferred resource of 1,300,000 t has grades of 7.5 percent Cu, 7.2 g/t Au, 37 g/t Ag, and 0.8 percent Zn at a 4.0 percent Cu cutoff. This is currently the best documented SMS deposit for which drill hole data are available.

From the size estimates of 62 deposits in the test group listed in Table 1 and from drilling information at deposits such as Solwara 1 (i.e., 2.1 Mt in a chimney zone covering 90,000 m2: Lipton, 2008), we can now construct a first-order tonnage curve for SMS. Each deposit in the test group has been classified into a size “bin” between 3,000 t and 10 Mt (Fig. 3), using the area versus tonnage relationship for Solwara 1 as a guide. About 33 percent of the deposits are considered to be smaller than 3,000 t and are not included in the plot. Six deposits have dimensions that indicate tonnages exceeding 2 Mt. The deposits in the largest size class (top 10%) include Middle Valley (˜10 Mt), Semyenov (˜9 Mt), Krasnov (˜3 Mt), Sunrise (˜3 Mt), the TAG Mound (2.7 Mt), Solwara 1 (2.1 Mt), and the Alvin zone (˜2 Mt). The sizes of three of these have been confirmed by drilling (TAG, Middle Valley, and Solwara 1); the others have “footprints” on the sea floor that are consistent with the indicated size class (e.g., Alvin zone, Sunrise, Krasnov, and Semyenov: Iizasa et al., 1999; Beltenev et al., 2007; Cherkashov et al., 2008). The total amount of massive sulfide in the test group of 62 deposits is estimated to be ˜50 Mt; about 70 percent of the total tonnage is contained in the top 10 percent of that group.

Fig.3.

Binned tonnages for 62 SMS deposits listed in Table 1. The size classes are estimated using the area vs. tonnage relationship for the Solwara 1 deposit as a guide. Only those deposits having surface areas larger than 100 m2 (˜3,000 t, n = 42) are included in the plot. The deposits in the largest size class (top 10%) include Middle Valley (˜10 Mt), Semyenov (˜9 Mt), Krasnov (˜3 Mt), Sunrise (˜3 Mt), the TAG Mound (2.7 Mt), Solwara 1 (2.1 Mt), and the Alvin zone (˜2 Mt). See text for discussion.

Fig.3.

Binned tonnages for 62 SMS deposits listed in Table 1. The size classes are estimated using the area vs. tonnage relationship for the Solwara 1 deposit as a guide. Only those deposits having surface areas larger than 100 m2 (˜3,000 t, n = 42) are included in the plot. The deposits in the largest size class (top 10%) include Middle Valley (˜10 Mt), Semyenov (˜9 Mt), Krasnov (˜3 Mt), Sunrise (˜3 Mt), the TAG Mound (2.7 Mt), Solwara 1 (2.1 Mt), and the Alvin zone (˜2 Mt). See text for discussion.

The binned data can also be plotted as a cumulative frequency “curve,” similar to that used in land-based resource assessments (Fig. 4). In this plot, the median size of the deposits is seen to be less than 100,000 t and closer to 50,000 t. No distinction is made between deposits on the mid-ocean ridges and deposits in arc and back-arc settings. So far, no deposits have been found in back-arc basins or on active volcanic arcs that are significantly larger than those on mid-ocean ridges, although the grades are different. Therefore, a separate tonnage model for deposits in subduction-related environments is probably not justified at this time. If we assume that these size classes are representative of deposits that remain to be discovered on the sea floor, then only one in 10 new deposits would be expected to contain more than 2 Mt. New information about the sizes of the known deposits is unlikely to change the median tonnage of the population; certain deposits may be moved from one size class to another, but the general features of the tonnage curve will remain the same. The discovery of one or more large deposits would change the proportion of massive sulfide in the 10th percentile but not the expected median size. An example of such a large deposit is the Atlantis II Deep in the Red Sea. However, this deposit is unique on the modern sea floor, both in terms of size and geologic setting (Hannington et al., 2005), and therefore should not be included in a tonnage model for SMS.

Fig.4.

Cumulative frequency plot of binned tonnages for 62 SMS deposits based on data listed in Table 1 and shown in Figure 3. The size classes are estimated using the area vs. tonnage relationship for the Solwara 1 deposit as a guide and plotted at the end points of each bin. Only the first 42 deposits are shown, accounting for 67% of the population. All other deposits are estimated to be smaller than 100 m2 in size (˜3,000 t; Table 1). The plotted points define a general tonnage curve (lognormal distribution), and approximate intercepts for the 50th and 10th percentiles of the lognormal distribution are indicated. Based on this model, the median size of the SMS deposits is seen to be less than 100,000 t and closer to 50,000 t; Endeavour Ridge is shown as an example of a median-sized deposit. The curve is strongly influenced by the largest deposits in Table 1, which have the least uncertainty in terms of size. With additional exploration or more accurate data from drilling, certain deposits may move from one size class to another, but the general features of the tonnage curve are unlikely to change significantly.

Fig.4.

Cumulative frequency plot of binned tonnages for 62 SMS deposits based on data listed in Table 1 and shown in Figure 3. The size classes are estimated using the area vs. tonnage relationship for the Solwara 1 deposit as a guide and plotted at the end points of each bin. Only the first 42 deposits are shown, accounting for 67% of the population. All other deposits are estimated to be smaller than 100 m2 in size (˜3,000 t; Table 1). The plotted points define a general tonnage curve (lognormal distribution), and approximate intercepts for the 50th and 10th percentiles of the lognormal distribution are indicated. Based on this model, the median size of the SMS deposits is seen to be less than 100,000 t and closer to 50,000 t; Endeavour Ridge is shown as an example of a median-sized deposit. The curve is strongly influenced by the largest deposits in Table 1, which have the least uncertainty in terms of size. With additional exploration or more accurate data from drilling, certain deposits may move from one size class to another, but the general features of the tonnage curve are unlikely to change significantly.

Mass Accumulation Rates

The small sizes of many deposits on the mid-ocean ridges are most likely related to the transient nature of the heat sources, which include narrow dike injections along eruptive fissures and faults. This is confirmed by uranium series measurements, which indicate that hydrothermal discharge at fast-spreading centers is episodic on time scales of only 10 to 100 yrs (e.g., Kim and McMurtry, 1991; Stakes and Moore 1991; Lalou et al., 1993b; Koski et al., 1994). Similar short-lived venting episodes have been documented at hydrothermal sites on arc volcanoes (e.g., de Ronde et al., 2005). The high rate of heat removal by individual black smokers in such environments ensures that venting associated with the shallow diking events is relatively short lived (e.g., Lowell et al., 1995). However, a typical black smoker chimney, even in the smallest deposit, can deliver a considerable amount of metal and sulfur to the sea floor each year. Discharge rates of about 1 to 2 kg/s of fluid containing more than 250 ppm of dissolved metals and sulfur have been measured (Converse et al., 1984; Von Damm, 1990). At these rates, a single vent can have a mass flux of more 10,000 kg of metal and sulfur per year. A large vent field, which may contain as many as 100 black smoker vents, would have a total mass flux of 100 to 200 kg/s or an annual flux of metal and sulfur on the order of ˜1,000 t/yr.

The growth of the largest deposits appears to have taken place over periods lasting 103 to 105 yr. The large TAG mound formed from repeated episodes of hydrothermal activity over many thousands of years (Rona et al., 1993a, b; Humphris and Tivey, 2000). The presently active mound sits on 100,000- to 200,000-yr-old crust, and hydrothermal activity in the area is thought to have begun about 130,000 yrs ago with the deposition of low-temperature Mn oxides. The present mound appears to have developed in the past 20,000 to 50,000 yrs during pulses of high-temperature hydrothermal activity every 4,000 to 6,000 yrs (Lalou et al., 1990, 1993a, 1998; You and Bickle, 1998). Based on a cumulative high-temperature output of ˜5,000 yrs over the past 20,000 yr, Hannington et al. (1998) estimated a growth rate of about ˜500 t/yr for the main massive sulfide lens (2.7 Mt), although higher rates of growth have been estimated from the amount of metal currently vented at this site (Humphris and Cann, 2000). Similar growth rates have been estimated for other large deposits on the Mid-Atlantic Ridge (Logatchev 1 and 2, Ashadze 1 and 2, and Krasnov: Kuznetsov et al., 2006; Cherkashov et al., 2008), based on the maximum ages and estimated tonnages of the deposits. The large sulfide deposits at Middle Valley are also situated on crust that is at least 300,000 yrs old, and the hydrothermal system is estimated to be at least 125,000 yrs old (Davis and Villinger, 1992). The protracted history of hydrothermal venting at these sites is a consequence of deep-seated magmatic activity followed by long periods of cooling and the release of heat from depth. The present location of the deposits, as much as 10 km from the neovolcanic zone, is viewed as a favorable setting for such a long-lived hydrothermal system because of the deep faulting at the margins of the axial valley (e.g., Rona et al., 1993a; de Martin et al., 2007).

For comparison, the largest accumulation of metals from sea-floor hydrothermal activity, now preserved as metalliferous mud in the Atlantis II Deep, formed over a period of about 15,000 yrs (Bäcker and Richter, 1973; Shanks and Bischoff, 1980). Assuming that the present resource (1.89 Mt Zn) began accumulating when the stable brine pool was first established, the metal accumulation rate would have been 126 t Zn/yr, equivalent to 100 percent efficiency of deposition from a brine having a Zn concentration of about 5 ppm and an influx rate of 670 kg/s (Anschutz and Blanc, 1996). This is similar to the expected flux of metal from several large mid-ocean ridge black smoker vent fields. The corresponding mass accumulation rate is on the order of 6,000 t/yr and roughly ten times that of black smoker deposits, reflecting the greater efficiency of metal deposition in the brine pool.

Toward an Estimate of Global Sea-Floor Massive Sulfide Potential

The preliminary models presented here provide the basis for a first estimate of the global abundance of SMS deposits and their contained metal. Following the three-part mineral assessment practice employed by the USGS, the first step in this assessment is an estimate of the tonnages and grades of undiscovered resources using the data from well-explored deposits that are considered to be representative of the total population. The second part involves an examination of geologic maps to determine the area that may be permissive for the discovery of new deposits. The third part is an estimate of the number of undiscovered deposits in that area based on an analysis of deposit densities. The essential criterion is that all deposits are represented by the same grade-tonnage model developed from a subset of well-explored examples. In this study, the grade and tonnage models of SMS have been generated from a validation set of 62 deposits (Table 1). Another validation set might be chosen from the global database, but it is unlikely that the results would be significantly different from those presented in Figures 3 and 4.

The deposit densities can be determined from control areas with known occurrences, chosen to represent similarly permissive areas elsewhere in the oceans. Hannington and Monecke (2009) conducted a preliminary analysis of deposit densities in 32 control areas of 5° lat by 5° long and containing a total of 106 deposits. This is similar to the number of control areas used by Mosier et al. (2007) to determine deposit densities for land-based VMS deposits. The map scale (1:2,500,000) was chosen so that the quality of map data was the same for all control areas. Permissive geology was identified from available bathymetric data and tectonic maps for each of the 32 areas. The total permissive area was assumed to extend no more than 50 km on either side of a spreading ridge or arc. Defined in this way, the total area of permissive geology for the oceans would be approximately 8,900,000 km2, based on a cumulative strike length of 89,000 km of ridges, arcs, and back-arc basins. At a global average full-spreading rate of about 3 to 4 cm/yr for mid-ocean ridges and back-arc spreading centers, this area of permissive geology would contain rocks having maximum ages of 2 to 3 Ma. In reality, most of the ridges are explored no more than 5 to 10 km from their axis, and it is unlikely that deposits much older than 1 Ma, now covered by marine sediments, would be included in any mineral potential estimates in the near future. The permissive area is also much larger than would be chosen for advanced exploration if high-resolution bathymetric maps were available for the whole of the oceans.

In standard mineral assessments, the deposit densities must be based on unambiguous definitions of what constitutes a deposit. For SMS occurrences described in the literature, this has included everything from a single vent or chimney to a large mound or cluster of vent complexes. The latter may cover areas of the sea floor ranging from tens to hundreds of meters in diameter, and individual mounds or chimney complexes may be separated by hundreds of meters up to several kilometers of barren sediment or volcanic rock. In the most recent example of a mineral assessment for land-based VMS deposits, a minimum spacing of 500 m between occurrences was used in the identification of single deposits (Mosier et al., 2007). Because of the scale of the maps used for the control areas in Hannington and Monecke (2009), the spacing between sulfide occurrences was generally much larger (e.g., eight vent complexes over a strike length of 10 km in the Main Endeavour Field were grouped as one deposit), thus eliminating the possibility of counting individual vents or chimney complexes as “deposits.” However, in other areas (e.g., the TAG Hydrothermal Field on the Mid-Atlantic Ridge), several large deposits of ˜1 to 2 Mt each occupy a relatively small area of 10 km2. In estimating deposit densities, Hannington and Monecke (2009) did not distinguish between a “deposit” that consists of a cluster of small vent complexes (e.g., the Main Field at Endeavour Ridge) and a cluster of larger mounds in the same-sized area (e.g., TAG, MIR, and Alvin deposits).

An example of one of the 32 control areas chosen by Hannington and Monecke (2009), the Central Indian Ridge, is shown in Figure 5. In this case, the permissive geology covers an area of ˜55,000 km2 (50 km on either side of the ridge axis) and contains five deposits. The selected area of permissive geology is close to the average for all 32 control areas (Table 6), although the range was from 110,000 to as little as 25,000 km2. The variance in the size of permissive areas is generally lower on the mid-ocean ridges than in the geologically more complex arc and back-arc systems. In some cases, the permissive areas are small because of the proximity to land; in others they are large, due to the presence of more than one major geologic feature of interest, such as areas containing both back-arc rifts and arc volcanoes. It is noteworthy that the average size of the permissive areas is about five times the size of the exploration permits and 20 times the size of a final lease specified in the “Regulations for Prospecting and Exploration of Seafloor Polymetallic Sulfides” currently being drafted by the International Seabed Authority.

Fig.5.

An example of one of the 32 control areas used to estimate deposit densities (modified from Hannington and Monecke, 2009). Permissive geology within roughly 5° lat by 5° long was identified from available bathymetric data and tectonic maps of each of the 32 control areas. A map scale of approximately 1:2,500,000 was chosen so that the quality of the map data was the same for all control areas. In this case, the permissive geology covers an area of 55,000 km2 of the Central Indian Ridge (roughly 50 km on either side of the ridge axis) and contains five deposits. The average spacing between deposits in this area is 108 km. Estimates of deposit densities for all 32 control areas are listed in Table 6. The sizes of possible “mining leases” (e.g., as defined in the Draft Regulations for Prospecting and Exploration of Polymetallic Sulfides; see Hannington and Monecke, 2009) are illustrated by deposit areas of 2,500 km2 (25 contiguous blocks of 10 × 10 km each) surrounding each of the known occurrences.

Fig.5.

An example of one of the 32 control areas used to estimate deposit densities (modified from Hannington and Monecke, 2009). Permissive geology within roughly 5° lat by 5° long was identified from available bathymetric data and tectonic maps of each of the 32 control areas. A map scale of approximately 1:2,500,000 was chosen so that the quality of the map data was the same for all control areas. In this case, the permissive geology covers an area of 55,000 km2 of the Central Indian Ridge (roughly 50 km on either side of the ridge axis) and contains five deposits. The average spacing between deposits in this area is 108 km. Estimates of deposit densities for all 32 control areas are listed in Table 6. The sizes of possible “mining leases” (e.g., as defined in the Draft Regulations for Prospecting and Exploration of Polymetallic Sulfides; see Hannington and Monecke, 2009) are illustrated by deposit areas of 2,500 km2 (25 contiguous blocks of 10 × 10 km each) surrounding each of the known occurrences.

Ninety percent of the control areas have densities of two or more deposits per 100,000 km2, 50 percent have densities of six or more deposits, and 10 percent have densities of ten or more deposits per 100,000 km2. The density of deposits is overall much lower than that estimated for land-based VMS deposits. This is clearly a reflection of the scale of the maps used to make the assessment. Mosier et al. (2007) and Singer (2010) showed that estimates of deposit densities are always inversely correlated with the map scale and the size of the control area because the larger areas result in overestimation of the permissive geology. The map scale of 1:2,500,000 used by Hannington and Monecke (2009) is the minimum scale recommended by Mosier et al. (2007); however, maps of the sea floor with a larger scale (i.e., higher resolution bathymetry) are not available for all of the oceans. In the absence of better maps, a more meaningful assessment of deposit densities can probably be derived from the linear dimensions of the ridge or arc segments because the vast majority of the deposits are restricted to the relatively narrow neovolcanic zones. In this case, the average spacing between occurrences in all 32 control areas is 98 km (Table 6). The spacing is greater on the slow-spreading ridges (167 km) than on the fast-spreading ridges (46 km), although the individual sulfide occurrences on the slow-spreading ridges are larger on average, as noted above. On the volcanic arcs, the spacing of the deposits is more closely linked to the spacing of volcanoes, but the distances between volcanic centers are remarkably similar to the spacing of vent fields on the mid-ocean ridges (de Ronde et al., 2003; Hannington et al., 2005).

To estimate the global abundance of deposits, the simplest definition of the permissive “area” is the cumulative strike length of the oceanic plate boundaries (i.e., 89,000 km). If we assume an average density of one deposit for every 98 km of ridge, arc, and back-arc spreading center, the total number of deposits expected for the volcanically active parts of the oceans is ˜900. Considering the range of deposit densities estimated in Table 6, we can expect at least 500 deposits (90th percentile) and not more than 5,000 (10th percentile). This result obviously has a large uncertainty. However, several independent estimates of the spacing of hydrothermal vents, at least on the mid-ocean ridges, confirm that the distance between sulfide occurrences is likely quite regular at the regional scale and closely approximates the ˜100-km average spacing determined by Hannington and Monecke (2009).

Estimates of the global abundance of SMS in the neovolcanic zones are testable because the formation of the deposits is directly related to processes that can be observed and measured at a global scale. For example, a variety of geophysical measurements provide an indication of the heat and mass fluxes of hydrothermal systems on the mid-ocean ridges from which the numbers of vents can be estimated. In order to remove the heat from newly formed crust, about 3 to 6 × 1013 kg/yr of seawater must be circulated through the axial zones of the world’s mid-ocean ridges and heated to a temperature of at least 350°C (Elderfield and Schultz, 1996; Schultz and Elderfield, 1999; Alt, 2003). Similar mass fluxes have been estimated from the geochemical budgets of a number of elements in the oceans (e.g., 3He, Mg, Sr, Mn, Li). These have ranged as high as 1.5 × 1014 kg/yr (see reviews by Baker et al., 1995, 1996; German and Angel, 1995; Kadko et al., 1995) and as low as 7 × 1012 kg/yr (Nielsen et al., 2006). The calculations assume that the component of diffuse flow, about 90 percent of the discharge, represents end-member fluid in which the elements have been conservatively diluted with seawater (Rona and Trivett, 1992; Schultz et al., 1992; Baker et al., 1993; Ginster et al., 1994). The discharge of high-temperature black smoker fluids (10% of the total flux) is estimated at 1012 to 1013 kg/yr. From this the flux of metals and sulfur is on the order of 106 t/yr, assuming a combined metal and sulfur concentration of about 200 to 500 mg/kg in the high-temperature fluids. As noted above, a single large vent field may contain as many as 100 black smoker vents having a total mass flux of high-temperature fluids of 100 to 200 kg/s. Thus, a global mass flux for the mid-ocean ridges of 1012 to 1013 kg/yr would be equivalent to ˜1,000 large vent fields.

Mottl (2003) and Sinha and Evans (2004) also estimated the heat transport associated with high-temperature convection at the ridge axis. Their estimates of 1.8 ± 0.3 × 1012 W were similar to earlier calculations by Elderfield and Schultz (1996), ranging from 1.1 × 1012 to 2.5 × 1012 W. Assuming a heat flux of 2 to 5 MW for a single black smoker vent (Converse et al., 1984; Bemis et al., 1993; Ginster et al., 1994), between 50,000 and 100,000 black smokers would be required to account for the high-temperature flux (10% of 1.8 ± 0.3 × 1012 W), or a density of at least one black smoker for every kilometer of ridge axis. Single large vent fields have measured heat outputs equivalent to 200 to 500 MW (e.g., Becker and Von Herzen, 1996; Kelley et al., 2001, 2002). Thus, a vent field of this size would be expected only every 50 to 100 km, again equivalent to ˜1,000 large vent fields. Although simplistic, these calculations are a useful first-order check on the estimated number of high-temperature hydrothermal systems along the ridges. There are no estimates of the hydro-thermal flux from volcanic arcs or back-arc spreading centers, although the total magmatic budget and magnitude of hydrothermal convection is likely proportional to their lengths.

Baker and German (2004) and Baker (2007) also showed that the spatial density of hydrothermal plumes detected in the water column along the lengths of the mid-ocean ridges is quite regular. Their database indicates a low value of about one plume every 200 km to a high of one every 25 km. The distance between plume sources was considered to reflect the length of the recharge distance and therefore the length of the melt lens beneath the ridges. On slow-spreading ridges, the vents were expected to be spaced farther apart, and perhaps active for only 5 percent of the time, whereas on fast-spreading ridges, the vents are smaller, closely spaced, and active for more than 50 percent of the time (Baker, 2007), consistent with observations of the mapped occurrences. From the plume densities, Baker and German (2004) concluded that the number of active vent fields on the global ridge system is ˜1,000.

The agreement between these estimates gives some confidence in the numbers of vent fields on the mid-ocean ridges. However, the plume data and the heat flow data do not apply to the volcanic arcs and back-arc spreading centers. A first-order extrapolation based on the lengths of volcanically active arc and back-arc segments would suggest as many as 500 additional vent sites or a total of ˜1,500 sites for the global ocean. Our analysis, based on the spatial distribution of known deposits on the mid-ocean ridges and in subduction-related environments (Table 6), suggests that this is likely a maximum, and the number of significant SMS occurrences is probably closer to ˜900, as noted above.

Once an estimate has been made of the number of deposits, it is possible to place some constraints on the total amount of massive sulfide present by assuming a size distribution similar to that represented in Figure 4. If we use ˜1,000 deposits with a minimum size of 100 t, a maximum size of 10 Mt, and a size distribution similar to that shown in Figure 4, the total amount of massive sulfide is on the order of 600 Mt. The top 10 percent of deposits would contain ˜400 Mt. Importantly, these estimates do not include extinct deposits that may be buried beneath sediments flanking the mid-ocean ridges and back-arc spreading centers. The oldest deposits in the current database are <300,000 yrs old and located within 5 to 10 km of the ridge axis. If present-day rates of massive sulfide formation on the ridges are extrapolated to older crust, then significant tonnages of massive sulfide might be expected beneath the off-axis sediments (possibly an additional 600 Mt/m.y.). However, deposits located more than 50 km off-axis are likely to be covered by as much as 100 m of pelagic sediment at typical oceanic sedimentation rates (on the order of 1−5 cm/1,000 yr).

The total contained metal is more difficult to assess, owing to the large bias in sampling of SMS deposits and the very high grades illustrated in Figure 2. Figure 6 compares the bulk compositions of SMS deposits at mid-ocean ridges and in arc-related settings to grades of VMS. Metal concentrations similar to those of samples collected from SMS deposits are found in only the highest grade VMS deposits on land. Grades of more than 10 wt percent combined Zn + Cu + Pb, such as those reported for many SMS, occur in only 10 percent of deposits in the bimodal-mafic class on land (Mosier et al., 2009). As discussed above, the bulk metal concentrations of SMS are probably much lower than suggested by the surface samples. Therefore, to estimate the total contained metal, we have chosen a median grade of no more than 5 wt percent combined Zn + Cu + Pb, which more closely matches that of VMS deposits on land (Table 2). Applied to the cumulative tonnage of SMS determined above, we estimate the total contained metal of the deposits in the neovolcanic zones to be about 30 Mt. The top 10 percent of deposits would contain about 20 Mt of Zn + Cu + Pb. For comparison, the metalliferous muds of the Atlantis II Deep contain 2.3 Mt of Zn + Cu + Pb.

Fig.6.

Base metal grades of land-based VMS deposits and SMS deposits in mid-ocean ridge and suprasubduction settings. Grades are indicated for bimodal-mafic (n = 272), felsic-dominated (n = 421), and mafic-dominated (n = 174) VMS deposits from Mosier et al. (2009) (Table 2). Data for the mid-ocean ridge and suprasubduction SMS deposits (n = 95) are listed in Table 1. Boxes are defined for the 25th and 75th quartiles and the median (center line). The horizontal line indicates the average values for all VMS and all SMS.

Fig.6.

Base metal grades of land-based VMS deposits and SMS deposits in mid-ocean ridge and suprasubduction settings. Grades are indicated for bimodal-mafic (n = 272), felsic-dominated (n = 421), and mafic-dominated (n = 174) VMS deposits from Mosier et al. (2009) (Table 2). Data for the mid-ocean ridge and suprasubduction SMS deposits (n = 95) are listed in Table 1. Boxes are defined for the 25th and 75th quartiles and the median (center line). The horizontal line indicates the average values for all VMS and all SMS.

Comparisons with Land-Based Mineral Deposits

Comparisons with land-based VMS deposits provide a number of additional clues to the likely abundance and distribution of SMS deposits. Based on the current lithostratigraphic classification of VMS deposits, it is possible to identify close analogs of many ancient deposits on the present-day sea floor (e.g., Franklin et al., 2005). The two most relevant models are those for so-called “Cyprus-type” deposits and the Kuroko deposits of Japan. Cyprus-type massive sulfide deposits have long been considered to be the best ancient examples of sulfide deposits on the mid-ocean ridges and in mature back-arc basin environments (e.g., Hannington et al., 1998). The Kuroko deposits are analogs of deposits that occur in volcanic arc environments (Ishibashi and Urabe, 1995; Glasby et al., 2008).

The data for VMS deposits in Cyprus (Mosier et al., 1983; Singer and Mosier, 1986a) indicate a median size of about 2 Mt (Fig. 7). This is significantly larger than the estimated median size of SMS deposits, mainly because the data for deposits on land include only those deposits that were of sufficient size to be mined or large enough to have justified drilling. In reality, large numbers of sulfide occurrences are either too small or of too low grade to have been mined and, therefore, are not considered in the total past production and current reserves. Among the Cyprus deposits, this is estimated to include more than 90 undeveloped prospects containing <100,000 t each and probably many more much smaller occurrences that were never even considered to be prospects (Hannington et al., 1998, and references therein). These are probably analogous to the many small isolated chimneys and mounds that are found along the mid-ocean ridges and back-arc spreading centers. If all of these smaller undeveloped prospects were included, then the median size of Cyprus-type massive sulfides would shift toward significantly lower tonnage, similar to that estimated for SMS. A revised model for all mafic-hosted VMS deposits (n = 174; Mosier et al., 2009) more closely matches that estimated for SMS, but the median size of 0.74 Mt is still an order of magnitude larger (Table 2, Fig. 7). The size distribution of the Kuroko deposits is also shown in Figure 7. During mining of the Kuroko deposits, careful records were kept of the physical dimensions of the orebodies, which are minimally deformed (Tanimura et al., 1983). The average surface area of the 44 mined deposits in the Hokuroku basin was about 200 × 200 m, which is similar to that of the largest SMS deposits.

Fig.7.

Comparison of the estimated tonnage curve for SMS deposits with tonnage curves for selected VMS deposit types on land. The SMS curve is based on binned data shown in Figure 4. The curve for VMS deposits in the Troodos ophiolite of Cyprus (n = 42) is from data in Mosier et al. (1983) and Singer and Mosier (1986a). The curve for Kuroko deposits in the Hokuroku district of Japan is from data in Tanimura et al. (1983). The more recent compilation for mafic-hosted VMS deposits (n = 174) from Mosier et al. (2009) is also shown. Intercepts for the 90th, 50th, and 10th percentiles of the lognormal distribution are indicated for the VMS deposits. The plotted data for VMS include only those deposits of sufficient size to have been mined economically; however, a majority of sulfide occurrences in the Troodos ophiolite and in the Hokuroku basin are either too small or of too low grade to be mined and are not included in the tonnage models. In Cyprus, there are estimated to be more than 90 undeveloped prospects containing <100,000 t each and probably many more much smaller occurrences that were never considered to be prospects (Hannington et al., 1998, and references therein). If undeveloped prospects are included, the curves for VMS deposits would shift toward significantly lower tonnages.

Fig.7.

Comparison of the estimated tonnage curve for SMS deposits with tonnage curves for selected VMS deposit types on land. The SMS curve is based on binned data shown in Figure 4. The curve for VMS deposits in the Troodos ophiolite of Cyprus (n = 42) is from data in Mosier et al. (1983) and Singer and Mosier (1986a). The curve for Kuroko deposits in the Hokuroku district of Japan is from data in Tanimura et al. (1983). The more recent compilation for mafic-hosted VMS deposits (n = 174) from Mosier et al. (2009) is also shown. Intercepts for the 90th, 50th, and 10th percentiles of the lognormal distribution are indicated for the VMS deposits. The plotted data for VMS include only those deposits of sufficient size to have been mined economically; however, a majority of sulfide occurrences in the Troodos ophiolite and in the Hokuroku basin are either too small or of too low grade to be mined and are not included in the tonnage models. In Cyprus, there are estimated to be more than 90 undeveloped prospects containing <100,000 t each and probably many more much smaller occurrences that were never considered to be prospects (Hannington et al., 1998, and references therein). If undeveloped prospects are included, the curves for VMS deposits would shift toward significantly lower tonnages.

The total tonnage of SMS deposits estimated here for seafloor neovolcanic zones (˜600 Mt) is small compared to the total past production and current reserves of VMS on land. The most recent databases of Franklin et al. (2005) and Mosier et al. (2009) indicate past and current geologic resources of ˜14,000 Mt in global VMS. The cumulative tonnage of mafic rock-hosted VMS deposits, which are considered to be the closest analogs of presently known deposits on the sea floor, is 831 Mt, but this includes deposits that are as old as 600 Ma. A more meaningful comparison with ancient VMS can be made by looking at rates of massive sulfide formation normalized to the area of preserved volcanic rocks (Franklin et al., 2005; Table 3).

Table 3 shows that some time periods, such as the Paleozoic, were clearly more favorable for the formation and preservation of large VMS deposits, reflecting environmental conditions such as major episodes of global anoxia. However, such conditions do not exist, except in very local areas, in the present-day oceans. Although there is evidence for variation in spreading rates and lithosphere production during the past 180 m.y. (e.g., Conrad and Lithgow-Bertelloni, 2007), it seems unlikely that hydrothermal activity in the modern oceans has been any more or less productive in terms of mass accumulation rates of metals than at other times since the Paleozoic. Taking into account the half-lives of oceanic crust and active marginal basins (e.g., Veizer, 1988) and the probability of preservation of the associated volcanic rocks (e.g., Eastoe and Gustin, 1996), it appears that the net addition of massive sulfide to the crust in the past few hundred million years has remained relatively constant. The global rate of massive sulfide formation, in terms of tons of discovered metal per km2 of preserved volcanic rock, was about 4 t of combined Zn + Pb + Cu per km2 in the Cenozoic and 15 t per km2 in the Mesozoic. If we assume that ˜600 Mt of massive sulfide is the minimum amount in 8,900,000 km2 of permissive geology in the modern oceans (i.e., considerably more metal might be found beneath off-axis sediments 50 km on either side of the neovolcanic zones), then the total metal accumulation would be at least 3.4 t per km2 (Table 3). This is similar to the amount of discovered metal in VMS deposits occupying a comparable area of the youngest volcanic rocks preserved on land (˜5 Mkm2 of Cenozoic volcanic belts) and further evidence to suggest that the estimated rates of SMS formation are of the right order of magnitude.

Another characteristic of VMS deposits on land is that they are highly clustered in districts that contain numerous closely spaced deposits. The Kuroko deposits, which included more than 90 Mt of massive sulfide, occupied a relatively small area of only 100 km2. In Canada, the typical massive sulfide mining district contained an average of 12 deposits in an area of 84 km2 (Sangster, 1980). The Noranda camp hosted nearly 100 Mt in 17 deposits, not including the giant zone 5 at Horne, in an area of less than 200 km2. Mosier et al. (2007) determined similarly high deposit densities in 38 different VMS districts worldwide, ranging in size from 24 to 82,000 km2, with a median area of 425 km2. Clusters of deposits have been found on the sea floor (e.g., the TAG, MIR, and Alvin zones of the TAG Hydrothermal Field; Solwara deposits of the eastern Manus basin), but the numbers of deposits and the total tonnage in a given area, so far, have been small by comparison. Using Mosier et al.’s (2007) equation developed for VMS deposits on land, the number of deposits expected in a permissive area of 55,000 km2, which was the average of 32 control areas analyzed by Hannington and Monecke (2009), would be 18. This is significantly larger than the number of deposits known in those areas (Table 6). Smaller areas, such as the TAG Hydrothermal Field or the area surrounding Solwara 1 in the eastern Manus basin, contain close to the expected number of occurrences (i.e., three deposits in 100 km2 at TAG and nine in 5,000 km2 surrounding Solwara 1). Nevertheless, it is clear that the resource potential represented by many VMS districts on land is substantially larger than that presently known for any similar-sized areas on the sea floor. Part of this might be explained by the fact that many of the major mining districts on land include deposits that are exposed at a number of different stratigraphic levels. However, large numbers of deposits in districts like the Hokuroku basin and in Noranda formed more-or-less contemporaneously at discrete horizons in the host volcanic successions (e.g., Main contact tuff in the Noranda camp: Franklin et al., 2005). Thus, the range of ages of the deposits in most VMS districts is not significantly greater than the 2- to 3-m.y. window of permissive geology considered by Hannington and Monecke (2009).

A major challenge for future explorers of SMS deposits will be to discover larger deposits that contain a much higher proportion of the total metal, similar to the large-tonnage VMS deposits on land, or districts with large numbers of smaller but closely spaced deposits. Nearly one-half of the total tonnage of VMS deposits on land is contained in just 50 large-tonnage deposits, each >50 Mt in size. Only one deposit of this size has been found on the modern sea floor, in the Atlantis II Deep of the Red Sea. Other large-tonnage deposits might eventually be found buried by off-axis sediments or forming by subsea-floor replacement in sediment-covered marginal basins. These settings contained some of the most important VMS deposits in the geologic record (e.g., Bathurst, New Brunswick, and the Iberian pyrite belt), but similar rifted continental margin environments in the present-day oceans, such as the Okinawa trough, have yet to be thoroughly explored. Subsea-floor replacement is considered to have been an important process in the formation of many of the world’s largest VMS deposits (e.g., Large et al., 2001). The depth of emplacement is poorly known in most cases but likely from 10 to 200 m below the sea floor; at greater depths, the sediments are progressively compacted, dewatered, and altered, and therefore less amenable to large-scale infiltration and replacement by hydrothermal fluids (Doyle and Allen, 2003). Such deposits may be difficult to find in modern rift basins, as they may lack an obvious sea-floor expression that would be immediately recognized during exploration. New technology is being examined to overcome this, including deep-towed magnetics, electromagnetics, seismics, heat flow, and gravity, but application of most of these geophysical tools to SMS exploration is still in its infancy.

Conclusions

Glasby (2000) provided a sobering reminder of the past economic forecasts for the recovery of metals from the oceans and the legal framework in which deep-sea mining might take place, suggesting that it was premature to judge the potential future economic significance of deep-sea mineral deposits. Still, the uncharted oceans are widely believed to host vast mineral resources (e.g., Rona, 2003). The recent advances in commercial exploration for SMS deposits have encouraged this first attempt to estimate the magnitude of the potential new resource. Also, various models have been proposed for multiyear exploitation of SMS deposits, although production rates and other technical aspects have not been fully tested or disclosed. These models are based on comparisons with mining on land, which is reasonable, as it must be assumed that any seabed mining would have to compete economically with operations on land. Comparisons with grade-tonnage models for land-based mineral deposits can be used to characterize SMS occurrences that are likely to be uneconomic, even if they were to be discovered. However, questions about the numbers and sizes of much larger SMS deposits continue to be raised. Using the general principle that the frequency distributions of deposit sizes and grades in well-explored areas of the sea floor can serve as models for tonnages and grades of undiscovered deposits, many of these questions can now be addressed. A growing database of SMS occurrences is beginning to provide the necessary input for the models, allowing deposit occurrence information to be translated into more meaningful assessments of the global abundance of SMS.

Although the reliability of these estimates is far from assured, the present state of knowledge allows some constraints on the total amount of SMS in the oceans, with implications for their future development as a source of base metals. In particular, a number of geologic considerations outlined in this paper place important limits on the global abundance of SMS and help to eliminate unreasonably optimistic or pessimistic views of the potential at an early stage. A variety of independent datasets, including global heat flow, circulation models for high-temperature vent fluids, geochemical budgets of certain elements in the oceans, and the incidence of hydrothermal plumes, also provide a measure of confidence in the estimated numbers of SMS deposits. The largest known deposits, excluding those in the Atlantis II Deep, are on the order of 10 Mt in size. However, the median deposit size is only about 70,000 t. The total tonnage in ˜1,000 deposits is expected to be on the order of 600 Mt, containing about 30 Mt of Zn + Cu + Pb. For comparison, the USGS National Mineral Resource Assessment (U.S. Geological Survey, 1996) estimated that the amount of metal remaining to be discovered in stratiform massive sulfide deposits in the conterminous United States is about 90 Mt of Zn + Cu + Pb.

New discoveries on the sea floor are certain to be made, and new information is likely to become available on the size and grade of known deposits, but it is unlikely that this will greatly change the grade and tonnage models proposed for the neovolcanic zones. If commercial companies or other consortia are prepared to explore beyond the neovolcanic zones, a significantly larger resource will likely be found, but this exploration must take place under cover and any proposed mining would have to occur under tens to hundreds of meters of sediment. As well, little is known about the long-term preservation of SMS deposits off-axis. Deposits far from the ridges are likely to be dissected by extensive faulting at the flanks of spreading centers and may undergo further degradation as a result of submarine weathering and the action of iron-oxidizing bacteria (e.g., Edwards, 2004).

Comparisons of SMS with ancient analogs assume that the conditions of ore formation and preservation have been uniform over geologic time. Extensive research has shown that this assumption is likely not valid for deposits older than a few hundred million years. However, calculated rates of massive sulfide formation on the modern sea floor are similar to those of the Cenozoic when normalized to the area of exposed volcanic rocks from that time. This suggests that the formation of massive sulfide deposits in the oceans has not changed much in the past hundred million years and that unique models for SMS are probably not required. A major difference, however, is that most of the tonnage of fossil VMS is contained in a small number of large deposits and districts. This aspect of ancient VMS appears to be fundamentally different from the presently known distribution of SMS and suggests that a few large-tonnage deposits containing a higher proportion of the total metal, or districts with large numbers of smaller but closely spaced deposits, remain to be discovered in the oceans. Volcaniclastic successions and sediment-filled marginal basins are obvious targets for future exploration, as these settings host some of the most important ancient VMS deposits. However, the same problem that challenges explorers on land, that is exploration under cover, will also be a challenge for future exploration in the oceans.

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Acknowledgments

Support for this research has been provided by the International Seabed Authority, which funded the creation of the global database of SMS deposits, through a Natural Sciences and Engineering Research Council of Canada Grant to MDH, and a Society of Economic Geologists Student Research Grant to John Jamieson. Jim Franklin, Bruce Gemmell, and Rich Goldfarb provided helpful comments that improved an earlier version of this paper.

Figures & Tables

Contents

GeoRef

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