Who would have considered that the next gold rush might be located up to 3,700 metres below the ocean's surface at tectonic spreading centres, where new crust is continually being formed?
The generation of hydrothermal fluids heated by the molten rock (magma) beneath the crust, has been observed to form 'black smokers' as they escape the seafloor at temperatures up to 400° Celsius. As these fluids mix with the cold surrounding seawater, metal sulphides in the water are precipitated onto the chimneys and nearby seabed. These sulphides, including galena (lead), sphalerite (zinc) and chalcopyrite (copper), accumulate at and just below the seafloor, where they form massive deposits that can range from several thousands to about 100 million tonnes.
High concentrations of base metals (copper, zinc, lead) and especially precious metals (gold, silver) in some of these massive sulphide deposits have recently attracted the interest of the international mining industry. Today, more than 100 sites of hydrothermal mineralization are known, including at least 25 sites with high-temperature black-smoker venting and associated mineral deposits. In addition, many polymetallic sulphide deposits are found at sites that are no longer volcanically active.
To-date, the best potential information available for any one vent-site in the South Atlantic is in a segment immediately south of the Ascension Fracture Zone. Water depth in this segment is >3000m yet it is situated <100km from the anchorage of Georgetown, Ascension Island.
• Prospecting has been limited – Currently, only ~ 5 percent of the 60,000 kilometres of ocean ridges around the world have been surveyed in any detail. The ocean ridges proximal to Ascension, St Helena and Tristan da Cunha are as yet un-surveyed.
• Deposits vary in size – For example the TAG hydrothermal mound on the Mid-Atlantic Ridge at 26°N and is reported to consist of ~million tonnes of ore and in comparison the Atlantis 11 deposit in the Red Sea consists of some 94 million tonnes.
• Metal contents are high – A detailed evaluation of nearly 1,300 seafloor sulphides reveals that different volcanic and tectonic settings have different metals concentrations. Deposits are generally rich in zinc, copper, iron, silver, lead and gold. Gold typically varies from 1.2g/tonnes to 26g/tonnes (highest is 230g/tonne)
• Ore processing is simple – As the deposits are predominantly metal sulphide (plus some silicates) it is relatively simple to separate gangue minerals using gravitational techniques. Subsequent recover of the metals from the ore is relatively straight forward using oxidative dissolution under acidic conditions followed by electrochemical extraction.
• Versatile mining operations – The technology for deep sea mining now exists (thanks to the oil and gas industry). The entire mining system is portable and can be moved from site to site. An investment in mining systems and ships would thus not be tied to a single location as in the case of land. Mining would be limited to relatively small areas of the seafloor and largely restricted to the surface (strip mining) and shallow subsurface (open cast mining) extraction.
• Value for the UK – The British territories of Ascension, St Helena and Tristan da Cunha are located close to the Southern Mid Atlantic ridge. Within the associated Economic Exclusion Zone there is a significant chance that seafloor sulphide deposits exist. Mining these deposits will add significant value to the economies of these islands through the provision of logistics, processing and distribution support to any mining operations. Such activities would generate employment, significant income and new infrastructure.
Chemical analyses of hundreds of seafloor samples reveal that deposits in different volcanic and tectonic settings have different concentration of metals. Relative to samples from sediment-starved mid-ocean ridges, massive sulphides formed in basaltic to andesitic environments of back-arc spreading centres (573 samples) are characterized by high average concentrations of zinc (17%), lead (0.4%) and barium (13%), but little iron. In general, the bulk composition of seafloor sulphide deposits in various tectonic settings is a consequence of the nature of the volcanic source rocks from which the metals are leached.
High concentrations of gold have recently been found in sulphide samples from back-arc spreading centres, whereas the average gold content for deposits at mid-ocean ridges is only 1.2g/t (1,259 samples). Sulphides from Lau back-arc basin have gold content of up to 29g/t with an average of 2.8g/t (103 samples). In the Okinawa Trough, gold rich sulphide deposits with up to 14g/t of gold (average 3.1g/t, 40 samples) occur in a back-arc rift within the continental crust. Preliminary analyses of sulphides in the Eastern Manus Basin reveal 15g/t with a maximum of 55g/t gold (26 samples). High gold content up to 21g/t has been found in barite chimneys in the Woodlark Basin. The most gold-rich seafloor deposit found to date is located at Conical Seamount in the territorial waters of Papua New Guinea, close to Lihir Island. Maximum gold concentrations in samples collected from the summit plateau of this seamount (2.8 km basal diameter at 1,600m water depth, summit at 1,050m) range up to 230g/t with an average of 26g/t (40 samples), which is about 10 times the average value for economically-minable gold deposits on land.