GEOCHEMISTRY


‘Nobody is planning to mine under the sediment – yet. We have not even found those deposits because they are hidden under cover.’ SMSD


exploitation, like


polymetallic nodule


mining, will use remotely operated mining machines


to undertake excavation of surface outcrops on the sea-bed


once the sediment overburden is removed. Yet the problem of how to economically remove the overburden is currently unresolved. The


machines will cut the ore from the ocean ridges,


crush it and then send it as slurry to a


support vessel via an enclosed riser and lifting system


similar to that currently used by the oil industry. The slurry will then be dewatered to collect all but the finest particles – down to ~10 µm – then the seawater and fine particles will be discharged back into the sea at a depth close to the seabed. The ore will be transported to land for further processing. While single mining sites may cover local areas as small as 100m2


, economically it will make


sense to exploit multiple sites in close proximity. The third economically important sub-sea mineral


resource is cobalt crusts. Cobalt crusts are hard, metallic layers that form on the flanks of submarine volcanoes at water depths of


between 600 and 7000m. The richest crusts are found in areas where ocean currents are strongest. This is because cobalt crusts are very porous and mineral precipitation occurs inside their micrometre-sized


pores. The stronger the water current, the less infilled by sediment will be the crusts. Cobalt crusts are mainly formed


through the deposition of iron oxide- hydroxide [FeO(OH)] and manganese oxide (vernadite, MnO₂). But cobalt crusts are host to other valuable minerals as well because various other metals are also deposited as the iron oxide-hydroxide and vernadite precipitate out on, and in, the crusts. Iron oxide-hydroxide is slightly


positively charged and thus attracts negatively charged ions such as molybdate (MoO4


2- ). Vernadite, on


the other hand, is slightly negatively charged and attracts positively charged ions such as cobalt (Co2+ copper (Cu2+


), ) or nickel (Ni2+ ). Most of the metal ions contained


in seawater originate from land. Over time they are washed out of the rocks and transported by rivers to the oceans. Iron and manganese, however, usually enter the ocean through volcanic sources on the sea floor called hydrothermal seeps. Like polymetallic nodules, cobalt


crusts grow very slowly, at a rate of 1 to 5mm/million years. Cobalt crusts vary in thickness and mineralogy in different areas as a function of seawater composition. Typically the crusts vary from 2cm to 26cm thick in the richest deposits. Mining cobalt crusts presents


different challenges to both polymetallic nodules and SMSDs because they are firmly attached to the rocky substrate and need to be mechanically separated from their bed-rock.


Ecological concerns One thing all these technologies have in common is that they are invasive. They damage the environment, first by extracting the material and, secondly, by returning processed sediment to the ocean. Several groups worldwide are


concerned. Nautilus Minerals has already been granted a licence to exploit the sea-bed in the coastal regions around Papua New Guinea but is being challenged by, among others, the Alliance of Solwara Warriors. Pastor Matei, a spokesman for the


alliance says: ‘People from the Pacific are custodians of the world’s largest oceans and it is these oceans that connect everyone in the Pacific. The


oceans are as important as land. They are sources of food and livelihoods and they are of strong cultural and spiritual importance. Experimental seabed mining threatens this.’ However, Nautilus Minerals points out it that it has complied with all regulatory requirements including the production of an Environmental Impact Statement. CEO Mike Johnston also explains that, although they will extract the copper and gold, a Chinese consortium will be on site to remove the iron and the sulphur, thereby reducing the tailings left behind by the operation. Nautilus expects to begin operations by the first quarter of 2019 and will initially run its extraction machines at only 25% output to shakedown the technology and ensure ecological damage is kept to a minimum.


Pros and cons However, Mills stresses that there are benefits to studying deep sea metal deposits beyond the purely economic. It is now known, for example, that the deposits of metals such as iron, cobalt and gold from deep sea vents do not all re-precipitate out in the area of their formation. They can be carried for hundreds of miles as superplumes and are an important source of raw materials for algal production. In impoverished sea- surface waters, they can fuel the oxygen supply to the atmosphere and the draw down of carbon dioxide. However, proponents of mining


stress that manganese nodules and cobalt crusts are present as thin layers lying directly on the sea floor or on the flanks of seamounts. In contrast to ore deposits on land, they are therefore a two-dimensional resource that can theoretically be extracted with relatively little effort and without deep pits. Marine mining, they believe,


would be comparatively benign because only the surface of the sea floor or the seamount would be removed; however, this overlooks the unique nature of the ocean – its interconnectedness – and the fact that sedimentary disturbance will therefore travel over vast areas. Broadly speaking then, it is still


unknown to what extent deep sea mining will change life in the sea and what the eventual consequences would be for people and fisheries.


08 | 2017 37


DIRK WIERSMA/SCIENCE PHOTO LIBRARY


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