Polymetallic massive sulphide deposits
Polymetallic massive sulphide deposits are most commonly formed along tectonic plate boundaries and volcanic provinces in water depths from <500 to 5000m. Seawater seeps into the volcanic rock making up the seafloor through cracks and fissures. As this fluid becomes heated it circulates through the sub-seafloor, dissolving metals and other elements from the surrounding rocks. Convection carries this hot (up to 450°C), acidic, metal-rich fluid back up towards the surface where it is expelled at the seafloor, creating hydrothermal vents (see cartoon below). As this hot fluid reacts with the cold seawater particles of metal sulphide minerals precipitate from the fluid and settle out on the seafloor, creating an 'apron' rich in copper, lead, zinc, precious and trace metals. As venting continues, metal sulphides and minerals such as anhydrite (calcium sulphate) can build up to form multiple large chimneys through which hydrothermal fluids are expelled, or the fluid flow can be more diffuse in nature without the development of chimneys. Over time, the build-up of particulate sulphides, collapsed chimneys and other vent debris form a mound of metal sulphide-rich material: a seafloor massive sulphide deposit. Deposits of this type can range in size from several thousand to several million tonnes, and it is estimated that around 600 million tonnes of massive sulphide deposits occur within the easily accessible neovolcanic zone of mid-ocean ridges.
Above: Cartoon showing the formation of hydrothermal mineral deposits at mid-ocean ridges.
Deep-sea hydrothermal vents - the chimneys from which hot, mineral-rich water is expelled at the seafloor - are home to unique ecosystems. Here, the environment is harsh and hostile, with extremes of high and low temperatures, toxic chemicals spewing into the water column and a lack of sunlight combining to make vent sites inhospitable places for most forms of life. However, in spite of this, hydrothermal vents are home to animals that have evolved to thrive in this environment. These highly adapted fauna are able to survive due to their ability to tolerate the conditions and exploit chemical energy in the place of sunlight.
The fluid coming out of a vent is rich not only in dissolved minerals but also in chemosynthetic bacteria. These bacteria process hydrogen sulphide to produce organic material through chemosynthesis, which then fuels the local food chain. All animals living on or around the hydrothermal vent ultimately depend on these bacteria for food: some co-exist in a symbiotic relationship with these bacteria, some graze the bacteria directly (limpets and mussels), whilst others such as crabs and octopus prey on the creatures that eat the bacteria. Perhaps the most iconic of vent creatures is the tube worm, which are found in great numbers at many hydrothermal sites. Tube worms, like many other vent-specific creatures, are entirely dependent on a symbiotic (mutually beneficial) relationship with chemosynthetic bacteria within their tissue to provide them with nutrition, and the bacteria are reliant on the tube worm to deliver hydrogen sulphide from the vent fluids.
Other classic vent fauna include clams, mussels, crabs and shrimp.
Many of the creatures living at deep-sea vents are endemic and are found nowhere else on earth. In fact, over 600 species of animal have been discovered at hydrothermal vents. Due to the dynamic physical nature of hydrothermal vent sites, these communities are constantly at risk of extermination from either a sudden lapse in the emission of hydrothermal fluids or a sudden outpouring of lava from the spreading ridge. To cope with this, many vent fauna species have reproductive strategies that allow them to colonise new sites quickly.
Colonisation of a new hydrothermal vent site takes place in stages. Initially, bacterial mats form around the vent site, which attract grazing animals such as copepods and amphipods, and the presence of chemosynthetic bacteria within the hydrothermal fluids allow vent worm larvae to colonise the site. Slowly, larger animals such as crabs, shrimps, snails and octopus move in to prey on the grazing animals.
Above: Video from University of Bremen/MARUM showing footage of hydrothermal vents at mid-ocean ridges
Potential impacts of mining
Present mining scenarios have focused on cooled, inactive massive sulphide deposits colonised by apparently sparse populations of sponges, cnidarians, echinoderms and fish. Active hydrothermal vents are of limited commercial interest at present because most contain only comparatively minor resources, but they provide an important habitat for unique and productive communities (see above).
The distribution of various species along mid-ocean ridges is largely unknown, but recent research has indicated that benthic species have a close affinity with those found on continental margins. However, there is little information on the connectivity between populations along a ridge or between ridges and margins. Connectivity between vent sites is largely unknown; diffuse venting may be widespread and may aid the migration of adults and larvae. More information is needed on whether vent organisms are adapted to ephemeral environments prone to natural episodic events, and their resilience to a high level of natural toxicity. An assessment is required of whether a series of localised impacts will affect the distribution of these species over larger areas. Most information to date has been derived from photographic surveys, so more extensive sampling using Remotely Operated Vehicles (ROVs) is required for a full inventory of organisms associated with ridges and sulphide deposits.
Sediment plumes created by mining operations may impact suspension feeders, such as sponges and corals. In addition, the pulverisation of massive sulphides on the ocean floor will produce highly reactive sulphide mineral surfaces, generate acid by-products and potentially release harmful major and trace metals into the environment. The image on the left shows the effects of a submarine eruption at El Hierro in the Canary Islands. The eruption has created an area of acidic, turbulent water that is considered to be similar to the anticipated effects of processing deep-sea sulphide mineral ore on mobile platforms.
Current exploration activity
Currently, most exploration activity for massive sulphide deposits is taking place within Exclusive Economic Zones (EEZs) in the SW Pacific (c. 1000-2000 m water depth). Mining of copper and gold from sulphides was due to commence off Papua New Guinea in 2014, but at present is on hold. Greater interest is now being shown on waters in and around the Azores archipelago where exploration for minerals is due to begin in 2013 or 2014, and other major reserves occur within the waters of the overseas territories of a number of European states. Political and commercial interest is increasing owing to rising metal prices and the need to secure metal supplies. Within the past three years China, the Russian Federation, France and South Korea have been granted exploration licenses by the ISA for massive sulphides. Most recently, new license applications have been submitted to the International Seabed Authority to carry out exploration for polymetallic massive sulphides in the central Indian Ocean (see our News section).