By Dan Oancea - Twitter
A few decades ago the prospect of offshore gas and oil production looked more like a Vernian fantasy. Nowadays, a large share of world’s oil and gas production come from the bottom of the seas.
What about minerals? Let’s start with the beautiful and hard carbon crystals: diamonds.
During the Cretaceous period a continental uplift generated a great deal of erosion for the South African land mass which hosts diamondiferous kimberlites deposits. Some 1,400 meters of kimberlite have been eroded and carried away by paleo river systems; with them an estimated 3 billion carats of diamonds found their way and a resting place in paleo river terraces along the Orange River, paleo beaches and marine deposits on the Namibian and South African coast of Atlantic. It is estimated that these marine deposits host 90% of the ‘fugitive’ diamonds’ total. The sorting and accumulation of diamonds took place on and near shorelines that are marked by numerous onshore and offshore old marine terraces (they witness sea level fluctuations). Depressions of the hard bedrock constituted traps, where rich pockets of diamonds have been found.
In 1908, over 40 years after the discovery of the first kimberlite pipe in South Africa, a railway construction worker found the first diamond near Luderitz, Namibia. A diamond rush ensued in the desert resulting in booming mining towns and in fencing of a large area, the Sperrgebiet (i.e.’ Forbidden Area’ in German). Note: A 2004 press release stated that as a result of an agreement with De Beers, the Sperrgebiet area now falls completely under the control of the Namibian government, which intends to declare it a National Park.
Old marine terraces had been found to be diamondiferous and beach deposits have started to be mined in the 1930s. In 1961 Sammy Collins, a Texan entrepeneur, did something that nobody believed to be possible: diamond mining of the shallow sea floor of South Africa.
Mining operations ceased in 1971 because of a slump in the price of diamonds, but small-scale shallow operations continued over the years. In the 1990s deep-water mining operations started with renewed vigor offshore the Namibian and South African coasts. Namibia has the richest marine diamond deposits in the world – 1.5 billion carats.
It was estimated that over 100 million carats have been produced in that area since 1908.
As there is no textbook for marine diamond mining, everything had to be invented and/or adapted – the exploration, sampling and mining technology and procedures. For exploration and sampling they used sonar equipment, shallow reflection seismic profiling, sonic rock drilling and grab sampling. For mining, water jets blast through the sediments which are then airlifted onboard, processed and X-ray sorted. The best method of monitoring the profitability is to calculate carats per hour of dredging. Grade is expressed in carats per square meter rather than carats per tonne (cpt). Adverse winds and sea swells are greatly affecting this type of marine operations.
De Beers used a 6 meter diameter drill head which cut into the sediments and air-lifted them on board of the ship and they also used seabed crawlers (remote-controlled tracked mining vehicles fitted with submersible dredge pumps and hydraulic jetting motors).
De Beers proved to be able to profitable run a relatively shallow offshore Namibian operation. At the same time they were the first to develop a suitable technology for mining deep-sea diamond deposits.
Earlier this year they announced that a new vessel will join their mining fleet. The “Peace in Africa” is under construction in Newcastle, UK and it will be the first ship to mine deep-sea diamond deposits on their offshore Sasa concession in South African waters. In Cape Town, DCD-Dorbyl offshore operation is building a 260 t undersea crawler; while another South African supplier is busy fabricating a 250 tph diamond treatment plant, which is going to be installed on board of the ship. The ship is supposed to dock in Cape Town before Christmas and the offshore mining operations are due to begin in the second quarter of 2007. The operation is supposed to generate a 250,000 ct/y production.
The Namibian offshore waters are also being scoured with mixed success by a group of determined exploration/mining companies.
Diamond Fields International operates its own vessel, the mv Diamond Fields Discoverer, a twin air-lift diamond mining vessel. Their Luderitz ML 111 property is a joint-venture exploration project with Bonaparte Diamond Mines. The latter operates the Fremantle mining ship, which boasts a proprietary seabed sampler: it has 5 m overall length and samples a 1.13 sq m area over a 5 m depth; a water jet liberate seabed sediments, which are then airlifted and processed on the board of the ship. The company also tested Western Australian offshore paleo river channels in the Cambridge Gulf area, with disappointing results to date.
Afri-Can Marine Minerals Corp. successfully explored their Namibian Block J marine diamond placer deposit and in October 2006 began the first phase of a resource delineation program.
Other minerals are also known to line up the sea bed. In 1978, Inco lifted on-board some 800 t of polymetallic nodules from a depth of almost 5,500 m. They are rich in manganese, copper and cobalt and represent metal concretions generated by a precipitation of metals from sea water. Subsequently, metal prices fell and nobody was interested in mining the ocean depths anymore.
In 1977 a small NOAA submersible stumbled upon something new: black smokers. They represent hydrothermal vents that riddle the sea floor in active plate zones along the Pacific “ring of fire”. Sea water percolates through crust, takes into solution metals and then a superheated pregnant solution vents through a chimney-like structure, sometime as tall as 30 m; metals spewed as a black smoke are deposited in the proximal area, thus forming sulphide crusts (i.e. ore deposits). It is a process similar with the one that was responsible for the genesis of nowadays land situated massive sulphide deposits. The black smoker ore deposits are rich in gold, copper and zinc.
In 1997, Julian Malnic, an Australian mining news writer acquired offshore exploration licences in Papua New Guinea; Nautilus Minerals Inc. was born. Subsequently, the company went public in Canada.
In 1999, Simon McDonald, an Australian geologist followed suit and established its Neptune Minerals, a London based company. Its target: the offshore waters of New Zealand.
The international organization in charge with regulating sea mining is the International Seabed Authority. Limited research revealed that the coastal waters of Papua New Guinea, New Zealand, Indonesia and Japan host almost 200 black smoker deposits, many of which contain some 5 to 10 million tones of ore. The problem is that they lie to an average depth of 2,100 m.
GNS Science a New Zealand Crown Research Institute posts a useful map called the Global Distribution of Arcs and Vents.
Another site that you could also visit is the International Marine Minerals Society’s site. It is a professional society whose members share a common interest in marine mineral deposits. They are also a sponsor for the Underwater Mining Institute - the annual conference on marine minerals.
In Papua New Guinea on their Solwara property, Nautilus looks for black smokers and their associated ‘ore fields’ by employing an underwater camera, which is also able to measure salinity, water temperature and magnetism. Sampling was done by a small submersible equipped with robotic arms and drills.
Good exploration results brought not only a well-deserved recognition for the small pioneer company but also some cash too: Giant miners stepped in and today Barrick owns a 9.59 per cent stake in Nautilus. On October 30, 2006 Anglo-American agreed to invest $25 million in Nautilus and to provide technical assistance.
On October 4, 2006 Nautilus announced that it had signed a deal with a Belgian dredging company specialized in building deep-sea mining ships. A 191 m vessel to be named Jules Verne will be delivered by 2009. That is the year when the company expects to begin mining its Papuan underwater mineral kingdom.
Nautilus envisages that two remote controlled vehicles would roll on the sea bed and grind some 400 t of ore per hour; the gold-copper ore will be then air-lifted on the vessel and subsequently loaded on barges. The ore will be processed on land. The company plans to produce 150,000 t of copper and some 400,000 ounces gold per year.
Neptune Minerals has exploration rights over 35,000 sq km of New Zealand waters. Their quest for what they call Seafloor Massive Sulphides (metal accumulations rich in copper, lead, zinc, gold and silver) was only partially successful. The 120 m to 1,800 m offshore exploration areas lie along two distinctive volcanic arcs: the Colville and Kermadec ridges. To date, samples collected from steep slopes returned no significant metal accumulations.
Even though technically possible the mining of the deep sea floor has to overcome another obstacle: a fair environmental assessment. A rich, diverse, unusual and little known association of plants, animals and bacteria live in this hostile environment. The mining companies claim that they will not mine the active vents; they are interested in mining the spent vents and their associated proximal layers of metal accumulation, so the disturbance will be minimal as there is almost no overburden to be removed. The same thing is claimed by deep sea diamond mining companies even though in their case a broad area has been disturbed by marine tailings discharge. The underwater miners stated that their operations are not causing more disturbance than those generated by offshore oil companies.
A novel exploration frontier is now open. Consider that water covers some 70% of the Earth’s surface and we know less about our oceans than about Moon or Mars. In the near future more geoscientists, miners and engineers will become aware of the new vast resource that lies underwater. And hopefully they will be more motivated in studying Marine Geology.
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