Farming mineral resources in artificial undersea hydrothermal deposits

Japan Agency for Marine-Earth Science and Technology Center
Mr. Tatsuo Nozaki

Can't we somehow make use of the resources lying at the bottom of the sea? This is a long-held dream of Japan, a country surrounded by oceans. Tatsuo Nozaki of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has a unique idea. As a mineral deposit scientist aboard the deep-sea exploration vessel Chikyu, he realized that high-quality ore could be artificially precipitated during seafloor drilling projects, and he is now working to realize a novel idea: "cultivating" seafloor mineral resources.

Chimney above artificial hydrothermal vent

It all started in 2010 with the Chikyu's search for microorganisms in the extreme environment of the Okinawa Trough. The Chikyu's high seafloor drilling capability has been utilized to collect sediments that make up the strata beneath the seafloor and to investigate the microbial habitat limits in the vicinity of hydrothermal vents. During this process, metal casing pipes were installed in some of the vents to protect the borehole walls from collapsing as they were dug, thus creating artificial hydrothermal vents where hydrothermal water spurts out artificially. After drilling, progress observation and sample collection surveys were conducted during a submersible survey with JAMSTEC's unmanned research vehicle (ROV) "Hyper Dolphin 3000" onboard. In the artificial hydrothermal vent, hydrothermal fluid continued to spout even after the drilling survey, but the pipe installed in the vent clearly looked strange. It was found that the pipe was clogged and minerals had precipitated on top, forming a chimney-like ore formation called a chimney. Similar chimney formation was observed in bare vents (bare holes) where no pipes were installed.

Young ore body brought back to earth

Mr. Nozaki took on the challenge of describing and chemically analyzing this chimney. Nozaki originally majored in mineralogy and has a track record of searching for events in Earth's history hundreds of millions of years ago through onshore deposits using the rhenium (Re)-osmium (Os) dating method, a radioactive isotope with a half-life of 41.6 billion years. The reason he wanted to work at JAMSTEC was that the seafloor contains younger, nearly freshly formed ore deposits than land-based mines. He had hoped that researching these deposits would provide insight into how they were formed on the seafloor before they were subducted with the plate and underwent high-temperature, high-pressure metamorphism. At such a time, a freshly formed ore deposit, which was created within two years of the drilling survey at the maximum, came to us as a research target. The sample was of great interest from a mineralogy point of view, and after several days of analysis, it became clear that it was a high-grade sample rich in copper, lead, and zinc.

Cooling mechanism in seawater

If a container (cell) can be placed completely over the vent and cooled in a manner that prevents direct contact between the hydrothermal fluid and seawater, it would be possible to precipitate high-purity ores in the cell and minimize the contamination of impurities (sulfate minerals) derived from seawater. Why were chimneys generated more rapidly in artificial hydrothermal vents than in naturally formed ones? This seems to be due to the fact that the Chikyu drilling formed large hydrothermal channels with a diameter of over 50 cm, which are difficult to form in nature, and the hydrothermal water rose "slowly" in the channels. This appears to lead to efficient mineral precipitation (rapid growth of chimneys), and it is believed that the hot water is cooled by the surrounding seawater, which in turn efficiently precipitates metallic elements. By utilizing this mechanism, it is possible to "cultivate" mineral deposits. For example, a pipe above an artificial hydrothermal vent can be fitted with a valve to control the flow velocity.

If the "slow cooling" and "slightly faster cooling" control can be achieved, it may be possible to selectively precipitate metals such as zinc and copper.

To develop collection and recovery methods

Mineral resources have required large-scale extraction and refining processes, for example, several grams from a ton of ore in the case of gold deposits. If selective "farming" of high-grade ores in the seabed becomes feasible, large quantities of rocks other than copper, lead, and zinc resources will not be produced as waste, and the weight to be recovered will be relatively small. Japan has the world's sixth largest exclusive economic zone (EEZ), which is said to contain submarine mineral resources classified as submarine hydrothermal deposits, manganese nodules, manganese crusts, and rare earth muds. With this new concept of resource development methods, we can become a resource-rich country even without land-based mines. The dream of utilizing seafloor resources is coming closer step by step.