In a review paper published June 23 online in the journal Mineralium Deposita, Cathles, Cornell professor of earth and atmospheric sciences, writes that while land-based deposits may be a dwindling source of valuable minerals, deposits on the ocean floor could power humanity for centuries.
The minerals, including sulfur, copper, zinc, iron and precious metals, are contained in volcanogenic massive sulfide (VMS) deposits that form on the ocean floor where tectonic plates pull apart and allow magma (molten rock) to invade the Earth’s 3.7-mile- (6 kilometer-) thick crust. The magma heats seawater to 662 degrees Fahrenheit (350 degrees Celsius) and moves it through the ocean crust via convection; and the seawater deposits the minerals where it discharges along the ridge axis.
Mineralium Deposita – What processes at mid-ocean ridges tell us about volcanogenic massive sulfide deposits
Episodic seafloor spreading, ridge topography, and fault movement at ridges find (more extreme) analogs in the arc and back-arc setting where the volcanogenic massive sulfide (VMS) deposits that we mine today were formed. The factors affecting sulfide accumulation efficiency and the extent to which sulfides are concentrated spatially are the same in both settings, however. The processes occurring at mid-ocean ridges therefore provide a useful insight into those producing VMS deposits in arcs and back-arcs. The critical observation investigated here is that all the heat introduced by seafloor spreading at mid-ocean ridges is carried out of the crust within a few hundred meters of the ridge axis by ∼350°C hydrothermal fluids. The high-temperature ridge hydrothermal systems are tied to the presence of magma at the ridge axis and greatly reduce the size and control the shape of axial magma intrusions. The amount of heat introduced to each square kilometer of ocean crust during its formation can be calculated, and its removal by high-temperature convection allows calculation of the total base metal endowment of the ocean basins. Using reasonable metal deposition efficiencies, we conclude that the ocean floor is a giant VMS district with metal resources over 600 times the total known VMS reserves on land and a copper resource which would last over 6,000 years at current production rates.
According to model simulations by Cathles and colleagues combined with heat flow measurements from the 1980s around the Galapagos Islands, the seawater convection cools the entire crust — “like a homeowner who lights a fire in his fireplace for the express purpose of cooling his house,” said Cathles.
That knowledge, along with the known thickness of the ocean crust, allows researchers to calculate the quantity of dissolved minerals that could be transported over each square meter of ocean floor.
If just 3 percent of the dissolved minerals precipitate — an estimate based on earlier studies — the ocean floor would hold reserves vastly greater than those on land, Cathles said.
In the case of copper — a key component in construction, power generation and transmission, industrial machinery, transportation, electronics, plumbing, heating and cooling systems, telecommunications and more — calculations show that just half of the total accumulated amount could be enough to bring the world’s growing population up to a modern standard of living and maintain it for centuries.
“I think there’s a good chance that it’s a lot more than 3 percent,” Cathles said. “But even just taking 3 percent, if you calculate how long the copper on the ocean floor would last, just half of it could last humanity 50 centuries or more.
With the necessary precautions, extracting the underwater deposits may also be a more environmentally friendly process than mining on land, Cathles said.
And it could provide other benefits, both scientific and psychological. Undersea exploration around ocean ridges could open doors to new research on the fundamental processes behind the formation of Earth’s crust, he noted; and a more positive outlook on the future could lead to fewer wars and more positive engagement.
“We are not resource limited on planet Earth. For a human on Earth to complain about resources is like a trillionaire’s child complaining about his allowance or inheritance. It just doesn’t have much credibility in my view,” he said.