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Scientists at the United States Geological Survey (USGS) have discovered that some metal-rich rocks recovered from the deep ocean can spontaneously combust when exposed to air and processed, raising unexpected safety concerns for a potential new mining industry.
The finding follows a 2022 research expedition to the Escanaba Trough, off the Oregon–California border, where two of 57 hydrothermal rock samples ignited during laboratory processing.
The reactions generated temperatures exceeding 100°C and completely oxidized the original material, according to a study published in Nature’s Scientific Reports.
“The production of heat through exothermic reactions has not been investigated in seafloor massive sulfide deposits but has the potential to cause rapid onset fires on ships,” the researchers wrote.
The discovery introduces another challenge for companies seeking to extract valuable metals from the ocean floor. While deep-sea sulfide deposits contain copper, zinc, gold and silver, their behaviour during recovery, transportation and processing remains poorly understood, the researchers noted.
The findings could influence safety standards as governments and mining companies assess the commercial potential of seabed resources.
Unexpected reactions
The USGS collected the samples in collaboration with the Bureau of Ocean Energy Management (BOEM) and the National Oceanic and Atmospheric Administration (NOAA), using a remotely operated vehicle to investigate four regions of the Escanaba Trough.
Located within the US Exclusive Economic Zone, the trough contains mineral deposits formed by hydrothermal activity that concentrates metals, including copper, zinc and iron, on the seafloor.
Most of the recovered rocks contained substantial quantities of metal sulfides. However, when researchers freeze-dried and crushed the samples in the laboratory, two underwent rapid oxidation and generated enough heat to ignite.
To determine what caused the reactions, scientists compared the samples’ mineral composition, chemistry and thermal behaviour.
They initially considered whether petroleum or tar trapped within the deposits could explain the combustion. Hydrothermal processes and heavy sedimentation can produce petroleum-like materials capable of self-heating when exposed to oxygen.
Instead, the investigation identified nanocrystalline marcasite, an unstable form of iron sulfide, as the likely culprit.
The scientists determined that exposure to oxygen and mechanical forces from crushing or grinding could trigger heat-generating oxidation in the rocks.
“The new research suggests that some seafloor massive sulfide rocks have physical and chemical properties that could make them behave very differently from comparable mineral deposits mined on land,” the USGS said in a statement.
The findings do not establish that all marcasite-bearing deposits present a combustion hazard. Rather, they identify specific mineral characteristics that may warrant closer examination before commercial extraction begins.
Mining implications
The research comes as interest in deep-sea mineral resources grows, driven by demand for metals used in infrastructure, manufacturing and energy technologies.
The International Seabed Authority (ISA) has awarded exploration contracts for polymetallic sulfide deposits along the Northern Mid-Atlantic Ridge and Southwest Indian Ridge, although commercial extraction under those contracts has not begun.
Unlike minerals that remain under relatively stable conditions on the seafloor, recovered sulfide rocks would encounter major environmental changes during mining operations.
“If seafloor massive sulfide mining occurs, rocks brought to the surface would experience conditions dramatically different from those on the seafloor, including exposure to oxygen and changes in temperature, pressure, and moisture,” the USGS said.
“Such changes could affect how sulfide minerals react and how much heat they generate.”
Those risks could require operators to assess mineral stability before extraction and develop procedures for handling, storing and transporting potentially reactive material. The possibility of heat buildup aboard vessels presents a particular concern where large quantities of crushed sulfide rock could be concentrated in enclosed spaces.
Deep-sea mining companies The Metals Company (Nasdaq: TMC) and Deep Sea Minerals (CSE: SEAS) did not respond to MINING.COM’s requests for comment on potential operational safety concerns or whether they had assessed similar risks.
The findings add an occupational safety dimension to an industry already facing scrutiny over its potential environmental effects.
Environmental groups have warned that seabed mining could damage fragile marine habitats, disturb sediments and threaten ecosystems that scientists have only begun to understand. The consequences of large-scale extraction remain uncertain, particularly in deep-ocean environments where recovery from disturbance may be slow.
For prospective operators, the USGS research suggests that conventional approaches to handling sulfide ores may not adequately address the conditions encountered during deep-sea mining.
Further testing will be needed to determine how widely reactive marcasite occurs in seabed deposits and whether the laboratory results translate into comparable hazards during industrial operations.
