Metal-rich rocks recovered from the Pacific Ocean began smoking and heating when researchers broke them apart in a laboratory, highlighting a potential fire hazard for mining seafloor sulphide deposits.
Two of 57 hydrothermal rock samples collected during a 2022 expedition to the Escanaba Trough, off the Oregon–California border, spontaneously combusted after freeze-drying. Temperatures exceeded 100 C and the original samples completely oxidized, according to a study led by the U.S. Geological Survey in Scientific Reports.
“The production of heat through exothermic reactions has not been investigated in SMS deposits but has the potential to cause rapid onset fires on ships,” the researchers wrote, referring to seafloor massive sulphide deposits, in research outlined Oct. 7 by the federal agency.
The findings highlight handling considerations specific to deposits formed around hydrothermal vents, where mineral-rich fluids meet cold seawater. These differ in composition and structure from the polymetallic nodules targeted by The Metals Company (Nasdaq: TMC) and Deep Sea Minerals (CSE: SEAS).
Unstable minerals
Researchers collected the rocks using a remotely operated vehicle to investigate four regions of the Escanaba Trough. Forty-one samples contained metal sulphides as a major component.
The two that combusted were dominated by nanocrystalline marcasite, a form of iron sulphide made up of extremely small crystals. It had replaced an earlier mineral, pyrrhotite, while preserving its blade-like shape.
That transformation left small crystals and pore spaces that could increase contact with oxygen and make the material more reactive.
The researchers examined whether petroleum-like material associated with the deposits could explain the combustion. Their analyses instead pointed to reactions involving the sulphide minerals.
The laboratory handling offers an important qualification. The rocks had been freeze-dried, and the two reactive samples began generating noticeable heat and smoke when broken by hand to fit inside a crushing vessel, before the mechanical crusher started.
The laboratory results offer an early indication of how particular seabed minerals may react during processing, with further testing needed to assess their behaviour under commercial operating conditions.
Shipboard risks
Self-heating and combustion of sulphide ores are already documented in terrestrial mining. The question is whether younger seabed deposits, with different mineral textures and environmental conditions, require different precautions.
The authors suggest they may. Modern seafloor sulphides have undergone much less compaction and geological alteration than ancient deposits mined on land, potentially leaving them more reactive.
How operators handle the material could also matter. Breaking rocks underwater could allow heat to dissipate and reduce the risk of self-heating. Further crushing aboard ships or excessive removal of water could increase it.
The authors call for dedicated risk assessments and engineering controls if mining of seafloor massive sulphide deposits proceeds, with the characteristics of individual deposits guiding precautions.
“The new research suggests that some SMS rocks have physical and chemical properties that could make them behave very differently from comparable mineral deposits mined on land,” the agency said.
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