Polymetallic Sulphides: Why Mining Here Means Mining at Hydrothermal Vents

Minerals Born from Volcanic Heat

Polymetallic sulphides are not scattered randomly across the ocean floor. They form in one place and one place only: at active or recently extinct hydrothermal vent fields, where superheated water (up to 400°C) vents through the seafloor and meets cold ambient seawater. The sudden temperature drop precipitates copper, zinc, lead, gold, silver, and iron sulphide minerals around the vent orifice, building chimney structures and mound deposits over decades to centuries.

This means there is no version of polymetallic sulphide mining that does not interact with hydrothermal vent ecosystems. The ore deposit and the ecosystem are the same object.

What the Numbers Show

The Abyssal Claims database tracks 504 ISA exploration contracts targeting polymetallic sulphides. These concessions concentrate along mid-ocean ridges — the volcanic spreading centres where new seafloor is created and hydrothermal activity is highest:

• Mid-Atlantic Ridge — the longest and most studied hydrothermal corridor, hosting dozens of named vent fields
• Central Indian Ridge and Southwest Indian Ridge — increasingly surveyed since the mid-2000s
• East Pacific Rise and Juan de Fuca Ridge — well-studied but with significant overlap with areas of existing research protections
• Back-arc basins in the western Pacific (Manus Basin, Lau Basin, Okinawa Trough)

The spatial analysis we run shows a higher rate of vent proximity for sulphide concessions than for any other resource type — unsurprisingly, since the vents are the deposit.

The Ecosystem at Stake

Hydrothermal vent communities are among the most scientifically remarkable ecosystems on Earth. Discovered only in 1977, they overturned a foundational assumption in biology: that all life on Earth depends, directly or indirectly, on sunlight.

Vent communities are powered by chemosynthesis — microbes that oxidise hydrogen sulphide to fix carbon, forming the base of a food web that includes tube worms (some exceeding 2 metres), chemosynthetic clams and mussels, ghostly vent fish, and shrimp with photoreceptors adapted to detect the faint infrared radiation emitted by the vent fluid itself.

Many vent species are found at only a handful of sites globally. The geological isolation of vent fields — separated by kilometres of cold, food-poor abyssal plain — produces high rates of endemism. Species found at the Logatchev vent field on the Mid-Atlantic Ridge may not exist at Lucky Strike, 600 km to the north. Mining a vent field is not a local disturbance; it is the potential extinction of species with no other address.

The Temporal Dimension

Hydrothermal vents are geologically ephemeral. Individual vent fields have lifespans measured in decades to centuries before the heat source beneath them shifts. After a vent goes extinct, the biological community collapses. After a new vent activates, recolonisation begins from dispersal — larvae in the water column, carried by deep currents to available substrate.

This natural dynamism is sometimes used to argue that vent communities are inherently transient and therefore not worth protecting. The argument fails on inspection. Natural vent extinction occurs on a timescale that allows regional larval dispersal and recolonisation from neighbouring active sites. Mining removes the physical substrate — the sulphide mound — along with the vent fauna. There is nothing left to recolonise.

Furthermore, mining operations would suspend fine sulphide particles as sediment plumes. These plumes, potentially spreading hundreds of kilometres in deep currents, could smother neighbouring active vent fields and the larval supply routes that connect vent populations across ocean basins.

Who Is Mining Where

The major contractors in the sulphide space include JOGMEC (Japan), IFREMER (France), BGR (Germany), COMRA (China), and several private-sector entities with ISA sponsorship through Pacific island states. Each holds licences covering multiple ridge segments, with exploration phases involving repeated survey expeditions, sample collection, and environmental baseline studies.

The ISA's regulations require contractors to identify and protect reference zones — areas of comparable habitat left unmined for monitoring purposes. Critics argue the reference zone concept is inadequate for sulphide deposits, where the spatial footprint of a mining operation could encompass an entire vent field with no comparable nearby alternative.

The Governance Gap

The ISA Mining Code — the set of regulations that would govern commercial extraction — has been under negotiation for over a decade and remains unfinished. In the interim, exploration licences continue to be issued. Environmental impact assessments are contractor-funded and contractor-submitted, raising questions about independence.

Several countries have called for a precautionary pause on sulphide (and all deep-sea) mining until independent scientific assessments are complete. The argument is straightforward: once a hydrothermal vent deposit is mined, the sulphide chimney is gone. So is everything living on it.

With 504 licences already issued, the question of whether that pause will come in time is open.

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See all polymetallic sulphide concessions, their proximity to hydrothermal vents, and biodiversity risk scores on the Abyssal Claims map.