The Deposits That Shouldn't Be Mined: Polymetallic Sulphides and the Vents They Destroy

There is a hierarchy of destructiveness in deep-sea mining, and polymetallic sulphide extraction sits at the top of it.

Unlike manganese nodule fields — which are mined from flat abyssal plains — sulphide deposits form at hydrothermal vents. Mining them doesn't just disturb the seafloor near a unique ecosystem. It removes the ecosystem's physical foundation entirely.

What Are Polymetallic Sulphides?

Hydrothermal vents form where seawater percolates down through cracks in the oceanic crust, gets superheated by underlying magma, and erupts back through the seafloor carrying dissolved minerals. When that superheated fluid meets cold seawater, minerals precipitate rapidly — building the towering chimneys and mounds characteristic of black and white smoker vents.

Over time, these precipitates accumulate into polymetallic sulphide deposits: concentrated ores of copper, zinc, lead, gold, and silver. The richer deposits can contain metal concentrations significantly higher than equivalent terrestrial ores.

The ISA has issued exploration licences for sulphide deposits in the Mid-Atlantic Ridge and the Indian Ocean ridge systems. Active and inactive vent fields along these ridges are the targets.

The Ecosystem Problem

Hydrothermal vents were unknown to science until 1977. In the nearly five decades since their discovery, they have overturned fundamental assumptions about where life can exist and how ecosystems function.

Vent communities — tube worms up to two metres long, blind shrimp, ghostly crabs, chemosynthetic bacteria — do not depend on sunlight. They run on chemical energy from the Earth's interior. They are, in a real sense, a parallel biosphere.

Most vent species are found nowhere else. Endemism rates at hydrothermal vents are extraordinarily high. The InterRidge Global Database catalogues 721 vent fields worldwide — active, inactive, and extinct. Many of the active fields have never been systematically surveyed for biodiversity.

Sulphide mining removes the vent structure itself. There is no restoration pathway. The physical substrate that took decades or centuries to form is gone.

The ISA Licences

Eleven ISA sulphide concessions are currently flagged as high environmental risk. The contractors include:

Russia holds blocks in the mid-Atlantic, including two that sit 817km and 1,107km from France's EEZ boundary. Russia has been an ISA contractor since the early 2000s and holds licences across multiple resource types.

IFREMER (France) — the French national oceanographic institute — holds two high-risk sulphide blocks, one of which is approximately 1,480km from French territory. This creates an uncomfortable situation: France is one of the countries most frequently cited as a voice for ocean conservation in international negotiations, while its own research institute holds high-risk extraction licences.

Government of India / NIOT holds three high-risk sulphide blocks near Mauritius's EEZ. India has been expanding its ISA footprint significantly in recent years, framing deep-sea mineral access as a strategic priority.

KGHM Polska Miedź S.A. — a Polish copper mining company, not a research institute — holds a high-risk sulphide block 1,061km from Portugal. KGHM is one of the world's largest copper producers. Its presence as an ISA contractor signals that the transition from exploration to commercial intent is well underway.

Active vs. Inactive Vents

The ISA and some industry proponents argue that inactive vent fields — sites where hydrothermal circulation has ceased — are less ecologically sensitive than active vents. This argument has some scientific basis: active vents support dense chemosynthetic communities that inactive sites do not.

But it understates the complexity. Inactive vent fields are not biological dead zones. They retain endemic species adapted to the residual chemistry of former vent activity. They may reactivate on geological timescales. And the sulphide deposits at inactive sites are chemically identical to those at active ones — the mining operation doesn't discriminate.

More fundamentally, we don't have adequate survey data to make confident biodiversity assessments for most target sites. The argument that "we've checked and it's low-impact" requires the checking to have actually happened.

The Monitoring Gap

The ISA requires environmental baseline studies before exploration licences proceed to exploitation. But the data quality and geographic coverage of existing baselines varies enormously. For some mid-Atlantic sites, detailed biological surveys have been conducted. For others, baseline data is thin or dated.

Meanwhile, the exploration licences are live. The contractors are mapping, sampling, and developing the technical case for eventual exploitation applications.

Real-time monitoring from Argo floats and ocean current modelling — the kind visualised on Abyssal Claims — can track how water column disturbances propagate from mining sites. But that monitoring infrastructure doesn't yet exist at the resolution needed to catch early-stage ecological damage at vent fields.

What Makes Sulphides Different

Nodule mining is harmful. Sediment plumes, habitat destruction, and the loss of slow-growing abyssal communities are serious concerns.

But there is a qualitative difference with sulphide extraction: you are removing something that cannot regenerate on any human timescale, at a site whose ecology is still being discovered, using a technology whose waste streams have never been tested at commercial scale in the deep ocean.

The eleven high-risk ISA sulphide licences represent a choice that hasn't been made transparently. That choice — whether the minerals are worth the ecosystems — deserves a public debate that has not happened.