Researchers have identified several mounds on the seafloor at approximately 13°N along the Mid-Atlantic Ridge, which is at a latitude similar to that of Senegal and Nicaragua. These mounds represent the largest known deposits of seafloor massive sulfides (SMS).
The mounds measure up to 300 meters in height and almost half a kilometer in width. Initial estimates suggest they contain at least 100 million tonnes of sulfide.
For comparison, the SMS deposit Mohn’s Treasure, located on the Mohn’s Ridge, is believed to contain about 2.2 million tonnes of sulfide ore, according to the Norwegian Offshore Directorate.
These mounds are part of the Semenov hydrothermal field cluster, which has been regularly visited by researchers involved in the ULTRA project.
The project members have collected a variety of data from the sites, using high resolution seafloor mapping, remotely operated vehicles, seafloor drilling, sediment coring, sub-seafloor imaging by seismic reflection and refraction, and sub-seafloor resistivity surveys using controlled source electromagnetics.
At the Deep Sea Minerals conference in Bergen in April, project leader Bramley Murton from the National Oceanography Centre will present the latest findings from ULTRA, based on investigations conducted at six sites.
Mantle exposed at the seabed
The researchers propose that the vast amounts of accumulated sulfide material can be explained by the presence of an oceanic core complex (OCC). OCCs form on the flanks of mid-ocean ridges where detachment faulting exposes ultramafic rocks (mantle rocks) at the seabed.
The study suggests that detachment faulting creates ideal conditions for large SMS deposits by sustaining geothermal activity, maintaining high temperatures, and keeping fluid pathways open. These processes make OCCs promising targets for future deep-sea mineral exploration.
OCC formation is common at slow-spreading ridges, such as the Mohn’s Ridge, suggesting excellent potential for sulfide deposits in the Norwegian exclusive economic zone.
Norwegian participants in Project ULTRA are the University of Bergen, Equinor, and Green Minerals. Other partners include the National Oceanography Centre, the British Geological Survey, the universities of Cardiff, Southampton, Leeds, and the Memorial University (Canada), as well as GEOMAR (Germany).
Deep Sea Minerals 2025 will take place in Bergen from 1-3 April. The program and registration page can be found on the conference website.