Sulphides are chemical compounds of sulphur and a number of different elements, which are deposited in contact with cold seawater.
In Norway, there are older sulphide deposits in multiple mines on land, and sulphides are currently being formed on the volcanic spreading ridge between Norway and Greenland.
Minerals are deposited
Seawater penetrates into the subsurface above magma chambers, and is heated to over 300 degrees C. The hot and corrosive seawater leaches metals from the rocks. The hot water rise up in thermal springs on the seabed where the minerals precipitates on the seabed after contact with the colder seawater.
This can cause the sulphides to accumulate into tall chimneys (hydrothermal vents). The chimneys eventually collapse and form piles of rubble on the seafloor. The Norwegian Petroleum Directorate (NPD) has mapped such piles and has taken many samples of them.
Photo: Sulphide sample, malachite precipitation, 2020, NPD.