Do you have a source for your assertion?
I saw it on a Smithsonian or National Geographic special years ago.
Did a quick search so don’t condemn the source. Look for yourself
https://www.cbsnews.com/news/mining-for-gold-on-the-ocean-floor/
“Scientists have long known about remarkably pure concentrations of metals found near some of the hydrothermal vents, nicknamed “black smokers” because they resemble underwater chimneys.”
Read up on "black smokers" around deep ocean volcanic areas. Literally belching out clouds of metal sulfides.