Some deposits form not around an intrusion but on the seafloor itself, from hot springs venting into the ocean — the famous black smokers.

VMS — Volcanogenic Massive Sulfide

Seawater seeps down into hot volcanic rock on the seafloor, gets heated, leaches out metals, and rises back up to vent. When the hot, metal-rich fluid hits cold seawater, it dumps its load as a mound of massive sulfide on the seabed. Buried and preserved over millions of years, these become VMS deposits — typically copper, zinc, and lead, with gold and silver. "Massive" means the ore is nearly solid sulfide.

SEDEX — the sediment-hosted cousin

SEDEX (Sedimentary Exhalative) is closely related. Here metal-rich brines vent into a quiet, muddy sea basin rather than a volcanic seafloor, and the sulfides settle in layers among the sediments — forming giant lead-zinc deposits.

Because they form on ancient seafloors, today these deposits are found in old volcanic-sedimentary belts that have since been uplifted onto land.