mineral systems
Seawater sinks into hot ocean crust, comes back up at 400 degrees, and drops its metal when it hits cold water. It is the one ore-forming system whose engine can be watched running today — which is exactly why its ancient products are so hard to read.
About: Volcanogenic massive sulfide system
Volcanogenic massive sulfide deposits form at or near the sea floor where circulating hydrothermal fluids discharge, and are important sources of copper, zinc, lead, gold and silver.[1,2]
The order of discovery is unusual and worth keeping. Laboratory experiments on basalt reacting with seawater indicated that convection driven by subseafloor magma chambers could produce ore-forming fluids; those experiments set the stage for the discovery of active vents on mid-ocean ridges, island arcs and back-arc spreading centres. The prediction came first.[1]
High-temperature black smoker vents discharge at about 350 degrees Celsius. They were first found on the East Pacific Rise in 1979, and studies of them form much of the basis for current models of the ancient systems — although the model notes that important questions about their formation and evolution remain.[1]
The vast majority of chemical and isotopic studies of fluid compositions, in both modern and ancient deposits, indicate that seawater or evolved seawater is the dominant mineralising fluid. Magmatic heat drives the convection; reaction with the rocks along the flow path changes the seawater into something that can carry metal, and it reaches up to about 400 degrees Celsius in the upflow zone and at the vent.[1,2,3]
A magmatic contribution is possible as well. Magmatic water and volatiles are identified as other potentially important contributors, with magmatic gases typically about 90 mole percent water, 5 to 10 mole percent carbon dioxide and 1 to 2 mole percent sulfur species. The model states this as potential, and Lithoen records it as inferred rather than established.[1]
Deposition is simple here in a way it is not elsewhere: cooling at the sea floor is regarded as the principal cause of sulfide precipitation, and wall-rock alteration is considered largely less significant to ore precipitation. Hot metal-bearing fluid meets cold seawater and the metal comes out.[1]
Surviving is the hard part. Sulfides on the sea floor oxidise rapidly, so ancient deposits are preserved only where something intervened: anoxic or sulfidic bottom water, a capping lava flow, or a chemical precipitate of silica or sulfate sealing the deposit from oxidising water.[1,2]
Often said
“Because we can watch black smokers form sulfide, we understand ancient VMS deposits.”
What the evidence supports
The model itself notes that important questions about the formation and evolution of modern vents remain, and ancient deposits have additionally been deformed, metamorphosed and preserved only where specific circumstances protected them from oxidation.
Supporting records: vms-black-smoker-temperature · vms-preservation-mechanisms
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-07.