The hydrothermal fluid may be:
● magmatic (originating in an intruding magma)
● circulating groundwater
● ocean water
Convective circulation of saline in ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and in other submarine volcanic areas. Patterns of hydrothermal alteration are used as guides in the search for deposits of valuable metallic ores.
[links: images: pyritized rocks (form a halo around the copper deposit) and result from percolation of chemically-reactive hot groundwaters through the strata]