Hydrothermal mineral deposits in oceanic crust include metalliferous sediments and encrustations and massive sulfides. The occurrence of these deposits is explained by the hypothesis that they are concentrated at the discharge zones of high intensity subsea-floor hydrothermal convection systems involving the circulation of sea water through oceanic crust and upper mantle at oceanic spreading centers.Criteria that have proven most useful for recognition of hydrothermal deposits in oceanic crust based on known deposits at oceanic spreading centers include petrology of the deposits and surrounding rocks; structural conditions that create exceptionally high permeability and thermal gradients; seismicity in the form of microearthquakes and earthquake swarms; geochemical properties of hydrothermal discharge ( 3 He, 222 Rn, ferric hydroxides, silica); contrasts in acoustic impedence between normal sea water and hydrothermal solutions; anomalous gravity as an indicator of geologic structure; electrical properties of the hydrothermal solutions and deposits; an associated low in residual magnetic intensity attributed to hydrothermal alteration of the magnetic mineral component of basalt; patterns of deposition in early rift and advanced oceanic ridge stages of opening of an ocean basin; and distribution of hydrothermal deposits both parallel and perpendicular to an oceanic spreading center.The seismic, geochemical, acoustic, thermal, and certain electrical criteria are applicable to recognition of active discharge zones of subsea-floor hydrothermal convection systems. The petrologic, structural, gravity, magnetic, and other electrical criteria are applicable to recognition of hydrothermal deposits in all of oceanic crust which underlies ocean basins covering two-thirds of the Earth and is emplaced on land as ophiolites.

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