Abstract

The increasing hydrostatic pressure of the water column with increasing water depth in subaqueous environments limits the ability of superheated volatiles to expand instantaneously against the ambient pressure. Explosive submarine eruptions are only likely in water depths less than 1 km, and generally less than 500 m, thus refuting models of highly explosive, very deep water calderas popularly proposed as the host volcanic centers for the kuroko volcanic-hosted massive sulfide (VHMS) deposits of Japan. Large volumes of blocky breccias can be produced by quench fragmentation during submarine lava eruptions, and such deposits may have been mistaken for pyroclastic deposits in many cases in the past. Pyroclastic debris may occur in the host-rock successions of some VHMS deposits and may even be very voluminous. However, facies characteristics and theoretical constraints indicate that these are usually the products of mass-flow deposition from shallow-water or basin margin volcanic centers. The maximum water depths for submarine explosive eruptions coincide approximately with the minimum pressures and water depths required to prevent boiling of mineralizing hydrothermal fluids in the stockwork before the fluids reach the sea floor. The key elements in evaluating the prospectivity of ancient volcanic successions for VHMS deposits appear to be deep-water sediments and lavas or shallow intrusions in an extensional basin setting. Pyroclastic debris, in many cases at least, appears to be an accidental, externally introduced component. There is little evidence that explosive submarine calderas are essential as host volcanic centers to VHMS deposits.

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