Published numerical calculations show that the convective systems which produce near-surface geothermal systems or ore deposits are likely to cool their intrusion heat source in at most a few tens of thousands of years, even if the intrusion is large. Generally, hydrothermal circulation, near-surface geothermal activity, and magmatic intrusion should be closely tied in time and space. Long-lived ( approximately 1 m.y.) hydrothermal activity normally suggests multiple pulses of intrusion with associated pulses of hydrothermal circulation. However, if the permeability of the intruded environment is just above that which allows convection, and the volume of the intrusion is very large, calculations presented here show that a single episode of intrusion can sustain hydrothermal circulation and near-surface geothermal activity for approximately 800,000 yr. For a deep sill heat source, these unusually long-lived hydrothermal systems can vent through a few very widely separated discharge sites and have the potential to produce isolated massive sulfide deposits of unusually large size.

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