New insights into the genesis of volcanic-hosted massive sulfide deposits on the seafloor from numerical modeling studies

摘要

Numerical computer simulations have been used to gain insight into the evolution of marine hydrothermal systems and the formation conditions of massive sulfide deposits in ancient and modern submarine volcanic terrains. Simulation results have been used to gain a better understanding of the formation of massive sulfide ore deposits, their location, zonation, size, and occurrence in various geotectonic settings. Most hydrothermal fluid discharging at the seafloor exhibits temperatures ranging from 200 deg C to about 410 deg C and average fluid discharge velocities of 1 to 2 m/s in agreement with seafloor observations. Mass calculations imply that average massive sulfide deposits may form in -5000 years while giant deposits take longer than 5000 years to accumulate; supergiant deposits either need much longer time to form (>35,000 years) or at least 100 ppm of metal in solution. Results indicate that supergiant deposits may only form in certain geotectonic environments where longevity and preservation potential of the hydrothermal system are high. An additional process (mineral precipitate cap) is proposed here to explain the zinc content of massive sulfide deposits. This cap would prevent the widespread dissolution of anhydrite and the 'washout' of zinc by subsequent hydrothermal fluid discharge.