DIVERSITY OF SEAFLOOR HYDROTHERMAL SYSTEMS ON SEDIMENT-FREE MID-OCEAN RIDGES

Investigations of seafloor hydrothermal systems along sediment-starved mid-ocean ridges over the past decade revealed important variations in their geological setting not previously recognized on the modern seafloor. Deposits have not only been found to be associated with basaltic (N-MORB) volcanism, but also with enriched E-MORB lavas, at least partially influenced by hot-spot volcanism, and with gabbros and serpentinites in areas of crustal uplift. Styles of mineralization and alteration vary dramatically, from small scale sulfide pinnacles, conventional mound-like, Cyprus-type massive sulfides, large steep-sided sulfide/sulfate chimneys directly forming on basaltic substrate, to massive talc-anhydrite deposits, and huge carbonate-rich towers occurring in water depths ranging from 4100 m to as shallow as 100 m.

Our understanding of the formation of massive sulfide deposits at the modern seafloor has always been limited by the fact that, until recently, only two dimensions of the hydrothermal systems were accessible. ODP drilling at the TAG mound (4Mt @ 2.1% Cu and 0.6% Zn) in 1994 provided the first opportunity to sample and model the interior of a VMS deposit formed on a modern, sediment-starved mid-ocean ridge. The overall predominance of breccias within the deposit, the occurrence of large amounts of anhydrite in the subseafloor, and the prominent zone refining of selected base and precious metals were unexpected and provide new insights into the development of modern and ancient massive sulfide deposits.

The diversity of hydrothermal activity along sediment-starved mid-ocean ridges allows for the study of water depth and source rock control on vent fluid chemistry, precipitation mechanisms (e.g. boiling), and precious metal content of the deposits. Long-term monitoring of hydrothermal systems supplies important information on the time scale of hydrothermal processes and the influence of magmatic and tectonic events on massive sulfide formation.