CONSTRAINTS ON THE WATER DEPTH OF ANCIENT VOLCANIC SUCCESSIONS HOSTING SEAFLOOR MASSIVE SULFIDE DEPOSITS

Establishing the water depth of ancient volcanic successions is one of the most challenging and controversial aspects of paleoenvironmental studies on volcanic-rock-hosted massive sulfide deposits. Distinctive volcanic facies characteristics develop only in shallow water where the hydraulic regime is strongly influenced by tidal currents, seasonal wind- and wave-induced currents, and marked variations between fair weather and storm conditions. Below the storm wave base, the environment in which most massive sulfide deposits are likely to have formed, the study of the host rock succession provides only few undisputed constraints on water depth.

Important constraints on water depth of massive sulfides in ancient volcanic successions can potentially be derived from the study of modern seafloor hydrothermal systems. Compilation of data available to date demonstrates that water depth of modern vent sites in arc environments correlates with plate tectonic setting and regional magmatic and volcanic setting. The shallowest hydrothermal systems are hosted by arc volcanoes. Massive sulfide occurrences in arc-related rifts are generally restricted to water depths from ca. 700 to 2000 m, with rifts developing within old arc crust clustering in the deeper part of this range. Sulfide occurrences at mature back-arc spreading centers proximal to arcs have been discovered at depths of ca. 1500 to 2000 m. The deepest hydrothermal systems occur along mature back-arc spreading centers distal to volcanic arcs where water depth ranges from ca. 2000 to 3700 m. The study of modern seafloor hydrothermal systems confirms that a continuous spectrum of deposit types, ranging from relatively deep marine massive sulfides to shallow marine and subaerial epithermal deposits, may occur in some suprasubduction settings.