Hydrothermal Exploration of Mid Ocean Ridges: Where Might the Largest Sulfide Deposits Occur?
C.R.German S.Petersen M.D.Hannington
This paper considers the relationship between the distribution of modern-day seafloor hydrothermal activity along the global ridge crest and the nature of the mineral deposits being formed at those sites. Since the first discovery of seafloor venting, a sustained body of exploration has now prospected for one form of hydrothermal actvity – high temperature “black smokers” - along ~20% of the global mid ocean ridge crest. While that still leaves 80% to be explored, some important trends have emerged. First, it is now known that submarine venting can occur along all ridge-crests and in all ocean basins. Further, from a water-column perspective, the abundance of currently-active venting scales linearly with available magmatic heat-flux (or, put more simply but less correctly) with seafloor spreading rate. What is increasingly recognized, however, is that there is also an “excess” of high temperature venting along slow and ultra-slow spreading ridges when compared to what was originally predicted from seafloor spreading / magmatic heat-budget models. Further, when examined in detail, hydrothermal systems tracked to source on the slow spreading Mid Atlantic Ridge have revealed that only approximately half of the sites responsible for the “black smoker” plume signals observed in the overlying water column are associated with neovolcanically-hosted systems comparable to those known from fast-spreading ridges. The other half of all known submarine systems on the Mid-Atlantic Ridge, by contrast, are hosted under tectonic control and appear to be both long-lived and to give rise to much larger sulfide deposits than their neovolcanically-hosted counterparts, presumably as a result of sustained fluid flow at these locations. Intriguingly, first evidence for hydrothermal activity along ultraslow ridges seems to depart, further, from this emerging slow-ridge pattern. Along ultra-slow ridges (with the exception of the Gakkel Ridge in the Arctic) there seems to be a similar 50:50 distribution of “black smoker” plume observations above seafloor that is dominated by either neovolcanic or tectonic activity. For the first three vent-sites tracked to source along these ultra-slow spreading ridges, however, what has been reported is that large polymetallic sulfide deposits, up to 100m in extent, have been found in unambiguously neovolcanic settings with no indication of the same forms of tectonic control that appear to be important in the formation of similar sized deposits on slow spreading ridges.
