Hydrothermal exploration of mid-ocean ridges: Where might the largest sulfide deposits be forming?

Abstract

44Here, we review the relationship between the distribution of modern-day seafloor 45 hydrothermal activity along the global mid-ocean ridge crest and the nature of the 46 mineral deposits being formed at those sites. Since the first discovery of seafloor 47 venting, a sustained body of exploration has now prospected for one form of 48 hydrothermal activity in particular -high temperature "black smoker" venting -49 along >30% of the global mid ocean ridge crest. While that still leaves most of 50 that ~60,000km continuous network to be explored, some important trends have 51 already emerged. First, it is now known that submarine venting can occur along 52 all mid-ocean ridges, regardless of spreading rate, and in all ocean basins. 53Further, to a first approximation, the abundance of currently active venting, as 54 deduced from water column plume signals, can be scaled linearly with seafloor 55 spreading rate (a simple proxy for magmatic heat-flux). What can also be 56 recognized, however, is that there is an "excess" of high temperature venting 57 along slow and ultra-slow spreading ridges when compared to what was 58 originally predicted from seafloor spreading / magmatic heat-budget models. An 59 examination of hydrothermal systems tracked to source on the slow spreading 60Mid Atlantic Ridge reveals that no more than half of the sites responsible for the 61 "black smoker" plume signals observed in the overlying water column are 62 associated with magmatic systems comparable to those known from fast-63 spreading ridges. The other half of all currently known active high-temperature 64 submarine systems on the Mid-Atlantic Ridge are hosted under tectonic control. 65These systems appear both to be longer-lived than, and to give rise to much 66 larger sulfide deposits than, their magmatic counterparts -presumably as a result 67 of sustained fluid flow. A majority of these tectonic-hosted systems also involve 68 water-rock interaction with ultramafic sources. Importantly, from a mineral 69 resource perspective, this subset of tectonic-hosted vent-sites also represents 70 the only actively-forming seafloor massive sulfide deposits on mid-ocean ridges 71 that exhibit high concentrations of Cu and Au in their surface samples (>10wt.% 72 average Cu content and >3ppm average Au). Along ultraslow-spreading ridges, 73 3 first detailed examinations of hydrothermally active sites suggest that sulfide 74 deposit formation at those sites may depart even further from the spreading-rate 75 model than slow-spreading ridges do. Hydrothermal plume distributions along 76 ultraslow ridges follow the same (~50:50) distribution of "black smoker" plume 77 signals between magmatic and tectonics settings as the slow spreading MAR. 78 However, the first three "black smoker" sites tracked to source on any ultra-slow 79 ridges have all revealed high temperature vent-sites that host large polymetallic 80 sulfide deposits in both magmatic as well as tectonic settings. Further, deposits 81 in both types of setting have now been reveal...

Exploring the Ecology of Deep-Sea Hydrothermal Vents in a Metacommunity Framework

“…One is that the high disturbance rates at these eastern Pacific sites likely result in species assemblages at individual fields that do not reach equilibrium. In contrast, sites of particular interest for exploitation include large sulfide deposits that may have been active for thousands of years (Tao et al, 2014;German et al, 2016;Cherkashov et al, 2017), thus accumulating species diversities that are at or near equilibrium. Additionally, the close proximity of source communities in the studies on eastern and northeastern Pacific explain, in part, the relative rapid recovery.…”

“…For example, long-lived vent fields have deep and stable conduits for hydrothermal circulation, whereas venting in the aftermath of volcanic eruptions may be short-lived. In general, at spreading ridges the shortest-lived vent fields occur at fastspreading ridges, where they are under magmatic control, and the longest-lived (i.e., "thousands of years"; German et al, 2016) at slow-spreading systems controlled by tectonics. The age of a vent (time since initiation of hydrothermal activity) differs from longevity (duration of most recent hydrothermal activity), since hydrothermal activity may wax and wane over time (Cherkashov et al, 2017).…”

“…Furthermore, the spatial variation of patch quality, as influenced by vent fluid composition, differs between ridges. Although some ridges may have similar composition of venting fluids across great distances (e.g., 1,000-km scale on the basalt-hosted EPR), others have diverse chemistries [e.g., 100-km scale due to the combination of basalt-and ultramafic-hosted systems on the Mid-Atlantic Ridge (MAR); Schrenk et al, 2013;German et al, 2016]. Arc/back-arc settings may have vent fields (Beaulieu, 2015), PB2002 plate boundaries (Bird, 2003), ETOPO1 bathymetry (Amante and Eakins, 2009).…”

Geological interpretation of volcanism and segmentation of the Mariana back‐arc spreading center between 12.7°N and 18.3°N

“…German et al . [] noted that hydrothermal activity on slow‐spreading ridges may, in fact, be significantly enhanced by the strong tectonic control on fluid flow. Deeply penetrating faults in these settings allow circulation of seawater to considerable depths and, in some cases, at some distance off axis, so that some unusually large hydrothermal systems are found on slow (20–50 mm yr −1 ) and ultraslow‐spreading (<20 mm yr −1 ) ridges where few were expected at all.…”

“…Recent discoveries along the spreading axis of every segment type indicate a primary magmatic control on venting. By comparison with slow‐spreading ridges [e.g., Devey et al ., ; German et al ., ], the greatest potential for large massive sulfide accumulations is in the type II segment at 17.0°N and the type III segment at 14.5°N. As hydrothermal outflow is strongly influenced by crustal permeability, our mapping of the fault patterns provides a means to examine the relationships between large‐scale tectonic processes (and associated volcanism) with hydrothermal venting as discoveries continue to be made.…”