Publication Date:
2022-05-25
Description:
© The Author(s), 2015. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Chemical Geology 420 (2016): 114-126, doi: 10.1016/j.chemgeo.2015.11.006.
Description:
Here, we review the relationship between the distribution of modern-day seafloor
hydrothermal activity along the global mid-ocean 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 activity in particular – high temperature “black smoker” venting -
along 〉30% of the global mid ocean ridge crest. While that still leaves most of
that ~60,000km continuous network to be explored, some important trends have
already emerged. First, it is now known that submarine venting can occur along
all mid-ocean ridges, regardless of spreading rate, and in all ocean basins.
Further, to a first approximation, the abundance of currently active venting, as
deduced from water column plume signals, can be scaled linearly with seafloor
spreading rate (a simple proxy for magmatic heat-flux). What can also be
recognized, however, is that there is 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. An
examination of hydrothermal systems tracked to source on the slow spreading
Mid Atlantic Ridge reveals that no more than half of the sites responsible for the
“black smoker” plume signals observed in the overlying water column are
associated with magmatic systems comparable to those known from fast-
spreading ridges. The other half of all currently known active high-temperature
submarine systems on the Mid-Atlantic Ridge are hosted under tectonic control.
These systems appear both to be longer-lived than, and to give rise to much
larger sulfide deposits than, their magmatic counterparts - presumably as a result
of sustained fluid flow. A majority of these tectonic-hosted systems also involve
water-rock interaction with ultramafic sources. Importantly, from a mineral
resource perspective, this subset of tectonic-hosted vent-sites also represents
the only actively-forming seafloor massive sulfide deposits on mid-ocean ridges
that exhibit high concentrations of Cu and Au in their surface samples (〉10wt.%
average Cu content and 〉3ppm average Au). Along ultraslow-spreading ridges,
first detailed examinations of hydrothermally active 74 sites suggest that sulfide
deposit formation at those sites may depart even further from the spreading-rate
model than slow-spreading ridges do. Hydrothermal plume distributions along
ultraslow ridges follow the same (~50:50) distribution of “black smoker” plume
signals between magmatic and tectonics settings as the slow spreading MAR.
However, the first three “black smoker” sites tracked to source on any ultra-slow
ridges have all revealed high temperature vent-sites that host large polymetallic
sulfide deposits in both magmatic as well as tectonic settings. Further, deposits
in both types of setting have now been revealed to exhibit moderate to high
concentrations of Cu and Au, respectively. An important implication is that ultra-
slow ridges may represent the strongest mineral resource potential for the global
ridge crest, despite being host to the lowest magmatic heat budget.
Description:
Preparation of this review has benefited from research
support to CRG, SP and MDH from the Woods Hole Oceanographic Institution,
USA, from GEOMAR and the Helmholtz Foundation, Germany and from NSERC,
Canada. The opportunity to discuss ideas and bring together our different
perspectives - from water column geochemistry and seafloor massive sulfide
studies - was facilitated by a Research Award from the Alexander von Humboldt
Foundation to CRG.
Description:
2016-11-14
Keywords:
Hydrothermal activity
;
Seafloor massive sulfides
;
Mid-ocean ridges
;
Exploration
;
Copper
;
Gold
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
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