Minerals Report - International Seabed Authority

also explain why the only other study of hydrothermal activity near a fresh
and active hot-spot -near the Amsterdam-St. Paul’s plateau of the South East
Indian Ridge (Scheirer et al., 1998) - also shows an anomalously low incidence
of hydrothermal venting for its measured spreading rate (cf the Juan de Fuca
Ridge; (Figure 7).
3.3 The significance of slow-spreading ridges
Whilst the long-term impact of hot-spot effects may remain
speculative for now, however, what is undeniable is that deep penetrating
faults which allow seawater to penetrate deep beneath the seafloor, and not
just fresh outpourings of lava onto the seafloor - can lead to high-temperature
circulation. Further, whilst volcanically hosted systems have a finite supply
of heat and, hence, a finite lifetime during which high-temperature circulation
can be established, the cross-cutting fault structures at segment ends have
great potential to both a) focus hydrothermal up-flow at a single point on the
seafloor and b) continuously reactivate themselves through movement along
the fault zone, reopening cracks and propagating into fresh unreacted ridgecrust.
It is these latter structures, therefore, which might be most productive
in terms of seafloor massive sulphide production and, again, it is probably no
coincidence that both the TAG and Rainbow hydrothermal fields host large
fault-controlled massive sulphide deposits distant from any fresh neovolcanic
activity (German & Parson, 1998).
Thus, although the majority of currently active hydrothermal fields in
the modern ocean may occur along fast spreading ridges, it may well be that
the most economically interesting (i.e. largest) concentrations of polymetallic
sulphides are actually produced in fault-controlled hydrothermal systems
along slow spreading ridges. And if faulting can play an important part in
controlling the distribution of hydrothermal venting at ridge crests, as well as
fresh volcanism, then the scope exists for there to be important hydrothermal
circulation systems throughout all the world’s ocean ridge systems and not
just along the fast spreading ridges - which only make up a minority of the
total 60,000km of global ridge-crest.
Spurred on by this finding, we have already located the first evidence
for hydrothermal activity along part of the South West Indian Ridge (German
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