Minerals Report - International Seabed Authority

oxide material, polymetallic sulphides material and biogenic material being
erupted from a range of vent-sites between 9°N, and 11°N, whose actual ventfluid
characteristics were only subsequently confirmed, at much greater cost,
following a dedicated series of sampling dives conducted by the deep
submersible research vehicle “Alvin”. Thus, careful, but reasonably
straightforward analysis of hydrothermal material collected as part of any
surface-ship investigation of a new hydrothermal field can provide important
and potentially insightful information concerning the nature of the sulphides
deposit forming at depth, at relatively little additional research cost.
Another key aspect of initial data gathering that can provide important
information is a regional (5-10km) scale side-scan sonar image of the area
hosting the detected hydrothermal source. I have described, previously, the
difference between volcanically-hosted and fault-hosted hydrothermal
systems and the likelihood that the latter should give rise to larger massive
sulphides deposits focussed at a single location whereas the former more
probably generate a series of much smaller sulphides deposits dispersed
across a wider area of the ocean floor within a mid-ocean ridge rift valley.
However, simple chemical analysis of a hydrothermal plume’s characteristics
would provide no such information that would allow one to distinguished
such obvious differences in resource potential. Nor, strikingly, would hullmounted
swath bathymetry collected from a surface ship survey – as was
demonstrated by the erroneous early identification of the non-transform
discontinuity between the AMAR and S. AMAR segments as an additional
volcanically active ridge segment. These important differences can readily be
discerned by co-registered collection of water column hydrothermal plume
data, however, together with sidescan sonar images of the underlying
seafloor. Figure 12 shows a portion of a typical volcanically active segment
centre from the Mid-Atlantic Ridge rift valley that might be expected to host a
short-lived (ca 10-100 yrs) hydrothermal system linked to the most recent
episode of volcanic eruption and measuring only some tens of metres in
diameter.
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