Minerals Report - International Seabed Authority

The morphology of the mineral edifices (chimneys, mounds,
sedimented layers, blocks, breccias, conglomerates, etc.) makes difficult the
estimation of the real volume of material. For the most compact
accumulations, such as mounds, which have a less complicated shape, the
thickness cannot be determine solely by visual observations or topographic
calculations from bathymetric or sonar surveys. The Bent Hill Massive
Sulphides (BHMS) of the Juan de Fuca spreading centre give a good example
of how misleading such an estimate could be [35]. Obviously, a sufficient
number of drill hole samples must be obtained before acquiring an
understanding of the third-dimension of the deposit’s geometry.
Moreover, the internal structure of the deposit may change with
depth, as can be deduced from comparisons with similar land-based deposits
[32-36-38]. The following succession can be predicted from surface to depth:
(1) oxidized mineral breccias, (2) clastic sulphides, (3) massive sulphides, (4)
veins and replacements of a deeply altered host-rock, (5) dense network of
small veins in unaltered host-rock (stockwork), and (6) large veins irregularly
distributed in the bedrock. The boreholes drilled during leg 169 of the Ocean
Drilling Program on the BHMS of the Juan de Fuca Ridge reveal such a
structure [35].
The assemblage of minerals is also complex and variable. The main
mineral is the ubiquitous pyrite, and its companion pyrrhotite, associated
with variable proportions of zinc (sphalerite, wurtzite) or copper (isocubanite,
chalcopyrite) minerals. Galena is present in large quantities in Kuroko types
of deposits such as found in Myojin-sho or Aeolian island arcs, where
sphalerite is predominant and copper minerals are less important [39-41]. It
may be a constant feature of back-arc deposits compared to spreading center
ones: Mariana and Okinawa deposits are other examples of the same mineral
association. Sulphates, as anhydrite and barite, are also present in variable
quantities, mainly near surface. Silica occurs in various forms - chert, opal and
quartz – associated with chlorite.
To determine the mass of the mineral body from the estimated
volume, one must make a tentative assumption of its density. Table 2 gives
the specific gravity of the different minerals. The average density, computed
after mineral reconstitution from the analyses of Table 3, varies from 2.77
INTERNATIONAL SEABED AUTHORITY 432