Minerals Report - International Seabed Authority
Minerals Report - International Seabed Authority Minerals Report - International Seabed Authority
4.4 Marine Polymetalic sulphides 4.4.1 What marine polymetalic sulphides comprise The majority of sub-sea polymetallic sulphides (PMS) are massive ore bodies containing varying proportions of pyrrhotite, pyrite/marcasite, sphalerite/wurtzite, chalcopyrite, bornite, and isocubanite. Some massive polymetallic sulphides located on spreading centres behind deep-ocean trenches also contain galena (lead sulphide) and native gold. Other minor sulphides of tin, cadmium, antimony, aresenic and mercury also occur in varying amounts at different localities (Rona and Koski, 1985; Herzig and Hannington, 1995). 4.4.2 How marine polymetalic sulphides deposits are formed Polymetallic mineral deposits on the seafloor are intimately related to the formation of new oceanic crust by seafloor spreading. At mid-ocean ridges, convection-driven circulation of seawater through the oceanic crust is the principal ore-forming process (Scott, 1985; Herzig and Hannington, 1995). Hydrothermal fluid leaches and transport metals and other elements from their host rock to the surface of the seafloor. As they discharge, at temperatures up to 350°C from the “black smoker” chimneys (at depths in excess of 2,500 m), metal sulphides deposits form at the seafloor (as mounds) or as sub-surface stock-works. Lower temperature systems are also present and generate mineralisation of considerable economic potential. In the southern Lau Basin, for example, the first examples of actively forming, visible primary gold in seafloor sulphides were documented at “white smoker” chimneys (Hannington, M.D. and Scott, 1988; Herzig et al., 1993). 4.4.3 Where marine polymetalic sulphides deposits are found Sub-sea massive polymetallic sulphide bodies are found along the earth’s major tectonic belts (and identified in Figure 10). INTERNATIONAL SEABED AUTHORITY 692
The main areas of occurrence are fast-, intermediate-, and slowspreading mid-ocean ridges, on- and off-ridge volcanoes and seamounts, in sedimented rifts adjacent to continental margins and in volcanoes and spreading ridges related related to deep-ocean trenches (Rona, 1988; Rona and Koski, 1985; Herzig, 1999; Herzig and Hannington, 2000). Hightemperature hydrothermal activity and large accumulations of polymetallic sulphides are known at some 25 different sites world-wide (Figure 10). Small oceanic basins related to deep-ocean trenches are important sites for PMS mineralisation. For example, the “PACMANUS” hydrothermal deposits are scattered along a 10 km-long crest of an active volcanic ridge in the Eastern Manus Basin, Papua New Guinea. Here, chimneys dominated by chalcopyrite and sphalerite, with barite and some bornite, have average compositions of 11 wt% Cu, 27% Zn, 230 ppm Ag and 18 ppm Au (Moss et al., 1997; Scott and Binns, 1995; Binns et al., 1993). The shallow depths (less than 1,500 m) and high gold content make such sites potentially viable for mining. INTERNATIONAL SEABED AUTHORITY 693
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4.4 Marine Polymetalic sulphides<br />
4.4.1 What marine polymetalic sulphides comprise<br />
The majority of sub-sea polymetallic sulphides (PMS) are massive<br />
ore bodies containing varying proportions of pyrrhotite, pyrite/marcasite,<br />
sphalerite/wurtzite, chalcopyrite, bornite, and isocubanite. Some massive<br />
polymetallic sulphides located on spreading centres behind deep-ocean<br />
trenches also contain galena (lead sulphide) and native gold. Other minor<br />
sulphides of tin, cadmium, antimony, aresenic and mercury also occur in<br />
varying amounts at different localities (Rona and Koski, 1985; Herzig and<br />
Hannington, 1995).<br />
4.4.2 How marine polymetalic sulphides deposits are formed<br />
Polymetallic mineral deposits on the seafloor are intimately related<br />
to the formation of new oceanic crust by seafloor spreading. At mid-ocean<br />
ridges, convection-driven circulation of seawater through the oceanic crust<br />
is the principal ore-forming process (Scott, 1985; Herzig and Hannington,<br />
1995). Hydrothermal fluid leaches and transport metals and other<br />
elements from their host rock to the surface of the seafloor. As they<br />
discharge, at temperatures up to 350°C from the “black smoker” chimneys<br />
(at depths in excess of 2,500 m), metal sulphides deposits form at the<br />
seafloor (as mounds) or as sub-surface stock-works. Lower temperature<br />
systems are also present and generate mineralisation of considerable<br />
economic potential. In the southern Lau Basin, for example, the first<br />
examples of actively forming, visible primary gold in seafloor sulphides<br />
were documented at “white smoker” chimneys (Hannington, M.D. and<br />
Scott, 1988; Herzig et al., 1993).<br />
4.4.3 Where marine polymetalic sulphides deposits are found<br />
Sub-sea massive polymetallic sulphide bodies are found along the<br />
earth’s major tectonic belts (and identified in Figure 10).<br />
INTERNATIONAL SEABED AUTHORITY 692
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