electrical measurement on a volcanic surface. When the Cyana reached the area
where the contact zone between the sulfide deposit and lava flow is presumed to
occur, then a second deployment of the cable was made by lowering the submersible to where it had visual contact with the sea floor. This contact between the
sulfide and the lava flow was found thanks to the fully automated acoustic navigation network deployed on the seamount and relayed to the ship and to the Transit
satellite navigation system. During two dives (Cy84-25 and Cy 84-31) conducted
in the area of the seamount containing a sulfide deposit, four geo-electrical
measurements were made. At the beginning of the last dive, after two measurements, it was decided to cut the cable and leave it on the bottom in order to free the
submersible from the constraints of the cable and so it would be better able to
continue exploring the sea floor.
The results of the geo-electrical measurements under the supervision of Tim
Francis carried out by submersible (Cyana) at four different locations on the
southern flank of the Southeastern Seamount helped in evaluating the extent of the
deposit. The results were astonishing since, for the first time, we could determine
the differences between the various types of formations through these measurements. Now the task was to compare the in situ readings with that obtained on the
different samples collected from the area and measured in the laboratory. When
matching the laboratory and the in situ results, these measurements gave an
average apparent resistivity for the sulfide of 0.214 X/m while that of the pillow
basalt was 0.552 X/m. In addition, it was noticed that the sulfide generates a selfpotential. Comparing the resistivity measurements on a Fe-sulfide sample of
known dimensions permitted us to estimate the thickness of the deposit to vary
from about 8 m up to 15 m thick (Francis 1985).
Using previous submersible and deep-towed camera stations, plus in situ
observations on the Southeastern seamount on the EPR near 12°43
0 N, enabled us to
assess the size of the sulfide deposit. Assuming that sulfide deposits cover the entire
surface of the explored triangle with a base of 900 m length, sides of 800 and 700 m
respectively, and an average thickness of 5 m when assuming a sulfide density of
2.9 gr/cm
3 , it is estimated that the deposit is less than 3.8 million tons. This is a
conservative estimate since the entire surface within the triangle was not entirely
explored and, as shown from the survey tracks, only a portion of the entire seamount was covered (Fig. 7.23). In terms of its size, this deposit is similar to some
of the small exploitable deposits found in Cyprus of 15,000 to 15 million tones
(Searle 1972) and if it were emerged, the seamount would be exploitable for ore.
The theory is that circular cones, because of their shape, are better suited to
canalize ore enriched fluids than along large fissures where a dispersion of matter
and energy is more prominent, such as on the spreading ridge axis. Indeed, when
considering the geometry of a seamount (an inverted funnel-shaped structure), the
concentration of energy sources is likely to be more efficient with centrally located
conduits, than that of the accreting plate boundary regions where the energy source
(heat release) to drive the system would seem to be more dissipated due to
extensive linear fissuring. Although most off axis seamounts are not volcanically
active, some structures that are located up to 50 km (i.e. at 21°N) from the ridge
228
7 Oceanic Spreading Ridges and Sea Floor Creation
where the contact zone between the sulfide deposit and lava flow is presumed to
occur, then a second deployment of the cable was made by lowering the submersible to where it had visual contact with the sea floor. This contact between the
sulfide and the lava flow was found thanks to the fully automated acoustic navigation network deployed on the seamount and relayed to the ship and to the Transit
satellite navigation system. During two dives (Cy84-25 and Cy 84-31) conducted
in the area of the seamount containing a sulfide deposit, four geo-electrical
measurements were made. At the beginning of the last dive, after two measurements, it was decided to cut the cable and leave it on the bottom in order to free the
submersible from the constraints of the cable and so it would be better able to
continue exploring the sea floor.
The results of the geo-electrical measurements under the supervision of Tim
Francis carried out by submersible (Cyana) at four different locations on the
southern flank of the Southeastern Seamount helped in evaluating the extent of the
deposit. The results were astonishing since, for the first time, we could determine
the differences between the various types of formations through these measurements. Now the task was to compare the in situ readings with that obtained on the
different samples collected from the area and measured in the laboratory. When
matching the laboratory and the in situ results, these measurements gave an
average apparent resistivity for the sulfide of 0.214 X/m while that of the pillow
basalt was 0.552 X/m. In addition, it was noticed that the sulfide generates a selfpotential. Comparing the resistivity measurements on a Fe-sulfide sample of
known dimensions permitted us to estimate the thickness of the deposit to vary
from about 8 m up to 15 m thick (Francis 1985).
Using previous submersible and deep-towed camera stations, plus in situ
observations on the Southeastern seamount on the EPR near 12°43
0 N, enabled us to
assess the size of the sulfide deposit. Assuming that sulfide deposits cover the entire
surface of the explored triangle with a base of 900 m length, sides of 800 and 700 m
respectively, and an average thickness of 5 m when assuming a sulfide density of
2.9 gr/cm
3 , it is estimated that the deposit is less than 3.8 million tons. This is a
conservative estimate since the entire surface within the triangle was not entirely
explored and, as shown from the survey tracks, only a portion of the entire seamount was covered (Fig. 7.23). In terms of its size, this deposit is similar to some
of the small exploitable deposits found in Cyprus of 15,000 to 15 million tones
(Searle 1972) and if it were emerged, the seamount would be exploitable for ore.
The theory is that circular cones, because of their shape, are better suited to
canalize ore enriched fluids than along large fissures where a dispersion of matter
and energy is more prominent, such as on the spreading ridge axis. Indeed, when
considering the geometry of a seamount (an inverted funnel-shaped structure), the
concentration of energy sources is likely to be more efficient with centrally located
conduits, than that of the accreting plate boundary regions where the energy source
(heat release) to drive the system would seem to be more dissipated due to
extensive linear fissuring. Although most off axis seamounts are not volcanically
active, some structures that are located up to 50 km (i.e. at 21°N) from the ridge
228
7 Oceanic Spreading Ridges and Sea Floor Creation