294 Resources from the Ocean Hoor
In the various discussions about the origin of ferromanganese, the trace element
content, as well as the iron-to-manganese ratio play an important role. Typical values
for Pacific, Atlantic, and Indian Ocean are given in Table 10.2. Note that the Mn/Fe
ratio is greater in the Pacific than in the Atlantic. Also, the content of trace elements
in Pacific nodules, on average, is about twice that of Atlantic ones.
What factors control the Mn/Fe ratio? Which control the trace element content?
The Mn/Fe ratio, on the whole, increases with the degree of oxidation and with
depth - the more "deep-sea" character the nodules have, the more manganese they
contain. Shallow water ferromanganese concretions (e. g., on continental slopes) are
generally iron-rich. In the economically interesting manganese zone north of the
central equatorial Pacific, Mn/Fe ratios are as high as ten. High manganese content
appears to be favored by both high biogenous sediment supply and low rates of
accumulation. Under these conditions, presumably, the carrier material dissolves but
leaves its content of trace elements. Also, in the "manganese zone" in the Pacific, the
underlying sea floor consists of dissolving biogenous sediment rich in trace metals.
The sediment was originally formed underneath the Equator, then moved northward
and downward due to plate motion. this motion brought the calcareous sediment from
a zone of accumulation into a zone of dissolution. Thus, one way to concentrate
manganese and trace elements is to have organisms precipitate the metals (which they
do very efficiently), bring them to the sea floor in shells and fecal matter, and
dissolve or oxidize these carriers to obtain a more nearly pure concentrate.
Other mechanisms for concentrating the metals also must exist. For example, the
element cobalt tends to be high (> 1 %) within ferromanganese accumulating on
seamounts under highly oxidizing conditions. Here the manganese precipitates extremely slowly out of seawater (- 1 mm/million years), at the same time scavenging
(co-precipitating catalytically) the chemically similar cobalt. Indeed, cobalt contents
of crusts near hydrothermal vents are two orders of magnitude lower, because they
are diluted by high Mn and Fe precipitation, as indicated by growth rates of more
than 1000 mm/million years.
10.4.4 Ores from Spreading Axes. For some of the ferromanganese deposits on the
deep-sea floor, the origin is hardly in doubt: those on the crest of active spreading
ridges. Here, seawater circulates through cracks in the newly formed crust, reacting
with the hot basalt (Fig. 10.12). Precipitation of sulfides (Fe, Mn, Cu, Zn, etc.)
originating there may become economically important in the distant future. Up to
now some 100 localities were discovered, by far the most in the Pacific. There is a
great variability in metal contents (Table 10.2).
Seawater penetrates the hot basalt and reacts with it. It takes up Si02 and metals,
gives up Mg to alteration products (smectites and other clay minerals) and gains Ca.
Seawater sulfate is stripped of its oxygen by the reactions with reduced iron in the
basalt, and sulfides precipitate accordingly. The hydrothermally active zone above the
magma chambers feeding the central rift is 3-5 km thick, and temperatures within the
fluids issuing from the vents reach some 350°C. The high temperatures, of course,
greatly accelerate all rates of reaction.
In the various discussions about the origin of ferromanganese, the trace element
content, as well as the iron-to-manganese ratio play an important role. Typical values
for Pacific, Atlantic, and Indian Ocean are given in Table 10.2. Note that the Mn/Fe
ratio is greater in the Pacific than in the Atlantic. Also, the content of trace elements
in Pacific nodules, on average, is about twice that of Atlantic ones.
What factors control the Mn/Fe ratio? Which control the trace element content?
The Mn/Fe ratio, on the whole, increases with the degree of oxidation and with
depth - the more "deep-sea" character the nodules have, the more manganese they
contain. Shallow water ferromanganese concretions (e. g., on continental slopes) are
generally iron-rich. In the economically interesting manganese zone north of the
central equatorial Pacific, Mn/Fe ratios are as high as ten. High manganese content
appears to be favored by both high biogenous sediment supply and low rates of
accumulation. Under these conditions, presumably, the carrier material dissolves but
leaves its content of trace elements. Also, in the "manganese zone" in the Pacific, the
underlying sea floor consists of dissolving biogenous sediment rich in trace metals.
The sediment was originally formed underneath the Equator, then moved northward
and downward due to plate motion. this motion brought the calcareous sediment from
a zone of accumulation into a zone of dissolution. Thus, one way to concentrate
manganese and trace elements is to have organisms precipitate the metals (which they
do very efficiently), bring them to the sea floor in shells and fecal matter, and
dissolve or oxidize these carriers to obtain a more nearly pure concentrate.
Other mechanisms for concentrating the metals also must exist. For example, the
element cobalt tends to be high (> 1 %) within ferromanganese accumulating on
seamounts under highly oxidizing conditions. Here the manganese precipitates extremely slowly out of seawater (- 1 mm/million years), at the same time scavenging
(co-precipitating catalytically) the chemically similar cobalt. Indeed, cobalt contents
of crusts near hydrothermal vents are two orders of magnitude lower, because they
are diluted by high Mn and Fe precipitation, as indicated by growth rates of more
than 1000 mm/million years.
10.4.4 Ores from Spreading Axes. For some of the ferromanganese deposits on the
deep-sea floor, the origin is hardly in doubt: those on the crest of active spreading
ridges. Here, seawater circulates through cracks in the newly formed crust, reacting
with the hot basalt (Fig. 10.12). Precipitation of sulfides (Fe, Mn, Cu, Zn, etc.)
originating there may become economically important in the distant future. Up to
now some 100 localities were discovered, by far the most in the Pacific. There is a
great variability in metal contents (Table 10.2).
Seawater penetrates the hot basalt and reacts with it. It takes up Si02 and metals,
gives up Mg to alteration products (smectites and other clay minerals) and gains Ca.
Seawater sulfate is stripped of its oxygen by the reactions with reduced iron in the
basalt, and sulfides precipitate accordingly. The hydrothermally active zone above the
magma chambers feeding the central rift is 3-5 km thick, and temperatures within the
fluids issuing from the vents reach some 350°C. The high temperatures, of course,
greatly accelerate all rates of reaction.