Fe-Oxyhydroxide Deposits
Fe-oxide and hydroxide rich deposits were first recovered on-site from the Mid
Atlantic Ridge in 1973 during the FAMOUS Project. They consist of about
40–80 % Fe 2 O 3 .H 2 O forming primary translucent spherules of goethite minerals.
These are primary precipitates and not an alteration state of pre-existing sulfides.
They differ from other Fe-rich deposits such as the gossans and clay-rich (nontronite) ochreous deposits by their low transitional metal contents
(Cu ? Zn ? Co \ 0.05 %), their low SiO2 (\20 %), and their higher degree of
iron oxidation (Fe3þ=Fe2þ ¼ 400 À 1500). These deposits are deprived of sulfur
(\0.2 %) and do not show any trace of secondary sulfide alteration. Also, the
depletion in Mn (0.5 %) content and the lack of manganese crust on newly formed
Fe-rich hydrothermal deposits indicate that the decoupling between Fe and Mn
precipitation is related to the source composition and to its degree of solubility in
water. The relatively high content of Fe with respect to Mn in basalt will enhance
the formation of Fe-rich precipitates rather than manganese. The manganese will
precipitate as hydrogenous products from the water column.
The primary source of Fe is from the minerals of titanomagnetite and ilmenite
in the leached source rocks with the formation of rutile. Rutile is encountered in
association with chlorite and serpentine in hydrothermally metamorphosed dolerite
and/or gabbro.
The formation of Fe-rich oxyhydroxide deposits could be due either to hydrothermalism or to a simple alteration of the surface of a hot lava flow at contact
with seawater. Hydrothermalism giving rise to Fe-rich oxyhydroxides takes place
at relatively shallow depths within the oceanic crust in comparison to the source of
the sulfide rich deposits. Fe-staining on the broken surface of pillow lavas indicates that a locally low-temperature reaction (at the site of alteration) at the
boundary layers between seawater and the cooling of a lava flow during its
emplacement on the ocean floor is a common phenomenon. Also, seawater trapped
in cavities above sills or dykes may be heated, similar to boiling water in a closed
pot on a stove, and the hot seawater will react with the country rock. When fluid
circulation in the oceanic crust remains at relatively shallow depths (\1 km), the
leaching of deep-seated material will be prevented, and the water/rock ratio in the
reactive zone will be higher. Experimental results of Hajash (1975) demonstrate
both the presence of copper-bearing sulfides and very high Fe and Mn concentrations in fluids that form oxides where the water has become more oxygenated.
The common occurrence of Fe-oxyhydroxide on seamounts of intraplate origin
may also be due to the porous nature of the edifice, which acts some what like a
sponge, absorbing seawater whose solution will then have an increased pH and
oxygen fugacity after being mixed with rising hydrothermal fluid. This may also
explain the lack of polymetallic sulfide deposits on many tall ([1500 m in height)
intraplate seamounts where sill and dyke intrusions occur at shallow depths within
the volcanic edifices.
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6 Hydrothermal Activity and Metalliferous Deposits
Fe-oxide and hydroxide rich deposits were first recovered on-site from the Mid
Atlantic Ridge in 1973 during the FAMOUS Project. They consist of about
40–80 % Fe 2 O 3 .H 2 O forming primary translucent spherules of goethite minerals.
These are primary precipitates and not an alteration state of pre-existing sulfides.
They differ from other Fe-rich deposits such as the gossans and clay-rich (nontronite) ochreous deposits by their low transitional metal contents
(Cu ? Zn ? Co \ 0.05 %), their low SiO2 (\20 %), and their higher degree of
iron oxidation (Fe3þ=Fe2þ ¼ 400 À 1500). These deposits are deprived of sulfur
(\0.2 %) and do not show any trace of secondary sulfide alteration. Also, the
depletion in Mn (0.5 %) content and the lack of manganese crust on newly formed
Fe-rich hydrothermal deposits indicate that the decoupling between Fe and Mn
precipitation is related to the source composition and to its degree of solubility in
water. The relatively high content of Fe with respect to Mn in basalt will enhance
the formation of Fe-rich precipitates rather than manganese. The manganese will
precipitate as hydrogenous products from the water column.
The primary source of Fe is from the minerals of titanomagnetite and ilmenite
in the leached source rocks with the formation of rutile. Rutile is encountered in
association with chlorite and serpentine in hydrothermally metamorphosed dolerite
and/or gabbro.
The formation of Fe-rich oxyhydroxide deposits could be due either to hydrothermalism or to a simple alteration of the surface of a hot lava flow at contact
with seawater. Hydrothermalism giving rise to Fe-rich oxyhydroxides takes place
at relatively shallow depths within the oceanic crust in comparison to the source of
the sulfide rich deposits. Fe-staining on the broken surface of pillow lavas indicates that a locally low-temperature reaction (at the site of alteration) at the
boundary layers between seawater and the cooling of a lava flow during its
emplacement on the ocean floor is a common phenomenon. Also, seawater trapped
in cavities above sills or dykes may be heated, similar to boiling water in a closed
pot on a stove, and the hot seawater will react with the country rock. When fluid
circulation in the oceanic crust remains at relatively shallow depths (\1 km), the
leaching of deep-seated material will be prevented, and the water/rock ratio in the
reactive zone will be higher. Experimental results of Hajash (1975) demonstrate
both the presence of copper-bearing sulfides and very high Fe and Mn concentrations in fluids that form oxides where the water has become more oxygenated.
The common occurrence of Fe-oxyhydroxide on seamounts of intraplate origin
may also be due to the porous nature of the edifice, which acts some what like a
sponge, absorbing seawater whose solution will then have an increased pH and
oxygen fugacity after being mixed with rising hydrothermal fluid. This may also
explain the lack of polymetallic sulfide deposits on many tall ([1500 m in height)
intraplate seamounts where sill and dyke intrusions occur at shallow depths within
the volcanic edifices.
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6 Hydrothermal Activity and Metalliferous Deposits