84
Y. Fouquet
the surface of deposits. Some elements such as selenium and cobalt are enriched at
certain sites.
Seawater/Basalt/Sediment Interaction (Deposits Rich in Cu, Zn, Pb, As) In certain
environments, such as the Gulf of California and the west coast of Canada, the ridge
is close to continents and can be buried under a layer of sediment. Hydrothermal
fluids must therefore make their way through the sediment, before emerging on the
ocean floor. As they rise, their acidity, which is extremely high to begin with, is
neutralised by reaction with carbonates. Certain elements, such as lead and arsenic,
which are enriched in sediments of continental origin, are dissolved through chemical reactions. A share of the other metals is deposited in the sediment, which acts as
an efficient trap. These environments are enriched in lead and arsenic, in relation to
sites formed directly on basalt.
Seawater/Andesite-Rhyolite Interaction (Deposits Rich in Zn, Cu, Pb, As, Ag,
Au) In back-arc basins, acidic volcanic rocks (andesite, rhyolite) are commonly
found. These environments are well known in the Western Pacific. Deposit compositions are midway between those of ridges and those formed within the context
of the continental crust. In particular, samples taken from the surface are extremely
high in zinc and rich in lead and arsenic in relation to sites associated with basalt.
This high zinc content is also visible in the composition of fluids. One particularity
of these sites is their high silver and gold contents, whose average concentrations
can exceed 10 gm per tonne for gold and 250 gm per tonne for silver.
Seawater/Basalt/Mantle Rock Interaction (Deposits Rich in Cu, Zn, Co, Au) Since
the late 1990s, many observations have shown that at slow-spreading ridges, the
Earth’s mantle is commonly exposed. On certain portions of the ridge, in particular
at the ends of volcanic segments, very few basaltic flows are observed. As plates
continue to relentlessly spread and diverge, the crack formed results in the exposure
of subjacent rocks, i.e. mantle rocks. On the Mid-Atlantic Ridge, five active sites,
Rainbow, Logatchev 1 and 2, and Ashadze 1 and 2, are currently known on mantle
bedrock. They result from the reaction between seawater and ultramafic mantle
rocks. Associated fluids are exceptional in terms of their high methane and hydrogen concentrations, formed by chemical reactions related to mantle rock hydration.
These reactions result in a 30 % increase in rock volume. In this case, the nature of
the bedrock is very marked in the chemical composition of fluids and hydrothermal precipitates. In particular, sulphides are characterised by high copper, zinc and
cobalt contents. Locally, gold concentrations of over 50 gm per tonne have been
measured. Here, like in back-arc basins, high copper concentrations lie directly on
the ocean floor at the surface of the deposits.
A Specific Case, Metalliferous Sediments in The Red Sea Metalliferous sediments
in the Red Sea today constitute the largest hydrothermal mineral deposit in the
world’s oceans. The Red Sea is an ocean in the early stages of ocean opening;
the basins are located in the deepest part of the volcanic axis. In1948, the Swedish research vessel, Albatross, identified abnormal temperature and salinity in the
Red Sea. The first metalliferous sediments were discovered in the 1960s in several
Y. Fouquet
the surface of deposits. Some elements such as selenium and cobalt are enriched at
certain sites.
Seawater/Basalt/Sediment Interaction (Deposits Rich in Cu, Zn, Pb, As) In certain
environments, such as the Gulf of California and the west coast of Canada, the ridge
is close to continents and can be buried under a layer of sediment. Hydrothermal
fluids must therefore make their way through the sediment, before emerging on the
ocean floor. As they rise, their acidity, which is extremely high to begin with, is
neutralised by reaction with carbonates. Certain elements, such as lead and arsenic,
which are enriched in sediments of continental origin, are dissolved through chemical reactions. A share of the other metals is deposited in the sediment, which acts as
an efficient trap. These environments are enriched in lead and arsenic, in relation to
sites formed directly on basalt.
Seawater/Andesite-Rhyolite Interaction (Deposits Rich in Zn, Cu, Pb, As, Ag,
Au) In back-arc basins, acidic volcanic rocks (andesite, rhyolite) are commonly
found. These environments are well known in the Western Pacific. Deposit compositions are midway between those of ridges and those formed within the context
of the continental crust. In particular, samples taken from the surface are extremely
high in zinc and rich in lead and arsenic in relation to sites associated with basalt.
This high zinc content is also visible in the composition of fluids. One particularity
of these sites is their high silver and gold contents, whose average concentrations
can exceed 10 gm per tonne for gold and 250 gm per tonne for silver.
Seawater/Basalt/Mantle Rock Interaction (Deposits Rich in Cu, Zn, Co, Au) Since
the late 1990s, many observations have shown that at slow-spreading ridges, the
Earth’s mantle is commonly exposed. On certain portions of the ridge, in particular
at the ends of volcanic segments, very few basaltic flows are observed. As plates
continue to relentlessly spread and diverge, the crack formed results in the exposure
of subjacent rocks, i.e. mantle rocks. On the Mid-Atlantic Ridge, five active sites,
Rainbow, Logatchev 1 and 2, and Ashadze 1 and 2, are currently known on mantle
bedrock. They result from the reaction between seawater and ultramafic mantle
rocks. Associated fluids are exceptional in terms of their high methane and hydrogen concentrations, formed by chemical reactions related to mantle rock hydration.
These reactions result in a 30 % increase in rock volume. In this case, the nature of
the bedrock is very marked in the chemical composition of fluids and hydrothermal precipitates. In particular, sulphides are characterised by high copper, zinc and
cobalt contents. Locally, gold concentrations of over 50 gm per tonne have been
measured. Here, like in back-arc basins, high copper concentrations lie directly on
the ocean floor at the surface of the deposits.
A Specific Case, Metalliferous Sediments in The Red Sea Metalliferous sediments
in the Red Sea today constitute the largest hydrothermal mineral deposit in the
world’s oceans. The Red Sea is an ocean in the early stages of ocean opening;
the basins are located in the deepest part of the volcanic axis. In1948, the Swedish research vessel, Albatross, identified abnormal temperature and salinity in the
Red Sea. The first metalliferous sediments were discovered in the 1960s in several