Minerals Report - International Seabed Authority

intrusions are responsible for the big copper deposits that occur in the coastal
ranges of South America, Central America and North America.
Professor Herzig provided illustrations of the products of a magma
chamber. These included fresh pillow lavas that are manifested at the seafloor
and covered with a glass crust due to the cooling of the hot lava by cold
seawater, and young pillow lavas that cover older pillow lavas.
Observing that no one has actually witnessed the formation of new
oceanic crust at seafloor spreading centres, Professor Herzig said that seafloor
spreading is directly related to the formation of polymetallic massive
sulphides deposits. Utilizing a slide illustrating a profile of an oceanic
spreading centre, Professor Herzig said that at specific locations seawater is
able to penetrate into the oceanic crust for one of two reasons: seismicity
occurring at a given location due to seafloor spreading that creates channel
ways; and, cooling of hot lava at the seafloor that also provides channel ways
because of the thermal contraction of the newly formed crust. Professor
Herzig also said that at the seafloor and through these channel ways, seawater
penetrates into the oceanic crust due to water pressure of about 200-300 bars.
Once the seawater has reached the vicinity of the magma chamber he
continued, it is converted into hydrothermal fluid in a high temperature
reaction zone. The first change he noted in the conversion process is seawater
being heated to temperatures of about 500˚ C. The second change that he also
noted is that the pH of seawater, which is about 7.8, is lowered to values as
low as 2 to 3 in the high temperature reaction zone. He stated that this is due
to a number of chemical processes involving the precipitation of Magnesium
and OH-groups from seawater. The third change in the process he further
noted is that oxygen contained in seawater, for example in sulphates, is
completely removed from it, converting it into a hot, aggressive hydrothermal
fluid, that doesn’t contain any free oxygen. The hydrothermal fluid reacts
with the surrounding rock and up wells to the seafloor. The rocks are leached
of metals such as manganese, copper, iron, zinc, gold and silver. Sulphur is
also leached from the rocks and is converted to hydrogen sulphide (H2S). The
reduced metals and hydrogen sulphide are then transported in solution to the
seafloor, with the precipitation of sulphides taking place below the seafloor.
INTERNATIONAL SEABED AUTHORITY 147