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ardent basalts at the depth of 1-2 km and subsequent elevation heated
hydrotherms, enriched with ore components, with discharging them at the
bottom of oceans under a cold strata of seawaters (Cronan, 1980; Lisitsyn et
al., 1990).
So we have seen that certain metals are emitted in great quantity during
the process of expansion of oceans, especially in relation to mid-oceanic
ridges. This phenomenon is so significant that it needs to be discussed in
terms of any concept presupposing a global view of
the geological evolution
of the Earth. It should be noted that none of the ore-forming processes
known at present could be considered responsible for the phenomenon in
the question. To explain these virtually inexhaustible resources, one is
obliged to impute a grandiose source, which is
by many orders of magnitude
larger than all known ore sources on the continents.
For this purpose we propose considering a process of silication as a
possible source of the ore materials in oceans. This process involves
intermetallic silicides in the interiors of mid-oceanic ridges. This
intermetallic compounds preserve some substantial part of the metallic
bond, and hence are capable of forming various alloys as well as solid
solutions. For that reason the silicides of magnesium, iron, and other
intermetals are able to retain in their lattices large admixtures of various
other metals and probably of metalloids as well: phosphorus, carbon,
sulphur, and e.t.c. Silicates, on the contrary, do not form alloys with metals,
and their ability to form solid solutions is substantially inhibited. The
rigidity of the Si-O bond prevents the formation of intrusive structures; and
the cation-anion character of the crystal matrix in many cases restricts
substitution between elements of comparable atomic radius, by the
incompatibility of outer electron shell structures with electronegativity.
Hence, the isomorphic capacity of silicate crystal lattices is rather small.
The regeneration of silicides into silicates must therefore be accompanied
by the evacuation of most other elements. This process releases a large
number of “extraneous” (non-petrogenetic) metals and, probably,
metalloids
as well, because they are not capable of substantial isomorphic substitution
in the silicon-oxygen lattice (Larin, 1980).
Oceanic ores exhibit quite remarkable compositions. Their siderophilic
elements are iron, manganese, cobalt, nickel, and vanadium. Their
chalcophilic elements are zinc, copper, lead, silver, and gold. In oceanic
sediments their main ore components are represented by the elements that
have small affinities for oxygen, a fact which cannot be regarded as mere
chance, because silication delivers, first of all, those elements whose
oxygen
bond energy is low, when oxygen is first available for reaction.
According to our model, the supply of ore material to the planetary
surface should be fully manifest as late as the mature stage of ocean
evolution. At this stage the median ridge has formed and the regeneration of
intermetallic compounds into silicates goes on under near-surface
conditions directly beneath the rift valley bottom. At first the zone of
silication is situated at a depth of more than 100
km and is hidden below the
asthenosphere, which prevents persistent zones of tectonic weakness from