The metamorphic process of changing a
rock’s composition or mineralogy by the gradual replacement
of one component by another through the movement and
reaction of fluids and gases in the pore spaces of a rock is
called metasomatism, and these processes are metasomatic.
Metasomatic processes are thought to be responsible for the
formation of many ore deposits, which have extraordinarily
concentrated abundances of some elements. They may also
play a role in the replacement of some limestones by silica,
and the formation of dolostones.
Many metasomatic rocks and ore deposits are formed in
hydrothermal circulation systems that are set up around igneous
intrusions. When magmas, particularly large batholiths,
intrude country rocks they set up a large thermal gradient
between the hot magma and the cool country rock. Any water
above the pluton gets heated and rises toward the surface, and
water from the sides of the pluton moves in to replace that
water. A hydrothermal circulation system is thus set up, and
the continuous movement of hot waters in such systems often
leaches elements from some rocks and from the pluton and
deposits them in other places, in metasomatic processes.
See also METAMORPHISM.
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