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bodies of magma through a process
called differentiation.
The
concentration of some simple high
temperature oxide minerals occurs
within the liquid magma itself. The
vast majority of minerals are
deposited in the surrounding rocks
from the cooling liquid by
replacement of wall rock
and by the
filling of the cavities. In addition,
important deposits may be formed by
the reworking
of older deposits of
magma origin, by weathering and
erosion acting at or near the earth's
surface.
(2)………Whatever the source of
energies involved in its formation,
the magma is very important in the
field of ore deposits since a great
bulk
of these deposits is found as a
direct result from magma activity.
The nature and location of deposition
is influenced by factors outside the
magma, such as structure, nature
of
rocks invaded,
depth below surface,
and abundance
of ground water.
The main source of mineral,
however, is the magma itself and
therefore, it must be of considerable
size in order to have sufficient
mineral present.
(3)………….The term "direct
magma deposits" refers to deposits
of mineral, which have formed within
the body proper of magma mass. As
magma ceases to rise and encroach
upon the rocks in which has been
invading, it begins to cool. The rate
of older material and reposition
or
reconcentration by mechanical,
chemical or organic means form
them. Sedimentary ore deposits may
be divided into the following:
• Mechanical deposits form by
concentrating valuable materials
with relatively high specific
gravities and comparative
resistance
to chemical and
physical breakdown.
These are
placer deposits. Gold, platinum,
tin, monazite, and gemstones
occur in mechanical deposits.
• Residual deposits form in place
by selective leaching
and
removal
of worthless material
while the valuable mineral is left
behind, thus building a
concentration that is worth
mining. Examples are iron,
manganese, nickel, and
aluminum deposits.
• Chemical deposits form by the
precipitation
of material from
solution into bodies of surface
water such as lakes or seas.
Generally, these deposits are
non-metallic, for example,
dolomite, gypsum and salt
deposits.
• Supergene enrichments form
by solution of minerals of
percolating
ground waters
above the water table and re-
precipitation at or near the water
table. Copper deposits of New
Mexico and Arizona are notable
examples.