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Metamorphism
and Sedimentary Processes
Metamorphism
Orogenesis is the geologic term for mountain building.
Subduction and continent-continent collision result in orogensis and
metamorphism. Convergent boundary plate movements result in
thrusting, folding, and faulting in the upper parts of the Earth's crust, and
plastic folding, metamorphism, and plutonism (formation of magma chambers at
depth) at greater depths. Metamorphism is the mineralogical, chemical, and
structural adjustment of solid mineral assemblages to changing physical and
chemical conditions. Typically but not always, metamorphism occurs due to
increasing temperature and pressure. Two types of metamorphism occur
during orogenesis: regional metamorphism and contact metamorphism.
 Regional
metamorphism is typically marked by increasing temperature and and
directed pressure over a large area. Regional metamorphism occurs in a
linear belt in the plate overriding the the subducting plate due to increasing
temperature and pressure caused by compression, thrusting, folding, and
intrusion of magmas from greater depths. The sediment cover and
basalts and ultramafic rocks that make up the subducting slab may also undergo
regional metamorphism. Initially blueschist facies metamorphism,
characterized by low temperature and high pressure, will occur. At higher
pressures and temperatures, regional eclogite facies metamorphism may
result. The Franciscan series of rocks in California are an example of
blueschist and eclogite facies rocks.
Contact
metamorphism is the localized metamorphism occurring immediately
adjacent to an igneous pluton (large mass of magma, roughly equivalent to a
magma chamber) caused by increasing temperature and/or possible
metasomatism (diffusion of elements and fluids between the magma and the rocks
surrounding the igneous pluton resulting in a change of composition).
Contact metamorphism can yield economic ore minerals such as those in skarns.
A skarn is the Swedish mining term describing a mineral assemblage of
calcium-bearing silicates, iron and sulfide ore minerals formed through the
contact metamorphism of impure limestones or dolomites by the introduction of
large amounts of Si, Al, Fe, and Mg (metasomatism). Skarn ore deposits
adjacent to or extensions of porphyry copper or other porphyry base-metals
deposits are common.
Metamorphic
minerals The finest quality gemstone rubies (red corundum),
from Burma (Myanmar), formed through the regional metamorphism of impure
carbonate rocks. These pigeon-blood rubies are the most valuable gemstones
in the world. Thai rubies occur as xencrysts (foreign crystals) in
basaltic lavas. Aluminous material was assimilated by the high-temperature
basalt and recrystallized as corundum due to the contact metamorphism of the
aluminous material. Most beryls, or emeralds and aquamarines, either
crystallize from pegmatites (igneous processes) or through regional metamorphism
of aluminous rocks to form talc and mica schists.
The worlds most valuable
emeralds from the Muzo and Chivor districts of Columbia formed in an unique
environment and by unique processes that are difficult to classify. The
Muzo and Chivor emeralds occur in veins of calcite and albite that occur in
organic-rich shales and limestones. These emeralds crystallized from 300oC
brines that had leached metals from sediments in sedimentary basins and later
metasomatically reacted with organic-rich black shales.
Sedimentary processes
 With
the exception of minerals such as calcite, quartz, gypsum, and celestite,
precipitated by aqueous solutions in cavities or caves in sediments or
sedimentary rocks near the Earth's surface, sedimentary minerals rarely are as
breathtakingly beautiful as igneous and metamorphic minerals. The biggest
exception to this rule is the precious opal precipitated from silica-bearing
waters in sediments in arid and semi-arid regions.
The sedimentary processes
of weathering, chemical alteration of surrounding minerals, erosion,
transportation, and subsequent deposition are important in placer
deposits. A placer deposit typically is a water-sorted and water-deposited
aggregate of sand or gravel that has concentrated gemstones, or other heavy
minerals of value. Diamonds, rubies, sapphires and other hard precious
gemstones and precious metals such as gold and platinum that were formed by
igneous, metamorphic, and hydrothermal processes are sometimes later
concentrated by the action of river or sea-waters into placer deposits, after
eroded from the original host rock.
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