Phyllic alteration is a
hydrothermal alteration zone in a
permeable rock that has been affected by circulation of
hydrothermal fluids. It is commonly seen in
copper porphyry
Porphyry copper deposits are copper ore bodies that are formed from hydrothermal fluids that originate from a voluminous magma chamber several kilometers below the deposit itself. Predating or associated with those fluids are vertical dikes of ...
ore deposits in
calc-alkaline rocks
The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic ma ...
. Phyllic alteration is characterised by the assemblage of
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
+
sericite
Sericite is the name given to very fine, ragged grains and aggregates of white (colourless) micas, typically made of muscovite, illite, or paragonite. Sericite is produced by the alteration of orthoclase or plagioclase feldspars in areas that hav ...
+
pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic Luster (mineralogy), lust ...
, and occurs at high temperatures and moderately acidic (low pH) conditions.
Hydrogen-ion
metasomatism is the process that causes phyllic alteration. While the
mineralogy
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proces ...
of the rock is altered throughout,
texture is preserved and primary porphyry structure (including position of original veins) may still be visible.
If a rock undergoes phyllic alteration, then
orthoclase feldspar,
biotite
Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...
and various silicates are altered in addition to plagioclase. Plagioclase will be altered to sericite (a fine-grained white mica) by
sericitic alteration, and mafic minerals are replaced by quartz.
Tourmaline
Tourmaline ( ) is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is a gemstone and can be found in a wide variety of colors.
The ...
may appear as radiating
aggregate
Aggregate or aggregates may refer to:
Computing and mathematics
* collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...
or prismatic crystals between the quartz-sericite assemblage.
Phyllic alteration is often closely associated with
argillic alteration Argillic alteration is hydrothermal alteration of wall rock which introduces clay minerals including kaolinite, smectite and illite. The process generally occurs at low temperatures and may occur in atmospheric conditions. Argillic alteration is re ...
, which occurs at lower temperatures and dominantly affects plagioclase.
Variation with depth
Phyllic alteration typically forms in the
base-metal zone of a porphyry system.
Alteration assemblages vary with depth and with degree of fluid interaction. In deep environments, the most highly altered areas are veins and thin ''selvages'', or halos, that surround them. The selvages are generally <10 cm in diameter and composed of major sericite and minor quartz. Vein orientation is preserved from original rock, but minerals within are mostly replaced by pyrite.
With decreasing depth, selvages widen (10 cm - 1m) and contain more quartz and pyrite.
Outside of selvages, most alteration occurs in replacement of mafic minerals by
chlorite
The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...
and of plagioclase by sericite.
References
Sericite at Mindat{{reflist
Geological processes