Phyllic alteration is a
hydrothermal alteration
Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a Rock (geology), rock by hydrothermal and other fluids. It is traditionally defined as metamorphism which involves a change in t ...
zone in a
permeable
Permeability, permeable, and semipermeable may refer to:
Chemistry
*Drug permeability
*Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion
*Vascular permeability, the movement of fluids a ...
rock that has been affected by circulation of
hydrothermal fluids. It is commonly seen in
copper porphyry ore deposits in
calc-alkaline rocks. Phyllic alteration is characterised by the assemblage of
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
+
sericite
Sericite is the name given to very fine, ragged grains and Aggregate (geology), aggregates of white (colourless) micas, typically made of muscovite, illite, or paragonite. Sericite is produced by the alteration of orthoclase or plagioclase feldsp ...
+
pyrite
The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic luster and pale brass-yellow hue ...
, and occurs at high temperatures and moderately acidic (low pH) conditions.
Hydrogen-ion
metasomatism
Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is traditionally defined as metamorphism which involves a change in the chemical com ...
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 mineralogy, optical) properties of minerals and mineralized artifact (archaeology), artifacts. Specific s ...
of the rock is altered throughout,
texture
Texture may refer to:
Science and technology
* Image texture, the spatial arrangement of color or intensities in an image
* Surface texture, the smoothness, roughness, or bumpiness of the surface of an object
* Texture (roads), road surface c ...
is preserved and primary porphyry structure (including position of original veins) may still be visible.
If a rock undergoes phyllic alteration, then
orthoclase
Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles ...
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 al ...
and various silicates are altered in addition to plagioclase. Plagioclase will be altered to sericite (a fine-grained white mica) by
sericitic alteration
Sericitic alteration or sericitization is a process of mineral alteration caused by hydrothermal fluids invading permeable country rock. Plagioclase feldspar within the rock is converted to sericite (sericite is not a mineral; it is a term that ...
, and mafic minerals are replaced by quartz.
Tourmaline
Tourmaline ( ) is a crystalline silicate mineral, silicate mineral group in which boron is chemical compound, compounded with chemical element, elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. This gemstone comes in a ...
may appear as radiating
aggregate 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 (oxyanion), 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 s ...
and of plagioclase by sericite.
References
Sericite at Mindat{{reflist
Geological processes