Types of Wall Rock Alteration
1. Potassic alteration
ØPotassic (or K-silicate) alteration is characterized by the formation of new K-feldspar and/or biotite, usually together with minor sericite, chlorite, and quartz.
ØAccessory amounts of magnetite/hematite and anhydrite may occur associated with the potassic alteration assemblage.
ØIt typically represents the highest temperature form of alteration (500–600°C) associated with porphyry Cu-type deposits, forming in the core of the system and usually within the granite intrusion itself.
ØPyrite and minor chalcopyrite and molybdenite are the only ore minerals associated with this alteration.
ØNot all K-feldspar alteration is characterized by the presence of reddish colouration.
2. Phyllic (or sericitic) alteration
ØThis alteration style is the most common in a variety of hydrothermal ore deposits and forms over a wide temperature range by hydrolysis of feldspars to form sericite (fine-grained white mica), with minor associated quartz, chlorite, and pyrite.
ØPhyllic alteration is associated with porphyry Cu deposits, but also with mesothermal precious metal ores and volcanogenic massive sulfide deposits in felsic rocks.
3. Propylitic alteration
ØPropylitic alteration is probably the most widespread form of alteration.
ØIntermediate argillic alteration affects mainly plagioclase feldspars and is characterized by the formation of clay minerals kaolinite and the smectite group (mainly montmorillonite). It typically forms below about 250°C by H+ metasomatism and occurs on the fringes of porphyry systems.
ØAdvanced argillic alteration is characterized by kaolinite, pyrophyllite, or dickite (depending on the temperature) and alunite together with lesser quartz, topaz, and tourmaline. This type of alteration is characteristic of many epithermal precious metal deposits and a smaller number of mesothermal deposits such as Butte, Montana.
ØSilication is the conversion of a carbonate mineral or rock into a silicate mineral or rock. It is the main process which accompanies the prograde stage in the formation of polymetallic skarn deposits which develop when a fertile, acidic, magmatic fluid infiltrates a carbonate host rock.
ØSilicification should not be confused with silication and refers specifically to the formation of new quartz or amorphous silica minerals in a rock during alteration.
6. Carbonatization (Dolomitization)
ØIs the formation of carbonate minerals (calcite, dolomite, magnesite, siderite,etc.) during alteration of a rock. As dolomite in association with amphibolite, siderite in a banded iron-formation, or calcite in a granitic host.
ØA process of hydrothermal alteration in which feldspar and muscovite are converted to an aggregate of quartz, topaz, tourmaline, and lepidolite (i.e., greisen) by the action of water vapor containing fluorine. Ref
ØMedium to high temperature alteration. Associated with many tin and gold deposits. Quartz-sericite-tourmaline veins and alteration common.
ØAlteration that is associated with oxidizing fluids often results in the formation of minerals with a high Fe3+/Fe2+ ratio and, in particular, hematite with associated K-feldspar, sericite, chlorite, and epidote.
ØA fenite is a quartzofeldspathic rock that has been altered by alkali metasomatism at the contact of a carbonatite intrusive complex. The process is called fenitization. Fenite is comprised mostly of alkalic feldspar, with some aegirine, subordinate alkali-hornblende, and accessory sphene and apatite. Chemically, fenites are Na- and K-rich silicate rocks which develop at the contact between alkaline (carbonatite) igneous intrusions and their surrounding country rocks.
ØChlorite may result from alteration of mafic minerals or introduction of Fe and/or Mg. Very common surrounding plumbing of sea-floor massive sulfides.
ØNot characterized by any specific mineral assemblage, but rather a color change between altered and unaltered rock. Generally the result of oxidation of Fe.
Alteration is a complex process of ion exchange whereby some constituents are removed, others are added and still others are merely redistributed. The physical effects of alteration include recrystallization, changes in permeability and changes in colour.
Carbonate rocks are characteristically recrystallized along the borders of a vein or near an igneous contact. Conversely argillization may reduce permeability of a rock, leaving the orebody enclosed within a relatively impermeable shell.
Colour changes include bleaching, darkening and production of aureoles (zones) of various colours. Pastel colours are especially prominent around certain ore deposits and may form conspicuous leads to the ore.
Pyrite is a standard alteration product around sulphide ore deposits (since iron is one of the most abundant metals in the earth's crust). Pyrite forms whenever sulfur is added to a host rock containing iron or ferro-magnesian minerals. Pyrite causes a striking colour change e.g. the pyritization of a red sandstone or shale will produce a bleached zone due to reduction of iron. Conversely, any pyritized rock is likely to be made conspicuous at the surface by oxidation of iron which will produce a red, brown-red or yellow weathered zone.
Unstable (not in equilibrium) rocks undergo physical and chemical changes (in order to attain equilibrium) in the presence of early ground preparing hydrothermal fluids of ore solutions.
The alteration may be very subtle (hydration of ferromagnesian minerals) to very intense (silicification of limestones).
Indeed replacement ores are merely commercially valuable products of wallrock alteration.
Wallrock alteration has been recognized a valuable tool in exploration, because the alteration haloes around many deposits, are widespread and easier to locate than the orebodies themselves.
At various distances from a vein, the conditions of temperature and chemistry are usually different. As a result of this different types of alteration are likely to be produced simultaneously at various distances from the vein or fissure. For example, in the outer fringes of the alteration zone, the ferromagnesians may have been slightly hydrated while the interior zone was being silicified or sericitized, and the intermediate zone argillized. The product of this is a zoning of different alteration products arranged symmetrically around the central vein. In some deposits this zoning is conspicuous and may be an excellent guide to ore.
TYPES OF WALL-ROCK ALTERATION
Now coming to the exact question raised, the types are varied and manifold. About 10 types are being named as under though numerous permutations and combinations often tend to produce mixed variables. To name the types : Argillic(kaolin+montmorillonite+dickite+pyrophyllite), Potassic(potash feldspar+biotite), Phyllic(quartz+sericite+pyrite), Propylitic(chlorite+epidote+calcite), Silicification (quartz+chert), Dolomitization, Feldspathization, Greissenisation, Fenitizaztion and Bleaching.
Ref: Wallrock Alteration - S. Farooq, Dept of Geology Aligarh Muslim University, India
www.geol-amu.org/notes/m7-1-8.htm: Extracted from Google page (Internet Search).