Physical Effects of Alteration:
of Wall Rock Alteration
Advanced argillic - characterized by the clays dickite, kaolinite and pyrophyllite (all hydrated aluminum silicates) and quartz. Sericite may be present as well as alunite and tourmaline. Alteration involves the extreme leaching of cations, especially the difficult to leach alkalis and calcium, and the concentration of H+. This type of alteration is characteristic of many epithermal precious metal deposits and a smaller number of mesothermal deposits such as Butte, Montana.
Potassic alteration - characterized by secondary kspar + biotite. Anhydrite may be present, but its susceptibility to solution generally results in its dissolution in near surface environments. Because it is characterized by common silicates, potassic alteration is often difficult to detect. Pyrite and minor chalcopyrite and molybdenite are the only ore minerals associated with this alteration.
Sericitization (Phyllic) - characterized by the assemblage quartz + sericite + pyrite. Generally the most common form of alteration. Sulfides present, in addition to pyrite, include chalcopyrite, bomite and a variety of less common copper sulfides.
Argillic - characterized by kaolinite + montmorillonite. Somewhat similar to advanced argillic alteration, but with a lesser degree of leaching of cations. Also unlike advanced argillic alteration which is associated with vein type deposits, argillic alteration is more closely associated with disseminated deposits, porphyry coppers in particular. Sulfides are less common in association with this alteration type.
Propylitic - characterized by the assemblage chlorite + epidote + calcite. Albite as well as other carbonates may be present. Due to presence of the green minerals chlorite and epidote this zone is usually easily recognizable by its color. Associated sulfides include pyrite, copper sulfides, galena, sphalerite and a host of complex arsenides. Often this zone can be quite large and is useful during mineral exploration. Unlike the previous types above which are characterized by leaching of cations this zone seems to represent the addition of cations.
Silicification - characterized by quartz or chert. Can be added by solutions as is the case in many low temperature deposits or the result of complete leaching of all cations plus aluminum.
Dolomitization - addition of magnesium to limestone to form dolomite. Common in Mississippi Valley type deposits.
Other alteration types:
Feldspathization - kspar + albite, forms in the deep zones of some porphyry copper deposits.
Greisenization - tourmaline + topaz + cassiterite + various tungsten-bearing minerals. Common form of alteration on association with porphyry tin deposits.
Fenitization - characterized by nepheline, alkali feldspar and Na-bearing amphiboles. Hematization - characterized by secondary hematite.
Bleaching - not characterized by any specific mineral assemblage, but rather a color change between altered and unaltered rock. Generally the result of oxidation of Fe.
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