Hydrothermal alteration in porphyry copper deposits
Refers to metasomatic processes that change the composition, mineralogy, and texture of rocks that constitute a porphyry copper system. In porphyry environments, wall-rock alteration is intimately linked to narrow veins, commonly 0.1 to 10 cm in width, that typically make up less than 1 to 5 volume percent of ore but also are present in other alteration zones.
Hydrothermal alteration is characterized by ionic metasomatism, including alkali metasomatism and hydrolytic (or acidic) reactions, oxidation-reduction reactions (including sulfidation), solubility-induced precipitation reactions, such as quartz precipitation, and hydration-carbonation reactions in which water or carbonate is added.
Major alteration types commonly present in porphyry copper
deposits are (1) potassic, (2) sericitic, (3) advanced argillic, (4) intermediate argillic, (5) propylitic, (6) sodic-calcic and sodic, (7) greisen, and (8) skarn.
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