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 ore deposits in calc-alkaline rocks. Phyllic alteration is characterised by the assemblage of quartz + sericite + pyrite, and occurs at high temperatures and moderately acidic (low pH) conditions.[1]
Hydrogen-ion metasomatism is the process that causes phyllic alteration. While the mineralogy of the rock is altered throughout, texture is preserved and primary porphyry structure (including position of original veins) may still be visible.[2] If a rock undergoes phyllic alteration, then orthoclase feldspar, biotite 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.[2] Tourmaline may appear as radiating aggregate or prismatic crystals between the quartz-sericite assemblage.[2] Phyllic alteration is often closely associated with argillic alteration, which occurs at lower temperatures and dominantly affects plagioclase.[3]