Phyllic alteration is characterised by the assemblage of quartz + sericite + pyrite, and occurs at high temperatures and moderately acidic (low pH) conditions.
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.
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.
Outside of selvages, most alteration occurs in replacement of mafic minerals by chlorite and of plagioclase by sericite.