phyllic

Phyllic is an adjective used in geology to describe a hydrothermal alteration style characterized by the development of phyllosilicate minerals, especially fine-grained sericite (white mica) and muscovite, together with quartz in the rock matrix. Phyllic alteration, often referred to as sericitic alteration, results from hot fluids altering feldspar minerals to mica and silica, producing a silica-rich, mica-bearing assemblage.

Phyllic zones are commonly identified in porphyry copper and related hydrothermal systems. They typically form at intermediate temperatures and are often found surrounding cores of higher-temperature alteration (such as potassic zones) and extending outward toward other alteration types. The mineral assemblage commonly includes quartz and sulfide minerals like pyrite, with variations depending on host rock and fluid chemistry.

Distinguishing features and relationships:

- Argillic alteration is dominated by clay minerals (illite, kaolinite) and lacks the abundant fine mica seen in phyllic zones.

- Propylitic alteration features chlorite, epidote, albite, and lower silica content.

- Sericitization can be used synonymously with phyllic alteration, though some sources distinguish the terms by emphasis on mica-rich assemblages.

In exploration, mapping and sampling of phyllic zones help delineate alteration halos around mineralized centers and guide drill targeting. Overall, phyllic describes a phyllosilicate-rich, quartz-bearing alteration assemblage indicative of hydrothermal activity at moderate conditions.