chloritization

Chloritization is a metasomatic or metamorphic alteration process in which primary minerals are partially or wholly transformed into chlorite-group phyllosilicates. It is common in low- to moderate-temperature environments, notably greenschist facies metamorphism and hydrothermal alteration zones. The reaction typically involves hydration and the introduction of magnesium- and iron-rich fluids that replace mafic minerals such as pyroxene, amphibole, and biotite, and can also alter olivine.

Chlorite forms as green, foliated masses that can outline grains, fill fracture networks, or create alteration halos around ore bodies. It often occurs alongside other alteration minerals such as epidote, quartz, and calcite. The chlorite produced can vary from magnesium-rich to iron-rich depending on fluid composition and temperature, which can influence its color and its appearance in petrographic analyses.

Significance: Chloritization records low-grade metamorphism or hydrothermal activity and helps define alteration zones around mineral deposits. The magnesium–iron ratio of chlorite can provide insights into temperature and fluid chemistry during formation. In exploration geology, chloritized halos are used to delineate ore-bearing zones and are commonly studied through petrographic examination, geochemical assays, and X-ray diffraction analysis.