Hydrothermal alteration

Hydrothermal alteration is chemical and mineralogical change in rock caused by hot, aqueous fluids moving through it. It can replace original minerals, fill fractures, and form new mineral assemblages.

Water–rock interaction: Chemical exchange between water and rock that changes the composition of one or both. Hydrothermal alteration is a high-temperature form of this exchange.
Hydrothermal fluid: Hot aqueous fluid circulating through rock and carrying dissolved substances. It supplies the heat and transported chemicals that drive alteration.
Porphyry copper deposit: A large copper deposit associated with intrusive igneous rocks and disseminated sulfide minerals. Its zoned alteration patterns help locate and interpret copper-bearing parts of the system.
Propylitic alteration: A common alteration assemblage containing minerals such as chlorite, epidote, and calcite. It commonly forms in cooler, more distal zones around hydrothermal systems.
Fluid–rock ratio: The amount of fluid that reacts with a given amount of rock. It helps determine how completely circulating fluids alter the host rock.
Permeability: The capacity of a material to transmit fluids through connected pores or fractures. Fluid access through permeable pathways governs where alteration develops.
Epithermal deposit: A mineral deposit formed from hot fluids at shallow crustal depths, commonly rich in gold or silver. Its alteration assemblages can guide exploration and indicate fluid chemistry.
Phyllic alteration: An alteration assemblage dominated by quartz, sericite, and commonly pyrite. It records acidic, potassium-bearing fluids in many porphyry systems.
Metasomatism: Chemical replacement of rock through the addition or removal of elements by fluids. Hydrothermal fluids can alter rocks by transporting elements into and out of them.
Chemical equilibrium: A state in which forward and reverse chemical reactions proceed at equal rates. Fluid and rock chemistry can approach equilibrium as they react during circulation.

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