Living Reference Work Entry

Encyclopedia of Engineering Geology

Part of the series Encyclopedia of Earth Sciences Series pp 1-2

Date: Latest Version


  • Yonathan AdmassuAffiliated withGeology and Environmental Science, James Madison University Email author 


Alteration is any mineralogic change to a preexisting rock through chemical reaction caused by hot circulating hydrothermal fluids.


Hydrothermal fluids, owing to temperature and pressure gradient, travel within a rock’s primary or secondary porosity. They react with country rock, alter original mineralogy, and produce new minerals. Hydrothermal fluids can be magmatic, meteoric, marine, or sedimentary (connate) in origin. They carry mobile elements, large ion lithophile elements (Li, Be, B, Rd, Cs), alkalies, alkali earths, and volatiles (Guilbert and Park 1986).

Alteration Processes

The fluids responsible for inducing alteration of minerals may eventually deposit ore minerals as a result of thermal and chemical changes. Therefore, mapping alteration halos is key to discovering hydrothermal mineral deposits that may or may not outcrop on the surface. Alteration is common with porphyry, s ...

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