The heat and minerals of hydrothermal waters may result in the chemical alteration of rocks that it comes in contact with. The minerals that result from this alteration may be evidence of past hydrothermal activity.
Opalization - alteration to opal.
Argillization- alteration to clay minerals such as smectite, illite, and kaolinite which often form caprocks.
Phyllic alteration - “Hydrothermal alteration typically resulting from removal of sodium, calcium, and magnesium from calc-alkalic rocks, with pervasive replacement of silicates, muting the original rock texture. It is a common style of alteration in porphyry base-metal systems around a central zone of potassic alteration” (reference: http://www.maden.hacettepe.edu.tr/dmmrt/dmmrt864.html).
Silicification - alteration to silica.
Hematitic alteration - alteration to hematite.