Any process associated with igneous activity involving the action of very hot waters. The waters involved can be derived directly from an igneous intrusion (i.e. juvenile water) as a residual fluid formed during the late stages of crystallization of the body, or can be external groundwater heated during crystallization of the intrusion. The hydrothermal fluids can react with and alter the rocks through which they pass, or can deposit minerals from solution. Hydrothermal reactions include serpentinization, chloritization (see chlorite), saussuritization (see saussurite), uralitization (see uralite), and propylitization; whilst hydrothermal vein and replacement mineral deposits include Cu, Pb, and Zn sulphides. Hydrothermal activity should not be confused with geothermal activity which involves the convection and movement of hot waters but is not necessarily connected with an igneous intrusion (see geothermal field; geothermal gradient).