A type of hydrothermal wall-rock alteration and a class of tin-tungsten deposits (so-called greisen deposits). Hydrothermal wall-rock alteration is the process whereby rocks on the margins of hydrothermal flow channels are changed from an original assemblage of minerals to a different one. This change occurs because of heat and mass exchange between water and rock. The term greisen was originally used by miners in Saxony with reference to relatively coarse-grained aggregates of quartz and muscovite found on the borders of tin veins in granite of the Erzegebirge.