Metasomatism is a mass transfer process, it is not restricted to the rocks which are changed by addition of chemical elements and minerals or hydrous compounds. In all cases, to produce a metasomatic rock some other rock is also metasomatised, if only by dehydration reactions with minimal chemical change. This is best illustrated by gold ore deposits which are the product of focused concentration of fluids derived from many cubic kilometres of dehydrated crust into thin, often highly metasomatised and altered shear zones and lodes. The source region is often largely chemically unaffected compared to the highly hydrated, altered shear zones, but both must have undergone complementary metasomatism.