Hydrothermal alteration of the quartz monzonite host rock in the Castle Dome copper deposit consists of three phases. Very weak propylitic alteration of biotite and plagioclase occurs in the marginal part of the mineralized area. Where mineralization is stronger most of the plagioclase and some of the orthoclase and biotite are replaced by a montmorillonitetype clay, resembling beidellite, and hydrous mica. The third phase is localized along quartz-pyrite veins where the wall rock, commonly as much as an inch from the vein, is replaced by quartz and seriate. In contrast to the localization of quartz-sericite alteration along the veins, clay alteration is general and can be seen between the veins wherever the rock is not completely sericitized. Thus these two types of alteration cannot be separated areally. The most intense clay alteration together with moderate quartz-sericite alteration occurs in the ore body, whereas the strongest quartz-sericite alteration, where pyrite veins are largest and most numerous, is in a zone along the north side of the ore body.