Hydrothermal alteration at the Phelps Dodge Archcan volcanogenic massive sulfide deposit has converted rhyodacite and rhyolite footwall rocks into the following mineral assemblages: quartz-chlorite-albite-epidote + or - sericite, quartz-chlorite + or - sericite, chlorite + or - sericite, and chlorite-talc + or - stilpnomelane. Alteration at constant Al was initiated by hydration, addition of Fe + Mg, silicification, and leaching of Ca + Na which produced Al-saturated chlorites with a wide range of Fe/Fe + Mg ratios within the assemblage quartz-chlorite-albite-epidote + or - sericite. With continued addition of Fe and less Mg, combined with extensive leaching of quartz, the chlorite became Al-undersaturated (quartz-chlorite-talc assemblage), and ultimately formed a rock composed essentially of chlorite. The chlorite-talc + or - stilpnomelane assemblage is stable in late crosscutting veinlets and in the ore zone.Chlorites in the Si-Al-Mg-Fe system form a rectangular solid solution plane with limited substitution of Al for Si and extensive substitution of Fe for Mg. In Si-Al space the solid solution plane is outlined by Al saturation (presence of other Al-rich mineral) and Al undersaturation (presence of Al-poor or Al-free mineral) boundaries. Al iv substitution for Si in the chlorites increases from 2.3 to 3.01 formula units (based on 8 Si + Al iv ) with Fe/Fe + Mg, whereas Al iv occupancy decreases from 2.88 to 2.65. The valence charge balance is compensated by octahedral site vacancies or by the conversion of Fe (super +2) to Fe (super +3) . The Al iv content of chlorite also increases with temperature. Chlorites on the Al-saturated boundary of the solid solution field form a potentially useful geothermometer: T degrees C = 106 Al iv + 18; Al iv is corrected for changes with the Fe/Fe + Mg ratio as: Al iv corrected = Al iv + 0.7 Fe/Fe + Mg.The Fe/Fe + Mg ratios of bulk rock and contained chlorite correlate well. Birefringence colors of the chlorite change from green to brown, violet, and Berlin Blue as Fe/Fe + Mg changes from 0.18 to 0.64. The Fe/Fe + Mg variation apparently depends largely on the proportions of Mg-rich seawater and Fe-rich hydrothermal fluid in the altering brine.