Biotites from hydrothermal alteration zones of the Casino porphyry Cu-Mo-Au deposit have a chemical composition that is distinct from biotites of the unaltered Dawson Range Batholith and Casino Plutonic Suite. Biotites from potassic, phyllic, and propylitic alteration zones contain distinctly higher XMg (XMg = Mg/(Mg+Fe)) values than biotites from the unaltered country rocks. The XMg values are positively correlated with Ti, F, Cl, Si, and negatively correlated with Al, Fe, Mn, Ba contents. The XF/XOH (X = mole fraction) values are positively correlated with XMg/XFe values and describe a positive linear trend from propylitic, to potassic, to phyllic zones. The distinct XMg, Ti, F, Cl, Al, Fe, Mn, and XF/XOH and XMg/XFe compositions of biotites from alteration zones may prove to be useful in understanding the evolution of fluid chemistry, and the identification of alteration zones in porphyry copper systems in the Dawson Range.