The phyllic alteration in a number of circum-Pacific porphyry Cu-Au deposits is related to high-temperature saline magmatic fluids. This contradicts the widely used genetic models wherein phyllic alteration formed as the result of mixing between magmatic and meteoric fluids. At the Endeavour 26 North porphyry deposit in eastern Australia, the transition from early potassic to the main-stage phyllic alteration is associated with fluids that with time decline in total salinity, NaCl/KCl, and temperature from ∼600 to ∼550 °C. Calculated and measured δ18O and δD compositions of fluids (5.1‰–8.5‰ δ18O, −57‰ to −73‰ δD) confirm a primary magmatic origin for both the early potassic and main- stage phyllic alteration. These results are consistent with other recent studies (e.g., El Salvador, Chile, Far Southeast, Philippines, and Panguna and Porgera, Papua New Guinea) and suggest that, rather than these results being unusual, a major revision of porphyry Cu genetic models is required.