Ar/Ar dating of samples comprising fresh porphyry phenocrysts and alteration minerals from the two deposits shows that muscovite from Au-quartz veins cutting porphyry in the largely carbonate-hosted El Hueso deposit (40.25 +/- 0.05 Ma) predates hornblende, primary and secondary biotite, and sericite from the Cobre porphyry (35.65 +/- 0.04 Ma). Mineralization that accompanied cooling of the Cobre porphyry from hornblende Ar closure (550 C, 35.83 +/- 0.21Ma) to biotite closure (250 C, 35.65 +/- 0.04 Ma) was accomplished in less than 300 ka. Advanced argillic alteration (36.23 +/- 0.07 Ma, alunite, with diaspore, pyrophyllite, dickite, and zunyite) at El Hueso significantly postdates Au mineralization there, attesting to the reutilization of structural fluid conduits by a later hydrothermal system. Other porphyries in the district, including the Norte Cu porphyry (36.63 +/- 0.04 Ma) and the barren Bochinche porphyry (37.91 +/- 0.06 Ma), show no detectable differences in age between phenocrysts and alteration minerals having high or low closure temperature, indicating rapid cooling and mineralization following emplacement of the stocks. While the spatial association between Cu and Au at Potrerillos appears to fit a popular genetic model, the temporal relations would require the existence of additional, unexposed porphyry stocks and related mineralization.