The lower Eocene Mount Skukum epithermal Au deposit, southwestern Yukon Territory (60 degrees 13' N, 135 degrees 28' W), exhibits both hypogene alteration and mineralization facies characteristic of the adularia-sericite deposit type, and barren advanced argillic alteration of the alunite-kaolinite type, the two being separated by only 750 m on the present erosional surface.The advanced argillic alteration forms a crudely cylindrical, possibly downward-tapering body, with an alunite-quartz core and surrounding pyrophyllite + or - kaolinitc zones, and is centered on a small (< or = 150-m diam)rhyolitic stock. The delta D and delta 34 S values of the alunite, which average -175 + or - 0.5 (1Sigma ) per mil and +2.93 + or - 0.43 (1Sigma ) per mil, respectively, suggest that this alteration conforms to the hypogene type formed by magmatic SO 2 mixing with meteoric water, an inference that is consistent with mineral associations and textures that imply fairly high temperatures (pyrophyllite stable). The 40 Ar- 39 Ar age spectra for alunite, generated by furnace heating, exhibit good multi-step plateaus and show that during slow heating with small-increment steps alunite releases virtually all of its argon through volume diffusion at temperatures below 540 degrees C. The preferred age for the alunitic alteration is 55.69 + or - 0.24 (2Sigma ) Ma, the error-weighted average of four plateau dates that range from 55.00 + or - 0.55 to 56.12 + or - 0.45 Ma. Calculated closure temperatures for alunite average 280 degrees + or - 20 degrees C (2Sigma ) at a cooling rate of 50 degrees C/m.y.Gold-bearing quartz-calcite(-adularia) veins are hosted by andesitic volcanic strata of the Eocene Skukum Group and by rhyolitic dikes. The mineralized veins conform entirely to the adularia-sericite class and were controlled by strike-slip faults and fracture zones. A wide rhyolite dike cut by the most important ore zone was emplaced at 56.3 + or - 0.4 (2Sigma ) Ma (earliest Eocene), as defined by U-Pb zircon dating. Step-heating 40 Ar- 39 Ar dating of vein adularia, with orthoclase structure, yields plateaus, which suggest that no significant Ar loss occurred. The error-weighted average of two plateau dates for the Lake zone and the lowest age steps in two slightly saddle-shaped plateaus for the Cirque zone, 54.05 + or - 0.31 Ma, is taken as the age of adularia-sericite-type mineralization, which is slightly but significantly younger than the alunite-kaolinite-type alteration. A swarm of intermediate dikes, some of which cut ore veins, was intruded over an interval that embraced the time of mineralization. One of these dikes, which cuts the wide rhyolite dike, yielded a U-Pb zircon age of 55.7 + or - 0.3 Ma, but this does not further constrain the age of the mineralization.The ages of alunite and adularia from Mount Skulmm indicate a succession from the barren alunite-kaolinite-type hydrothermal system to the gold-bearing adularia-sericite-type system, both of which formed during ongoing magmatism in the Mount Skukmn Volcanic Complex.