THE TIMING OF ADULARIA-SERICITE-TYPE MINERALIZATION AND ALUNITE-KAOLINITE-TYPE ALTERATION, MOUNT-SKUKUM EPITHERMAL GOLD DEPOSIT, YUKON-TERRITORY, CANADA - AR-40-AR-39 AND U-PB GEOCHRONOLOGY

The lower Eocene Mount Skukum epithermal Au deposit, southwestern Yuko n Territory (60 degrees 13' N, 135 degrees 28' W), exhibits both hypog ene alteration and mineralization facies characteristic of the adulari a-sericite deposit type, and barren advanced argillic alteration of th e alunite-kaolinite type, the two being separated by only 750 m on the present erosional surface. The advanced argillic alteration forms a c rudely cylindrical, possibly downward-tapering body, with an alunite-q uartz core and surrounding pyrophyllite +/- kaolinite zones, and is ce ntered on a small (less than or equal to 150-m diam)rhyolitic stock. T he delta D and delta(34)S values of the alunite, which average -175 +/ - 0.5 (1 sigma) per mil and +2.93 +/- 0.43 (1 sigma) per mil, respecti vely, suggest that this alteration conforms to the hypogene type forme d by magmatic SO2 mixing with meteoric water, an inference that is tex tures that imply fairly high temperatures (pyrophyllite stable). The A r-40-Ar-39 age spectra for alunite, generated by furnace heating, exhi bit good multi-step plateaus and show that during slow heating with sm all-increment steps alunite releases virtually all of its argon throug h volume diffusion at temperatures below 540 degrees C. The preferred age for the alunitic alteration is 55.69 +/- 0.24 (2 sigma) Ma, the er ror-weighted average of four plateau dates that range from 55.00 +/- 0 .55 to 56.12 +/- 0.45 hla. Calculated closure temperatures for alunite average 280 degrees +/- 20 degrees C (2 sigma) at a cooling rate of 5 0 degrees C/m.y. Gold-bearing quartz-calcite(-adularia) veins are host ed by andesitic volcanic str ata of the Eocene Skukum Group and by rhy olitic dikes. The mineralized veins conform entirely to tile 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 emp laced at 56.3 +/- 0.4 (2 sigma) Ma (earliest Eocene), as defined by U- Pb zircon dating. Step-heating Ar-40-Ar-39 dating of vein adularia, wi th orthoclase structure, yields plateaus, which suggest that no signif icant Ar loss occurred. The error-weighted average of two plateau date s for the Lake zone and the lowest age steps in two slightly saddle-sh aped plateaus for the Cirque zone, 54.05 +/- 0.31 Ma, is taken as the age of adularia sericite-type mineralization, which is slightly but si gnificantly younger than the alunite-kaolinite-type alteration. A swar m of intermediate dikes, some of which cut ore veins, was intruded ove r an interval that embraced the time of mineralization. One of these d ikes, which cuts the wide rhyolite dike, yielded a U-Pb zircon age of 55.7 +/- 0.3 Ma, but this does not further constrain the age of the mi neralization. The ages of alunite and adularia from Mount Skukum indic ate a succession from the barren alunite-kaolinite-type h hydrothermal system to the gold-bearing adularia-sericite-type system, both of whi ch formed during ongoing magmatism in the Mount Skukum Volcanic Comple x.