PETROLOGY AND 40Ar/39Ar GEOCHRONOLOGY OF GRANITIC ROCKS AND GREISENS AT THE SLEITAT MOUNTAIN TIN-SILVER-TUNGSTEN DEPOSIT, ILIAMNA DISTRICT, SOUTHWEST ALASKA

A small, 2 km², peraluminous, composite granite pluton intrudes Mesozoic flysch at Sleitat Mountain in the Taylor Mountain A-3 quadrangle of southwest Alaska. Significant quartz-tourmaline-topaz –tin-silver-tungsten deposits occur in east-west trending zones in the Sleitat pluton. The Sleitat stock consists of a biotite-Kspar-zinnwaldite-bearing granite core rimmed by a muscovite biotite binary granite. Anomalous tin and silver also occur in quartz-axinite veins cutting an extensive 8 km² hornfels aureole surrounding and overlying the pluton. Field evidence suggests several mineralizing events following crystallization of the stock that include: 1) early albitization of granite over wide areas near hornfels contacts and propylization of biotite in the binary granite phase ; 2) early high temperature greisen formation (topaz-quartz-coarse grained muscovite - cassiterite - tourmaline); 3 ) low temperature greisen event with tourmaline - chlorite - quartz muscovite greisen followed by a chalcopyrite - arsenopyrite - wolframite - quartz assemblages ; and 4) pervasive potassic alteration over much of the intrusion. Mineral separates from four samples of greisens and granite were subjected to ⁴⁰Ar/³⁹Ar step-heating analyses. Biotite ages for the two intrusions are 59.4 ± 0.2 Ma and not significantly different from one another at the 95% confidence level although the biotite-muscovite granite is slightly younger than the biotite-Kspar –zinnwaldite granite, which is consistent with observed field relationships. Muscovite ages from greisens are slightly younger than the age of plutonic biotite, also consistent with field relationships. More significantly, greisen muscovites show evidence of excess argon, indicative of a protracted history which was not observed in muscovite grains of the two-mica granite phase. Although the data do not conclusively constrain the timing of greisen formation, a single greisen muscovite age of 58.2 ± 0.4 Ma may indicate that the duration of that activity was on the order of 1.0 million years, consistent with the progressive evolution of other tin - granite systems reported in the literature. To our knowledge, this is the only tin greisen event to be precisely dated from the early Tertiary ‘McKinley plutonic suite’ of southern Alaska.