Fenwick Creek (105D/3) and Alligator Lake (105D/6) map areas, located southwest of Whitehorse, Yukon, were mapped at 1:50 000 scale during the 1987 field season. The map areas are within the Teslin Plateau and the Boundary Ranges physiographic regions.
Cretaceous Coast Plutonic Complex and Upper Triassic volcanic and sedimentary rocks of the Whitehorse Trough are separated by the 40 km long 140 degree trending "Tally-Ho Shear Zone." East of the shear zone, limestone containing late Upper Trassic conodonts is interbedded with volcanics previously mapped as Mesozoic volcanics, and now considered part of the Lewes River Group.
Mount Skukum and Bennett Lake are two Eocene volcanic complexes in the map area. The volcanic stratigraphy of the Skukum Complex has been reinterpreted and a number of previously unreported rhyolite porphyry plugs and related flows were mapped in other areas outside the main volcanic complexes.
U-Pb age dating on zircon concentrates from three plutons was completed. Megacrystic feldspar granodiorite, dated at 220 +/- 5 Ma, intrudes the Lewes River Group volcanics. The granodiorite forms a regionally extensive northwest trending late Triassic batholith and is interpreted as the plutonic root of the Lewes River Arc. The Ibex Alaskite, a high level discordant pluton that intrudes late Cretaceous (?) felsic volcanics is 58 +/- 1Ma. Mt. Anderson granite-granodiiorite is 119 +/-5 Ma.
Three types of mineral occurrences have been recognized in the area:1. epithermal gold-silver veins, 2. silver-antimony +/- gold viens, and 3. gold-silver telluride veins. Epothermal gold-silver veins in faults cutting late Triassic to Tertiary granites and the gold-silver-telluride veins are spatially related to the "Tally-Ho Shear Zone."