CONTROLS AND DISTRIBUTION OF AU-CU MINERALIZATION AT THE ISLAND MOUNTAIN PROSPECT, WHISTLER PROJECT, ALASKA

Island Mountain is one of numerous mineralized magmatic-hydrothermal gold systems within the ~545 km² Whistler property, located ~150 km northwest of Anchorage, Alaska. The deposit is classified as a “reduced” Au-Cu porphyry, a newly recognized and economically important deposit type in which the processes of alteration, fluid evolution and gold mobility are poorly understood. Petrographic, textural and geochemical data generated through logging of more than 10,000 meters of drill core are part of an ongoing study to better understand the genesis and metal partitioning in this and other reduced-type porphyry Au-Cu deposits.

Extensive drilling at Island Mountain has outlined a cluster of mineralized magmatic-hydrothermal breccias pipes hosted in equigranular diorite, porphyritic monozodiorite and hornfels rock types. Sodic-calcic (albite-scapolite-actinolite +/- pyrrhotite) alteration assemblages are the main product of hydrothermal brecciation, which deposited metals in two primary forms: 1) Au-Cu-bearing breccias consisting of heterolithic clasts supported by an actinolite-pyrrhotite cement, and 2) dominantly Au-only zones of stockwork pyrrhotite veins and disseminations, both proximal and distal to the Au-Cu breccias. The highest Cu/Au ratios are found where sodic-calcic veins and breccias crosscut zones of massive orthoclase-quartz, a potassic alteration spatially and temporally related to the monzodiorite porphyry. In contrast, Au-only mineralization (very low Cu/Au ratios) is found within sodic-calcic hydrothermal breccias and massive pyrrhotite zones located outward of potassic phases and overprinted by lower temperature propylitic alteration assemblages. Overprinting of a Au-bearing sodic-calcic high-T hydrothermal fluid on a previously Au-Cu mineralized porphyry likely remobilized disseminated metals into high concentrations, upgrading the monzodiorite porphyry and bounding areas, while low-T Au-only deposition occurred within the barren peripheral host rocks. These paragenetic and lithogeochemical results suggest that elevated gold grades (up to 4 g/t) resulted by amassing multiple intrusion-related Au-Cu events, which included “reduced” Au-bearing hydrothermal breccias overprinting older monzodiorite-hosted porphyry-style Au-Cu mineralization.