Geologic description
Magnetite Island consists of metamorphosed volcanic rocks of Early Jurassic Talkeetna Formation and an Upper Triassic marble that has been subjected to contact metamorphism by Jurassic quartz diorite associated with the Alaska-Aleution Range batholith (Grantz, 1956). There are two magnetite occurrences on the island. The western occurrence consists of a single lens of magnetite garnet skarn. The skarn appears to be about 35 feet wide and is exposed for about 55 feet along strike. A reconnaissance dip -needle traverse indicated the total strike length of the body is about 110 feet. This exposed skarn strikes N 40 E and dips 40 SE. The hanging wall of the skarn is a white medium grain marble of Upper Triassic age which has been recrystallized by contact metamorphism. The foot wall of this occurrence is hornfelsed siltstone and volcanic rock of the Talkeetna Formation. The contact between the marble and the hornfels is unclear but it may be a fault (Grantz, 1956). The eastern occurrence consists of magnetite disseminated in hornfels, probably derived from volcanic rocks. The magnetite occurs in two parallel zones. The upper zone is about 30 feet thick and is separated from the lower zone by 20 feet of hornfelsed volcanics. The lower zone is at least 10 feet thick but its base is not exposed. The mineralized zones strike approximately N 35 to 45 E and dip 45 SE. The magnetite in the eastern zone is disseminated in a matrix of garnet, diopside, chlorite, and plagioclase. No zoning of the skarn minerals are reported. An assay from the upper zone contained 25.8 percent iron and 0.01 percent sulfur. However, about one-fifth of the iron may occur in silicate minerals (Grantz, 1956). |
