Trachsel

Occurrence, Inactive

Alternative names

Snug Harbor

Commodities and mineralogy

Main commodities Fe
Ore minerals magnetite
Gangue minerals garnet; pyroxene

Geographic location

Quadrangle map, 1:250,000-scale KN
Quadrangle map, 1:63,360-scale A-8
Latitude 60.246
Longitude -152.851
Nearby scientific data Find additional scientific data near this location
Location and accuracy Magnetic Island is at the mouth of Tuxedni River, which is on the west side of Cook Inlet, due west of Ninilchik. The island can be accessed by small boat or seaplane. However, the trip should only be attempted at high tide and under favorable weather conditions since most of Tuxedni Bay is dry at low tide. There are two magnetite occurrences on the island, one on the east and one on the west side. This is Cobb's (1972, MF-377) location 1 and is accurate to within 300 feet.

Geologic setting

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).
Geologic map unit (-152.85325558529, 60.2454003000605)
Mineral deposit model Fe skarn (Cox and Singer, 1986; model 18d).
Mineral deposit model number 18d
Age of mineralization Jurassic; hornfels related to Intrusion of Jurassic quartz diorite.
Alteration of deposit Hornfels and metasomatic alteration are present in this occurrence.

Production and reserves

Workings or exploration Small excavation in sea cave at the western deposit contains 25 to 50 percent iron.
Indication of production None
Reserve estimates Up to several thousand tons in zones with 20 to 75 percent magnetite (Grantz, 1956).

References