Unnamed (Astrolabe Peninsula)

Occurrence, Inactive

Commodities and mineralogy

Main commodities Fe; Ti
Ore minerals ilmenite; magnetite

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale B-3
Latitude 58.353
Longitude -136.89
Nearby scientific data Find additional scientific data near this location
Location and accuracy A magnetite deposit crops out on the east side of the southernmost peak on the Astralobe Peninsula. The deposit is exposed from about 1200 to 1600 foot elevation. The coordinates are for the approximate center, as inferred from sample numbers 38-39 of Rossman (1963, B 1121-F). Another deposit, sample 34 of Rossman, is approximately 0.3 mile southwest of the southernmost peak at an elevation of about 1000 feet. The site is equivalent to location 73 of MacKevett and others (1971) and location 9 of Cobb (1972, MF-436). The Rossman 38-39 locations are about the same as those in Kimball and others (1978, fig. C35).

Geologic setting

Geologic description

These iron-titanium occurrences are in the Astralobe-DeLangle stock, the southernmost of four layered mafic intrusives in the Fairweather Range in Glacier Bay National Park and Preserve (Brew and others, 1978). The layered gabbro is mainly composed of plagioclase (An content of 55-70 percent), monoclinic and orthorhombic pyroxene, and olivine. Magnetite and ilmenite make up as much as 20 percent of some layers of the stock, and there are accessory amounts of apatite, sulfides, and graphite (Rossman, 1963, B 1121-F, p. F23-25, F44-45). Chemical analyses of four rocks typical of the stock are also given by Rossman (1963, table 2). Approximately 2000 feet of the stock is exposed without significant compositional variation (Rossman, 1963, p. F32).
The mineral occurrences exposed near the south peak of the Astralobe Peninsula are approximately central in the stock, assuming that the stock goes offshore at least as far south as Sugarloaf Island. Rossman (1963, p. F44-45) proposed that the magnetite-ilmenite rich layers were at the top or side of the stock, and that they occurred in approximately 1000 stratigraphic feet of the stock and extended through the mountain.
The deposit in the vicinity of samples of Rossman's (1963) 38-39 series contain as much as 12.5 weight percent ilmenite and 14 percent magnetite, as determined in mineral separates. The average content of 10 samples of the 38 series was approximately 8.3 weight percent magnetite and 3.6 weight percent ilmenite. Rossman believed that the deposit was very large.
Geologic map unit (-136.891897713697, 58.3526373997805)
Mineral deposit model Similar to Bushveld Fe-Ti-V (Cox and Singer, 1986; model 3). Magmatic segregations in synorogenic layered mafic igneous intrusive of Tertiary age (Foley and others. 1997).
Mineral deposit model number 3
Age of mineralization Tertiary.

Production and reserves

Workings or exploration There are no mine workings, all data are from samples collected from natural exposures. At Rossman's (1963) location 38, magnetite averages 8.3 percent and ilmenite (nine of ten samples) averages 3.6 percent. The rock at nearby sample site 34 contains as much as 13.5 percent magnetite; two of four samples average 2 percent ilmenite. Maximum magnetite content at the 34 site is 22 percent; maximum ilmenite is 12.5 percent (Rossman,1963, table 8, p. F45).
Indication of production None
Reserve estimates A resource exists based on Rossman's (1963) estimate of a 1000-foot thickness of the layered gabbro. Based on 1000 x 1000 x 500-foot (depth) and 10 cu-ft /ton , there is, conservatively, about 50 million tons of iron-titanium enriched gabbro at the upper site (samples in the 38-39 series of Rossman).

Additional comments

Chugach terrane. The iron and titanium rich sites are in Glacier Bay National Park and Preserve.