Geology of the eastern part of the Mount Fairweather Quadrangle, Glacier Bay, Alaska

Rossman, Darwin

Rocks on the W. side of Glacier Bay in southeastern Alaska are predominantly sedimentary, and consist chiefly of thin-bedded and massive limestones and some argillaceous rock. The 5 formations recognized, from oldest to youngest - the Willoughby Limestone, Tidal Formation, Pyramid Peak Limestone, Rendu Formation, and Black Cap Limestone - have a total exposed thickness of about 26,000 ft, an range in age from Late Silurian to Middle Devonian. Several isolated sedimentary rock units, whose age and relative stratigraphic position are unknown, also crop out. The most abundant igneous rock, diorite, underlies about one-third of the mapped area. This rock varies widely in composition and may include rocks of more than one age and mode of origin. Other igneous rocks include quartz diorite, granite, gabbro, and mafic and silicic dike rock. Highly recrystallized stratified rocks, which are now banded amphibolite gneisses, lie in the southern part of the mapped area. A zone containing migmatite dikes also lies in the southern part of the mapped area. The zone is only a few hundred feet wide, but it extends at least 45 mi. The migmatite dikes show both intrusive and replacement relations to the host rock. Tactite, containing well-developed crystals, lies near a diorite contact in the northern part of the area. Glacial advances occurred in the Glacier Bay area 7000, 4000, and 500 years ago. During the last advance, which was the most extensive, ice reached as far S. as the entrance to Glacier Bay. Only a few mineral deposits have been found. A small but rich Ag deposit (now mined out) lay near the W. shore of Rendu Inlet, and a molybdenite-bearing zone exists on the E. shore of Muir Inlet. Some placer Au has been found at the foot of Brady Glacier and in the upper part of Dundas River. A few Au-bearing veins crop out between Dundas Bay and Brady Glacier and on the S. shore of Gilbert Island. Some sulfide minerals are present in massive limestone on Willoughby, Francis, and North Marble islands. Hydrothermally altered rock is abundant along numerous fault zones in the northern part of the mapped area. Most of the alteration zones contain no quartz or minerals of economic importance, but the few that do contain quartz also contain some free Au. Paligorskite crops out in and on massive limestone on Lemesurier Island.

Geology of the eastern part of the Mount Fairweather Quadrangle, Glacier Bay, Alaska

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