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Anchorage

Reboudt

Valdez

Willow

Yenta

The region includes much of the Alaska Range, which culminated at 20,300—foot Mount McKinley; the northern slopes of the western Chugach Mountains; and most of the Talkeetna Mountains These areas are separated by poorly drained, lake-dotted lowlands the most extensive of which extends northward from the head of Cook Inlet.

The oldest geologic units in the region are Paleozoic clastic andcarbonate rocks, exposed mainly in the Alsaka Range. Mesozoi volcanic and clastic rocks, in which considerable limestone is interbedded locally, make up the bulk of the bedded rocks in the Chugach Mountains. Recent investigations in a neighboring area (Plafker and MacNeil, 1966) indicate that some of the clastic and interbedded volcanic rocks of the Chugach Mountains probably are early Tertiary in age. Large granitic batholiths of Jurassic Cretaceous, and Tertiary age invaded the older sedimentary and volcanic rocks in the Talkeetna Mountains and Alaska Range (Dutro and Payne, 1957; Grantz and others, 1963; Reed and Elliott, 1970), and there are smaller plutons in the Chugach Mountains and other parts of the region. Part of a discontinuous bell of small ultramafic bodies of probable late Mesozoic age that extends from the southern tip of the Kenai Peninsula nearly to the Wrangell Mountains is in the Anchorage district.

Tertiary continental deposits underlie Cook Inlet and large areas in the Susitna Lowland and Matanuska Valley. Tertiary and Quaternary volcanic rocks have been found in the Matanuska Valley I west of Anchorage, and in the southern Alaska Range, where Augustine Island and some of the highest peaks are active volcanoes. Most of the lowlands are underlain by thick glacial and alluvial deposits. Except along the shore of Cook Inlet, in the lower Matanuska Valley, and in a belt on both sides of the Susitna River below Curry, most of the region is underlain by permafrost. Ice completely covered the Cook Inlet-Susitna River region during the Pleistocene, spreading from the Alaska Range far out to sea Ice still covers the highest parts of the mountains, and valley glaciers extend many miles from their source areas; one of them I the Kahiltna Glacier, is more than 35 miles long.

Gold and silver have been recovered from lodes in many part I of the Cook Inlet-Susitna region and a little copper has been re covered from deposits in the Redoubt and Valdez Creek districts. These lodes and others have been investigated as possible source of antimony, iron, chromite, molybdenum, copper, lead, and zink (Berg and Cobb, 1967, p. 16—37, figs. 5—9; Reed and Eberleir I 1972). Reconnaissance studies in the southern Alaska Range j I 1969 (Reed and Elliott, 1970) indicate that metallic sulfide min¬erals are common in and near granitic plutons. Float samples col¬lected in the vicinity of the Mount Estelle pluton in the south¬western part of the Yentna district contained as much as 60 parts per million (about 1.7 fine ounces per ton) gold associated with chaleopyrite, arsenopyrite, and other sulfides. The only large-scale production was from the Willow Creek area north of Palmer where, between 1909 and World War II, gold-bearing quartz veins in the southern border zone of the Talkeetna batholith were the source of about 404,425 fine ounces of gold, about 5 percent of Alaska’s total lode gold output.

Placer gold was discovered in the Cook Inlet-Susitna region in the late 19th century, and mining has been continuous since the early 1900’s. Total placer gold production from the region probably has been about 250,000 fine ounces, or less than 40 percent of the region’s lode-gold production. Production from the Valdez Creek district cannot be confidently separated from that of the neigh¬boring Chistochina district of the Copper River region. The total for the two is somewhat less than 200,000 fine ounces, of which probably about 20 percent should be credited to the Valdez Creek district. The combined placer production of the Willow Creek and Yentna districts through 1960 was about 204,350 fine ounces; an unknown, but small amount has been produced since that time. As the main gold-producing part of the Anchorage district has historically been considered a part of the Kenai Peninsula region, its output cannot be stated accurately; probably it was not more than a few thousand ounces. Scant data suggest that not more than 275 ounces of gold was recovered in the Redoubt district, where a single stream was worked for a few years.