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DISTRICTS

Aniak

Bethel

Goodnews Bay

McGrath

The Kuskokwim River region (p1. 1, figs. 12, 13, 16) includes Nunivak and Nelson Islands and the mainland area drained by streams flowing into Baird Inlet, Etolin Strait, and Kuskokwim Bay. It comprises the Aniak, Bethel, Goodnews Bay, and McGrath districts.
The Kuskokwim River region is dominated by the Kuskokwim Mountains, a succession of rounded northeast-trending ridges 1,500—2,000 feet in altitude surmounted locally by rugged moun¬tains as much as 2,000 feet higher. Other upland areas include the Kilbuck Mountains and parts of Nunivak Island. The eastern boundary of the region is the crest of the southern Alaska Range, most of which is more than 6,000 feet in elevation; the highest peak is Mount Foraker (17,395 ft). About a third of the region consists of lowlands less than 1,000 feet in altitude along the major rivers.
The following summary of the geology of the region is based mainly on reports by Cady and others (1955), Hoare (1961), Hoare and others (1968), and Reed and Elliott (1970), and on informal discussions with William H. Condon, Joseph M. Hoare, and Bruce L. Reed.
The oldest rocks, a narrow belt of gneiss and schist about 75 miles long in the western part of the r~egion, may be Precambrian in age. Paleozoic sedimentary rocks range in age from Cambrian to Devonian in the Alaska Range and from Devonian to Permian in the Kuskokwim Mountains. A great mass, possibly as much as 5 or 6 miles thick, of Carboniferous, Mesozoic, and Tertiary gray¬wacke, shale, conglomerate, volcanic rocks, and limestone under¬lies most of the Kuskokwim River region west of the Alaska Range and its foothills. These rocks were displaced by major northeast-trending zones of strike-slip faulting, some of which can be traced far beyond the boundaries of the region (Grantz, 1966).
Upper Cretaceous and Tertiary plutons, dikes, and sills that range in composition from ultramafic to felsic intruded the older rocks in nearly all parts of the region. Tertiary and Quaternary basaltic lava flows and associated tuffs cover most of Nunivak and Nelson Islands.
Quaternary fluvial and glacial deposits that locally have been reworked by wave and wind action floor the lowlands and valleys. Most of the surficial deposits are permanently frozen except near large bodies of water; many of the mountainous parts of the re¬gion are in areas underlain by discontinuous or isolated masses of permafrost. Only the Alaska Range, the mountains in the southwestern part of the region, and isolated summits of the Kuskokwim Mountains have nourished Pleistocene glaciers, rem¬nants of which are preserved in cirques and valleys in the Alaska Range.
Lodes in the Kuskokwim River region have been the source of most of the mercury mined in Alaska; a total of somewhat more than 35,000 76-pound flasks was produced between 1902 and 1967 (Alaska Division of Mines and Minerals, 1967, p. 8). Some gold and a little antimony ore as a byproduct have also been mined. Other lodes, some fairly extensively prospected, contain gold and various copper, lead, zinc, molybdenum, tungsten, bismuth, anti¬mony, mercury, manganese, and uranium minerals (Berg and Cobb, 1967, p. 88—97, figs. 16—18). Reed and Elliott (1968a, 1968b, 1970) described many occurrences of base and precious metals in the eastern parts of the Aniak and McGrath districts. Some in bedrock, others consist of mineralized float in and near zones around granitic plutons.
Gold lodes north of Medfra (near bc. 5 and 6, fig. 16) were the source of 40 to 60 thousand ounces of gold and a little silver.
Lode cinnabar was discovered by the Russians in the Kusko¬kwim River region about 1838 and prospectors looking for gold passed through the region as early as 1889, but no placer deposits were found until about 1900, when a number of men from Nome participated in a stampede set off by vague rumors of a discovery on a stream called “Yellow River,” said to be somewhere in the Kuskokwim Valley (Maddren, 1915, P. 299—300). From 1908 through 1960 about 650,000 fine ounces of gold (3.2 percent of the total Alaskan placer-gold production) was recovered from placers in the region. Mining has been reported in every year since 1960, but production data have not been made public. More than half a million ounces of platinum-group metals have been recovered from placers in the Goodnews Bay district (Mertie, ~ p. 77, 7~, 87). Small amounts of cinnabar and scheelite were mined from streams draining lodes that carry these minerals.