Placer Deposits of Alaska GEOLOGICAL SURVEY BULLETIN 1374
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 region, 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 graywacke, shale, conglomerate,
volcanic rocks, and limestone underlies 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
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 region 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, remnants 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, antimony, 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.
US Geological Paper 610
The Kuskokwim region, which includes the country drained by the Kuskokwim River, is roughly 400 miles long and 75 to 100 miles wide extending from the mouth of the Kuskokwim River, in southwest Alaska, to the northwest slopes of the Alaska Range, in south-central Alaska. Important goldproducing districts are Georgetown, Goodnews Bay, McKinley, and Tuluksak-Aniak. The area southwest of the town of Aniak is underlain predominantly by Quaternary sands and gravels, but the more mountainous regions east and northeast of Aniak are underlain by bedded rocks that range in age from Ordovician ( ?) to Tertiary (Cady and others, 1955, pl. 1). Only parts of the region have been geologically studied in any detail; much of it remains to be mapped. The Kuskokwim River, particularly its lower reaches, was penetrated first by Russians who in 1829 began exploring the area and later established trading posts along the river (Cady and others, 1955, p. 3--4). The first report of gold in this re- gion was by Spurr (1900, p. 259-261) who, in 1898, noted that gold was present both in veins and in stream gravels at various points along the Kuskokwim. These reports were of mere occurrences rather than of bonanza deposits; thus prospectors were reluctant to enter this relatively unknown region. It was not until 1908 that the first gold was produced (Smith, 1933, table facing p. 96). Placers have been the principal producers from this region, yielding substantially even in the 1950's. Production from 1908 through 1959 totaled 640,084 ounces, of which only 41,598 ounces was from lode mines.