Placer Deposits of Alaska GEOLOGICAL SURVEY BULLETIN 1374
The Copper River
region includes the area drained by the Copper River and
its tributaries, the area east of the divide between Prince William
Sound and Cook Inlet, the area drained by streams flowing into
the Gulf of Alaska between the Copper River and longititude 141
W., and offshore
islands, including Middleton Island. It is divided into five districts.
The region includes parts of the Alaska Range, Wrangell, St. Elias,
Kenai-Chugach Mountains and extensive lowlands along the Copper
and Chitina Rivers.
The mountains, which rise to summits more than
16,000 feet in the Wrangells, and to more than 18,000 feet at Mount
St. Elias, support and nourish the largest icefields and piedmont
ice lobes and some of the longest valley glaciers in North America,
all remnants of even more extensive Pleistocene ice that covered
of the region. The lowlands along the Copper River are floored
by thick accumulations of
Pleistocene and Holocene glacial, lacustrine, and fluvial deposits
that are frozen to depths of several hundred feet. The islands
and most of the shores of Prince William Sound, the extensive Copper
River Delta, and the lowlands and low mountains bordering the Gulf
of Alaska in the Yakataga district are generally free of permafrost.
The following summary of the geology of the Copper River region
is based mainly on reports and maps by Brabb and Miller (1962),
and MacKevett (1969), MacKevett and Smith (1968), Moffit (1938a,
1954b), Plafker (1967), Plafker and MacNeil (1966), and Smith and
Bedrock in the Copper River region ranges in age from late Paleozoic
to Quaternary. The bulk of the rocks are of Mesozoic age and include
large masses of graywacke, slate, and greenstone and lesser amounts
of carbonate rocks. Recent work has shown that some of the rocks
Prince William Sound previously considered to be Mesozoic are Tertiary
in age. In late Mesozoic and early Tertiary time, plutons, some
of batholithic dimensions, were emplaced in many parts of the region.
They range in composition from granite to dunite but most are
granodiorite, quartz diorite, and related rock types.
In the Yakataga district, complexly deformed Cenozoic marine and
continental rocks underlie the area between the crest of the Chugach
and the Gulf of Alaska and may be continuous with similar coeval
rocks in Cook Inlet and on Kodiak Island. Middleton Island is
composed of slightly indurated marine clastic sediments that were
in part by floating ice and are correlative with generally similar
rocks exposed on the main-land. The most extensive formation in
the Wrangell Mountains is a thick pile of Tertiary and Quaternary
flows and associated rocks. The crater of Mount Wrangell (14,005
still emits steam and ash.
Lodes in many parts of the Copper River region contain copper,
gold, silver, molybdenum, antimony, nickel, chromite, lead, and
only copper, gold, and byproduct silver were mined commercially
(Berg and Cobb, 1967, P. 37—73, figs. 10—13).. The
famous Kennecott mines near McCarthy in the Nizina district and
mines in the southwestern
and northeastern parts of Prince William Sound accounted for most
of the copper produced in Alaska. Gold worth $2 or $3 million and
smaller amounts of silver were produced from mineralized quartz
and calcite veins and byproducts of copper mining in the Prince
Sound district. Similar veins near Golconda Creek (27)
and in the southeastern part of the Nelchina district were mined
on a small scale, but the entire region was not a statistically
significant contributor to the total lode-gold production of Alaska.
Placer deposits have been worked in all districts of the Copper
River region, but the total production probably was no more than
fine ounces of gold and a few ounces of platinum.
the head of the Chistochina River and near Slana in the northern
parts of the Chistochina district acccounted for an estimated 150,000—160,000
ounces of gold and all the platinum; deposits in the north-central
part of the Nizina district accounted for about the same amount
gold; and beach and stream placers in the Yakataga and Nelchina
districts, practically all the remainder. Placer-gold production
from the Prince
William Sound district probably did not exceed 500 ounces.
US Geological Paper 610
The elliptical~shaped Copper River region, which includes a large part of the drainage basin of the Copper River, is in southern Alaska, bounded by the Alaska Range on the north, the Chugach Mountains on the southwest, and the Wrangell Mountains on the northeast. The region lies roughly between lat 61 °00' and 63°10' N. and long 142°00' and 146°00' W., and it includes the major gold districts of Chistochina and Nizina. Gold mining began in this region in 1900 in the Chistochina district, but prospectors were active in the Copper River country as early as 1898 (Schrader, 1900, p. 421). The first locations were in auriferous gravels along the Chisna, one of the main tributaries of the Chistochina River. Productive placers were discovered along the upper part of the Nizina River and its tributaries in 1902 (Mendenhall, 1905, p. 118). Minor discoveries were made elsewhere in the Copper River region about this time, and in 1914 the Nelchina placers were discovered (Chapin, 1918, p. 59)-but the bulk of the gold production came from the placers of Chistoc~ina and Nizina. In the Copper River region, especially the Chitina district, copper deposits were worked extensively by the Kennecott Co. during 1900-38 (Moffit, 1946, p. 93), but they yielded little gold. Frqm 1900 to 1959 the Copper River region produced 2,400 ounces of lode gold, 295,000 ounces of placer gold, and 5,600 ounces of gold undifferentiated as to source-a total of 303,000 ounces. From World War II through 1959 only a few hundred ounces per year were produced. The ·geology of the region is summarized here from a more detailed account by Moffit (1938, p. 19-107). Throughout most of the region the low-lying areas are blanketed qy glacial sands and gravels of Quaternary age. In the higher areas, a thick succession of bedded rocks range in age from early Carboniferous to Recent. The oldest rocks consist of schist and slate associated locally with altered limestone, tuff, and basalt flows, and they include the Mississippian Strelna Formation and Dadina Schist and the Carboniferous or older Klutina Series. Overlying these rocks are layers of lava flows, tuff, volcanic breccia, shale, limestone, sandstone, and conglomerate of Permian age; these are overlain by the Nikolai Greenstone, a thick .sequence of basaltic lava flows of Permian and Triassic ( ?) age. The post-Triassic Mesozoic rocks in the Copper River region are not fully understood because of the correlation problems imposed by variable lithology, exposures in disconnected areas, and lack of diagnostic fossils. Tuffaceous beds of Middle Jurassic age occupy a small area near the mouth of the Chitina River. Upper Jurassic rocks occur in a few places in the central part of the Copper River basin along the north tributaries of the Chitina River. Along the north side of Chitina River valley a thick series of bedded sedimentary rocks of varied lithology is Jurassic or Cretaceous in age. Black shale and sandstone, conglomerate, and sandy shale considered to be of Early Cretaceous age overlie Triassic rocks in the Nizina district. The Chugach Mountains, in the southern part of the region, are underlain by dark slate and graywacke considered to be Cretaceous or older(?). These are equivalent to the Valdez and Orca Groups of earlier reports. The Tertiary rocks are dominantly of volcanic origin and include sev~ral thousand feet of lavas and tuffs interbedded with fresh-water conglomerate, clay, sandstone, and shale. These rocks compose the higher parts of the Wrangell Mountains.