By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
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 61Â°00' and 63Â° 10' N. and long 142Â°00/ and 146Â°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 (Schra-der, 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 (Menden-hall, 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)â€”but the bulk of the gold production came from the placers of Chistochina 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.
From 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 undifferen-tiated as to sourceâ€”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 by 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 several 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.
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