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DISTRICTS

Chistochina

Nelchina

Nizina

Prince William Sound

Yakatanga

The Copper River region (p1. 1, fig. 7) 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 long 1410 W., and offshore islands, including Middleton Island. It is divided into five districts: Chistochina, Nelchina, Nizina, Prince William Sound, and Yakataga. The region includes parts of the Alaska Range, Wrangell, St. Elias, and Kenai-Chugach Mountains and extensive lowlands along the Copper and Chitina Rivers. The mountains, which rise to sum¬mits more than 16,000 feet in the Wrangells, and to mor 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 most 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 eSound, 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 re¬gion is based mainly on reports and maps by Brabb and Miller (1962), Jones and MacKevett (1969), MacKevett and Smith (1968), Moffit (1938a, 1954a, 1954b), Plafker (1967), Plafker and MacNeil (1966), and Smith and MacKevett (1970).
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 near 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 com¬position 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 Mountains and the Gulf of Alaska and may be con¬tinuous with similar coeval rocks in Cook Inlet and on Kodiak Island. Middleton Island is composed of slightly indurated marine clastic sediments that were deposited 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 basaltic flows and associated rocks. The crater of Mount Wrangell (14,005 ft) still emits steam and ash.
Lodes in many parts of the Copper River region contain copper, gold, silver, molybdenum, antimony, nickel, chromite, lead, and zinc, but 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 William Sound dis¬trict. Similar veins near Golconda Creek (27, fig. 7) 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 350,000 fine ounces of gold and a few ounces of platinum. Placers near the head of the Chistochina River and near Slana in the northern and northeastern parts of the Chistochina district ac¬counted 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 of gold; and beach and stream placers in the Yakataga and Nelchina districts, prac¬tically all the remainder. Placer-gold production from the Prince William Sound district probably did not exceed 500 ounces.