Precambrian rocks of Alaska

Abstract

Eleven widely separated areas in Alaska contain rocks of Precambrian or probable Precambrian age. The age assignment in five areas is based on radiometric dating; in the other six areas stratigraphic evidence is used to infer a Precambrian age. All known Alaskan Precambrian rocks are of Late Proterozoic age except those constituting the Kilbuck terrane of southwestern Alaska and an area in the Yukon-Tanana upland of eastern Alaska, which are Early Proterozoic, and the schist of the northeastern Kuskokwim Mountains in central Alaska, which is Middle and Late Proterozoic. No evidence exists at this time for the presence of rocks of Archean age in Alaska. The Tindir Group along the Yukon River in east-central Alaska, which is considered equivalent to parts of the Windermere, Belt, and Purcell Supergroups of Canada, is the only group of Precambrian rocks in Alaska that can be definitely related to Precambrian stratigraphic sections in other parts of the North American Cordillera. The Neruokpuk Quartzite and underlying strata of northern Alaska are Precambrian on the basis of stratigraphic evidence. The Tindir(?) Group along the Porcupine River in east-central Alaska and low-grade metamorphic rocks in the Livengood-Crazy Mountains region of central Alaska are probably Precambrian, although their precise ages are not known.Tectonomagmatic events in the interval 1,100 to 600 million years ago are recorded in schist of the Yukon-Koyukuk region in west-central Alaska and in gneiss on the Seward Peninsula. The Wales Group of the Alexander terrane of southeastern Alaska and the Kilbuck metamorphic terrane of southwestern Alaska are allochthonous and appear to belong to exotic terranes accreted to the North American craton during Phanerozoic time.Although mineral deposits are known to occur in several areas of Precambrian rocks, many of these deposits were produced during Phanerozoic mineralization episodes. However, banded iron-formation within the Tindir Group of east-central Alaska and certain volcanogenie, stratabound base-metal deposits within the Wales Group in southeastern Alaska are believed to have been formed during the Precambrian era.