Geology, economic geochemistry, and placer gold resources of the western Clearwater Mountains, east central Alaska

Abstract

Metamorphosed bedrock underlying the western Clearwater Mountains comprises two main rock sequences — a thick section of low grade meta- basalts of Triassic age, and an overlying pelitic succession, zonational from prehnite- and pumpellyite-bearing argillite in the south to sillimanite-bearing gneisses north of Valdez Creek. Combined, the sequences aggregate nearly 50,000 feet in thickness and are exposed in a broad northeasterly trending belt about 17 miles wide and extending at least 70 miles along the south flank of the Alaska Range. The pelitic rocks, deposited in Late Triassic and Jurassic tine, have been intruded by alkali gabbro of Late Jurassic age (143 MY), and by a magma series of intermediate composition during Cretaceous and Early Tertiary time (61-66 MY). Metamorphism of the bedrock sequence, also in the Cretaceous and Early Tertiary, has produced an inverted zonation — of the Barrovian Series — increasing in grade upsection and northward. A steep thermal and dynamic gradient during metamorphism, now recorded by index mineral assemblages, rotational microtextures, and intrafolial folds, may be attributed to differential uplift and ductile overthrusting. Higher P-T assemblages on the north were apparently transported up and over lower grade rocks to the south.