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A Dissertation Submitted in Partial Fulfillment Of the Requirements for the Degree of Doctor of Philosophy

Metamorphosed bedrock underlying the western Clearwater Mountains conprises two main rock sequences — a thick section of low grade metabasalts of Triassic age? and an overlying pelitic succession, zonational from prehniteand 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 zcnation — 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 lcwer grade rocks to the south. Two centers of gold miineralization in the Clearwater Mountains have been identified during a recent geochemical sampling program in the Healy A-l, B-l quadrangles, Alaska. Both the Timber line Creek center and the Black Creek center are located near the same E-W trending fault system. Sites of structural weakness along the zone have apparently guided the emplacement of small stocks and hypabyssal bodies that acted as hosts for