Reconnaissance geology, mineral occurrences, and geochemical anomalies of the Yentna district, Alaska

Abstract

The Yentna district, in south-central Alaska, is underlain by slightly metamorphosed Mesozoic sedimentary rocks, and by sandstones, conglomerates and coaly minerals of the Tertiary Kenai Formation. The bedrock is locally covered by extensive surficial deposits of Quaternary and Recent (Holocene) age. The Mesozoic strata are cut by a quartz monzonite batholith in the Tokositna Mountains and by alaskitic dikes and plugs in the Peters and Dutch Hills. A silica-carbonate dike, which formed by alteration of a mafic or ultramafic dike, was noted in the Peters Hills. The major ore deposits are gold placer deposits of several types, including stream and bench deposits of Recent (Holocene) age, glacial-fluviatile deposits of Quaternary age, and conglomerates of Tertiary age. Quartz-rich conglomerates and breccias have also been productive and are interesting and controversial genetically. The present study indicates that they are closely related to shear zones containing quartz veins and highly altered rocks; previously the origin of the associated altered rocks had been ascribed to deep weathering. The report also contains descriptions of a few gold lode prospects and of geochemically anomalous areas such as those at Bunco Creek and near Mount Goldie.