Diagenesis and Uranium Mineralization in Kootznahoo Formation, Southeastern Alaska

K. A. Dickinson

The lower Tertiary Kootznahoo Formation, a clastic unit, crops out in scattered areas in the Admiralty Trough, southeastern Alaska. The formation consists mainly of arkosic sandstone, conglomerate, and lesser amounts of coal and shale. Samples were collected and outcrops described from four main outcrop localities in an area beginning on the western side of Zarembo Island and the northern end of Prince of Wales Island and extending north to the Kootznahoo Inlet area on the west-central side of Admiralty Island. X-ray diffraction, optical mineralogy, scanning electron micrographs, and chemical analyses were used during sample studies.

Three stages of diagenesis were recognized. In order of increasing intensity, they are (1) the formation of calcite and dolomite and the destruction of plagioclase, (2) the formation of siderite in addition to calcite and dolomite, and (3) the formation of kaolinite partly from K-feldspar and of the carbonates as in stages 1 and 2. In stage 3, chlorite and muscovite were destroyed.

Uranium mineralization associated with carbonaceous material has been found only where diagenesis to stage 3 has occurred. Scattered carbonized wood fragments enriched to as much as 0.02% uranium were found in Kootznahoo Formation sandstone near Kadake Bay on Kuiu Island, but the extent of the deposit is not known, and it is probably subeconomic. Slight mineralization was also found in coal at California Bay. The source of the uranium at Kadake Bay appears to be a heavy-mineral zone within the sandstone. Parts of the Kootznahoo where stage 3 diagenesis has occurred are favorable uranium-exploration targets if the sandstone is rich in heavy minerals and carbonaceous material.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988