MINERAL RESOURCES OF ALASKA

In the summer of 1945 an investigation was made for possible placer deposits of radioactive minerals on Cache and upper Peters Creeks in the Yentna district, southern Alaska. Five gravel types of different age or origin Eocene, late Tertiary, Quaternary glacio-fluvial, Quaternary bench, and the present floodplain deposits (including tailings from placer mining) were examined for their content of radioactive minerals. Radioactivity was measured by gamma count, as detected by a portable Geiger-Mueller counter, at 455 field stations, and in 526 rough-screened samples from 8 to 20 per minute; the rough-screened sample gamma counts from 8 to 21. Calculations from counts of carefully screened samples and gravity concentrates tested in the field laboratory show a maximum of 0.009 percent equivalent uranium even where concentration ratios are 500: 1 or greater. Sluice-box concentrates with an undetermined but extremely high concentration ratio obtained in 1945 have a maximum of 0.064 percent equivalent uranium. The equivalent-uranium content of the samples may be due either to particles of radioactive minerals in the mineral aggregate of pebbles or other rock fragments, or to individual mineral grains. Interest in 1945 was focussed on mineral grains, the only material recoverable by normal placer-mining methods. Mineralogic study indicates that the radioactivity is due chiefly to uranium and thorium in zircon, rnonazite, and uranothorianite.