Reconnaissance for radioactive deposits in the Nixon Fork mining district, Medfra quadrangle, central Alaska, in 1949 disclosed the occurrence of allanite in sampled containing as much as 0.05 percent equivalent uranium from the dump of the Whalen mine; the presence of radioactive parisite (a rare-earth fluocarbonate) in a highly altered limestone containing about 0.025 percent equivalent uranium near the Whalen shaft; and radioactive idocrase in samples of altered garnet rock with about 0.025 percent equivalent uranium, form the Crystal shaft of the Nixon Fork mine. This radioactivity is due mostly to thorium rather than uranium. Placer concentrates
from Ruby and Eagle Creeks contain 0.078 and 0.26 percent equivalent uranium respectively,
in which the radioactivity is due chiefly to uraniferous thorianite. The
bedrock source of the uraniferous thorianite was not located primarily because much of
the area is overlain by a relatively thick mantle of vegetation (mostly moss) which
limited the effectiveness of radiometric surveying. The uraniferous thorianite is
believed to occur in a restricted zone or zones at or near the contact of limestone with
monzonite similar to the gold-copper ores of the district and the deposits of radioactive
parisite and garnet rock at the Whelan and Crystal shafts respectively.
