Spectral gamma-ray logs in the McArthur River area in eastern Athabasca Basin indicate anomalously high thorium content in the conglomeratic Bird Member and the lower part of the Collins Member of the Manitou Falls Formation. The distribution of Th in such
coarse-grained fluvial strata is usually associated with accessory minerals in the heavy-mineral fraction. Instead, petrographic, chemical, and
SEM analyses reported here show that Th in the Manitou Falls Formation conglomerate beds resides in a Th-rich aluminophosphate mineral. It belongs to the solid-solution series of the crandallite group, including crandallite (CaAl3(PO4)2(OH)5·H2O) and goyazite (SrAl3(PO4)2(OH)5·H2O).
Crandallite-group minerals form in highly acidic environments and are associated with clay minerals such as illite. A more extensive investigation is required to determine the relationships between the Th-rich crandallite-group phase, clay minerals, and the hydrothermal processes that operated
during the formation of uranium deposits in the Athabasca Basin.
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