At the Sandstone mine, there is an anomalous orebody that lacks the characteristic coloring and gamma counts associated with known uranium ore at Ambrosia Lake. The orebody occurs along the downdip edge of a tongue of hematitic sand in the basal sand unit of the Westwater Canyon Member.
The orebody is white to light gray, most likely because of a lack of indigenous humic material. The abundance of pyrite indicates the uranium is in the tetravalent state, probably coffinite. Preliminary analysis also indicates the presence of uranophane evidently altered from the coffinite. Equivalent U3O8 by gamma determination is usually 30 to 60% of actual U3O8, indicating that this orebody is relatively recent.
Oxidizing meteoric water, forming a geochemical cell, remobilized the uranium minerals in the preexisting trend orebodies and deposited them downdip from the farthest extent of this cell. Post-Dakota deformation influenced the course of the migrating meteoric water and the extent of the redox interface controlling the orebody.
As lower grades of U3O8 become economical, the potential for unknown reserves adjacent to the redox interface should not be overlooked. Areas of low-grade ore should be sampled to become aware of any equilibrium imbalances. An established sampling program coupled with more sophisticated beta-gamma instrumentation should remedy the inaccuracy of present-day gamma evaluations.