The Midnite mine is located approximately 35 air mi. NW. of Spokane, Washington, near the confluence of the Columbia and Spokane rivers on the Spokane Indian Reservation. U mineralization occurs in a zone along the contact between the Cretaceous Loon Lake granite and Precambrian metasediments. Ores appear to be found within and near associated fractures and shear zones. Secondary, oxidized U minerals, principally autunite and meta-autunite, lie above a fluctuating water table, while below this water table there occurs a zone of partially oxidized sooty and compact uraninite. Sulfide mineralization is associated with the uraninite. Associated zones of argillic alteration that contain kaolinite, illite, montmorillonite, and intermingled adularia further indicate the action of hydrothermal solutions. Botryoidal masses of marcasite and shrinkage cracks in the uraninite imply colloidal precipitation in the formation of the primary U ore. Ore deposition is inferred to have taken place at temperatures on the order of 300 degrees C. in a neutral or slightly acidic environment.

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