Geology and ore deposits of Johnny M Mine, Ambrosia Lake District, New Mexico

The Johnny M mine is one of very few mines in the Ambrosia Lake district with uranium ore in two members of the Jurassic Morrison Formation; these members are the Westwater Canyon sandstone and the Brushy Basin shale. The Westwater Canyon ore is contained in the two upper sandstone units of the member, and the Brushy Basin ore is contained in the Poison Canyon sandstone. The sedimentary features and structures in the Westwater Canyon sandstone indicate that the sediments were deposited by a system of aggrading braided streams, possibly at the distal end of coalescing alluvial fans. The Poison Canyon sandstone was probably the result of deposition in a complex environment of meandering and braided streams. Paleocurrent direction indicators, such as fossilized-log orientation, foreset azimuths, and the axis of cross-beds and channel scours, suggest that the local paleostream flow was to the east and southeast. The uranium mineralization is closely associated with (1) local accumulations of carbonaceous (humate) matter derived from the decay of organic material, such as trees and plants; and (2) paleostream channels preserved in the rocks. The ore elements were derived from the leaching of volcanic air-fall tuffs and ash, which were introduced into the fluvial system during volcanic activity in the western United States. The mobile ore element ions were reduced and concentrated by humic acids and bacteria present in the fluvial system, and ultimately remobilized in the system into the forms present today. The uranium is thus envisioned as forming either essentially on the surface as the sediments were being deposited or at very shallow (20 ft; 6 m) depth.