Argillic Alteration and Uranium Emplacement on the Colorado Plateau

Abstract

Argillic alteration is recognized in association with uranium deposits on the Colorado Plateau and along border areas. Alteration and mineralization are geometrically related in sedimentary strata where solution conduits result from lithologic and structural changes. The association also exists in collapse features and in breccia pipes. Occasional diatremes bearing uranium point to hypogene solution activity following igneous invasion as one original source for uranium ions. In several places ascending fluids apparently were confined under sufficient pressure to blast consolidated strata into vertical chimneys of brecciated masses. Pipes and fracture systems which transect porous strata provide paths along which hypogene solutions may ascend from depth and migrate horizontally. Alteration may extend both vertically and horizontally, encircling such areas.

Occasionally silica or carbonate precipitation accompanies uranium. Where found, well-marked masses of carbonate or silica replace sedimentary strata. Field conditions demonstrate upward migration of solutions. Often silica and carbonate may be dissolved from country rock and redeposited at higher levels. The silica minerals involved are quartz, chalcedony, and opal; the carbonates, calcite, dolomite and siderite.

The identification of mica polymorphs close to ore, corresponding to those created at elevated temperatures in the laboratory; the recognition of dickite in gouge zone veinlets associated with primary sulphides; uranorganic complexes; and replacement textures involving multiple pseudomorphs after a single mineral, further indicate alteration by hypogene solution.

Uranium deposition is favored by precipitating agents peculiar to certain strata. Hydrogen sulphide, produced by decaying fossil plant detritus, or petroliferous material, appears to have been an effective precipitating agent. Strata often involved are members of the Chinle or Morrison Formations. These units provide an environment with sustained reducing conditions favorable for uranium precipitation. The reduction of hexavalent uranium in solution occurs to the less soluble tetravalent form. Accompanying bleaching of red beds also suggests reducing conditions.