Model for uranium, humate, and silica fixation in lacustrine carbonaceous sediments at Anderson Mine, western Arizona

Sedimentation at the Anderson mine in western Arizona was dominated by subaerial and subaqueous debris-flow and lacustrine-turbidite processes. The typical debris flow at the mine is 30 to 100 cm thick, massive, internally chaotic, composed of various amounts of reworked tuff, mica, quartz, and silicified plant and shelly detritus, and has undergone varying degrees of zeolitization and silicification. These debris flows grade laterally to thinner debris flows with graded bedding and laminated lacustrine turbidites that are rich in carbonaceous organic detritus. The more distal flows and turbidites are the principal uranium host at the mine. Uraniferous silica, amorphous uranium silicate, and coffinite are commonly associated with internally structureless though crudely stratified organic matter (humate?) in the carbonaceous siltstones. Uranium is not associated with fragments of coaly plant debris. Uranium, silica, and humic(?) material were probably leached from proximal subaerial and subaqueous debris-flow facies and transported to the anoxic lake-bottom environment where reducing conditions, low permeability, and porosity prevailed. In the anoxic environment, uranium, silica, and humate all tended to precipitate from open water at an Eh-pH boundary zone either at the sediment-water interface or at some depth in the sediment. The carbonaceous laminated lacustrine turbidites were the favorite host, but individual orebodies cross lithologic boundaries, suggesting that uraniferous silica apparently precipitated wherever reducing conditions existed on the lake bottom.