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Petrographic studies of outcrop and drill core samples in the Westwater Canyon Member of the Morrison Formation along the southern and western margins of the San Juan basin reveal a close spatial relationship among altered (iron-leached) detrital magnetite and ilmenite (FeTi oxides), depositional facies in the overlying Brushy Basin Member, and distribution of primary uranium deposits. Iron leaching of FeTi oxides resulted from passage of solutions containing soluble organic material; concentrations of this organic material are the sites of the primary uranium orebodies. Along the southern and western parts of the basin, FeTi oxides typically have been leached in the upper Westwater Member, but are unaltered in the lower Westwater; however, locally, leaching occurred thro ghout the Westwater. This zone of leaching systematically thins northward to zero, where unleached FeTi oxides occur throughout the Westwater.
Regional patterns of alteration of FeTi oxides correspond to regional facies distribution in the overlying Brushy Basin Member. Extensive FeTi oxide leaching characterizes the Westwater beneath the smectite-rich mud-flat facies of the Brushy Basin, whereas negligible leaching characterizes the Westwater beneath the zeolite-rich playa facies of the Brushy Basin. This correspondence between facies and alteration patterns suggests that solutions responsible for solubilization of organic material, which in turn leached FeTi oxides in the Westwater, originated from the mud-flat facies of the Brushy Basin. Organic material that precipitated from these solutions concentrated uranium to form primary uranium orebodies; therefore, distribution of the Brushy Basin mud-flat facies may define, and restrict, distribution of primary orebodies in the Westwater.
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