Uranium mineralization in the Guindani Canyon area of the northern Whetstone Mountains, Cochise County, Arizona

Rain and snow contain dissolved oxygen because of equilibration with atmospheric oxygen. In contrast, groundwater deep in the Earth, especially at depths of hundreds to thousands of meters, contains little dissolved oxygen. As erosion gradually lowers the Earth’s surface, oxygen-bearing groundwater gradually gains access to rock within the Earth that had not previously been exposed to oxidizing conditions. Uranium is generally stable under anoxic conditions, but is dissolved and transported by oxidizing waters. Gradual downward movement of uranium with a lowering groundwater table may produce a uranium enrichment zone at depth (supergene enrichment) overlain by rocks that have been depleted in uranium. Uranium may also be carried laterally for great distances away from sites where it was dissolved, and may be concentrated in secondary, supergene deposits.