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Some Speculations on the Genetic Geochemistry and Hydrology of Roll-Type Uranium Deposits
Water in equilibrium with the atmosphere can dissolve no more than about 7 or 8 parts per million free oxygen. The large tongues of altered sandstone associated with roll-type uranium deposits were created when pre-existing pyrite in the rocks was oxidized by these minuscule quantities of dissolved oxygen. It is estimated that about 4,000 volumes of water are required to oxidize one volume of pyrite-bearing rock in a typical ore-host rock.
The limitation on concentration of dissolved oxygen places equally restricting limitations on the concentrations of soluble products that result when solid constituents of the rocks and orebodies react with oxygen. On the basis of requirements necessary to create known oxidized zones that have areal extents of tens of square miles, coupled with the creation of associated ore deposits that contain uranium and lesser amounts of selenium, vanadium, and molybdenum, it is possible to recreate an internally consistent hypothetical model of the ore-forming process that takes into account the hydrologic properties of the system, the geologic setting, and even the time at which the process took place.
The hypothetical model discussed in this paper would have taken about 700,000 years to form the altered tongue and about 50,000 years to form associated uranium ore deposits; it presumably developed when the host rocks were still relatively young, unconsolidated, and permeable. By far the most realistic source that can be visualized for the uranium at that time seems to be tuffaceous sediments that were being deposited on the eroded and exposed surfaces of the host rocks.
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