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The relation between regional groundwater flow, hydrochemistry, and uranium distribution in the Powder River basin indicates that uranium was deposited during the Tertiary Period in groundwater recharge areas where the groundwater changed from a sulfate-bicarbonate water to a bicarbonate-rich water.
The regional recharge and discharge areas of present groundwater-flow systems have about the same locations as the recharge and discharge areas of the Tertiary groundwater-flow systems. The present groundwater is recharged in the eastern, western, and especially the southern margins of the basin and is discharged in the valley of the Powder River, especially in the north. Flow nets for the groundwater were constructed on the basis of piezometric data from existing water wells in the Powder River basin.
The groundwater chemistry of this area during the Tertiary was probably similar to that of today because the groundwater flowed through the same sediment as present groundwater. Anions in the present groundwater undergo the following sequence of hydrochemical changes along the regional flow path from the southern recharge area to the northern discharge area:
HCO3- + SO4- - ^rarr SO4- - + HCO3- ^rarr HCO3-.
Major unoxidized uranium deposits in the Powder River basin occur near the transition zone between the SO4- - + HCO3- and HCO3- facies. The uranium is transported in solution by groundwater in the HCO3- + SO4- - and the SO4- - + HCO3- facies and precipitated in the transition zone between the SO4- - + HCO3- and HCO3- facies. Precipitation occurs where strong reducing conditions exist around abundant organic material in which sulfate-reducing bacteria may live and multiply.
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