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Abstract

Early diagenesis in the Morrison Formation resulted in the formation of the world’s largest sandstone-hosted uranium deposits. Distribution of diagenetic alterations in ore-bearing sandstones of the Westwater Canyon Member suggests that these alterations were strongly influenced by pore waters expelled from fine-grained units in the overlying Brushy Basin Member. A moderately high pH created by hydrolysis and dissolution of volcanic ash enabled these fluids to dissolve and mobilize humate in lower Brushy Basin and upper Westwater Canyon sediments. When these fluids mixed with connate water in sandstones of the middle to lower parts of the Westwater Canyon Member, tabular uranium orebodies were formed. A strong diagenetic overprint related to Laramide tectonism and late Tertiary oxidation obscured early alteration patterns and resulted in the local redistribution of primary uranium ore and dissolution of previously formed authigenic cements.

Similarities between ore mineralogy and postdepositional alterations in the Morrison Formation of the Grants uranium region and in the Morrison of the northern part of the Colorado Plateau suggest that these ore deposits have a common genesis. The apparent replacement of the organic matrix in the Grants region by chlorite and locally by a chlorite-coffinite mixture in ore zones suggests that, where a chlorite- dominated assemblage is now present, carbonaceous uranium ore once existed. This observation leads to the hypothesis that the organic-carbon-rich, locally chlorite- bearing Grants uranium ore and the organic-carbon-poor, chlorite-rich ore of the northern Colorado Plateau are end members of the same mineralization process.

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