Abstract Diagenesis and weathering of the Morrison Formation have played a major role in determining the present aspect of the nonopaque heavy-mineral assemblage, which is now a mature garnet-zircon-apatite-tourmaline suite. For instance, the presence of authigenically etched to skeletal garnet and staurolite in cores of holes drilled across the Grants uranium region implies that entire grains have been destroyed. Comparison of mineralogic data from cores with that from measured sections indicates that near-surface weathering has destroyed acid-sensitive minerals such as apatite. As a result of intense diagenetic processes, stratigraphic intervals in which postdeposi- tional processes strongly affected the detrital mineralogy had to be identified before provenance interpretations could be made. The extent of heavy-mineral alteration zones may define the movements of fluids related to the concentration of and/or redistribution of uranium. Stratigraphic variations in mineral species and mineral diversity in Morrison sandstone units reveal that an igneous (rhyolitic) component, characterized by euhedral zircon and subhedral apatite, increases upward. Complementing this trend, the assemblage of well-rounded grains of recycled material in the Recapture Member changes to a complex mixture dominated by first-cycle, angular grains that is present upward through the Westwater Canyon Member and continues into the tuffaceous Brushy Basin Member. The Morrison Formation was derived from a variety of litholo- gies in the source area. A significant continuing input came from low- to medium- grade metamorphic and plutonic rocks, a lesser component from sedimentary material, and significant amounts from rhyolitic to dacitic volcanic detritus. Sedimen- tologic and mineralogic data suggest that the ancient Mogollon highlands, characterized by widespread Triassic and Jurassic volcanism, were the primary Morrison source area; however, a more distant, active orogenic belt along the Mesozoic continental margin may have also contributed material to the San Juan basin.

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.