Relationship of Detrital, Nonopaque Heavy Minerals to Diagenesis and Provenance of the Morrison Formation, Southwestern San Juan Basin, New Mexico
Published:January 01, 1986
Paula L. Hansley, 1986. "Relationship of Detrital, Nonopaque Heavy Minerals to Diagenesis and Provenance of the Morrison Formation, Southwestern San Juan Basin, New Mexico", A Basin Analysis Case Study: The Morrison Formation, Grants Uranium Region, New Mexico, Christine E. Turner-Peterson, Elmer S. Santos, Neil S. Fishman
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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.
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A Basin Analysis Case Study: The Morrison Formation, Grants Uranium Region, New Mexico
This volume summarizes results of a U.S. Geological Survey multidisciplinary basin analysis research effort that encompasses all aspects of the geology of the Morrison Formation in the Grants uranium region, located in the San Juan basin of northwestern New Mexico, U.S.A. Tectonic, stratigraphic, sedimentologic, structural, petrographic, mineralogic, geochemical, and resource studies are drawn together to provide a geologic synthesis of the Jurassic Morrison Formation, the main uranium host rock in the region, and to provide background data for the formulation of genetic models for ore genesis. The result is a compendium of 21 papers that incorporates many recent and significant advances in our understanding of factors that favored uranium mineralization in the Morrison, and several new genetic models that incorporate these recent advances. The basin analysis approach used here has proved fruitful in that ore genesis can now be viewed in the context of the evolution of a sedimentary basin rather than as an isolated event. This approach to ore genesis has application not only to uranium deposits but also to all sediment-hosted ore deposits.