GeoScienceWorld
Volume

A Basin Analysis Case Study:

The Morrison Formation, Grants Uranium Region, New Mexico

Edited by Christine E. Turner-Peterson, Elmer S. Santos and Neil S. Fishman

Abstract

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.

  1. Page 1
    Abstract

    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. 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 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.

  2. Page 7
    Abstract

    The Middle Jurassic San Rafael Group and Upper Jurassic Morrison Formation of the San Juan basin consist of complexly interrelated conglomerate, sandstone, silt- stone, mudstone, limestone, and gypsum. The San Rafael Group, consisting of the Entrada Sandstone, Wanakah Formation, Cow Springs Sandstone, and sandstone at Mesita, was deposited in eolian, sabkha, minor fluvial, and marine or possibly lacustrine environments. The Morrison Formation, consisting of the Salt Wash, Recapture, Westwater Canyon, and Brushy Basin Members, was deposited in widespread fluvial, lacustrine, and eolian environments.

    The presence of eolian rocks in both the San Rafael Group and Morrison Formation led to mapping and correlation problems in the southern half of the basin. The chief problem was that a widespread eolian facies of the Recapture Member of the Morrison was considered a part of the eolian Cow Springs Sandstone by earlier workers. Recent work in the southern and western parts of the basin shows that eolian beds of the two units can be distinguished by lateral relationships and sedimentologic features including sorting, sedimentary structures, and crossbed dip-vector resultants. Separation of the two units yields an improved understanding of depositional processes and paleoenvironmental distributions. Problems in this interval still exist in the southeastern part of the basin where the relationship of the San Rafael Group to the Recapture Member of the Morrison remains unclear.

  3. Page 27
    Abstract

    In the Middle and Late Jurassic, the structural configuration of the area occupied by the San Juan basin was different from the present-day configuration, which developed in response to Laramide deformation in the latest Cretaceous to early Tertiary. An isopach map indicates a thickening of Middle to Upper Jurassic strata near the southern margin of the area. The thickening is attributed to downwarping, mainly in the Jurassic, which may have controlled depositional patterns and influenced the distribution of primary uranium ore deposits in the host rocks of the Jurassic Morrison Formation. Paleotectonic reconstruction indicates the existence of a magmatic arc immediately west of the Colorado Plateau in the Jurassic. Jurassic folding in the basin and magmatic activity in the arc have important implications bearing on the uranium geology of the area. The folding probably exerted some control on depositional trends in the host rocks which, in turn, influenced the distribution of uranium deposits in them. Volcanism in the magmatic arc supplied silicic ash that may be the source of some, if not all, of the uranium in the host rocks.

  4. Page 35
    Abstract

    A 118,535 km2 (45,750 mi2) area that encompasses the San Juan basin and surrounding structural features is contoured on the base of the Dakota Sandstone of Cretaceous age, using a 100-m (330-ft) contour interval at a scale of 1: 500, 000. Faults shown on the map are those that have been mapped at the surface. The structure contours were generated from compilations of structure contours and from large-scale original mapping; in areas of little or no surface data, the structure contours were generated from logs of drill holes.

  5. Page 47
    Abstract

    The Westwater Canyon Member of the Morrison Formation, the main ore-bearing sandstone in the San Juan basin, consists of a sequence of vertically stacked braided stream deposits. Three fluvial units within the sequence can be delineated in the basin. Volcanic pebbles are abundant in the middle fluvial unit, in a zone that forms a crude time line.

    A pronounced thickening of sandstone in the Westwater Canyon Member north of Gallup, once believed to be the apex of a large alluvial fan, is now thought to merely reflect a greater accumulation of sediment in response to downwarping of the basin in that area. Provenance studies suggest that highlands that contributed detritus to Westwater Canyon streams were located several hundred kilometers to the west and southwest of the San Juan basin, and thus fan apices would also have been several hundred kilometers upstream. The fluvial units recognized in the basin may well be coalesced distal fan deposits, but are probably best interpreted as vertically stacked braided stream sequences.

    Facies changes in fine-grained interbeds of the Westwater Canyon probably have greater significance in terms of localizing ore than any special attribute of the fluvial sandstones themselves. Uranium ore generally occurs in sandstones that are inter- bedded with greenish-gray lacustrine mudstones. Pore waters that were expelled from these mudstones are thought to have been the source of the pore-filling organic matter (humate) associated with primary uranium ore in nearby sandstones.

  6. Page 77
    Abstract

    The Brushy Basin Member and the upper part of the Westwater Canyon Member of the Morrison Formation in northwest New Mexico are nonmarine sedimentary rocks of Late Jurassic age. This stratigraphic interval consists of as many as four lithofacies deposited in fluvial and playa-lake environments. Lithofacies A is composed of crossbedded feldspathic sandstone and was deposited by braided streams on an alluvial plain. Lithofacies B is composed of crossbedded feldspathic sandstone and tuffaceous mudstone, and was deposited by braided and anastomosing streams at the distal end of the alluvial plain. Lithofacies C is composed of calcareous, tuffaceous mudstone and was deposited on a mudflat between the alluvial plain and a playa lake. Lithofacies D is composed of zeolitic, tuffaceous mudstone and was deposited in a playa lake.

    The distribution of diagenetic facies in mudstones and tuffs in the Brushy Basin Member and upper part of the Westwater Canyon Member reflects the pH and salinity gradients common to fluvial/playa-lake systems. The abundant vitric ash in the sediments reacted to form montmorillonite in the fluvial facies. Calcite and montmorillonite were the reaction products where the fluvial and outermost playa facies met. Vitric ash reacted to form clinoptilolite and heulandite along the playa margins. In the center of the playa facies, analcime replaced clinoptilolite, an early zeolite. These early diagenetic minerals were replaced by albite, quartz, and mixed-layer illite-montmorillonite where the Brushy Basin Member and upper part of the Westwater Canyon Member have been deeply buried in the San Juan basin.

  7. Page 93
    Abstract

    A thin, discontinuous, fluvial, locally conglomeratic sandstone at the base of the Dakota Sandstone in the vicinity of the southeastern San Juan basin, New Mexico has been named the Encinal Canyon Member of the Dakota Sandstone. In the past, the sandstone beds, placed here in the Encinal Canyon, have been included in the Jackpile sandstone, (an economic unit in the Morrison Formation), in the Burro Canyon Formation, or in the Oak Canyon Member of the Dakota Sandstone. Distinction between the Encinal Canyon Member and the Jackpile sandstone, which are separated by an unconformity that probably spans most of the Early Cretaceous, is economically important. The Jackpile is a primary uranium exploration target, whereas the Dakota contains little known uranium.

    In the past, the sub-Dakota erosional surface in the southeastern San Juan basin generally was thought to be at the base of the marine and paralic Oak Canyon Member of the Dakota Sandstone, which overlies the Encinal Canyon Member. The unconformity is shown here to be at the base of the fluvial rocks of the Encinal Canyon Member.

    Local relief at the base of the Encinal Canyon indicates that the sub-Dakota erosional surface formed during a time of regional degradation. Easterly flowing streams scoured underlying units and in some places cut completely through the Jackpile sandstone and the Burro Canyon Formation. The Encinal Canyon was deposited in response to the initial transgression of the Dakota sea. As the sea inundated the area, a transgressive erosional surface formed, and the overlying paralic and marine sediments of the Oak Canyon Member were deposited.

  8. Page 105
    Abstract

    Approximately 1,800 geophysical logs and 100 measured sections provided data for several types of isopleth maps of the Westwater Canyon and Brushy Basin Members of the Upper Jurassic Morrison Formation in the southern San Juan basin, New Mexico. These types of maps include: isopach, sandstone:mudstone ratio, percent sandstone, net sandstone, and average number of mudstone interbeds per 100 ft (30 m) of section. We also constructed a paleotopographic map on the base of the Westwater Canyon and a structure contour map on the base of the Upper Cretaceous Dakota Sandstone.

    These maps illustrate depositional unit geometry, sandstone depocenter distribution, and large-scale lithofacies variations within the units. The Westwater Canyon is thinner and less sandy over paleotopographic highs and is thicker and sandier along paleotopographic lows, which suggests active structural control of facies distribution during deposition. Sedimentation of the Brushy Basin Member was also affected by some of the same active structural elements. Detailed reflection seismic studies have defined basement faults that were periodically reactivated since the Precambrian. These faults exerted a significant influence on depositional patterns in the Morrison Formation.

    Primary uranium ore in the Westwater Canyon +Member is restricted to sandstone depocenters associated with east-southeast-trending isopach thicks. Remnant ore deposits are relict primary deposits that lie in oxidized ground updip from a regional oxidation-reduction (redox) interface. Sedimentologic controls seem to be similar to those for primary ore, and in general these deposits have been preserved from oxidation by stratigraphic variations and by structures. Redistributed ore deposits are also concentrated in the vicinity of isopach thicks, but in rocks with relatively low sand-stoneimudstone ratios. However, the location of redistributed ore is much more closely related to the position of the regional redox interface. The geographic form of this interface was influenced regionally and locally by Laramide structures.

  9. Page 145
    Abstract

    A seismic reflection survey, using conventional field techniques and a computer-intensive data processing program, has identified basement structures that had an effect on the deposition of the uranium-bearing Upper Jurassic Morrison Formation. The structural style is one of block faulting defined by northwest- and northeast-trending high-angle normal faults with major periods of movement occurring in the Late Permian and Late Jurassic. The seismic survey, combined with detailed subsurface stratigraphic studies, shows that thickness and sand content of the Westwater Canyon Member increase within a downdropped block. Most of the deposits of the Church Rock uranium district are located within rocks of the Westwater Canyon Member in this block.

  10. Page 161
    Abstract

    Statistical treatment of the chemical data for samples from the Church Rock, Smith Lake, Ambrosia Lake, and Laguna districts, all in the Grants uranium region, San Juan basin, indicates that primary ore-forming processes concentrated copper, iron, manganese, molybdenum, selenium, vanadium, yttrium, arsenic, organic carbon, and sulfur, along with uranium. The initial uranium and vanadium mineralization occurred before compaction of the host rocks. A barium halo associated with all of these deposits formed as a result of secondary processes. Calcium and strontium were also enriched in the ores by secondary processes. Comparison of the chemical characteristics of redistributed deposits in the Church Rock district with those of primary deposits in the Grants uranium region indicates that calcium, manganese, strontium, yttrium, copper, iron, molybdenum, lead, selenium, and vanadium are chemically separated from uranium during redistribution of the deposits in the Church Rock district. Comparisons of the chemical characteristics of the Church Rock deposits with those of the secondary deposits in the Ambrosia Lake district suggest some differences in the processes that were involved in the genesis of the redistributed deposits in these two areas.

  11. Page 171
    Abstract

    Preliminary studies were made on the organic matter associated with fossil logs and uranium-rich ores of the Morrison Formation in the San Juan basin, New Mexico, using solid-state 13C nuclear magnetic resonance, stable carbon isotope ratios, and elemental analyses. These studies show that information about the structure of the organic matter can be obtained that could lead to a better understanding of its origin and nature. The nuclear magnetic resonance data indicate that the apparently non- cellular organic matter in these ores is highly aromatic, and confirm elemental analyses that show low atomic H/C ratios and high O/C ratios. Information from the nuclear magnetic resonance spectra, as well as the elemental data, led us to believe that this organic matter is more like a medium-rank coal than like the “amorphous carbon” described by previous workers. Trends in the nuclear magnetic resonance, elemental, and carbon isotope data are consistent with the hypothesis that the organic matter originated as humic acids. The trends also suggest that factors other than radiation damage could be responsible for the chemical structural nature and partially responsible for the stable carbon isotope composition of the organic matter. These factors include the early oxidation of plant material to form humic acids, which imparts structural characteristics different from those of the parent material, and the possibility that different types of plant material are involved in the formation of ores.

  12. Page 185
    Abstract

    The Section 23 mine is one of about 18 large uranium mines opened in sandstones of the fluvial Westwater Canyon Member of the Jurassic Morrison Formation, in the Ambrosia Lake uranium district, during the early 1960s.

    Two distinct types of unoxidized ore occur in the mine. One type consists of uranium-rich authigenic organic matter that impregnates parts of the reduced sandstone host rocks. This type of ore occurs as peneconcordant layers which are typically elongate east-southeast, subparallel both to the sedimentary trends and to the regional strike of the host rock. A second type of ore is essentially devoid of organic matter and occurs in thick zones which conform to interfaces that separate oxidized from reduced parts of the host rocks. Genesis of the second type of ore is similar to that of roll-type deposits in Tertiary rocks of Wyoming and Texas.

    Organic matter in the primary ores was probably introduced into the host rock as humic acids that precipitated in the pores of the sandstone. This material is inferred first to have fixed uranium as urano-organic compounds but, with further diagenesis, to have released some of the uranium to form coffinite. Vanadium, molybdenum, and selenium are associated with primary ore and may also have been fixed by the organic matter. The secondary or roll-type ores contain uranium mostly in the form of coffinite and only rarely as uraninite. They also contain vanadium and selenium but are virtually devoid of molybdenum.

  13. Page 211
    Abstract

    The Mariano Lake uranium deposit, hosted by the Brushy Basin Member of the Jurassic Morrison Formation, occurs in the Smith Lake district of the Grants uranium region, New Mexico. The orebody, contains abundant amorphous organic material, which suggests that it represents a primary-type deposit; however, the orebody is close to a regional reduction-oxidation interface, which suggests that uranium was secondarily redistributed by oxidative processes.

    Uranium contents correlate positively with organic carbon contents. Petrographic evidence points to uranium residence in amorphous organic material that was post- depositionally introduced in the diagenetic history of the host sandstone. Uranium mineralization was preceded by precipitation of pyrite (δ34S values of — 11.0 to — 38.2 per mil), mixed-layer smectite-illite clays, and quartz and potassium feldspar overgrowths; and also partial dissolution of some detrital feldspars. Alterations associated with uranium mineralization include precipitation of the organic material, microcrys- talline quartz, and pyrite and marcasite (δ34S values of -29.4 to -41.6 per mil), and the destruction of detrital Fe-Ti oxide grains. Following mineralization, calcite, dolomite, barite, and kaolinite were precipitated, and some iron disulfides were replaced by ferric oxides.

    Geochemical data and petrographic observations both indicate that the Mariano Lake orebody is a primary-type deposit. Oxidative processes have not noticeably redistributed uranium in the immediate vicinity of the deposit, nor have they greatly modified geochemical characteristics in the ore. Impedance of ground-water flow by local folds and the lower porosity characteristics of ore zones may have helped to preserve the deposit.

  14. Page 227
    Abstract

    This drilling project included 12 holes along a north-south-trending line from Mariano Lake to Lake Valley, New Mexico, near the southern margin of the San Juan basin. Of a total 33,075 ft (10,088 m) drilled, 4,550 ft (1,388 m) were cored in the strati- graphic interval that included the basal part of the Dakota Sandstone, the Brushy Basin and Westwater Canyon Members of the Morrison Formation, and the upper part of the Recapture Member of the Morrison Formation.

    The project objectives were (1) to provide cores and geophysical logs for study of the sedimentology, petrography, geochemistry, and mineralization in the uranium- bearing Westwater Canyon Member; (2) to provide control for a detailed seismic study of Morrison stratigraphy and basement structures; (3) to define and correlate the stratigraphy of Cretaceous coal-bearing units; (4) to supply background data for studies of ground-water flow pattern and ground-water quality; and (5) to provide data to aid resource assessment of uranium and coal.

    The project design included selection of (1) drill-hole locations to cross known ore and depositional trends in the Morrison Formation; (2) a coring interval to include the uranium-bearing unit and adjacent units; geophysical logs for lithologic correlations, quantitative evaluation of uranium mineralization, qualitative detection of coal beds, preparation of synthetic seismograms, and magnetic susceptibility studies of alteration in the Morrison; and (3) a high-salinity mud program to enhance core recovery.

    A regional stratigraphic correlation chart, brief lithologic descriptions, and descriptions of geophysical log responses of units in and adjacent to the cored interval aided in the interpretation of a geophysical log correlation diagram. Uranium mineralization was detected in four drill holes; coal beds and carbonaceous shales were encountered in all holes; and artesian flows of oil mixed with water were encountered in two holes.

  15. Page 241
    Abstract

    Geophysical well logs were obtained in eight deep holes drilled and cored by the U.S. Geological Survey to examine the geology of the Mariano Lake-Lake Valley area in the southern part of the San Juan basin, New Mexico. The logs were made to determine the petrophysical properties of the rocks penetrated by the holes, to aid in making stratigraphic correlations between the holes, and to estimate the grade of uranium enrichment in mineralized zones. The logs can be divided into six categories—nuclear, electric, sonic, magnetic, dipmeter, and borehole conditions. Examples of these logs are presented and related to lithological and petrophysical properties of the cores recovered. Gamma-ray and prompt fission neutron logs were used to estimate uranium grade in mineralized zones. Resistivity and spontaneous potential logs were used to make stratigraphic correlations between drill holes and to determine the variability of the sandstone:mudstone ratios of the major sedimentary units. In one drill hole a dipmeter log was used to estimate the direction of sediment transport of the fluvial host rock. Magnetic susceptibility logs provided supportive information for a laboratory study of magnetic mineral alteration in drill cores. This study was used to infer the geochemical and hydrologic environment associated with uranium deposition in the project area.

  16. Page 257
    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.

  17. Page 277
    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.

  18. Page 303
    Abstract

    Petrographic study of the Mariano Lake-Lake Valley cores reveals three distinct zones of postdepositional alteration of detrital Fe-Ti (iron-titanium) oxide minerals in the Westwater Canyon Member of the Upper Jurassic Morrison Formation. In the uranium-bearing and adjacent portions of the Westwater Canyon, these detrital Fe-Ti oxide minerals have been thoroughly altered by leaching of iron. Stratigraphically lower parts of the Westwater Canyon and the underlying Recapture Member are characterized by preservation of Fe-Ti oxide grains, primarily magnetite and ilmenite, and of hematite, and by an absence of uranium concentrations. Partly destroyed Fe-Ti oxide minerals occupy an interval between the zones of destruction and preservation. Alteration patterns of the Fe-Ti oxide minerals are reflected in bore-hole magnetic susceptibility logs. Magnetic susceptibility response in the upper parts of the Westwater Canyon Member is flat and uniformly < 500 /xSI units, but at greater depths it fluctuates sharply, from <1,000 to nearly 8,000 μSI units. The boundary between uniformly low and high magnetic susceptibility response corresponds closely to the interval that divides the zone of completely altered from the zone of preserved detrital Fe-Ti oxide minerals. The alteration pattern suggests that solutions responsible for destruction of the Fe-Ti oxide minerals originated in the overlying Brushy Basin Member of the Morrison Formation. Previous studies indicate that these solutions were rich in soluble organic matter and perhaps in uranium. Uranium precipitation may have been controlled by a vertically fluctuating interface between organic-rich solutions and geochemically different fluids in which the detrital Fe-Ti oxide minerals were preserved.

  19. Page 315
    Abstract

    Preliminary examination of the distribution, texture, and chemical composition of clay minerals in the Morrison Formation suggests that the sandstone of the Westwater Canyon served as a conduit for potassium- and aluminum-rich, possibly warm, fluids that moved updip from the center of the basin toward the basin margin, giving rise to mineral zonation within the sandstone over a lateral distance of approximately 35 km.

    The observed patterns include: (1) Pure smectite occurs as grain coatings in the Westwater Canyon Member in the shallowest core; this smectite is texturally similar to mixed-layer illite-smectite found in the deeper cores. (2) Expandability of the illite-smectite decreases toward the center of the basin and is more expandable near the upper and lower sandstone-mudstone contacts than at the center of the sandstone. This illite-smectite is generally highly ordered and frequently exhibits well-defined superlattice peaks. (3) Iron-rich chlorite occurs texturally on top of the smectite and illite-smectite and therefore must be later. (4) Smectite in the overlying Brushy Basin Member and in the underlying Recapture Member remains 100% expandable through the area analyzed. (5) Kaolinite, the latest clay mineral to form, is most abundant in the middle cores, decreasing in the shallow cores and in the deeper cores.

    The fluid movement event probably occurred subsequent to the basin formation during Laramide time. Diagenetic reactions are more extensive in the deeper sections, near the basin center, and have obliterated some chemical and mineralogical relationships which are still observable in the shallower cores.

  20. Page 331
    Abstract

    A newly developed data-directed numerical method is used to estimate the undiscovered uranium endowment of the Westwater Canyon Member of the Morrison Formation in the San Juan basin, New Mexico. Data from 1,800 geophysical logs, chip samples from 40 test wells, and 40 measured surface sections provided the basic geologic information. Using these data, we evaluated the favorability of uranium concentration in each of 2,068 cells defined within the basin. Favorability was based on the correlated similarity of the geologic characteristics of each cell to the geologic characteristics of five deposit models. Estimates of the undiscovered endowment for each cell were categorized according to deposit type, depth, cutoff grade, and resource area. The undiscovered uranium endowment of the Westwater Canyon Member is estimated at 2.6 x 106 tonnes U3O8 with an estimated error equal to 0.25 x 106 tonnes of U3O8. This estimate is roughly twice that obtained by the U.S. Department of Energy in the NURE program. The total uranium endowment, which is the sum of the discovered and undiscovered endowments, is estimated to be 3.5 x 106 tonnes of U3O8.

  21. Page 357
    Abstract

    A synthesis of current geologic knowledge of the Morrison Formation in the Grants uranium region, incorporating results of the multidisciplinary studies presented in this volume, permits genetic modeling of uranium mineralization in the broad context of basin analysis. The result is that primary uranium mineralization can be viewed as a product of a unique series of tectonic, depositional, hydrologic, and diagenetic processes that acted in concert at a specific, critical time during basin evolution.

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