Major Uranium Deposits Research Articles

Biostimulation as a sustainable solution for acid neutralization and uranium immobilization post acidic in-situ recovery

Major uranium (U) deposits worldwide are exploited by acid leaching, known as ‘in-situ recovery’ (ISR). ISR involves the injection of an acid fluid into ore-bearing aquifers and the pumping of the resulting metal-containing solution through cation exchange columns for the recovery of dissolved U. Rehabilitation of ISR-impacted aquifers could be achieved through natural attenuation, or via biostimulation of autochthonous heterotrophic microorganisms due to the associated acid neutralization and trace metal immobilization.In this study, we analyzed the capacity of pristine aquifer sediments impacted by diluted ISR fluids to buffer pH and immobilize U. The experimental setup consisted of glass columns, filled with sediment from a U ore-bearing aquifer, through which diluted ISR fluids were flowed continuously. The ISR solution was obtained from ISR mining operations at the Muyunkum and Tortkuduk deposits in Kazakhstan. Following this initial phase, columns were biostimulated with a mix of molasses, yeast extract and glycerol to stimulate the growth of autochthonous heterotrophic communities. Experimental results showed that this amendment efficiently promoted the activity of acid-tolerant bacterial guilds, with pH values rising from 4.8 to 6.5–7.0 at the outlet of the stimulated columns. The reduction of sulfate, nitrate, and metals as well as dissimilatory nitrate reduction to ammonia induced the rise in pH values, in agreement with geochemical modelling results. Biostimulation efficiently promoted the complete immobilization of U, with the accumulation of up to 3343 ppm in the first few centimeters of the columns. Synchrotron analysis and SEM-EDS revealed that up to 60% of the injected hexavalent U was immobilized as tetravalent non-crystalline U onto bacterial cell surfaces. 16S rDNA amplicon analysis and qPCR data suggested a predominant role played for members of the Phylum Firmicutes (from the genera Clostridium, Pelosinus and Desulfosporosinus) in biological U reduction and immobilization.

Geophysics of the Patterson Lake South Uranium Deposit, northwestern Saskatchewan

The Athabasca Basin’s major uranium deposits and mines are generally proximal to graphitic conductors (reducing environment) and accompanied by an alteration ‘halo’ which is usually a resistive low, but can also be silicified (resistivity high). The sandstone environment is normally highly resistive which makes things ideal for EM detection of weaker graphitic conductors at depth. The Triple R deposit on Fission Uranium Corp’s Patterson Lake South Property is located in Canada’s Athabasca Basin, home to the world’s richest uranium mines. It is the only major, high-grade deposit in the region that is potentially open-pitable and is the largest mineralized trend in the region - currently standing at over 3 km in length. Patterson Lake South is unconventional in that virtually all the major high grade uranium deposits have been inside the basin margins and were on the east side of the Athabasca Basin; whereas Patterson Lake South lies outside the basin margins and is on the west side of the Basin. The Patterson Lake South discovery is chronicled from the initial airborne radiometric and EM surveys, to ground follow-up using DC resistivity and induced polarization, horizontal loop EM, moving loop TEM and radon survey, leading up to the discovery holes. The deposit shows excellent correlation with a VTEM conductive ‘bright spot’, an interpreted conductor and a resistivity low segment. Also significant is the evidence of cross structure seen in the resistivity at the west side of the displayed deposit outline. The continued success of the resource delineation and expansion is attributed to the dedicated Fission staff for their work to bring the project forward. From Discovery to Resource Estimate, the Triple R Deposit was achieved in just two years of drilling.

Uranium in South Africa: Exploration and Supply Capacity

The major uranium deposits types are quartz-pebble conglomerate of the Central Rand Group (Witwatersrand Supergroup) and the Dominion Group, sandstone of the Karoo Uranium Province, surficial deposits of the Namaqualand region, as well as coal of the Springbok Flats Basin. Recent exploration activities (both greenfields and near-mine) have increased the resource potentials of these deposits. South Africa's Council for Geoscience has conducted high resolution magnetic and radiometric surveys in the Namaqualand region. Witwatersrand Basin is the only deposit where uranium mining is active at present in South Africa. Total uranium production in 2015 was 448 t. However, with production expected to increase in the Witwatersrand Basin, production is expected to reach 700 t U by the end of 2018. Furthermore, with uranium production envisaged to begin in the Karoo Uranium Province, uranium production may exceed 1000 t U by the year 2020.

Development of experimental approach to examine U occurrence continuity over the extended area reconnoitory boreholes: Lostoin Block, West Khasi Hills district, Meghalaya (India)

Exploratory drilling was undertaken in the Lostoin block, West Khasi Hills district of Meghalaya based on the geological extension to the major uranium deposit in the basin. Gamma ray logging of drilled boreholes shows considerable subsurface mineralization in the block. However, environmental and exploration related challenges such as climatic, logistic, limited core drilling and poor core recovery etc. in the block severely restricted the study of uranium exploration related index parameters for the block with a high degree confidence.The present study examines these exploration related challenges and develops an integrated approach using representative sampling of reconnoitory boreholes in the block. Experimental findings validate a similar geochemically coherent nature of radio elements (K, Ra and Th) in the Lostoin block uranium hosting environment with respect to the known block of Mahadek basin and uranium enrichment is confirmed by the lower U to Th correlation index (0.268) of hosting environment.A mineralized zone investigation in the block shows parent (refers to the actual parent uranium concentration at a location and not a secondary concentration such as the daughter elements which produce the signal from a total gamma ray measurement) favoring uranium mineralization. The confidence parameters generated under the present study have implications for the assessment of the inferred category of uranium ore in the block and setting up a road map for the systematic exploration of large uranium potential occurring over extended areas in the basin amid prevailing environmental and exploratory impediments.

In-situ SIMS uraninite U–Pb dating and genesis of the Xianshi granite-hosted uranium deposit, South China

The southeastern part of the Nanling metallogenic province, China is host to numerous granite-hosted vein-type hydrothermal uranium deposits. The geology and geochemistry of these deposits have been extensively studied. However, accurate and precise ages for the uranium mineralization are scarce because the uranium minerals in these deposits are usually fine grained, and may have formed in several stages. Therefore, the ages previously obtained by the bulk dating techniques are possibly a mixed age.The Xianshi uranium deposit, located in the southeastern part of the Guidong granite complex, is a major uranium deposit in South China. The uranium mineralization from this deposit is mainly fine grained uraninite in quartz or calcite veins which are spatially associated with the Cretaceous mantle-derived mafic dykes. Micro-Raman spectroscopy and X-ray diffraction analyses indicate that the dominant uranium mineral occurs as a rare form of uraninite (U3O7). Three distinct generations of uranium minerals have been identified based on petrographic and field relations. Stage 1 uraninite has the lowest UO2 and highest PbO contents whereas Stage 3 uraninite has the highest UO2 and lowest PbO contents.Uraninite from the Xianshi deposit was dated using an in-situ SIMS U–Pb dating technique. The results show three distinct age groups: 135±4Ma, 113±2Ma and 104±2Ma, which are in excellent agreement with the ages of three episodes of mantle-derived mafic dykes. Therefore, the Xianshi uranium deposit has experienced at least three hydrothermal events that are responsible for the deposition of uranium ores, which are genetically related to the emplacement of three sets of mafic dykes.

Geochemical evidence and hydrodynamic modeling of two fluid systems involved in sandstone-hosted uranium mineralization in the northeast of the Ordos basin, China

The Ordos basin is a Meso-Cenozoic intracontinental basin developed upon Paleozoic sedimentary rocks, which in turn are underlain by Precambrian basement belonging to the North China Craton.With an area of about 2.5×10 km, the Ordos basin represents one of the top oil, gas and coal producers in China and is increasingly recognized as an important uranium mineralization province. Major uranium deposits have been found in the Dongsheng area in the north-eastern margin of the Ordos basin, where uranium mineralization occurs as dissemination in sandstones of the Jurassic Zhiluo Formation. Geologic, petrographic and fluid inclusion studies suggest that the uranium mineralization took place at temperatures from 100 to 200 °C, which indicates either deep-burial or hydrothermal rather than early diagenetic environments. δCVPDB and δOVSMOW values of calcite associated with uranium mineralization range from −2.7‰ to −14.0‰ and 18.4‰ to 20.0‰, respectively, and δOVSMOW and δDVSMOW values of hydrothermal kaolinite are 12.6‰ to13.8‰ and −116‰ to −133‰, respectively. These data suggest that oxidation of organic matter was involved in the formation of the uranium deposits. Based on zoning ofmineral assemblages indicating upward increase in oxidation, it is inferred that organic matter was derived from an

Methodology for Rare Earth Element Determinations of Uranium Oxides by Ion Microprobe

A methodology for the determination of the rare earth elements in uranium oxides by ion microprobe has been set up on a Cameca ims‐3f instrument. An uranium oxide reference material from a syn‐metamorphic uranium deposit related to albitisation has also been developed for this type of analysis. Applications of the methodology are presented for a series of uranium oxides selected from some major uranium deposit types: from the world's highest grade unconformity‐related uranium deposit from the Athabasca Basin (Saskatchewan, Canada; the Shea Creek and the McArthur River examples), a perigranitic vein‐type deposit (Pen Ar Ran, Vendée, France) and a volcanic caldera‐related deposit (Streltsovkoye, Transbaikalia, Russia). Each type of uranium deposit appears to have a specific REE signature. All REE patterns from the Shea Creek and the McArthur deposits are characterised by bell‐shaped patterns centred on Tb‐Dy and similar to those already published for uranium oxides from unconformity‐related deposits from Australia. Such bell‐shaped REE patterns centred on Tb‐Dy may therefore be considered as a typical signature of uranium oxides from Mesoproterozoic unconformity‐related deposits. A smoother bell shape pattern centred on Eu characterises the syn‐metamorphic albitisation related deposit of Mistamisk selected for the reference material. The REE patterns from the Pen Ar Ran deposit show a fractionation from LREE to HREE with anomalously high abundances of Sm, Eu and Gd with respect to the other REEs, similar to the REE patterns of uranium oxides from the volcanic‐related deposits of Streltsovkoye.

Fluid extraction from quartz in sheeted leucogranites as a monitor to styles of uranium mineralization: an example from the Rössing area, Namibia

The Rössing Mine area in Namibia contains uranium mineralization primarily within crustally derived post-collisional sheeted leucogranites. Adjacent to the mine in the SH area, a small 1-km long granite cupola of coalesced sheets is also characterized by elevated U levels. In the Rössing pit, in the SJ area, U is hosted by uraninite and secondary U minerals including beta-uranophane. Conversely, in the SH area, U is hosted principally by the pyrochlore-group mineral, betafite. Manometric determination of the proportions of H 2 O, CO 2 and non-condensables (mostly methane) has been carried out on inclusions within quartz from leucogranite samples from both the SH and SJ anomalies. This method has been able to distinguish between leucogranites of the SH and SJ areas which has not been possible with thermometric fluid inclusion studies or by major and trace element geochemical investigations. Fluid geochemistry of sheeted leucogranites from the SH area exhibit lower absolute H 2 O contents, lower H 2 O/CO 2 ratios and low total fluids compared to that from the SJ area. Fluid geochemistry and large ion lithophile (LIL) element data suggest that the occurrence of the SJ anomaly as a major uranium deposit is linked to the high H 2 O and total fluid contents. This study illustrates the usefulness of this technique to distinguish between economic mineralization and the sub-economic SH anomaly.

Monoclines and palaeochannels: evidence for syntectonic sedimentation in the Beaufort Group of the Karoo basin, South Africa

In the south-western part of the Karoo basin, southward-dipping monoclines apparently had an influence on sedimentation patterns in fluvial deposits of the Late Permian-Early Triassic Beaufort Group. Palaeocurrents in this area are generally directed towards the northeast but deviate sharply towards the east on the steep flanks and towards the north on the gently dipping flanks of all five monoclines examined. Lithofacies also show statistically significant differences on the two flanks of the DR-3 monocline which is associated with a major uranium deposit. Structural evidence suggests that the monoclines probably formed after compaction of the sediments, therefore, an earlier phase in their development must have been responsible for the characteristic sedimentation patterns. It is suggested that shallow synclines, related to compressional paroxysms of the Cape orogeny, deflected streams parallel to their axial trends. As uranium occurs preferentially in thick channel sandstones, their association with monoclines can therefore be used as an exploration tool in the Karoo.

Geological environment of the Cigar Lake uranium deposit

The Cigar Lake uranium deposit occurs within the Athabasca Basin of northern Saskatchewan, Canada. Like other major uranium deposits of the basin, it is located at the unconformity separating Helikian sandstones of the Athabasca Group from Aphebian metasediments and plutonic rocks of the Wollaston Group. The Athabasca Group was deposited in an intra-continental sedimentary basin that was filled by fluviatile terrestrial quartz sandstones and conglomerates. The group appears undeformed and its actual maximum thickness is about 1500 m. On the eastern side of the basin, the detrital units correspond to the Manitou Falls Formations where most of the uranium deposits are located. The Lower Pelitic unit of the Wollaston Group, which lies directly on the Archean basement, is considered to be the most favourable horizon for uranium mineralization. During the Hudsonian orogeny (1800–1900 Ma), the group underwent polyphase deformation and upper amphibolite facies metamorphism. The Hudsonian orogeny was followed by a long period of erosion and weathering and the development of a paleoweathering profile.On the Waterbury Lake property, the Manitou Falls Formation is 250–500 m thick and corresponds to units MFd, MFc, and MFb. The conglomeratic MFb unit hosts the Cigar Lake deposit. However, the basal conglomerate is absent at the deposit, wedging out against an east–west, 20 m high, pre-Athabasca basement ridge, on top of which is located the orebody.Two major lithostructural domains are present in the metamorphic basement of the property: (1) a southern area composed mainly of pelitic metasediments (Wollaston Domain) and (2) a northern area with large lensoid granitic domes (Mudjatik Domain). The Cigar Lake east–west pelitic basin, which contains the deposit, is located in the transitional zone between the two domains. The metamorphic basement rocks in the basin consist mainly of graphitic metapelitic gneisses and calcsilicate gneisses, which are inferred to be part of the Lower Pelitic unit. Graphite- and pyrite-rich "augen gneisses," an unusual facies within the graphitic metapelitic gneisses, occur primarily below the Cigar Lake orebody.The mineralogy and geochemistry of the graphitic metapelitic gneisses suggest that they were originally shales. The abundance of magnesium in the intercalated carbonates layers indicates an evaporitic origin.The structural framework is dominated by large northeast–southwest lineaments and wide east–west mylonitic corridors. These mylonites, which contain the augen gneisses, are considered to be the most favourable features for the concentration of uranium mineralization.Despite the presence of the orebody, large areas of the Waterbury Lake property remain totally unexplored and open for new discoveries.

Descriptive models of major uranium deposits in China

Four major types of uranium deposits occur in China: granite, volcanic, sandstone, and carbonaceous-siliceous-pelitic rock. These types are major sources of uranium in many parts of the world and account for about 95 percent of Chinese production. Descriptive models for each of these types record the diagnostic regional and local geologic features of the deposits that are important to genetic studies, exploration, and resource assessment. A fifth type of uranium deposit, metasomatite, is also modeled because of its high potential for production. These five types of uranium deposits occur irregularly in five tectonic provinces distributed from the northwest through central to southern China.

Proterozoic Unconformity-Related Uranium Deposits in Canada and Australia--Geology, Genesis, and Impact on World Uranium Production: ABSTRACT

Fifteen major unconformity-related uranium deposits, cumulatively containing more than 1350 million lb U/sub 3/O/sub 8/, have been discovered since 1966 in almost identical geologic settings in the Pine Creek geosyncline, Northern Territory, Australia, and the Athabasca basin, Saskatchewan, Canada. A polygenetic mineralization model is preferred, involving primary syngenetic uranium accumulation in lower Proterozoic euxinic shallow marine environments near an Archean provenance; concentration as structure-stratabound mineralization during metamorphism 1800 Ma; and, subsequent to deposition of the middle Proterozoic sandstone, remobilization and reconcentration of uranium by sandstone-derived diagenetic-hydrothermal fluids. The latter upgrading process operated more strongly in the Athabasca basin, resulting in higher grades and the extensive sandstone-hosted mineralization. Grades average about 2% U/sub 3/O/sub 8/ in Canadian unconformity deposits and 0.3% U/sub 3/O/sub 8/ in Australian examples. Sizes range from restricted occurrences of a few hundred thousand lb U/sub 3/O/sub 8/ to concentrations of up to 450 million lb U/sub 38/. The five unconformity-related deposits presently in production in Australia and Canada are low-cost producers that have made substantial inroads into western world uranium markets. From 1975 to 1984, when total western world uranium production doubled from 49.6 to 100.3 million lb U/sub 3/O/sub 8/, the contribution of unconformity uranium depositsmore » in Canada and Australia increased from 0.72 to 26.2 million lb (1.4 to 26%).« less

The time-bound character of uranium mineralising processes, with special reference to the Proterozoic of Gondwana

It is more useful to consider the time-bound character of uranium mineralising processes than the time-bound character of uranium deposits. This is because present uranium deposits are the result of a series of mineralising steps which occurred over hundreds of millions of years. We have identified five periods which are characterised by moderately distinct mineralising processes. 1. (1) Before 2800 Ma. Uranium was fractionated into the upper crust during late Archaean—Early Proterozoic cratonisation by magmatic and metamorphic processes. The granitic terranes were an important source of uranium for subsequent mineralising events. 2. (2) 2800-2200 Ma. Although the atmosphere was probably not reducing at this time, the level of oxygen was sufficiently low for uraninite to be transported and concentrated in sedimentary basins. Cratonisation was a prerequisite for the formation and preservation of long-lived basins. Rapid burial and reducing conditions during diagenesis were important for preserving the uranitite. The uranium was mobilised during diagenesis and metamorphism. 3. (3) 2200-1200 Ma. After ∼ 2000 Ma, when the oxygen level in the atmosphere increased markedly, the movement of uranium in solution as uranyl ions became more important. The first marine shelf sequences containing chemical and organic sediments to be deposited on the cratons formed excellent sites for the syn-sedimentary, diagenetic or metamorphic concentration of uranium. In some cases this mineralisation was economic; in other cases it may have been just a step in a succession of mineralising events. Continental clastic sediments which overlie these shelf sediments were excellent hosts for large hydrothermal circulation systems which could remobilise the uranium from either granites or sediments. The lack of organic and argillaceous material in these continental sequences resulted in the precipitation of uranium near the basal unconformity, where there were reducing sediments. 4. (4) 1200-400 Ma. With the start of modern plate tectonics there was greater potential for the thermally activated remobilisation of uranium. The period 1200-900 Ma marked the final mineralisation in some important deposits. The Pan-African and Brazilian events (700-400 Ma) mobilised uranium of uncertain origin and concentrated it in previously deposited marginal marine sediments. 5. (5) 400-0 Ma. The incorporation of organic material into continental clastic sediments allowed uranyl ions to be precipitated before they reached the sea, to form the important sandstone-type of deposits. Any time-bound mineralising process may be either the final or an intermediate step in forming an economic uranium deposit. Which of these it is depends on the subsequent history of the area; on whether the uranium is preserved, dispersed or reconcentrated. That many major uranium deposits occur on late Archaean—Early Proterozoic cratons emphasises the importance of the long-term preservation of sedimentary sequences, even though the uranium may be remobilised several times within the sediments.

Chemical Characteristics of Some Major Uranium Deposits in Western USA: ABSTRACT

Multi-element chemical analyses of several thousand samples were retrieved from the U.S. Geological Survey's computerized Rock Analysis Storage System and used to estimate the average abundances of various elements in each of several types of uranium deposits, in altered rocks associated with some of these deposits, and in unmineralized parts of the various host rocks. Deposits for which results are presented include the tabular deposits in the Morrison Formation, Ambrosia Lake district, New Mexico; secondary deposits in the Ambrosia Lake district; tabular deposits in the Morrison Formation of the Henry Mountains, Utah; tabular deposits in the Chinle Formation in Utah and Colorado; roll-type deposits in Tertiary rocks from the Texas Gulf district; roll-type deposits in th Tertiary basins of Wyoming; tabular deposits in the Entrada Sandstone in Colorado; and a vein-type deposit in crystalline rocks of the Front Range of Colorado. Statistical treatment of the data identified elements that were notably more or less abundant in the deposits and altered rocks than in the unmineralized parts of the host rocks. Comparisons of the mean abundances of elements in the deposits show that the chemical composition of roll-type deposits varies greatly even among deposits in the same district. By contrast, the chemical characteristics of tabular deposits display little variation; the Ambrosia Lake tabular deposits and those of the Henry Mountains district are particularly similar. The data place some constraints on the geochemical aspects of genetic models and suggest c rtain elements as potential prospecting guides. End_of_Article - Last_Page 531------------

In-Situ Leaching of Crownpoint, New Mexico, Uranium Ore: Part 1 Mineralogical Frame of Reference

Vogt, T.C.,** SPE, Mobil Research and Development Corp. Dixon, S.A.; Mobil Research and Development Corp. Strom, E.T.; Mobil Research and Development Corp. Johnson, W.F.; SPE, Mobil Research and Development Corp. Venuto, P.B.; SPE, Mobil Research and Development Corp. Summary Before assessing potential for in-situ leaching, it is advantageous to identify the nature of probable reactants in the ore horizon, including both the uranium species and those in the gangue material. We report mineralogical data on an areally broad suite of samples from the Westwater Canyon member of the Jurassic Morrison formation in the Crownpoint area of the Grants mineral belt. Optical microscope, electron microprobe, X-ray diffraction, and spectroscopic and wet chemical analyses were conducted. In these samples, the uranium occurs predominantly as either coffinite or uraninite. The host predominantly as either coffinite or uraninite. The host rock is arkosic sandstone comprising mainly detrital quartz and feldspar. Kaolinite and chlorite are present in claystone clasts, and calcite occurs as a cement. Major trace elements include molybdenum, vanadium, iron, selenium and sulfur. There is carbonaceous organic material in the ore, and it is intimately associated with the uranium mineral when it is coffinite. Geochemical, pyrolytic and 13C nuclear magnetic resonance (NMR) pyrolytic and 13C nuclear magnetic resonance (NMR) analyses indicate that the organic material is probably a very mature, coaly kerogen derived from plant components. Introduction An understanding of reactions involved in leaching chemistry is a key aspect of successful in-situ uranium leaching. Mineralogic characterization of samples from the ore deposit provides identification of reactive uranium and gangue minerals and shows the association and distribution of labile and relatively inert and nonreactive species. Certain criteria have been established to judge the suitability of an ore deposit for uranium recovery by in-situ leaching. The ore deposit must be beneath the water table and must be restricted by impermeable strata above and below the permeable ore zone. The uranium minerals must be contacted easily by leachate injected into the deposit. Subsurface conditions should allow oxidation of the uranium and formation of a soluble complex, by the oxygenated leachate containing a complexing agent, so that the uranium solution can be pumped to the surface for recovery. pumped to the surface for recovery. We are among the first to test in-situ leaching in New Mexico and have pioneered leaching at depths of 2,000 ft (609.6 m); underground or strip mining are the conventional techniques in use in New Mexico.Our Crownpoint project areas are in McKinley County, NM, and are part of the Grants mineral belt (Fig. 1). The origin and the nature of these uranium deposits have been subjects of continuing research. The leases lie to the west and east of the town of Crownpoint. These leases are on the southern edge of the San Juan basin, about 40 miles (64 km) northeast of Gallup and about as far to the northwest of Grants. The surface of the properties is essentially flat, with an elevation of about 6,800 ft properties is essentially flat, with an elevation of about 6,800 ft (2073 m). The major uranium deposits in this area occur in the Westwater Canyon member of the Jurassic Morrison formation (Fig. 2). The subsurface ore depth is about 2,000 ft (609.6 m) in the project areas where the Westwater member is about 260 ft (79.2 m) thick and consists of interbedded sandstone and claystone. The sandstones are crossbedded and contain pebbles and silicified log fragments. The Westwater member is the major water-bearing member of the Morrison formation. The uranium mineralization typically is confined by impermeable claystone strata.The sandstone of the Westwater Canyon member can be classified compositionally as arkose. Detrital components of the sandstone are quartz, feldspar, rock fragments, and carbonaceous debris. JPT P. 2200

Tertiary volcanic history of the Sierra del Gallego area, Chihuahua, Mexico

Mapping of ∼3,400 km2 in the Sierra del Gallego area, 110 km north of Ciudad Chihuahua, Mexico, has revealed about 1,000 m of volcanic rocks unconformably overlying Lower Cretaceous limestone of the Chihuahua Tectonic Belt. The regional structure is dominated by Basin and Range faults that trend north-northwest and cut all volcanic units. Whole-rock and feldspar K-Ar age determinations, combined with whole-rock chemical analyses, show that subalkalic volcanism took place 45 to 29 m.y. ago, except for a conspicuous hiatus between 36 and 29.5 m.y. Between 45 and 39 m.y. ago, the area was covered by thin widespread blankets of rhyolitic ash-flow tuff derived from distant sources. These ash flows appear to correlate with units associated with a major uranium deposit about 50 km to the south, in the Sierra Pena Blanca. At Sierra del Gallego, these ash flows were followed by a 300-m thickness of extensive andesitic lava flows formed by magma mixing, and about 38 m.y. ago by a rhyolitic lava flow unit approximately 150 m thick. Around 36 m.y. ago, numerous local rhyolite flows and domes were emplaced. Approximately 29 m.y. ago, a renewed surge of volcanic activity produced a widespread 125-m thick group of basalt flows accompanied by a thin rhyolitic ash flow. Volcanism at Sierra del Gallego is broadly contemporaneous with and is gradational in composition between voluminous calc-alkalic activity in the Sierra Madre Occidental to the west and alkalic volcanism to the east in Trans-Pecos Texas. Tertiary rhyolitic volcanism throughout this region was controlled by tectonic patterns that preceded the profound episode of crustal extension so obvious in the present regional topography.

Uranium Deposits in Channel Curves of Salt Wash Sandstone Member of Morrison Formation (Late Jurassic), Utah: ABSTRACT

The Salt Wash Member of the Morrison Formation in southeastern Utah is characterized by fluvial sandstone in distinct paleochannels. The deposits are part of a large delta-fan with an apex in the vicinity of the western Grand Canyon. The exhumed channel fills, which excellently expose directional sedimentary structures, were deposited by streams of slight to moderate sinuosity, with occasional sharp bends. Such curves were loci of more conspicuous entrapment and burial of carbonaceous material. Later these areas became highly reducing environments where uranium ores precipitated from ground water percolating through permeable interconnecting channels. Major uranium deposits are more likely to occur in areas where such conditions existed. To determine areas with strong randomness of paleocurrent directions, the vector summation method was applied to the analysis of 2,638 paleocurrent measurements. Because this method is a sensitive measure of dispersion, it was possible to outline areas displaying high and low vector strength values. Areas of high scatter commonly yield low values and polymodal distribution, and areas with low scatter are characterized by high values and unimodal distribution. Thus, areas of high channel sinuosity were successfully outlined. Major known uranium deposits occur most commonly in areas of low vector strength values. Such areas were compared with those containing known ore deposits and those considered favorable for future discoveries, and the results support the conclusion. Although the ve tor method has been used in many paleocurrent studies, this is the first time it has been applied in conjunction with other geologic criteria to delineate and select areas favorable for uranium exploration in sandstone deposits. The results show an important relationship among low paleocurrent vector strength values, carbonaceous material, channel curves, and uranium occurrences in the sandstone channels. End_of_Article - Last_Page 1696------------

Hydrostratigraphic Framework and Flow Dynamics of Uraniferous Aquifer-Oakville Sandstone of South Texas Coastal Plain: ABSTRACT

Synthesis of geologic, hydrochemical, and hydrologic data for the Oakville Sandstone provides a framework for effective hydrochemical exploration, leach mine planning, and evaluation of mining impact. Quantitative lithofacies maps were combined with structural and well-pump test data to produce a regional Oakville transmissivity map. Interpreted groundwater flux patterns within the resultant three-dimensional transmissivity framework were mapped, and quantitatively illustrate several hydrodynamic principles. (1) The complete groundwater system includes meteoric, elisian (compactional), and thermobaric connate zones. Boundaries separating zones are dynamic and evolve through time. (2) Recharge of meteoric systems occurs directly at outcrop by seepage through overlying confining units, and by seepage from underlying aquifers. (3) Flow occurs within a hierarchy of cells, ranging from unconfined local (n × 10 sq mi) to intermediate confined (n × 103 sq mi) to regional confined (n × 104 sq mi) flow cells. Shallow groundwater may move in any direction with respect to dip. (4) Discharge occurs by direct groundwater flow upward to the water table and by vertical seepage, commonly localized by fault zones, into adjacent aquifers. (5) Topography and the three-dimensional permeability framework localize recharge and discharge zones and control flow direction. Recharge occurs in topographically high areas; discharge in lowest areas. Groundwater flux in the Oakville is highly complex. Major uranium deposits occur within the realm of local (Ray Point district), intermediate (George West district), and regional (Duval County) flow cells. Extant flow direction commonly does not agree with geochemically-inferred mineralization-front polarity. End_of_Article - Last_Page 711------------

The mode of occurrence and distribution of uranium deposits

Uranium deposits occur in association with igneous, sedimentary and metamorphic rocks. The bulk of low-cost reserves, however, occurs in Precambrian rocks or in Phanerozoic sediments immediately overlying the basement. In basement rocks, as well as in more recent rocks, major uranium deposits are spatially associated with leucogranites. In Phanerozoic sediments, close to the basement uranium is enriched in continental clastic formations under reducing conditions. Favourable lithologies are alternating horizons of clay and sandstone containing carbonaceous matter. Metamorphic developments are associated with zones of crustal thickening with a world-wide era of concentration at around 1900-1700 Ma ago. Uranium is also enriched in more recent metamorphosed clastic sediments. Deposits directly associated with igneous rocks tend to occur in unsaturated facies rich in volatiles. Granitic and alaskitic pegmatites also carry economic amounts of uranium. The most important secondary deposits of recent origin are those occurring in carbonateor sulphate-cemented sediments.

Geologic setting and concepts on origin of uranium deposits in the East Alligator River region, N.T., Australia

Major uranium deposits, including Jabiluka, Ranger, Koongarra, and Nabarlek, plus numerous smaller prospects, occur in metamorphosed lower Proterozoic psammatic, pelitic, and chemical sediments of the Cahill Formation (South Alligator Group). These transitionally overlie migmatic metasediments of the Mt. Partridge Formation that occur marginal to Archean granitic complexes. The migmatites are the result of anatectic rejuvenation of the Archean complexes during the late lower Proterozoic and these events were accompanied by regional metamorphism of the Cahill Formation. After metamorphism, pegmatites were intruded, followed by basic intrusives. A period of retrograde metamorphism, uplift, and erosion preceded middle Proterozoic deposition of the Kombolgie Formation. There is a marked spatial relationship of discovered uranium occurrences to the erosional margin of the lower Proterozoic-middle Proterozoic unconformity. Mineralization typically occurs in Cahill Formation chlorite and/or graphite schists which have undergone structural disruption. The main ore mineral is pitchblende which typically occurs in a massive, amorphous form with lesser amounts of disseminated crystalline uraninite. There is pervasive chlorite alteration of the host rocks and chlorite, quartz, and sericite are the main gangue minerals. Associated sulfides include pyrite with lesser chalcopyrite and galena. Gold occurs locally in economic concentrations. The ore deposits are considered to be of postprograde metamorphism in origin. The main stages of uranium concentration consisted of remobilization and introduction into open-space environments. The initial stage of uranium remobilization coincided with regional metamorphism around 1,700 m.y. ago. The main uranium mineralizing events are dated at approximately 900 and 500 m.y. and postdate deposition of the Kombolgie Formation. The latter sediments appear to have protected the deposits from surface oxidizing and dispersing processes until the present time.