Sandstone-hosted Uranium Deposits Research Articles

The Nuheting deposit, Erlian Basin, NE China: Synsedimentary to diagenetic uranium mineralization

Sedimentary units deposited during the post-rift stage of the Erlian Basin located in northeast China present an alternation of sandstone and mudstone layers. This sedimentological architecture is at the origin of confined permeable reservoirs hosting sandstone-type uranium deposits. The study of the Nuheting deposit offered the opportunity to identify synsedimentary/early diagenetic uranium concentrations and diagenetic mineralization hosted in mudstone-dominated layers of the Erlian Formation, indicating that a stock of uranium was present in the basin prior to the genesis of sandstone-hosted uranium deposits. Therefore, this pre-existing stock may constitute a significant source of uranium for the formation of roll front deposits present in other parts of the Erlian Basin.Detailed petrographic and geochemical study of drill-core samples from the Nuheting deposit led to the characterization of different stages related to the formation of the uranium ore bodies and allowed to propose a new metallogenic model. Uranium mineralization of the Nuheting deposit is mainly hosted in dark gray silty mudstone of wetland depositional environment of the Late Cretaceous Erlian Formation. Petrographic observations and EMP analyses evidenced that a significant amount of uranium was associated with clay minerals (interstratified clays, smectite, chlorite, palygorskyte, illite and kaolinite), either adsorbed on mineral surfaces as U (VI) ions or reduced mainly as UO2 nano to microcrystals disseminated in the clayey matrix, which corresponds to synsedimentary/early diagenetic concentrations. Trace elements on pyrite analyzed by LA-ICPMS, petrographic observations and whole-rock geochemical data led to the characterization of a diagenetic uranium mineralization. High As (1–50ppm), Mo (10–500ppm) and Se concentrations in the whole rock and the incorporation of these elements in pyrite highlight reducing conditions within the host-rocks during the diagenesis of the Erlian Formation. During the early diagenetic stage, uranium was either desorbed from clay minerals and organic materials to be reduced or directly reduced and precipitated as P-rich coffinite and pitchblende on pyrite crystals. During the late diagenetic stage, uranium was redistributed in situ and locally deposited mainly as coffinite on pyrites. Finally, an epigenetic stage of cementation was identified with sulfate and carbonate minerals, which may enclose some uranium minerals. This epigenetic stage of fluid circulation may be responsible for a minor uranium remobilization. Therefore, the Nuheting deposit experienced three main stages: (i) a synsedimentary/early diagenetic uranium concentration and mineralization, (ii) a late diagenetic in situ uranium remobilization and deposition on pyrite and (iii) an epigenetic cementation. Rock-Eval pyrolysis indicates that the organic matter contained in host-rocks of the Nuheting deposit is of type IV, inherited from land plant, and do not contain free hydrocarbons (very low S1). Therefore, our results do not support that migrated hydrocarbons were involved as a reducing agent for uranium mineralization.

Paleovalley-related uranium: exploration criteria and case-studies from Australia and China

Uranium exploration and mining in Australia and China are significant and increasingly important sectors of each country's respective mineral industry. Here we focus on similarities in the geology of paleovalley-related uranium mineralising systems, which can be used to refine strategies for exploration. Paleovalley-related uranium resources are developed as sandstone-hosted and surficial deposits within paleovalley-fills, deposited either on incised crystalline bedrock, or incised sedimentary cover that is often of similar age to the sediments that host the mineralisation. With respect to Sino-Australian examples, paleovalley-related uranium occurs mostly around the margins of Mesozoic and Cenozoic basins; often the mineralisation is hosted within sands contained within paleovalleys developed upon, or proximal to, Precambrian crystalline rock that contains primary uranium mineralisation. Numerous examples are known in Australia and China where granitic rocks of pre-Mesozoic age contain uranium in the range 10-100 ppm, concentrations significantly above the crustal average of 2.8 ppm uranium. Crystalline rocks with above-background levels of uranium that are incised by paleovalleys are often the inferred source of the sandstone-hosted uranium. In these regions, extensive fluvial systems developed particularly during Mesozoic and early Cenozoic times, eroded weathered basement rocks to physically disperse uranium into the cover sediments, with subsequent chemical remobilisation, dispersion and concentration of uranium occurring throughout the late Cenozoic. Paleovalley-related uranium deposits are typically hosted within unlithified sand, lignitic clays or calcreted channel-fill, deposited in continental or marginal marine environments. Sandstone-hosted uranium deposits in many cases contain impermeable clay layers, intercalated within the sedimentary sequence; impervious layers often form the upper and lower bounding strata of individual deposits. Uranium deposits are associated with reduced lithologies; mineralisation is developed where oxidising fluids (carrying dissolved U) were modified by interaction with reduced sands, the latter commonly containing pyrite and dispersed organic matter and/or seams of lignite. Mineralised bodies, often tabular or roll-shaped, formed along the interface between clay horizons and the sandy facies of paleovalley sediments. Geological, geophysical and geochemical features of the paleovalleys and related uranium deposits have been used to construct models to understand ore genesis and assist exploration for paleovalley-hosted uranium deposits. Australia and China remain highly prospective for the discovery of new paleovalley-related uranium deposits. The precise geometric definitions of the basin margin and paleovalley architecture are important in identifying exploration targets and improving the effectiveness of drilling. This requires the integration of various geoscientific data sets. Refinements in remote sensing, geophysical techniques and data processing, in combination with sedimentological and depositional interpretations, provide an efficient approach for outlining the principal drainage patterns and channel dimensions. To help reduce risk, an exploration strategy should combine these technologies with a detailed understanding of the physicochemical parameters of uranium reduction, mobilisation and preservation.

Applying regional airborne electromagnetic (AEM) surveying to understand the architecture of sandstone-hosted uranium mineral systems in the Callabonna Sub-basin, Lake Frome region, South Australia

The Frome airborne electromagnetic (AEM) survey was designed to provide reliable pre-competitive AEM data to aid the search for energy and mineral resources around the Lake Frome region of South Australia. Flown in 2010, a total of 32,317 line kilometres of high-quality airborne geophysical data was collected over an area of 95,450 km2 at a flight line spacing mostly of 2.5 km, opening to 5 km spaced lines in the Marree–Strzelecki Desert area to the north. The Lake Frome region hosts a large number of sandstone-hosted uranium deposits with known resources of ∼60 000 tonnes of U3O8 including the working In Situ Recovery operations at Beverley, Pepegoona, Pannikin and Honeymoon, and deposits at Four Mile East, Four Mile West, Yagdlin, Goulds Dam, Oban, East Kalkaroo, Yarramba and Junction Dam. The aims of the Frome AEM Survey were to map and interpret critical elements of sandstone-hosted uranium mineral systems including basin architecture, paleovalley morphology, sedimentary facies changes, hydrological connections between uranium sources and uranium deposition sites and structures. Interpretations of the data show the utility of regional AEM surveying for mapping crucial elements of sandstone-hosted uranium mineral systems as well as for mapping geological surfaces, structures and depth of cover over a wide area. Data from the Frome AEM Survey allow mineral explorers to put their own high-resolution AEM surveys into a regional context. Survey data were used to map and interpret a range of geological features that are associated with, or control the location of, sandstone-hosted uranium mineral systems and have been used to assess the uranium prospectivity of new areas to the north of the Flinders Ranges.

Fractionation of 238U/235U by reduction during low temperature uranium mineralisation processes

Investigations of ‘stable’ uranium isotope fractionation during low temperature, redox transformations may provide new insights into the usefulness of the 238U/235U isotope system as a tracer of palaeoredox processes. Sandstone-hosted uranium deposits accumulate at an oxidation/reduction interface within an aquifer from the low temperature reduction of soluble U(VI) complexes in groundwaters, forming insoluble U(IV) minerals. This setting provides an ideal environment in which to investigate the effects of redox transformations on 238U/235U fractionation. Here we present the first coupled measurements of 238U/235U isotopic compositions and U concentrations for groundwaters and mineralised sediment samples from the same redox system in the vicinity of the high-grade Pepegoona sandstone-hosted uranium deposit, Australia.The mineralised sediment samples display extremely variable 238U/235U ratios (herein expressed as δUCRM145238, the per-mil deviation from the international NBL standard CRM145). The majority of mineralised sediment samples have δUCRM145238 values between −1.30±0.05 and 0.55±0.12‰, spanning a ca. 2‰ range. However, one sample has an unusually light isotopic composition of −4.13±0.05‰, which suggests a total range of U isotopic variability of up to ca. 5‰, the largest variation found thus far in a single natural redox system. The 238U/235U isotopic signature of the mineralised sediments becomes progressively heavier (enriched in 238U) along the groundwater flow path.The groundwaters show a greater than 2‰ variation in their 238U/235U ratios, ranging from δUCRM145238 values of −2.39±0.07 to −0.71±0.05‰. The majority of the groundwater data exhibit a clear systematic relationship between 238U/235U isotopic composition and U concentration; samples with the lowest U concentrations have the lowest 238U/235U ratios. The preferential incorporation of 238U during reduction of U(VI) to U(IV) and precipitation of uranium minerals leaves the groundwaters enriched in 235U, resulting in a progressive shift in 238U/235U towards lighter values in the aqueous phase as U is removed. These data can be modelled by a closed system Rayleigh fractionation model, with a fractionation factor (α, representing the 238U/235U composition of the groundwater relative to the solid uranium minerals) ranging from ∼0.9996 to 1.0000, with the majority of datapoints ranging from α values of 0.9998 to 0.9999.The sense and magnitude of the results of this study imply that 238U/235U fractionation is likely to be controlled by volume-dependent nuclear field shift effects during the reduction of U(VI) to U(IV) during mineralisation processes. These findings support the use of the 238U/235U isotopic system as a tracer to constrain the nature and timing of palaeoredox conditions.

Australian sandstone-hosted uranium deposits

Australia’s sandstone-hosted uranium deposits occur in sedimentary basins of Carboniferous, Cretaceous and Tertiary age; these include some of Australia’s largest and highest grade uranium deposits. The conventional model for sandstone-hosted uranium deposits has proved robust and a predictive model leading to the discovery of many deposits in Australia. The location of deposits is strongly influenced by the presence of Mesoproterozoic and Archaean age leachable uranium-rich source rocks in the headwaters of channels draining into basins developed in the Northern Territory, Western Australia and South Australia. Australia’s production of uranium from sandstone-hosted deposits is limited to two in-situ leach operations and is relatively minor when compared to production from Kazakhstan and the USA. Nevertheless, Australia has a large inventory of uranium resources in 19 undeveloped sandstone-hosted deposits, amounting to 123 kt U3O8. The average grade of all 21 Australian deposits that have resources is 0·15% U3O8. Tertiary palaeochannels host the greatest number of deposits and include the largest and highest grade deposits. The Callabonna Sub-basin in South Australia is the most richly endowed basin, accounting for 62·4 kt U3O8, being 38% of Australia’s sandstone-hosted resource inventory. Australia remains highly prospective for the discovery of new palaeochannel hosted uranium deposits, with regional airborne geophysical surveys likely to be of great assistance in continuing to define palaeochannel systems that may host uranium in basins draining uranium rich source rocks.

Determining uranium concentration in boreholes using wireline logging techniques: comparison of gamma logging with prompt fission neutron technology (PFN)

The continuous movement of ground water in sandstone hosted uranium deposits renders them subject to disequilibrium between uranium and its daughter products. This is important when wireline gamma logging alone is used to quantify the presence of uranium which can be over- or under-estimated by a significant percentage. The problem can be overcome by logging using the prompt fission neutron (PFN) tool which directly measures the presence of uranium through neutron activation. PFN technology is also superior to core drilling and assay as it provides a larger sample, is less expensive and is instantaneous, allowing drilling programs to proceed uninterrupted. Examples are presented from uranium deposits in Australia and the USA demonstrating disequilibrium, and the use of PFN to map uranium throughout a deposit and to set the screens in in situ leach mining.

Sandstone-hosted uranium deposits amenable for exploitation by in situ leaching technologies

Sandstone uranium deposits represent uranium concentrations formed by low-temperature hydrothermal processes, usually of diagenetic to epigenetic origin. The deposits are commonly hosted in arkosic sandstone and are therefore referred to as sandstone-type uranium. Globally, this is the most abundant type of uranium mineralisation, containing approximately 28% of the world’s uranium resources and including several giant deposits with resources exceeding 100 kt of uranium. The main uranium minerals are pitchblende and coffinite, and uranium is recovered from host rocks by conventional hydrometallurgical technologies using sulphuric acid or alkaline leach. Host sediments were deposited in many different geological environments including continental intracratonic basins, intermontane depressions, coastal-plains and palaeo-river channels. Mineralisation is mostly stratabound and localised in the permeable sandstone at the redox interfaces where oxidised uranium-rich fluids have intersected with relatively reduced basin lithologies. Sandstone-type uranium mineralisation can also be distributed along permeable fault zones cutting sedimentary sequences. Deposits are subdivided into four groups: roll front (roll-type), tabular, basal channel and tectonic-lithologic types. Many sandstone uranium deposits cannot be exploited by conventional mining technologies because of low grade and difficult geotechnical conditions created by the presence of poorly consolidated wallrocks and the location of ore bodies below the water table. However, because of high permeability of the host sediments and the favourable uranium mineralogy, these deposits are well-suited to exploitation by in situ leach (ISL) technique. ISL mining is defined as the process of uranium extraction from the host sandstone in situ by injecting the chemical leach solutions directly into the ore zone. The pregnant solutions containing leached uranium are transported to the surface through production wells. Uranium is recovered from the solutions to produce yellowcake.

A continent-wide study of Australia's uranium potential: Part I: GIS-assisted manual prospectivity analysis

This paper describes the approach to, and outcomes of, a manual analysis (i.e., a cognitive assessment of spatial and non-spatial data) of the uranium potential of 90 geological regions in Australia. For this analysis, the 14 principal uranium deposit types recognized by the International Atomic Energy Agency were grouped in six uranium systems models (i.e., surficial, sedimentary, igneous-related, metamorphic/metasomatic, unconformity-related, and vein-related uranium systems) on the basis of similar genetic processes, environments of ore formation and ingredients mappable at the regional to continent scale. The newly proposed uranium systems models are structured according to the mineral systems approach and focus on the critical mineralization processes that must occur for a uranium deposit to form in a particular region. Our manual prospectivity analysis employed this approach to assess the probability of the critical genetic processes having occurred in each geological region. In this semi-quantitative, probabilistic evaluation, technical, quality and opportunity ranking schemes were used to rank each geological region based on the probability of occurrence of and potential for high-quality uranium deposits and opportunity for securing prospective ground. Based on this assessment, the geological regions with the greatest potential for discovery of potentially recoverable uranium resources are the Ashburton, Broken Hill, Litchfield, McArthur, Money Shoal, Murphy, Paterson, Pine Creek and Northeast Tasmania regions (i.e., quality ranking of 10.0), the Gawler and Polda regions (i.e., 9.0), and the Amadeus, Georgetown, Stuart, Tanami regions (i.e., 8.1). Most of these regions contain known unconformity-related or sandstone-hosted uranium deposits, although some of them are pure conceptual plays that have received relatively little attention in terms of uranium exploration. Maps based on the numerical output of the prospectivity analysis helped to inform area selection decisions and detailed follow-up studies, and focus time and resources. The template developed in this study can easily be modified to suit prospectivity analyses for other metals or a similar investigation in another country. As illustrated in Part II, the best possible approach to a complex, continent-wide prospectivity analysis is to harness the strengths of both manual and automated (i.e., sophisticated computational techniques applied to spatial data) approaches as these methodologies essentially address each other's limitations.

Evolution of Uranium Fractionation Processes through Time: Driving the Secular Variation of Uranium Deposit Types

Uranium deposit types have evolved considerably from the Archean to the Present. The major global drivers were (1) change of geotectonic conditions during the Late Archean, (2) strong increase of atmospheric oxygen from 2.4 to 2.2 Ga, and (3) development of land plants during the Silurian. Other significant variations of uranium deposit types are related to unique conjunctions of conditions such as those during phosphate sedimentation in the Cretaceous. Earth’s uranium fractionation mechanisms evolved through four major periods. The first, from 4.55 and 3.2 Ga, corresponds to formation of a thin essentially mafic crust in which the most fractionated trondheimite-tonalite-granodiorite (TTG) rocks attained uranium concentrations of at most a few parts per million. Moreover, the uranium being essentially hosted in refractory accessory minerals and free oxygen being absent, no uranium deposit could be expected to have formed during this period. The second period, from about 3.1 to 2.2 Ga, is characterized by several widespread pulses of highly fractionated potassic granite strongly enriched in U, Th, and K. Late in this period peraluminous granite was selectively enriched in U and to a lesser extent K. These were the first granite and pegmatite magmas able to crystallize high-temperature uraninite. The erosion of these granitic suites liberated thorium-rich uraninite which would then be concentrated in placer deposits along with pyrite and other heavy minerals (e.g., zircon, monazite, Fe-Ti oxides) within huge continental basins (e.g., Witwatersrand, South Africa, and Bind River, Canada). The lack of free oxygen at that time prevented oxidation of the uraninite which formed the oldest economic uranium deposit types on Earth, but only during this period. The third period, from 2.2 to 0.45 Ga, records increased oxygen to nearly the present atmospheric level. Tetravalent uranium from uraninite was oxidized to hexavalent uranium, forming highly soluble uranyl ions in water. Uranium was extensively trapped in reduced epicontinental sedimentary successions along with huge quantities of organic matter and phosphates accumulated as a consequence of biological proliferation, especially during the Late Paleoproterozoic. A series of uranium deposits formed through redox processes; the first of these developed at a formational redox boundary at about 2.0 Ga in the Oklo area of Gabon. All known economically significant uranium deposits related to Na metasomatism are about 1.8 Ga in age. The high-grade, large tonnage unconformity-related deposits also formed essentially during the Late Paleoproterozoic to early Mesoproterozoic. The last period (0.45 Ga-Present) coincided with the colonization of continents by plants. The detrital accumulation of plants within continental siliciclastic strata represented intraformational reduced traps for another family of uranium deposits that developed essentially only during this period: basal, roll front, tabular, and tectonolithologic types. However, the increased recognition of hydrocarbon and hydrogen sulfide migration from oil or gas reservoirs during diagenesis suggests potential for sandstone-hosted uranium deposits to be found within permeable sandstone older than the Silurian. Large uranium deposits related to high-level hydrothermal fluid circulation and those related to evapotranspiration (calcretes) are only known during this last period of time, probably because of their formation in near-surface environments with low preservation potential.

Interaction of two fluid systems in the formation of sandstone-hosted uranium deposits in the Ordos Basin: Geochemical evidence and hydrodynamic modeling

The Ordos Basin is one of the top oil, gas and coal producers in China and is increasingly recognized as an important uranium mineralization province. Uranium deposits occur near the margin of the basin and are mainly hosted in sandstones of the Jurassic Zhiluo Formation. The orebodies are associated with alterations which indicate a transition from oxidizing to reducing conditions, accompanied by a change of color from red to greenish grey and grey. Previous studies of fluid inclusions in epigenetic minerals associated with uranium mineralization yielded homogenization temperatures from 60 to 180 °C, indicating either deep-burial or hydrothermal rather than early diagenetic environments. The δ 13C, δ 18O and, δD, of the fluids related to mineralization, inferred from the C–H–O isotopes of calcite and kaolinite associated with mineralization, range from − 14.0 to − 2.7‰, − 9.1 to 8.8‰, and − 130 to − 94‰ V-SMOW, respectively. The C and H isotope compositions indicate that significant proportions of the carbon and hydrogen in the mineralizing fluids were derived from oxidation of hydrocarbons. Numerical modeling of fluid flow in the basin indicates that two fluid flow systems were developed in the Jurassic and Cretaceous time when the margins of the basin were relatively uplifted and the strata of the basin were gently inclined. The upper flow system was driven by gravity in relation to topographic relief and the flow direction was downward and from the basin margin toward basin center. The lower flow system was driven by overpressures related to sediment compaction and the flow was upward and from the basin center toward basin margins. We propose that the location of uranium mineralization is associated with the mixing of the two fluid flow systems: uranium was leached from the Cretaceous and Jurassic strata and carried by the descending meteoric fluids, and was reduced from U + 6 to U + 4 by hydrocarbons carried by the ascending basinal fluids and precipitated at the interface of the two fluid systems. The location of the interface between the two flow systems is dependent on the magnitude of the topographic relief. At the end of the Cretaceous sedimentation (65 Ma), the interface was located in the Upper Jurassic (the main stratigraphic interval of uranium mineralization) if the topographic relief was 350 m from the basin center to margin. Such an interface location can also be reached for a lower topographic relief at a later time due to gradual dissipation of the overpressures. Thus, a combination of decreasing topographic relief and dissipation of overpressures after the Cretaceous sedimentation was favorable for mineralization because the interface of the two fluid flow systems could be maintained in the same stratigraphic interval for a prolonged period of time.

Hydrochemistry and isotope compositions of groundwater from the Shihongtan sandstone-hosted uranium deposit, Xinjiang, NW China

Groundwaters and surface water in the Shihongtan sandstone-hosted U ore district, Xinjiang, NW China, were sampled and analyzed for their major-, and trace element concentrations and oxygen, hydrogen, boron and strontium isotope compositions in order to assess the possible origins of the waters and water–rock interactions that occurred in the deep aquifer system. The waters in the study district have been grouped into three hydrochemical facies: Facies 1, potable spring-water, is a pH neutral (7.0), Na–Ca–HCO 3 type water with low total dissolved solids (TDS; 0.2 g/l, fresh) and has δ 18O of − 8.3‰, δD of − 48.2‰,δ 11B of 1.5‰, and 87Sr/ 86Sr of 0.70627. Facies 2 groundwaters are mildly acidic to mildly alkaline (pH of 6.5–8.0, mean 7.3), Na–Ca–Mg–Cl–SO 4 type waters with moderate TDS (8.2 g/l–17.2 g/l, mean 9.3 g/l, brackish) and haveδ 18O values in the − 5.8‰ to − 9.3‰ range (mean − 8.1‰), δD values in the − 20.8‰ to − 85.5‰ range (mean − 47.0‰),δ 11B values in the + 9.5‰ to + 39.1‰ range (mean + 17.1‰), and 87Sr/ 86Sr values in the 0.70595 to 0.70975 range (mean 0.70826). Facies 3, Aiting Lake water, is a mildly alkaline (pH = 7.4), Na–Ca–Mg–Cl–SO 4 type water with the highest TDS (249.1 g/l, brine) and has δ 18O of − 2.8‰, δD of − 45.8‰,δ 11B of 21.2‰, and 87Sr/ 86Sr of 0.70840. The waters from the study district show a systematic increase in major, trace element and TDS concentrations and δ 11B values along the pathway of groundwater migration which can only be interpreted in terms of water–rock interaction at depth and strong surface evaporation. The hydrochemical and isotopic data presented here confirm that the groundwaters in the Shihongtan ore district are the combined result of migration, water–rock interaction and mixing of meteoric water with connate waters contained in sediments.

Applications of Natural Radiation-Induced Paramagnetic Defects in Quartz to Exploration in Sedimentary Basins

ABSTRACT Quartz grains in contact with uranium-bearing minerals or fluids are characterized by natural radiation-induced paramagnetic defects (e.g., oxygen vacancy centers, silicon vacancy centers, and peroxy radicals), which are amenable to study by electron paramagnetic resonance (EPR) spectroscopy. These natural radiation-induced paramagnetic defects, except for the oxygen vacancy centers, in quartz are concentrated in narrow bands penetrated by α particles: (1) in halos around U- and Th-bearing mineral inclusions and (2) in outer rims or along fractures. The second type of occurrence provides information about uranium mineralization or remobilization (i.e., sources of uranium, timing of mineralization or remobilization, pathways of uranium-bearing fluids). It can also be used to evaluate sedimentary basins for potential of uranium mineralization. In particular, the peroxy radicals are stable up to 800 °C and, therefore, are useful for evaluating metasedimentary rocks (e.g., Paleoproterozolc metasedimentary sequences in the central zone of the North China craton). EPR study of the Changcheng Series can focus on quartz from the sediment-basement unconformity and faults to determine the presence and types of natural radiation-induced paramagnetic defects, with which to identify and prioritize uranium anomalies. Other potential applications of natural radiation-induced paramagnetic defects in quartz include uranium-bearing hydrocarbon deposits in sedimentary basins. For example, the Junggar, Ordos, and Tarim basins in northwestern China all contain important oil and natural gas fields and are well known for elevated uranium concentrations, including economic sandstone-hosted uranium deposits. Therefore, systematic studies on the distribution of natural radiation-induced paramagnetic defects in quartz from host sedimentary sequences are expected to provide information about the migration of oil and natural gas in those basins.

Uranium-series disequilibria as a means to study recent migration of uranium in a sandstone-hosted uranium deposit, NW China

Uranium concentration and alpha specific activities of uranium decay series nuclides 234U, 238U, 230Th, 232Th and 226Ra were measured for 16 oxidized host sandstone samples, 36 oxic-anoxic (mineralized) sandstone samples and three unaltered primary sandstone samples collected from the Shihongtan deposit. The results show that most of the ores and host sandstones have close to secular equilibrium alpha activity ratios for 234U/ 238U, 230Th/ 238U, 230Th/ 234U and 226Ra/ 230Th, indicating that intensive groundwater-rock/ore interaction and uranium migration have not taken place in the deposit during the last 1.0 Ma. However, some of the old uranium ore bodies have locally undergone leaching in the oxidizing environment during the past 300 ka to 1.0 Ma or to the present, and a number of new U ore bodies have grown in the oxic–anoxic transition (mineralized) subzone during the past 1.0 Ma. Locally, uranium leaching has taken place during the past 300 ka to 1.0 Ma, and perhaps is still going on now in some sandstones of the oxidizing subzone. However, uranium accumulation has locally occurred in some sandstones of the oxidizing environment during the past 1 ka to 1.0 Ma, which may be attributed to adsorption of U(VI) by clays contained in oxidized sandstones. A recent accumulation of uranium has locally taken place within the unaltered sandstones of the primary subzone close to the oxic–anoxic transition environment during the past 300 ka to 1.0 Ma. Results from the present study also indicate that uranium-series disequilibrium is an important tool to trace recent migration of uranium occurring in sandstone-hosted U deposits during the past 1.0 Ma and to distinguish the oxidation-reduction boundary.

Magnetic Ilmenite-Hematite Detritus in Mesozoic-Tertiary Placer and Sandstone-Hosted Uranium Deposits of the Rocky Mountains

Mesozoic and Tertiary sandstones throughout the Rocky Mountains region of the United States contain a de-trital Fe-Ti oxide mineral that is generally regarded as unusual and is preserved wherever these sandstones have escaped destructive diagenesis. This mineral, a medial ilmenite-hematite solid solution, is highly magnetic and has sometimes been mistaken for magnetite. We have encountered this mineral in paleomagnetic studies and in investigations of sandstone-hosted uranium deposits and Cretaceous shoreline placer deposits. It is known to occur elsewhere in intermediate volcanic rocks, consistent with quenching from hypersolvus high-temperature conditions. The detrital occurrences we describe are of economic importance in uranium deposits, where they are among the reductants that precipitate uranium, and in Cretaceous placer deposits, whose exploitation would require a separation procedure tailored to this mineral. The distribution of detrital hematite-ilmenite sandstones in Cretaceous rocks suggests a contemporaneous volcanic source in the northern Rocky Mountains.

Magnetic Ilmenite-Hematite Detritus in Mesozoic-Tertiary Placer and Sandstone-Hosted Uranium Deposits of the Rocky Mountains

Magnetic Ilmenite-Hematite Detritus in Mesozoic-Tertiary Placer and Sandstone-Hosted Uranium Deposits of the Rocky Mountains

Phase relations among selenides, tellurides, and oxides; II, Applications to selenide-bearing ore deposits

The stability fields of binary selenide minerals calculated thermodynamically relative to native elements, sulfide, telluride, and oxide minerals as a function of temperature have been used to characterize the genesis of selenide-bearing deposits. Selenide-bearing hydrothermal deposits are restricted mainly to four major types: selenide vein-type deposits, unconformity-related deposits, sandstone-hosted uranium deposits, and Au-Ag epithermal volcanic-hosted deposits. The selenide minerals in telethermal selenide vein-type and selenide-bearing unconformity-related uranium deposits formed from hydrothermal fluids at conditions within the stability field of hematite, between 5.8 and 7 log units in the f (sub O2(g)) above the hematite-magnetite (HM) buffer (at 100 degrees C), and with an f (sub Se2(g)) /f (sub S2(g)) ratio higher than unity. These high f (sub O2(g)) conditions enhance the separation of selenium from sulfur, leading to a high f (sub Se2(g)) /f (sub s2(g)) ratio and deposition of a diverse group of selenides. Selenide deposition was not significantly influenced by wall-rock buffers, but late reequilibration and dissolution of some earlier formed selenides, such as krutaite (CuSe 2 ), probably occurred in some selenide-bearing unconformity-related uranium deposits. Mineral parageneses in sandstone-hosted uranium deposits suggest that they formed at higher f (sub Se2(g)) and lower f (sub O2(g)) (0-5.8 log units above the hematite-magnetite buffer at 100 degrees C) than did unconformity-related uranium deposits and selenide vein-type deposits. The selenide-bearing Au-Ag epithermal, volcanic-hosted deposits were formed from hydrothermal fluids with f (sub O2(g)) below or near the hematite-magnetite buffer and at an f (sub Se2(g)) /f (sub S2(g)) ratio lower than unity. Low f (sub Se2(g)) /f (sub S2(g)) ratios and the presence of sulfide assemblages that buffer f (sub Se2(g)) and f (sub S2(g)) prevent the enrichment of selenium in the hydrothermal fluid and deposition of selenide minerals except for silver selenides. Therefore, a selenium-rich, relatively reduced (e.g., below the hematite-magnetite buffer) hydrothermal fluid can deposit only silver selenides; no other selenide minerals can deposit from a fluid of this type. An oxidizing environment (e.g., close to the anglesite-galena buffer) is essential to form most selenide minerals. The actual mineralogy and the amount of selenide minerals that form from such an oxidizing fluid is a function of f (sub Se2(g)) and ultimately reflects the concentration of selenium in the fluid. A selenium-rich fluid alone can account for the presence of silver selenides in epithermal Au-Ag deposits, but both an oxidizing and a selenium-rich fluid is required to form the selenide mineral assemblages observed in telethermal selenide vein, unconformity-related, and sandstone-hosted uranium deposits.

Hydrogeology of an ancient arid closed basin: Implications for tabular sandstone-hosted uranium deposits

Hydrogeologic modeling shows that tabular-type uranium deposits in the grants uranium region of the San Juan basin, New Mexico, formed in zones of ascending and discharging regional ground-water flow. The association of either lacustrine mudstone or actively subsiding structures and uranium deposits can best be explained by the occurrence of lakes at topographic depressions where ground water having different sources and compositions is likely to converge, mix, and discharge. Ascending and discharging flow also explains the association of uranium deposits with underlying evaporites and suggests a brine interface. The simulations contradict previous suggestions that ground water moved downward in the mudflat.

The solubilities of some major and minor element minerals in ground waters associated with a sandstone-hosted uranium deposit

Des échantillons d'eaux de subsurface ont été récoltés dans 41 puits atteignant les grès de base Oakville d'âge Miocène dans le Sud du Texas et ont été analysés pour les éléments majeurs et les éléments en traces afin de mesurer les variations chimiques liées à la proximité de minéralisations uranifères. L'échantillonnage s'étend sur une surface de 240 km2 qui contient plusieurs gisements d'uranium liés à des failles. Les facteurs qui influent sur le système hydrochimique de la région étudiée sont : (1) les perméabilités relativement fortes des sédiments enfouis issus de chenaux fluviatiles, (2) l'écoulement vers le haut des saumures le long de failles vivantes dans l'aquifère, (3) la formation d'une interface redox (Eh=0 volt) dans l'aquifère et (4) le climat semi-aride. Les variations des indices de saturation (SI) pour les minéraux réduits d'U, As, Mo, Se et pour les minéraux associés tels que la pyrite déterminent la position des gisements connus. Les indices de saturation tendent vers zéro (la valeur d'équilibre) lorsque l'on approche les gisements à des distances de 3 à 4-5 km en amont et décroissent de nouveau en aval. Les données de saturation pour la calcite dans les eaux de subsurface sont fortement corrélées avec la position de la limite redox actuelle. Les valeurs des indices de saturation pour les minéraux d'U, As, Mo et Se se sont révélées beaucoup plus utiles pour localiser les gisements que les concentrations des éléments prises individuellement. Les traceurs radiogéniques Ra et Rn sont fortement anomaliques dans le minerai, mais descendent rapidement au niveau du fond à de faibles distances du minerai. Une intense anomalie en hélium sous forme de traînée est observée dans la direction de circulation de l'eau de subsurface. L'approche décrite dans cet article constitue un guide d'exploration efficace à l'échelle régionale qui devrait conduire à un programme de sondages d'évaluation plus sûr.

Sandstone-hosted uranium deposits in northern Saskatchewan as natural analogs to nuclear fuel waste disposal vaults

Proterozoic U deposits have been preserved in sandstone formations on several Precambrian Shields, and give testimony to the compositions and conditions required for the safe disposal of nuclear fuel wastes. The unconformity-type U deposits in the Athabasca Basin of northern Saskatchewan are discussed here in detail. Conditions for ore formation in the Saskatchewan deposits were similar to those expected around a waste package in terms of pressure, temperature and salinity of the solutions. Secondary dispersion of U and other elements of interest, resulting from groundwater interactions, is very limited. Only radiogenic Pb appears to have been dispersed to any extent. Two processes involved in the ore formation have a direct bearing on the stability of fuel waste and of buffer and backfill in the nearfield. These are redox control of groundwater by methane and quartz dissolution, with the former process still active.

Application of mineral-solution equilibria to geochemical exploration for sandstone-hosted uranium deposits in two basins in west central Utah

A geochemical survey utilizing mainly ground waters was conducted in the Milford and Beaver basins. Waters were collected mainly from wells and springs at 100 sites and analyzed for major and minor elements. A computer model (WATEQ3) was used to calculate the redox potential and the state of saturation of the waters with respect to uraninite, coffinite, and other mineral phases. Several areas in the basins were shown to have ground-water environments of reducing redox potentials which are favorable for precipitation of reduced uranium minerals. In addition, the ground waters in some of these areas were shown to be saturated or near saturation with respect to uraninite and coffinite. These areas do not necessarily coincide with areas containing the highest concentrations of uranium in ground water. The methods described in this study for hydrogeochemical exploration for sandstone-hosted uranium deposits may have wide application, particularly throughout those filled basins in the western United States where nearby sources for uranium exist.--Modified journal abstract.