Athabasca Basin Research Articles

Effective utilization of seismic reflection technique with moderate cost in uranium exploration

ABSTRACTIn participation with numerous industrial partners, the Seismic Laboratory of the University of Saskatchewan has conducted a variety of active seismic reflection experiments; both on the west and east sides of the Athabasca Basin. Results of the investigations at Shea Creek, McArthur River and Keefe Lake illustrate that the seismic investigations deliver effective, highly relevant primary structural images of the subsurface, with resolution that no other geophysical technique can match. Correlation of similar seismic signatures, on several distant but inter‐related seismic sections, allowed spatial extension of promising exploration target zones previously unrecognized. Within the three‐dimensional seismic volume, comparable reflectivity patterns defined the complex areal distribution of mineralization‐related fault systems. Beyond these novel contributions, extended analysis of seismic signal attributes (amplitude and frequency), optical televiewer, and full‐wave sonic data offer detailed lithological characterization, including alteration zones, clay content, as well as porosity and fracture density information. Although these structural and geologically relevant anomalies are primary indicators of mineralization, presenting novel exploration advantages, the seismic method is still not a standard component of the Athabasca Basin exploration approach, due to the negative perception that ‘it is very expensive’. Comparing the costs of all geophysical techniques to the cost of a single logged drill hole illustrates that the results of a properly designed seismic data acquisition program not only leads to more effective planning of a drilling program, but also would lead to a much quicker recognition of the major mineralized zone(s), and a reduction in the number of required exploration boreholes. This integrated approach to exploration would then translate into a significant reduction of the total exploration expenditures. Unquestionably, the drilling of boreholes provides the most explicit, reliable information to a certain depth, but only within a very small area. Directly connecting the borehole information to seismic results extends the local reliable data; permitting reduction of the number of boreholes to create accurate two‐dimensional or three‐dimensional subsurface images and reduction of the expenditures of the total exploration program.

Geology, alteration system, and uranium metallogenic potential of Pasha-Ladoga Basin, Russia

The Mesoproterozoic Pasha-Ladoga Basin is located at the margin of the Karelian Craton and Svecofennian Belt in northwest Russia. It is the only basin in Russia that is currently recognized to host unconformity-type uranium mineralization (e.g., Karku deposit). Mineralization is associated with the Priozersk Formation which comprises a basal sequence of immature arkosic sandstones and conglomerates interpreted as high-energy braided stream channel and channel bar facies within an alluvial fan depositional environment. Diagenetic and hydrothermal alteration of the Priozersk sandstones is similar to that observed in unconformity-related uranium deposits of the Athabasca Basin (Canada) and includes three main stages: (1) an early diagenetic assemblage comprising illite-smectite (K-rectorite), calcite, minor chlorite, and kaolinite precipitated at 100–150 °C; (2) a peak diagenetic-hydrothermal assemblage composed of kaolinite-dickite, Fe- and Fe-Mg chlorite, calcite, and sulfides formed at 150–250 °C; (3) late diagenetic-hydrothermal event involving fracture-controlled kaolinite and late hematite. Similar to the Athabasca Basin type uranium mineralization, the Karku deposit exhibits anomalous concentrations of LREE, Nb, Pb, and positive correlations between U and Cu, Zn, Ag, and Pb that is a geochemical signature of unconformity-type uranium mineralization. Based on oxygen and hydrogen stable isotope study, it was found that fluids in equilibrium with early and peak diagenetic clays were consistent with oxidized basinal brines and reduced basement fluids. In contrast to the Athabasca Basin, mineralizing fluids at the Karku area were more reduced and potassic. A new geological model for the Karku deposit is proposed based on petrographic observations and sandstones geochemistry. The proposed model envisages the permeable rocks of paleoregolith and the coarse-grained basal part of the Priozersk Formation as the main pathway for migration of uraniferous fluids responsible for the formation of Karku uranium deposit. From the perspective of the geological setting, the Karku deposit differs from deposits of the Athabasca Basin in that it occurs in a rapidly deposited rift-related basin with associated mafic magmatism, the host sandstone sequence is relatively thin (e.g., Priozersk Formation) and exhibits abrupt lateral facies variations, and the widespread presence of early diagenetic interstitial swelling clays. Based on these sedimentological characteristics, it is considered that the Karku deposit setting precludes optimum conditions to create high tonnage high-grade unconformity-type U deposits as found in the Athabasca basin.

Overburden characterization with formation pore pressure and anisotropic stress field estimation in the Athabasca Basin, Canada

One of the challenges encountered during the life cycle of an oil-sand thermal-production reservoir is the prediction of the formation pore pressure and in situ stress regime during the assessment phase of the reservoir development and, more importantly, during the development phase. We have investigated the state of formation pore pressure and stress in the overburden — represented by the Clearwater Formation, Grand Rapids Formation, and Colorado Group — of a preproduction oil-sands reservoir situated in the Athabasca Basin of Alberta, Canada. Our methodology integrates pressure data from piezometers, stress data from mini-frac (MF), dipole sonic logs, and elastic properties obtained from multicomponent 3D seismic inversion data. It combines the Terzaghi effective stresses with the Schoenberg and Sayers elastic stiffness matrix for horizontal transversely isotropic fractured materials. The total principal stresses (vertical, minimum, and maximum horizontal stresses) are expressed as functions of the normal fracture weakness (anisotropic correction factor), formation pore pressure, seismic data (Lamé constants), and the Biot-Willis coefficient. The effective principal stresses are estimated from the equivalent total principal stresses and the formation pore pressure multiplied by the Biot-Willis coefficient. On all three overburden intervals analysed, the relations between principal stresses indicate a normal stress regime. The estimated total minimum horizontal stress matches the MF values within 10%. The formation pore pressure, along with the 3D seismically derived estimates of the total and effective principal stresses, allows for better assessment of the caprock integrity and for operational savings based on a reduced number of MF tests. It can also be used for stress estimation within the formations hosting aquifers, which is so important for thermal production. Understanding the subsurface on the reservoir area is important for efficient production, but knowing the subsurface of the overburden is equally important for reducing potential issues due to production.

Uranium-rich diagenetic fluids provide the key to unconformity-related uranium mineralization in the Athabasca Basin

The Proterozoic Athabasca Basin is well known for its unusually large-tonnage and high-grade ‘unconformity-related’ uranium (U) deposits, however, explanations for the basin-wide U endowment have not been clearly identified. Previous studies indicate that U-rich brines with up to ~600 ppm U and variable Na/Ca ratios (from Na-dominated to Ca-dominated) were present at the sites of U mineralization, but it is unknown whether such fluids were developed solely in the vicinity of the U deposits or at a basinal scale. Our microthermometric and LA-ICP-MS analyses of fluid inclusions in quartz overgrowths from the barren part of the basin indicate that U-rich brines (0.6 to 26.8 ppm U), including Na-dominated and Ca-dominated varieties, were widely developed in the basin. These U concentrations, although not as high as the highest found in the U deposits, are more than two orders of magnitude higher than most naturally occurring geologic fluids. The basin-scale development of U-rich diagenetic fluids is interpreted to be related to several geologic factors, including availability of basinal brines and U-rich lithologies, and a hydrogeologic framework that facilitated fluid circulation and U leaching. The combination of these favorable conditions is responsible for the U fertility of the Athabasca Basin.

Geochemistry of uranium mill tailings in the Athabasca Basin, Saskatchewan, Canada: A review

The Athabasca Basin, located in northern Saskatchewan, Canada, is a major source of global U and an important economic driver for the province and country. Athabasca Basin U deposits consist of uraninite and pitchblende dominated by quartz and aluminosilicates and varying amounts of sulfide and arsenide minerals associated with varying concentrations of As, Se, Mo, Ni, and 226Ra (elements of concern; EOCs). Processing these U ores results in tailings that are often enriched in EOCs. Mill treatment processes are designed to generate tailings that minimize the long-term environmental impact of U tailings, although many challenges exist in reaching this goal. Many studies have contributed to our understanding of the geochemistry of these tailings and EOCs and their potential impact on the surrounding hydrosphere. Using nearly two decades of data from tailings samples, mill sampling campaigns, and laboratory experiments, this review provides a comprehensive analysis of the geochemistry and long-term behavior of U tailings in the Athabasca Basin and develops a geochemical model of the tailings. Results of this review are applicable to tailings generated from other milling operations with comparable acid leaching hydrometallurgical processes. This holistic review also highlights the limitations to our current understanding of U tailings geochemistry.

The Application of Seismic Techniques in the Development of the Millennium Uranium Deposit, Northern Saskatchewan, Canada

The Millennium uranium deposit is located within the Athabasca Basin of northern Saskatchewan, Canada. The deposit is situated within moderately dipping Paleoproterozoic supracrustal rocks unconformably overlain by greater than 500 m of porous Mesoproterozoic to Paleoproterozoic Athabasca Group sandstones. An unconformity, which is also a major fluid conduit, separates the basement lithologies from the overlying sandstone. Rock quality surrounding the deposit is compromised because of the effects of the significant hydrothermal alteration that has occurred syn- and postmineralization. Post-Athabasca structures, together with the unconformity, provide direct conduits for the migration of large amounts of basinal fluids to the general deposit area. To mitigate the risk involved with mining in such a complex hydrogeological environment several geotechnical surveys were proposed as part of a pre-feasibility study for potential mine development. Of these, seismic methods were identified as the best tool to potentially map the location of the unconformity, identify structurally complex zones and alteration at both the mine and property scales. Subsequently, comprehensive surface and borehole seismic programs were completed in an attempt to increase confidence in shaft sinking, delineate engineering hazards related to mine development, and provide information on the structural setting of the geology hosting the deposit. The program consisted of a surface 3D survey and multiple borehole seismic surveys. This was the first time a seismic program of this scale was undertaken for geotechnical studies prior to mine development in the Athabasca Basin. All surveys successfully mapped the unconformity and post-Athabasca structures at various scales.

Seismic reconstruction of the in situ anisotropic stress field for caprock integrity in the Athabasca oil-sands basin, Canada

In the Athabasca Basin of Alberta, Canada, the Lower Cretaceous McMurray Formation reservoir is overlain by the Clearwater Formation, a regionally continuous layer composed predominantly of shales with interbedded mudstones. The shales form the reservoir caprock and have the role of blocking the vertical migration of the steam from thermal oil production by confining the stresses and the deformations. We have developed a new method for the 3D seismic reconstruction of the anisotropic stress field, which accounts for the formation pore pressure and the effective stress. We integrated anisotropy estimated from dipole sonic logs with formation pore pressure data from piezometers and elastic properties obtained from multicomponent seismic inversion. The method combines the Terzaghi effective stress with the Schoenberg and Sayers elastic stiffness matrix for horizontal transversely isotropic (HTI) fractured materials. The key points of this method are the estimation of the formation pore pressure in the abnormal regime of the Clearwater Formation, the normal fracture weakness parameter (based on constraints on the compressional velocity of the intact rock under the HTI assumption), and the 3D seismic anisotropic stress field. We expressed the total vertical stress as the weight of the overlying formations, and the total minimum and maximum horizontal stresses as a combination of the total vertical stress, normal fracture weakness, formation pore pressure, Biot-Willis coefficient, and Lamé elastic constants. The effective principal stresses are estimated from the equivalent total principal stresses and the formation pore pressure multiplied by the Biot-Willis coefficient. We observed excellent consistency between the calculated total minimum horizontal stress and mini-frac values. This new method for the 3D seismic reconstruction of the anisotropic stress field allows for the assessment of the caprock integrity and for operational savings based on a reduced number of mini-frac measurements, and it can be used for time-lapse stress estimation within the thermal production reservoir.

Post-crystallization alteration of natural uraninites: Implications for dating, tracing, and nuclear forensics

Abstract This study presents systematic chemical (U, Pb, Ca, Si, Fe) mapping coupled with in situ analyses of major, minor and trace elements, U/Pb, 207Pb/206Pb, and O isotopic compositions of natural uraninites (UO2) from two samples of the high-grade uranium ore from the Cigar Lake unconformity-related uranium deposit (Athabasca Basin, Saskatchewan, Canada). The studied uraninites are characterized by major chemical and isotopic heterogeneities expressed at small scale (µm to tens of µm), from almost pristine zones to strongly altered material. The 206Pb/238U and 207Pb/235U ratios of the different areas are widely spread and depict two similar and well-defined Discordia, providing an upper intercept age of crystallization at ca. 1300 Ma (1299 ± 4 and 1308 ± 14 Ma, respectively) and lower intercepts at 38 ± 13 and 72 ± 22 Ma, respectively. The freshest areas are characterized by sub-concordant 206Pb/238U and 207Pb/235U ratios, identical chemical compositions and similar very low δ18O values (−39.3 to −31.4‰). These data indicate that the two uraninites both crystallized at ca. 1300 Ma, from the same fluid and under identical physico-chemical conditions. Alteration is characterized by (i) the progressive incorporation of Ca, Si, and Fe, reaching several wt.%, which substitute to the radiogenic Pb and cause a progressive decrease in the Pb/U isotopic ratios. The radiogenic Pb is also substituted by water during the alteration, (ii) concomitant variations in trace element contents (As, Mn, V, LREEs, Sr, Th, B, Ba, Nb, for example) and (iii) heavier δ18O signatures (−22.5 to −8.91‰), typical of meteoric waters, in the altered zones. This combined approach demonstrates that fluid-driven post-crystallization exchanges affected each uraninite during recent fluid flow events (ca. 40 Ma and 70 Ma respectively). The relatively high dispersion of the Pb/U ratios in relation to the Discordia for both samples is considered as linked to a local (nm to µm-scale) differential mobility between lead and uranium within the uranium oxides. The chemical changes affecting elements previously considered as immobile in uraninite, such as REEs, indicate that these elements are not preserved during the post-crystallization alteration process studied here. Alteration processes may therefore have a major impact on the classical geochemical tracers, such as REE patterns or LREE/HREE ratios, currently used in nuclear forensic studies. The isotopic and chemical tracers currently used to track back the origin, age and history of natural uraninites should therefore be considered with a high degree of caution to avoid misleading and erroneous conclusions. Moreover, the comparison of calculated U-Th-Pb chemical, 207Pb/206Pb, and Pb/U isotopic ages shows that the use of age clustering for determining U-Th-Pb chemical ages and 206Pb/207Pb ages is not appropriate for constraining crystallization stage(s) of altered uraninites and for deciphering the different fluid events that potentially altered or recrystallized the uraninites over time. This study also indicates that estimation of the crystallization age of uraninite from substitution trends of Pb to Ca is not applicable to unconformity-related U deposits and results in overestimated ages because of an initial integration of calcium in the uraninite lattice at the time of crystallization.

Electron paramagnetic resonance study of quartz grains: Implications on uranium and thorium mineralization

Electron Paramagnetic Resonance (EPR) study of quartz grains of Athabasca basin of Wheeler river of Canada have been investigated at room temperature. EPR spectrum of WR 2522 quartz sample exhibits resonance signals at g˜ 4.24, 2.15760, 2.08503, 2.04243, 2.00933, 1.92389, and 1.87331. The resonance signal at g˜ 4.24 has been attributed to Fe3+ ions. The resonance signals at g˜ 2.15760, 2.08503, 2.04243, 2.00933, 1.92389, 1.87331have been attributed to radiation induced paramagnetic defects such as the centers C', 4,3, E1', E2' and E4'. EPR study of WR 2571 quartz sample exhibits resonance signals at g˜ 8.96, 4.25, 2.22, 2.05143, 2.03501, 2.01817, and 2.01884. The resonance signals at g˜ 8.96, 4.25, and 2.22 have been attributed to Fe3+ ions. The remaining resonance signals have been attributed to defect centers such as C', 4, 3, E1', respectively. The solid state EPR parameters have been evaluated for the two samples and implications of uranium and thorium mineralization have been explained.

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.

Geochemical footprint of the Millennium unconformity-type uranium deposit, Canada: implications for vectoring new targets

The Millennium deposit, a c. 650-m deep monomineralic uranium deposit, is located in the southeastern Athabasca Basin, Saskatchewan, a region containing numerous high-grade unconformity-type U deposits. 3D modelling of the whole-rock lithogeochemistry of sandstones above the deposit reveals a distinct footprint with select major and trace elements showing increased concentrations towards mineralization. Molar Mg/K ratios increase from background levels 10 km north of the deposit, along the B1 conductive trend, whereas Mo, Co, Rb, and Ga exhibit elevated concentrations immediately above the deposit, extending vertically from the unconformity to surface. Lead, Ag, Bi, Sb, REE, and Y exhibit elevated concentrations up to 650 m above the main mineralized body. Increasing Mg/K values indicate the transition from diagenetic to hydrothermal alteration with shifts from illitic (K-dominant) to chloritic and dravitic (Mg-dominant) alteration, with the latter increasing more proximal to mineralization. Trace element enrichment patterns highlight that fractures and faults were conduits for fluid flow from the basement into the basin, both during ore formation and through tectonically driven post-depositional remobilization. Key indicators such as molar element ratios (Mg-K-Al) and trace elements related to redox reactions provide scalable vectors at the Millennium deposit that are likely applicable to similar unconformity-type U deposits elsewhere.

Muon Tomography Applied to a Dense Uranium Deposit at the McArthur River Mine

AbstractThis article reports on the first application of muon tomography (a novel method for imaging density contrasts underground using cosmic rays) for imaging dense uranium deposits within the Athabasca Basin in Canada. We present a qualitative assessment of the radiographic imaging and 3‐D density inversion of a uranium deposit using muon tomography. The muon tomography survey summarized in this article was performed at the McArthur River mine. We demonstrate the validity of muon tomographic imaging with data acquired at a depth of about 600 m underground. The statistical significance of the uranium deposit signature in the muon data was larger than 5 standard deviations, and the corresponding 3‐D density inversion compared well with drill assay data from the deposit.

Fluid P-T-X characteristics and evidence for boiling in the formation of the Phoenix uranium deposit (Athabasca Basin, Canada): Implications for unconformity-related uranium mineralization mechanisms

The Phoenix uranium deposit in the southeastern Athabasca Basin (Canada) is a typical unconformity contact-hosted uranium deposit, characterized by an association with reactivated basement faults, graphite-rich rocks in the underlying basement, and a pervasive clay alteration halo surrounding the mineralized zones. Petrographic results suggest that the mineralizing hydrothermal system was characterized by alternating desilicification and silicification events. Uraninite and clay-size minerals (tourmaline, kaolinite, illite and minor chlorite) mainly precipitated in the desilicification periods whereas hydrothermal quartz (mainly drusy quartz) formed during the silicification periods. Primary fluid inclusions in the hydrothermal quartz are inferred to represent the ore-forming fluids even though quartz did not co-precipitate with uraninite. The coexistence of multiple types of fluid inclusions (liquid-dominated biphase, vapor-dominated biphase, vapor-only and halite-bearing triphase) within individual fluid inclusion assemblages is interpreted to indicate fluid boiling and heterogeneous trapping. Bulk fluid inclusion volatile analysis by mass spectrometry indicates H2O as the dominant species, with less than 1 mol% non-aqueous volatiles that show a compositional trend typical of fluid boiling. Microthermometric and cryogenic Raman spectroscopic analyses indicate that the mineralizing fluids are of H2O-NaCl-CaCl2 ± MgCl2 composition, with salinities ranging mainly from 23.4 to 31.1 wt%. The liquid-dominated biphase inclusions (excluding those interpreted to have resulted from heterogeneous trapping) have homogenization temperatures from 90 to 157 °C. These data are generally consistent with the classical diagenetic-hydrothermal model in which basinal brines that extracted uranium from the basin or the basement were channeled along reactivated basement faults, and precipitated uraninite near the unconformity through reaction with reducing agents. However, the relatively low fluid inclusion homogenization temperatures documented in this paper, together with fluid boiling inferred from fluid inclusion assemblages, suggest that the deposit may have formed in a shallower environment (∼2.5 km) than assumed in the conventional diagenetic-hydrothermal model (>5 km). Abrupt fluid pressure drops during episodic faulting may have resulted in fluid boiling (flash vaporization) changing the fluid pH and promoting uraninite precipitation, with the alternating liquid-vapor conjugates of the boiling system resulting in alternating precipitation of quartz and uraninite respectively.

Numerical simulation of strain localization and its relationship to formation of the Sue unconformity-related uranium deposits, eastern Athabasca Basin, Canada

Unconformity-related uranium deposits in the Athabasca Basin (Canada) are spatially associated with reactivated basement faults that cut the unconformity surface between Archean to Paleoproterozoic basement and Paleoproterozoic to Mesoproterozoic sedimentary rocks of the Athabasca Group. The Sue deposits (Sue A, B, C, D, and E) are located in the eastern Athabasca Basin along a 2-km-long north-northeast-trending structural corridor and have both sandstone-hosted (Sue A and Sue B; to the north) and basement-hosted (Sue C, Sue D, and Sue E; to the south) orebodies. All the deposits are structurally controlled by north-northeast-trending basement faults that demonstrate different degrees of reverse displacements of the unconformity surface. However, it is not clear why the mineralization is distributed in such a pattern along the corridor. In this study, both 2-dimensional (2D) and 3-dimensional (3D) numerical modeling of deformation were conducted to examine the relationship between the distribution of strain and uranium mineralization. The modeling shows that dilation (positive volumetric strain), thought to correlate with the development of extensional fracture systems and dilational jogs that represent potential mineralization sites, is primarily controlled by the rheological contrasts in basement rocks and the degree of deformation. The presence of faulted graphitic pelitic gneiss associated with the sandstone-hosted Sue A and Sue B deposits favours dilation localized within the sandstone at low degrees of deformation mainly due to the overlying competent hangingwall granitic gneiss. In contrast, the basement-hosted Sue C, Sue D and Sue E deposits are associated with dilation zones localized along the contact between the faulted graphitic pelitic gneiss and competent footwall silicified gneiss at higher degrees of deformation. The modeling results highlight the importance of both graphitic basement structures and strong rheological contrasts between basement rocks in uranium exploration in the Athabasca Basin.

Effects of uranium mining and milling on benthic invertebrate communities in the Athabasca Basin of Northern Saskatchewan

Aquatic environments downstream of uranium operations (mining/milling) in Northern Saskatchewan are exposed to a variety of chemical and physical disturbances. There is extensive regulatory documentation of monitoring and effects assessments for modern uranium operations, though little is available in the scientific literature. The reference condition approach was used to predict expected benthic invertebrate community metrics for unexposed and exposed lakes. This approach was used to identify impacted communities that differed significantly from expected natural regional compositions. The number of taxa at the lowest practical level (taxon richness) downstream of approximately half of the uranium operations’ effluent release points was lower compared to the number of taxa observed in reference conditions. The taxon richness residuals were strongly correlated with about half of the measured concentrations of sediment contaminants and always negatively, such that richness was lower in lakes with higher concentrations of metals or radionuclide activities in the sediments. This was consistent for both reference and exposure lakes, implying a natural background influence of some metals and radionuclides on taxon richness. This exercise produced models that can be used by practitioners in the north of Saskatchewan in the design and delivery of monitoring programs not only for uranium mines, but for regional cumulative effects studies in support of sustainable resource planning and management.

Formation of U-rich mineralizing fluids through basinal brine migration within basement-hosted shear zones: A large-scale study of the fluid chemistry around the unconformity-related Cigar Lake U deposit (Saskatchewan, Canada)

The evolution of basinal brines through percolation into basement rocks is studied in the Paleo- to Mesoproterozoic Athabasca Basin and its underlying basement. This specific geological context includes the evolution of a NaCl-rich basinal brine to a U-mineralizing CaCl2-rich brine that is of particular importance for the understanding of the formation of giant U deposits. The objective of this study is to determine the physico-chemical parameters (P-T-X) of the basinal brines and their changes both within the Athabasca Group sandstones and the underlying basement lithologies, on the scale of several km around the world class unconformity-related Cigar Lake U deposit.Particularly, this study focuses on the basement-hosted fault systems that are considered to be the main fluid flow pathways. A comprehensive study including microthermometry, Raman spectroscopy, and LA-ICP-MS analyses was carried out on fluid inclusions, as was a semi-quantitative estimation of the microfracture paleoporosity based on the surface density of fluid inclusion planes.Initial NaCl-rich brines were trapped during early diagenesis (c.a. 1.7–1.65 Ga) in dickite-rich sandstones, under a three to seven kilometers thick sedimentary cover at around 90 to 150 °C under hydrostatic pressure, or at 120–210 °C assuming lithostatic conditions. At the time of U deposits formation (ca. 1.46–1.29 Ga), these brines percolated through the entire basement, but preferentially within the damage zones of some graphite-rich faults, formed during the ca. 1.8 Ga Trans-Hudson Orogeny and active during the subsequent retrograde metamorphism and exhumation (ca. 1.8–1.72 Ga). The circulation of the basinal brines into these structures was enhanced and structurally-controlled by the microfracture network inherited from earlier brittle reactivation during basement uplift. The resulting high microfracture paleoporosity in the vicinity of some, but not all, of these structures favored intense interaction between the brines and basement rocks that induced the formation of illite-sudoite alteration haloes. In the basement, the percolating brines scavenged Mn, K, Ba, Sr, and especially U, and evolved from the original NaCl-dominant composition toward a higher salinity CaCl2-rich composition. The resulting U-bearing brine mixed with the unmodified diagenetic NaCl-rich brine in, above, and all along the reactivated faults, either during thermal convection in isobaric settings and higher temperatures (110–200 °C) or through episodic tectonic reactivation in isothermal settings through episodic pressure fluctuation. In direct vicinity of the U deposits, the interactions between the brines and the basement rocks were the strongest, which is marked by the highest intensity of alteration of the host rocks.Thus, the formation of ore-forming fluids resulted from the interaction between NaCl-rich basinal brines and highly-microfractured basement rocks. This study reinforces the hypothesis that interaction with basement rocks enhanced by strong fluid flow in cataclastic zones, can significantly modify the physico-chemical characteristics of basin-derived brines, leading to the formation of mineralizing fluids and the genesis of giant ore deposits, such as the unconformity-related U deposits.

Hydrothermal Rare Earth Element (Xenotime) Mineralization at Maw Zone, Athabasca Basin, Canada, and Its Relationship to Unconformity-Related Uranium Deposits—A Reply

Hydrothermal Rare Earth Element (Xenotime) Mineralization at Maw Zone, Athabasca Basin, Canada, and Its Relationship to Unconformity-Related Uranium Deposits—A Reply

Hydrothermal Rare Earth Element (Xenotime) Mineralization at Maw Zone, Athabasca Basin, Canada, and Its Relationship to Unconformity-Related Uranium Deposits—A Discussion

Hydrothermal Rare Earth Element (Xenotime) Mineralization at Maw Zone, Athabasca Basin, Canada, and Its Relationship to Unconformity-Related Uranium Deposits—A Discussion

The Potential for Surface Mining a Heavy-Oil Reservoir: The Example of the Ratqa Lower Fars in the State of Kuwait

SummarySurface mining of hydrocarbon deposits is not a new technique, but its application has been mainly limited to the excavation of oil sands in the Athabasca Basin of Alberta, Canada, where the mining method has proved to be commercially successful, although in a narrow set of geological and environmental conditions. This paper discusses the scope for a broader application of a surface-mining approach and builds on the results of a conceptual study that examined the possibility of surface mining the viscous crude oil of the Ratqa Lower Fars (RQLF) reservoir in northern Kuwait. The study findings indicate that a large rate of crude oil might be profitably and sustainably produced for many decades through a surface-mining approach.

Mesoproterozoic rift sedimentation, fluid events and uranium prospectivity in the Cariewerloo Basin, Gawler Craton, South Australia

ABSTRACTThe Cariewerloo Basin formed in the Mesoproterozoic following assembly of the Gawler Craton, South Australia, and was filled by arenaceous redbeds of the Pandurra Formation. While previous regional-scale work reveals a basin with similar size and sedimentary fill to the Proterozoic Athabasca and Kombolgie basins that host unconformity-related uranium deposits, few details of the Cariewerloo Basin are known. In this study, stratigraphy, petrography, lithogeochemistry, stable isotope geochemistry and 40Ar/39Ar geochronology are integrated to clarify the depositional history of the Pandurra Formation, and to assess fluid events in the basin that could be linked to the formation of uranium deposits. In the study area, the Pandurra Formation was deposited in two eastward-thickening packages that terminate at faulted basement uplifts, interpreted as half-grabens that formed in a continental rift system as the eastern Gawler Craton underwent extension. Deposition occurred between 1575 Ma (latest Hiltaba Suite age) and ca 1490 Ma, the 40Ar/39Ar age of diagenetic illite in the basal Pandurra. Diagenesis involving fluids having δ18O and δ2H values between –2.1 and 3.6‰, and between –66 and –8‰, respectively, occurred at around 150°C. Protracted diagenesis preferentially occurred in the upper Pandurra Formation based on petrography and Pearce Element Ratios that show complete replacement of detrital lithic and feldspathic grains by diagenetic phyllosilicates, and younger 40Ar/39Ar ages between ca 1330 and 1200 Ma that record fluid events later into basin history. Conversely, the basal Pandurra Formation shows better preservation of detrital grains, and older 40Ar/39Ar ages around 1450 Ma that suggest these strata became closed to fluid flow earlier in basin history. Although, based on O-isotope ratios, fluid–rock interaction did not occur in the Cariewerloo Basin to the same extent as that in the Athabasca or Kombolgie basins, it is possible that a uranium deposit formed where the upper Pandurra Formation was in contact with metasedimentary basement units outside the present basin margins.