Athabasca Group Research Articles

Mineralogy and petrogenesis of fracture coatings in Athabasca Group sandstones from the McArthur River uranium deposit

ABSTRACT The McArthur River unconformity-related uranium deposit, located in the Athabasca Basin of Saskatchewan, Canada, is structurally hosted near the unconformity between Archean to Paleoproterozoic metasedimentary basement and the Proterozoic Athabasca Group sandstones. In this study, the mineralogy and geochemistry of fracture materials within the entire ca. 550 m thickness of the Athabasca Group sandstones and the metasedimentary (host) rocks from the McArthur River area were used to determine the paragenetic sequence and origin of minerals in and near the fractures. Our work sought to determine if the host minerals record elements associated with the uranium deposit at depth and if they could be used to guide exploration (vectoring). Fracture orientations indicate that most are moderately dipping (<50°) and provided permeable pathways for fluid movement within the basin, from below, and through the overlying sedimentary rocks. Many of the fractures and adjacent wall rocks record evidence of multiple distinct fluid events. Seven types of fracture fillings were identified from drill core intersecting the Athabasca Basin and present distinct colors, mineralogy, and chemical features. Brown (Type 1) and pink (Type 7) fractures host paragenetically late botryoidal goethite, Mn oxide minerals, and poorly crystallized kaolinite that formed from relatively recent low-temperature meteoric fluids, as indicated by poor crystallinity and low δ2H values of –198 to –115‰. These minerals variably replaced higher temperature minerals that are rarely preserved on the fractures or in wall rock near the fractures. Hydrothermal alteration associated with the mineralizing system at ca. 200 °C is recorded in assemblages of dickite, well-crystallized kaolinite, and spherulitic dravite in some white and yellow (Type 2) and white (Type 3) fractures, as reflected by the crystal habits and variable δ2H values of –85 to –44‰. Fibrous goethite in white and yellow (Type 2) and black and orange (Type 5) fractures and microfibrous Mn oxy-hydroxide minerals in black (Type 4) fractures also crystallized from hydrothermal fluids, but at temperatures less than 200 °C. White and yellow fractures (Type 2) containing fibrous goethite reflect fracture networks indicative of hydrothermal fluids associated with the mineralizing system during primary dispersion of pathfinder elements and therefore extend the deposit footprint. Brown (Type 1) and pink (Type 7) fractures have low δ2H values in botryoidal goethite and poorly crystallized kaolinite and are indicative of the movement of meteoric waters. Secondary dispersion of elements from the deposit to the surface on some fractures is evidence that fractures are pathways for element migration from the deposit to the surface, over distances exceeding ∼500 m.

Geochemistry of fracture coatings in Athabasca Group sandstones as records of elemental dispersion from the McArthur River Uranium deposit

A mineralogical and geochemical study of minerals on seven types of fracture fillings from near and above the McArthur River unconformity-related uranium deposit has been carried out to determine if fracture filling materials and wall rocks record evidence of primary and secondary dispersion of elements from the deposit. This study shows that Pb extracted by weak acid leaching (2% HNO3) provides Pb-Pb model ages near 1100 Ma for fractures with any of the three REE patterns: 1) LREE-enriched patterns, 2) REE patterns that are concave and similar to uraninite in the deposit, and 3) fractures with flat REE patterns. The REE patterns and ages indicate the fractures are an open system to post-mineralization dispersion of radiogenic Pb from a U-rich source, with a major mobilization of Pb from the uranium deposit at ca. 1100 Ma.Brown (Type 1) fractures, which host Mn-Fe oxides and clay minerals, provide the best evidence of U mineralization through post-mineralization dispersion, containing elevated concentrations of pathfinder elements Co, Ba, Tl, Mn and radiogenic Pb near the surface and at depth. Post-mineralization dispersion is also observed within brown (Type 1) and pink (Type 7; clay minerals + hematite) fractures and wall rock, showing low 207Pb/206Pb values of 0.29–0.50 that were subsequently overprinted by fluids with more common Pb, as recorded in fractures with higher 207Pb/206Pb values of 0.51–0.84.Fracture fillings were analyzed by continuous leach inductively coupled plasma mass spectrometry (CL-ICP-MS) to determine the relationship between trace elements and elemental release from specific mineral phases. CL-ICP-MS results indicate that common Pb (with 207Pb/206Pb values > 0.7) and anomalous Co (360 ppm) and V (220 ppm) contents are resident in organic phases that are dissolved during the 10% H2O2 leach phase. CL-ICP-MS also indicates that radiogenic Pb, as well as Ni and V, are resident in clay minerals and Fe-Mn oxy-hydroxides that break down during leaching with 30% HNO3. Uranium contents are low in the leachate during release of Fe, Al, Mn, and Pb using the 10% and 30% HNO3 leaches, reflecting post-mineralization dispersion of radiogenic Pb, but not U. Thus, secondary dispersion of elements into fractures enhances the size of the deposit footprint, particularly in deep fractures near the P2 fault, but also in some fractures near the surface. The results of this study strongly suggest that fractures did indeed act as conduits for dispersion of elements from the deposit, reaching all the way from deposit to the surface, providing viable exploration targets for U mineralization in sedimentary basins.

Geochemistry of the Athabasca Basin, Saskatchewan, Canada, and the unconformity-related uranium deposits hosted by it

ABSTRACTA large data set comprising near-total digestion analyses of whole rock samples from the Athabasca Basin, Saskatchewan, Canada (based principally on the Geological Survey of Canada open file 7495), containing more than 20,000 analyses, was used to define the average chemical composition of Athabasca Group sandstones and of unconformity-related uranium deposits hosted by the basin.The chemical composition of unaltered and un-mineralized Athabasca Group sandstones is dominated by Al (median Al2O3 of 1.14 wt.%), Fe (median Fe2O3 of 0.24 wt.%), and K (median K2O of 0.11 wt.%; Si was not measured), corresponding mostly to the presence of kaolin, illite, and hematite, in addition to the most-abundant quartz. The median concentration of U in the barren sandstones is 1 ppm, with 5 ppm Th, 3 ppm Pb, and 56 ppm ΣREE. Other trace elements present in significant amounts are Zr (median of 100 ppm), Sr (median of 69 ppm), and B (median of 43 ppm), corresponding to the presence of zircon, illite, and dravite.The elements most enriched in a typical Athabasca Basin unconformity-related uranium deposit relative to the barren sandstone are U (median enrichment of ×710), Bi (×175), V (×77), and Mg (×45), followed by five elements with enrichment factors between 20 and 30 (Co, Mo, K, As, and Ni). These correspond to the presence in the ore bodies of alteration minerals (dravite, kaolinite, illite, chlorite, aluminum-phosphate-sulfate minerals, and a suite of sulfide minerals) and are similar to what has been observed before. These elements are similar to the typical pathfinder elements described above known deposits, but their usefulness has to be assessed based on their relative mobility in the predominantly oxidizing Athabasca Basin sandstones.

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.

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.

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.

Radioactivity recorded by clay minerals in the Shea Creek area, Athabasca Basin (Canada): Implications for uranium transfers and exploration

The understanding of uranium mobility in the geosphere is a prerequisite for the modelling of high-level nuclear waste repositories and economic uranium deposit genesis. To complement more classical geochemical and mineralogical approaches, this understanding can be improved by measuring the record of past cumulative radioactivity as stable radiation-induced defects in clay mineral structure. This study focuses on world-class unconformity-related uranium deposits of the Athabasca Basin (Canada) for which the source, timing, and paths of the uranium-bearing fluids are still matters of debate. A set of 46 samples collected from the Athabasca Group sandstones in the Shea Creek area of the western Athabasca Basin, up to 634 m above either the unconformity (barren drill hole) or uranium mineralization, was selected in order to locate the paleo-occurrences of radioelements. A relevant three-dimensional view is shown by plotting (i) the concentrations of radiation-induced defects (RID's) in clay minerals, (ii) the present dose rate, and (iii) the distance to the mineralization or unconformity. The results clearly reveal different cases, such as geochemical background, equilibrated dose rate, late accumulations of radioelements, and/or records of their past temporary occurrence. Noticeable paleo-occurrences, now leached away, are revealed within 100 m of the structures hosting present-day mineralized bodies, which is in line with a recent model of long range lateral paleofluid flow in a basinal permeable formation, and may be useful for exploration, albeit within a proximal range. Such results rely on the detection of RID's in clay minerals, as chemical analysis or gamma counting alone detect only the present concentration of radioelements and are unable to distinguish between accumulations, equilibration of transfers, or temporary occurrences of uranium. This study represents a first step toward spatial 3D quantitative reconstruction of U transfers, which will require time constraints and artificial dosimetry to improve models of genesis of high-grade unconformity-related deposits and to identify paleo-pathways of U migration.

Multielement statistical evidence for uraniferous hydrothermal activity in sandstones overlying the Phoenix uranium deposit, Athabasca Basin, Canada

The Phoenix U deposit, with indicated resources of 70.2 M lb U3O8, occurs along the unconformity between the Proterozoic Athabasca Group sandstones and the crystalline basement rocks. Principal component analysis (PCA) is applied to the compositions of sandstones overlying the deposit. Among PCs, PC1 accounts for the largest variability of U and shows a positive association of U with rare earth elements (REEs) + Y + Cu + B + Na + Mg + Ni + Be. The evidence suggests that U was dispersed into sandstones together with these elements during the uraniferous hydrothermal activity. Uranium shows an inverse association with Zr, Hf, Th, Fe, and Ti. Since they are common in detrital heavy minerals, such heavy minerals are not the major host of U. The elements positively associated with U are high in concentrations above the deposit, forming a “chimney-like” or “hump-like” distribution in a vertical section. Their enrichment patterns are explained by the ascent of basement fluids through faults to sandstones and the circulation of basinal fluids around the deposit. The Pb isotope compositions of whole rocks are similar to expected values calculated from the concentrations of U, Th, and Pb except for sandstones close to the deposit. The data suggest that in situ decay of U and Th is responsible for the Pb isotope compositions of most sandstones and that highly radiogenic Pb dispersed from the deposit to the proximal sandstones long after the mineralization. This secondary dispersion is captured in PC8, which has low eigenvalue. The data suggests that the secondary dispersion has minor effect on the overall lithogeochemistry of sandstones.

Graphite-Bearing Pelitic Schists and Their Altered Equivalents In the Dufferin Lake Zone, South-Central Athabasca Basin, Saskatchewan: Constraints On Graphite Formation and Destruction, and Implications For Uranium Mineralization

Abstract Unconformity-type uranium deposits from the Athabasca Basin area are considered to be the result of mixing between oxidized basinal brines and basement-derived reduced fluids/gases, and/or reduced basement rocks. Graphite and its breakdown products are suggested to be responsible for uranium mineralization by acting as a reductant that could trigger deposition of uranium. In addition, the presence of well-connected graphite and sulfides within graphitic pelitic rocks is considered to be potentially indicative of basement structures, as the minerals are often concentrated along the structures, which hence become electromagnetic (EM) conductors. Thus, exploration for uranium deposits is often focused on the search for EM conductors. Variably graphitic pelitic schists with a steeply dipping fabric underlie the sedimentary rocks of the basin in the Dufferin Lake zone, south-central Athabasca Basin (Saskatchewan, Canada). Up dip, and just below the unconformity with the Athabasca Group, the pelitic schists are replaced by texturally similar rocks within “graphite-depleted zones”. These zones consist of chlorite and hematite alteration (Red/Green Zone; RGZ) and a bleached zone immediately adjacent to the unconformity. Both zones are characterized by a lower concentration of carbon and sulfur, with the bleached zone showing higher concentrations of uranium and boron, the latter corresponding to a high dravite content. The major elements composition of the graphite-bearing pelitic schists and altered equivalents (RGZ) are similar. Raman analyses indicate that well-ordered carbon species (graphite to semi-graphite) are present in the pelitic schists, with both types more common within shear zones. In contrast, only rare low-ordered carbon species (carbonaceous matter) were detected in the graphite-depleted samples within the RGZ. This variation is interpreted to be the result of graphite consumption by oxidizing fluids migrating downward from the Athabasca Group. This graphite consumption may have resulted in the production of a mobile reductant (gas or fluid), which may have played a subsequent role in the deposition of uranium mineralization.

Linked thermal convection of the basement and basinal fluids in formation of the unconformity‐related uranium deposits in the Athabasca Basin, Saskatchewan, Canada

AbstractWorld‐class unconformity‐related U deposits in the Athabasca Basin (Saskatchewan, Canada) are generally located within or near fault zones that intersect the unconformity between the Athabasca Group sedimentary basin rocks and underlying metamorphic basement rocks. Two distinct subtypes of unconformity‐related uranium deposits have been identified: those hosted primarily in the Athabasca Group sandstones (sediment‐hosted) and those hosted primarily in the underlying basement rocks (basement‐hosted). Although significant research on these deposits has been carried out, certain aspects of their formation are still under discussion, one of the main issues being the fluid flow mechanisms responsible for uranium mineralization. The intriguing feature of this problem is that sediment‐hosted and basement‐hosted deposits are characterized by oppositely directed vectors of fluid flow via associated fault zones. Sediment‐hosted deposits formed via upward flow of basement fluids, basement‐hosted deposits via downward flow of basinal fluids. We have hypothesized that such flow patterns are indicative of the fluid flow self‐organization in fault‐bounded thermal convection (Transport in Porous Media, 110, 2015, 25). To explore this hypothesis, we constructed a simplified hydrogeologic model with fault‐bounded thermal convection of fluids in the faulted basement linked with fluid circulation in the overlying fault‐free sandstone horizon. Based on this model, a series of numerical experiments was carried out to simulate the hypothesized fluid flow patterns. The results obtained are in reasonable agreement with the concept of fault‐bounded convection cells as an explanation of focused upflow and downflow across the basement/sandstone unconformity. We then discuss application of the model to another debated problem, the uranium source for the ore‐forming basinal brines.

Composition of Large Zoned Aluminum Phosphate Sulfate Minerals: Implications For Fluid Evolution In the Centennial Uranium Deposit Area, Athabasca Basin, Saskatchewan, Canada

Abstract Aluminum phosphate sulfate (APS) minerals associated with the Athabasca Basin are compositionally part of the alunite group. Typically 5–20 μm in size, they accompany the regional diagenetic-hydrothermal illite-kaolinite-dickite assemblage in the Athabasca Group sandstones, the illite-sudoite assemblage of the altered paleo-regolith, and alteration zones surrounding uranium deposits. In the vicinity of the Centennial deposit, where illite and sudoite replace coarse-grained aluminosilicate porphyroblasts in the basement phyllitic pelites, cubic APS crystals are as large as 80 μm. Detailed petrography indicates that the APS crystals form broadly coeval to illite. Backscattered electron imaging, elemental mapping, and compositional analysis reveal complex zoning as well as later hydrothermal alteration of the APS crystals. Growth-zoned crystals are a solid solution between crandallite-goyazite-svanbergite [Sr 0.17–0.32 Ca 0.0.18–0.28 LREE 0.0.25–0.46 (Al 2.78–2.92 Fe 0.01–014 )(PO 4 ) 1.82–2.03 (SO 4 ) 0.14–0.35 (OH) 6 ], whereas the later alteration of the APS minerals results in a compositional shift to endmember florencite [Sr 0.15 Ca 0.11 LREE 0.57 (Al 2.86 Fe 0.02 )(PO 4 ) 1.98 (SO 4 ) 0.15 (OH) 6 ]. Fluids responsible for the alteration of APS to florencite are paragenetically linked to late hydrothermal fluids associated with mafic Mackenzie dikes and do not appear to be related to proximity to uranium mineralization. Both zoned and altered portions of the crystals have bulk compositions that overlap with APS minerals in other areas of the basin suggesting a common genetic origin. However, it is critical to link the paragenetic context to observed compositional changes. An increase in MREE from early to late stages of zoned crystal growth correlates with the greatest concentration of REE found in uraninite from unconformity-related uranium deposits. This could be a link between broader APS growth and uraninite precipitation.

Fluid compositions and P-T conditions of vein-type uranium mineralization in the Beaverlodge uranium district, northern Saskatchewan, Canada

The Beaverlodge district in northern Saskatchewan is known for “vein-type” uranium mineralization. Most of the uranium deposits are spatially related to major structures, and hosted by ca. 3.2–1.9Ga granitic rocks (and albitite derived from them) and by ca. 2.33Ga Murmac Bay Group amphibolite, all of which are unconformably overlain locally by deformed but unmetamorphosed redbeds of the ca. 1.82Ga Martin Group, and by the flat-lying ca. 1.75–1.5Ga Athabasca Group. The uranium mineralization is mainly hosted in fault rocks (breccias) and carbonate±quartz±albite veins, referred to as breccia-style and vein-style mineralization, respectively, with the latter being the focus of this study. Most of the mineralized veins occur in the basement rocks, although some crosscut the Martin Group. This study examines the field, petrographic, fluid inclusion and C-O isotope characteristics of mineralized and non-mineralized veins from 19 deposits/occurrences as well as from the Martin Group, with an aim to better understand the mineralizing environment and processes.The coexistence of liquid-dominated (L+V), vapour-dominated (V+L) and vapour-only (V) fluid inclusions within individual fluid inclusion assemblages (FIAs) in the veins suggests fluid immiscibility and heterogeneous trapping. The L+V inclusions with the lowest homogenization temperatures (Th) within individual FIAs are interpreted to represent homogeneous trapping of the liquid phase, which yield Th values from 78° to 330°C (mainly 100° to 250°C), and salinities from 0.2 to 30.8wt.% NaCl equivalent. Mass spectrometric analysis of bulk fluid inclusions shows that the volatiles are dominated by H2O (average 97.2mol%), with minor amounts of CO2, CH4, H2, O2, N2, Ar and He. Fluid pressures were estimated to be <200bars based on the inference of fluid immiscibility, fluid temperatures of 100° to 250°C, and low concentrations of non-aqueous volatiles (<3mol%). The δ18OVPDB and δ13CVPDB of carbonate minerals associated with mineralization range from −20.5 to −8.9‰ and −10.1 to −0.9‰, respectively. The δ18OVSMOW values of the parent fluids calculated using the Th values range from −9.6 to +17.0‰, with the majority from 0 to +5.0‰. O isotopes of paired equilibrium quartz and calcite, analyzed by secondary ion mass spectrometry (SIMS), yield temperatures from 161° to 248°C, which are consistent with the fluid inclusion data.The new fluid inclusion and stable isotope data are inconsistent with a metamorphic or magmatic-hydrothermal model as proposed in some previous studies (for breccia-style and vein-style mineralization), but rather support a model in which the vein-type uranium mineralization took place at relatively low temperature (100° to 250°C) and shallow (<2km) conditions, with fluid pressure fluctuating between hydrostatic and sub-hydrostatic regimes, possibly related to episodic faulting. The mineralizing fluids were mainly sourced from the Martin Lake Basin, and uraninite was precipitated as a result of mixing between this basin-derived fluid and fluids carrying reducing agents (Fe2+, CH4) derived from the basement, although fluid-rock reactions and fluid immiscibility may have also played a role.

Fluids preserved in variably altered graphitic pelitic schists in the Dufferin Lake Zone, south-central Athabasca Basin, Canada: implications for graphite loss and uranium deposition

The Athabasca Basin (Canada) contains the highest grade unconformity-type uranium deposits in the world. Underlying the Athabasca Group sedimentary rocks of the Dufferin Lake Zone are variably graphitic, pelitic schists (VGPS), altered to chlorite and hematite (Red/Green Zone: RGZ). They were locally bleached near the unconformity during paleoweathering and/or later fluid interaction. Overall, graphite was lost from the RGZ and the bleached zone relative to the original VGPS. Fluid inclusions were examined in different generations of quartz veins, using microthermometry and Raman spectroscopy, to characterize and compare the different fluids that interacted with the RGZ and the VGPS. In the VGPS, CH4-, and N2-rich fluid inclusions, which homogenize into the vapor phase between −100 and −74 °C, and −152 and −125 °C, respectively, and CO2-rich fluid inclusions, homogenizing either into vapor or liquid between 20 and 28 °C, are present. Carbonic fluids could be the result of the breakdown of graphite to CH4 + CO2, whereas N2-rich fluid is interpreted to be the result of breakdown of feldspars/micas to NH4 ++N2. In the RGZ, the presence of fluid inclusions with low ice melting temperature (−38 to −16 °C) reflect the presence of CaCl2, and fluid inclusions with halite daughter minerals that dissolve between 190 and 240 °C indicate the presence of highly saline fluids. These fluids are interpreted to be derived from the Athabasca Basin. The circulation of carbonic fluids and brines occurred during two different events related to different P-T conditions of trapping. The carbonic fluids interacted with basement rocks during retrograde metamorphism of the basement rocks before deposition of the Athabasca Basin, whereas the brines circulated after the deposition of the Athabasca Basin. These latter fluids are similar to brines related to uranium mineralization at McArthur River and thus, in addition to possibly being related to graphite depletion in the RGZ, they could be linked to uranium mineralization.

Regional Setting, Geology, and Paragenesis of the Centennial Unconformity-Related Uranium Deposit, Athabasca Basin, Saskatchewan, Canada

The Centennial unconformity-related uranium deposit represents the first significant uranium mineralization along the Snowbird tectonic zone in the south-central part of the Athabasca Basin. The deposit is associated with a steeply WNW-dipping contact between the Virgin Schist Group and mylonitic granite of uncertain age. Virgin Schist Group rocks in the deposit area include phyllites, impure quartzite, and quartzite. Medium- to coarse-grained microcline “augen” porphyroclasts characterize the granite. The quartzite, being resistant to weathering, forms a paleotopographic high flanked by a paleotalus which formed prior to Athabasca Group sedimentation. Diabase, petrographically and geochemically similar to the 1.27 Ga Mackenzie dikes, intrudes the mineralized trend along brittle structures in the basement rocks and Athabasca Group. A detailed paragenetic study of the deposit area reveals a protracted history that is related to the episodic reactivation of brittle structures and associated fluid movement along this significant structural corridor. Retrograde metamorphism converted biotite to chamosite at temperatures between 335° to 350°C. Weathering caused the breakdown of microcline and the dissolution of quartz prior to erosion and deposition of the Athabasca Group, which started at about 1750 Ma. Compaction and diagenesis resulted in quartz overgrowths and development of a diagenetic clay assemblage of illite and sudoite at temperatures between 150° and 200°C, which was broadly synchronous with primary mineralization. Clinochlore, euhedral quartz, carbonate, and pyrite was developed after the intrusion of the diabase under reducing conditions, primarily at temperatures between 270° to 320°C. Uranium remobilization and alteration of uraninite to coffinite appears to have taken place after the emplacement of the diabase dikes. Kaolinite and uranophane form last in the paragenetic sequence and probably form at relatively low temperatures.

An interpretation of surface and borehole seismic surveys for mine planning at the Millennium uranium deposit, northern Saskatchewan, Canada

The Millennium uranium deposit is located within the Athabasca Basin, in northern Saskatchewan, Canada. The deposit is hosted within moderately dipping Paleoproterozoic gneisses that are unconformably overlain by more than 500 m of flat lying, porous Paleoproterozoic to late Mesoproterozoic Athabasca Group sandstones. The deposit is associated with the sandstone-basement unconformity, post-Athabasca structure, and hydrothermal alteration. These features combine to create a complex 3D hydrogeologic setting that presents challenges with respect to mine development, production, and safety. In 2007, as part of a prefeasibility study for potential mine development, a seismic program consisting of a 3D surface survey, vertical seismic profiling, moving source profiling, and side-scan surveys was undertaken to map the complex geology. The geometry and resolution of these different seismic surveys allowed for direct imaging of the geologic targets of interest, regardless of orientation and size. After integration with drill-defined geology, the program successfully imaged the location and character of the unconformity, the post-Athabasca structural setting at camp and deposit scales, and the alteration around the deposit. This information increased the understanding of geotechnical aspects of the geology hosting the deposit, and is currently being used to help minimize risk and costs associated with mine development. Seismic surveys are now viewed as an integral part of risk reduction associated with mining in the Athabasca Basin.

Formation and Evolution of the Centennial Unconformity-Related Uranium Deposit in the South-Central Athabasca Basin, Canada

The Centennial U deposit is situated in the south-central Athabasca Basin (Canada) and straddles the unconformity between early Paleoproterozoic to Archean metasedimentary and metavolcanic rocks and granitoids, and the clastic sediments of the Paleoproterozoic Athabasca Group. Although it has most characteristics of an unconformity-related uranium deposit, the Centennial deposit is atypical in that it is not directly associated with an electromagnetic conductor (there is a paucity of graphite in the basement) or with a major reverse fault zone; it is distal from a major fluid conduit (ca. 300 to 400 m from the Dufferin Lake Fault), has low Ni, Cu, Co, Zn, and Pb contents, and contains an unusually large amount (up to 5%) of secondary uranyl minerals. Additionally, a network of diabase dikes and sills is observed at Centennial, seemingly intruding the main U mineralization of massive uraninite, based on the relatively sharp contacts between the diabase dike and the high-grade U mineralization. The pre-U alteration assemblage at Centennial includes kaolinite, illite, and sudoite, which have been formed by fluids with isotopic and chemical compositions that are comparable with those from other sandstone-hosted unconformity-type U deposits in the Athabasca Basin. Pre-U illite-related fluids have δ 18O of ca. 3‰ and δ D of ca. −40‰, whereas pre-U chlorite-related fluids have δ 18O between 1.7 and 4.3‰ and δ D between −18 and 1‰. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U/Pb dating of the various U phases indicates that initial mineralization, represented by disseminated uraninite found directly to the north-northeast of the Centennial deposit, occurred at ca. 1.6 Ga. The main U mineralization, represented by massive and strongly altered uraninite, followed at an unknown time. A minor (<5%) unaltered uraninite formed from the local remobilization of the main massive uraninite at ca. 380 Ma. The main uranyl mineral, uranophane, formed last, at ca. 2 Ma. The recurrence of local U remobilization might have been facilitated by the persisting high permeability of the sandstones in the area due to the nearby Dufferin Lake Fault and to the emplacement of the diabase dikes. The usefulness of Pb isotopes for exploration is demonstrated at the Centennial deposit, with strongly radiogenic Pb isotope signatures close to the deposit and a common Pb signature observed at a distance of a few km from the deposit.

A comparison of airborne electromagnetic data with ground resistivity data over the Midwest deposit in the Athabasca basin

ABSTRACTThe Midwest deposit in the Athabasca basin of Saskatchewan, Canada, lies 200 m below an arm of South McMahon Lake. The rocks between the lake and the deposit are sediments of the Athabasca group while the deposit is hosted by graphitic gneisses or metapelites in the basement. Geological logs of holes drilled by AREVA indicate that the Athabasca sediments are strongly altered. In the Athabasca basin, alteration in combination with conductive graphitic zones in the basement is a strong indicator of the presence of economic amounts of uranium. The purpose of the airborne surveys is to determine whether airborne methods can detect the alteration and the graphitic conductor.The airborne methods tested are broad‐band fixed‐wing time‐domain electromagnetic (FTEM) systems (TEMPEST and GEOTEM) and helicopter frequency‐domain electromagnetic (HFEM) systems (RESOLVE). All the lakes in the area, particularly the one above the deposit, are mapped as conductive near‐surface features with a high resolution HFEM system. On a resistivity section, the HFEM can delineate the bottoms of the conductive lakes quite well. The fixed‐wing FTEM sections are unable to resolve sharp changes in near‐surface resistivity; however, they clearly map the steeply dipping basement conductor and they also see another flat‐lying conductor close to the basement/basin unconformity (perhaps a conductive paleoregolith or other conductive basement lithologies).Comparison of ground and airborne resistivity maps indicates there are some similar features but also some important differences. At intermediate depths, where an alteration response is expected, the ground resistivity data indicate two relatively conductive zones, one at Midwest and smaller zones to the north, including one at the Mae zone (a smaller deposit north of Midwest). Resistivity sections derived from the airborne electromagnetic data do not always show these features clearly. The HFEM sections tend to show the lakes and possibly the alteration, while the FTEM results show the lakes, the bedrock conductor and the conjectured paleoregolith. There is a hint of the alteration at Midwest and Mae on the FTEM on‐time data but this might also be a response associated with the lakes. Modelling shows that the alteration response at Midwest is subtle and unlikely to be detected: at early time the response is dominated by the overlying lake sediments, at late time by the basement conductor. The modelling suggests that alteration could be detected at delay times less than 0.2 or 0.3 ms but only if there is no near‐surface conductive feature like overburden, lake sediments, etc.

Detection of alteration at the Millennium uranium deposit in the Athabasca Basin: a comparison of data from two airborne electromagnetic systems with ground resistivity data

ABSTRACTThe Millennium uranium deposit is located within the Athabasca Basin of northern Saskatchewan. The basement rocks, comprised primarily of paleo‐Proterozoic gneisses, are electrically resistive. However, the deposit is associated with highly conductive graphitic metasediments that are intercalated with the gneisses. An unconformity separates the basement rocks from the overlying, horizontally stratified, Proterozoic sandstones of the Athabasca Group (which are also highly resistive). The strike extents of the graphitic metasedimenary packages are extensive and therefore electromagnetic (EM) survey techniques are successful at identifying these zones but do not identify the specific locations where they are enriched in uranium. Through drilling it has been noted that hydrothermal processes associated with mineralization has altered the rocks in the vicinity of the deposits, which should in theory result in a resistivity low. A significant resistivity low has been mapped coincident with the Millennium deposit using ground resistivity survey techniques.However, a comparison of the airborne EM and ground resistivity results reveals that the two data sets have imaged different features. The resistive‐limit (on‐time) windows of the MEGATEM data show conductive features corresponding to lakes located to the west and south of the deposit. The late‐time windows show a feature to the east of the deposit, interpreted as being associated with the east‐dipping graphitic basement conductors (similar to that observed in historical ground EM data collected in this area). The early‐time TEMPEST windows (delay times less than 0.2 ms) show a broad resistivity low located at approximately the same location as where the alteration has been identified through drilling. Modelling the data is not easy but a response that decays prior to 0.3 ms is consistent with 500 Ωm material in the sandstone, a resistivity value close to the lower limit with respect to the hydrothermally altered Athabasca group sediments in this area. The MEGATEM system does not see a conductive zone over the alteration as clearly but the high signal‐to‐noise ratio in the late‐time MEGATEM data means that the conductive material at a greater depth is more coherently imaged.

An EPR study of native radiation-induced paramagnetic defects in sudoite (di–trioctahedral Al–Mg chlorite) from the alteration halo related to unconformity-type uranium deposits

Natural radiation-induced defects were identified in specimens of sudoite (Al–Mg di-trioctahedral chlorite) related to unconformity-type uranium deposits at the base of the Athabasca Group (Saskatchewan, Canada), using electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band frequencies. X-band spectra indicate the presence of a main native defect, named the As-center, whose EPR signal is dominated by an axially distorted spectrum with apparent principal components as follows: g// = 2,051 and g⊥ = 2,005, and a secondary defect with apparent component g = 2,025. The study of oriented specimens shows that the main defect has its g// component perpendicular to the (ab) plane of sudoite. The As-center corresponds to an electron hole located on oxygen atoms of the structure and is likely associated with Si, according to the lack of hyperfine structure. The As-center in sudoite has EPR parameters similar to the A-center in kaolinite and dickite, and the Ai-center in illite. The saturation behavior of EPR spectra as a function of power demonstrates that native defects of sudoite are different from those known in other clays, such as kaolinite, dickite or smectite, but are similar to those of illite. The isochronal annealing data suggest that the main defect in sudoite is stable to more than 300°C. The corresponding defects characterized in sudoite may have the potential for tracing past radionuclide migration around unconformity-type uranium deposits.

Critical Geochemical and Mineralogical Factors for the Formation of Unconformity-Related Uranium Deposits: Comparison between Barren and Mineralized Systems in the Athabasca Basin, Canada

The Spring Point barren alteration system and the Maurice Bay subeconomic uranium deposit, situated on the northwest rim of the Athabasca Basin, northern Saskatchewan, are hosted by the sandstones and conglomerates of the Fair Point Formation of the Athabasca Group. Macroscopic observations indicate two different styles of alteration for the two systems: chlorite-dominated, partly reduced alteration at Spring Point, and illite-dominated, strongly oxidized alteration at Maurice Bay. The paragenetic sequences developed for each area are similar, but the crystal chemistry of the alteration minerals differs markedly, indicating that the alteration fluids at Spring Point were richer in Mg and Ca than at Maurice Bay; this is consistent with a predominantly basement origin for the former. Stable isotope analyses of clay alteration minerals indicate that, in particular, the chlorite-related fluids from Spring Point have a basement signature (δ 18 O = 3.5‰ and δD = −13‰), whereas chlorite from Maurice Bay is more consistent with a mixture of basement and basinal fluids (δ 18 O = 5.2‰ and δD = −28‰). The absence of uranium-transporting basinal fluids at the critical time when reducing basement fluids were producing chlorite alteration is likely the single most important cause for the lack of uranium mineralization at Spring Point.