McArthur River Uranium Deposit Research Articles

Forward modeling and 3D inversion of electromagnetic data collected over the McArthur River uranium deposit in the Athabasca Basin, Canada

Detection and assessment of the deeply buried high-grade uranium deposits in the Athabasca Basin rely on geophysical methods to map conductive rocks. Variable Quaternary surface cover can mask the anomalous signals from depth and affect the interpretation of inverted conductivity models. We present the analysis of several electromagnetic (EM) modeling studies and two field data sets to demonstrate the effects of varying Quaternary cover resistivity and thickness, on the ability to resolve the parameters of underlying sandstone, alteration, and basement conductors. Synthetic data, assuming a typical shallow EM sounding system and realistic resistivities found in the Athabasca Basin, indicate that resistivity and thickness parameters of the Quaternary cover can be separately recovered in cases in which this cover is more conductive than the underlying sandstone, but not when the cover is significantly more resistive. A 3D modeling study indicates that by using airborne EM data, it is possible to detect a basement conductor of 20 S at a depth of at least 600 m below the surface, even in the presence of Quaternary cover thickness variations of the up to 20% (40–60 m). Furthermore, although Quaternary cover variations and deeper sandstone alteration can produce comparable anomalous signal amplitudes in a time-domain EM response, their effects are most visible in distinctly separate time windows. Ground-penetrating radar and other data to characterize the Quaternary cover in the McArthur River area indicate that this cover consists mostly of sandy tills ranging in thickness from 0 to 117 m. Constrained 3D inversion of an airborne EM data set from the same area indicates basement conductors consistent with the depth and location of a known fault. Elevated conductivity in the sandstone by up to a factor of two over the background values could indicate possible alteration.

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.

Exploration geochemistry of surficial media over the high-grade McArthur River uranium deposit, Saskatchewan, Canada

ABSTRACT An orientation survey using surficial media was performed over the high-grade McArthur River unconformity-related U deposit (Saskatchewan, Canada) to test whether or not secondary dispersion of elements related to the ore body or alteration zone can be detected at the surface more than 500 m above the deposit. Organic-rich Ah-horizon soils, Fe-rich B-horizon soils, C-horizon soils, tree cores of Jack pine (Pinus banksiana), and glacially dispersed boulders of Manitou Falls Formation sandstone that host the U deposit were collected in four sampling grids near the mine site. Two of the grids overlaid the trace of the P2 fault that hosts the deposit and extends nearly to the surface, one grid overlaid both the P2 fault and one of the high-grade ore bodies (Zone 4), and one grid was located 2.5 km away from the ore body surface trace in the barren hanging wall of the P2 fault. The grid overlying the Zone 4 ore body had the highest proportion of samples with elevated U and low 207Pb/206Pb ratios, the latter indicative of radiogenic Pb from a high-U source, measured in two size fractions of Ah-horizon soils using Na pyrophosphate leach, pine tree cores using total digestion, and sandstone boulders using 2% HNO3 leach. A handful of pathfinder elements, such as As, Co, Ni, and Pb, are variably associated with the U and radiogenic Pb. Sandstone boulders with an assemblage of dravite + kaolinite ± illite, determined using shortwave infrared (SWIR) spectroscopy and matching the alteration mineralogy in the Manitou Falls Formation above the U deposit, were prevalent in the grid above the Zone 4 ore body and in the adjacent grid in the direction of glacial dispersion. A coarse fraction of the B-horizon soils, leached with 5% HNO3, highlighted the grid above the Zone 4 ore body to a lesser extent, whereas HNO3 leaches and aqua regia digests of C-horizon soil separates did not highlight the P2 fault or ore body trace due to influence by parent till mineralogy. Results of environmental monitoring at the mine site, which was active at the time of sampling, suggest that dust containing U, Pb, and radionuclides from waste rock piles and a ventilation shaft could influence A-horizon soil geochemistry near the mine site, and that U and radiogenic Pb anomalies in B- and C-horizon soils near the water table are close to a treated mine effluent discharge point. However, older trees that record elevated U and radiogenic Pb in annual rings that pre-date mining activity, and alteration mineralogy and geochemistry of boulders that are less susceptible to the influences of mining activity, add confidence that the geochemical anomaly in diverse surficial media above the Zone 4 ore body represents secondary dispersion from the underlying U deposit.

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.

The origin of Ti-oxide minerals below and within the eastern Athabasca Basin, Canada

Abstract Titanium oxide minerals along the P2 fault in the eastern Athabasca Basin are characterized to constrain their origin and the geological history of the area. Two types of rutile are recognized in the basement rocks. Early rutile is disseminated in graphitic metapelite and quartzite, and it formed during regional metamorphism and post-metamorphic hydrothermal activity. Late rutile occurs as a needle-like alteration product of mica and likely formed during retrogression of the basement. In graphitic metapelite, early rutile commonly occurs with an assemblage of oxy-dravite, quartz, graphite, zircon, pyrite, biotite, and muscovite. In quartzite, rutile occurs with quartz, sillimanite, muscovite, and zircon. Metamorphic rutile is characterized by high Nb/Ta ratios (up to 47) with high concentrations of U (up to 126 ppm) and V4+ (up to 1.44 wt%; V valance calculated from EPMA data). Hydrothermal rutile contains distinctly low Nb/Ta (as low as 4.80) with high Ta (≤3050 ppm), and relatively low V (as V 3+; as low as 0.02 wt%) and U (as low as 9.06 ppm), reflecting fluids in reduced oxidation conditions. Anatase forms small anhedral (rarely coarse and euhedral) grains in the basal sandstones and altered basement rocks. In sandstones, anatase occurs with the late diagenetic mineral assemblage, whereas in basement rocks it commonly occurs with the clay-sized minerals related to uranium mineralization. In both rocks, anatase likely formed through the dissolution of rutile and/or other Ti-bearing minerals. Anatase is characterized by variably high Fe (up to 0.99 wt%; possibly contributed by hematite micro-or nanoinclusions) and U (up to 180 ppm). The mineral assemblages and composition of anatase suggest its protracted crystallization from relatively low temperature, oxidizing, acidic, uraniferous fluids of the sandstones during late diagenesis and hydrothermal activity. Therefore, the occurrence of anatase records the incursion of basin fluids into the basement, and the interaction of basement rocks with fluids responsible for the formation of the McArthur River uranium deposit. The results of this study confirm that Ti-oxides are useful in unraveling the geological history of an area that underwent prolonged hydrothermal activity.

Paragenesis and Composition of Tourmaline Types Along the P2 Fault and Mcarthur River Uranium Deposit, Athabasca Basin, Canada

Abstract The P2 fault, a 13 km-long steeply dipping reverse fault, is the main structural control of the McArthur River uranium deposit in the eastern Athabasca Basin, northern Saskatchewan, Canada. Three types of tourmaline were observed in the metasedimentary basement rocks along the P2 fault: early oxy-schorl [(Na 0.47 □ 0.37 Ca 0.16 )(Fe 2+ 1.30 Al 0.91 Mg 0.72 Ti 0.07 )Al 6 (Si 5.79 Al 0.21 O 18 )(BO 3 ) 3 OH 3 (O 0.63 OH 0.29 F 0.08 ), where □ = vacancy] of metamorphic-anatectic origin, hydrothermal oxy-dravite [(Na 0.57 Ca 0.23 □ 0.18 K 0.02 )(Mg 1.93 Fe 2+ 0.62 Al 0.29 Ti 0.15 )Al 6 (Si 5.93 Al 0.07 O 18 )(BO 3 ) 3 OH 3 (O 0.57 OH 0.23 F 0.20 )], and magnesio-foitite [(□ 0.77 Na 0.20 Ca 0.02 K 0.01 )(Mg 1.99 Al 0.92 Fe 3+ 0.07 )Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH 3 )(OH 0.71 O 0.25 F 0.04 )]. Oxy-schorl formed in granitic pegmatites, a partial melt product of the metasediments during peak metamorphism. Oxy-dravite formed from hydrothermal fluids after the peak metamorphism but before deposition of the Athabasca sandstones, whereas magnesio-foitite is a product of later, low-temperature hydrothermal activity. Both oxy-schorl and oxy-dravite are coarse-grained (from 500 μm up to 1 cm), whereas magnesio-foitite occurs as radial aggregates of fine, prismatic crystals ( apfu ) and high Al at its Y-site (0.70–1.12 apfu ), suggesting that magnesio-foitite likely replaced pre-existing high-Al phases, such as kaolin and sudoite. The occurrence of magnesio-foitite along the entire P2 fault, in both areas of mineralization and apparently barren areas, suggests chemically similar fluids travelled along the entire P2 fault, but only produced ore in localized areas.

Compositional variation and timing of aluminum phosphate-sulfate minerals in the basement rocks along the P2 fault and in association with the McArthur River uranium deposit, Athabasca Basin, Saskatchewan, Canada

The Athabasca Basin hosts world class uranium deposits, such as the McArthur River deposit. This paper presents the occurrence of aluminum phosphate-sulfate (APS) minerals in the metasedimentary rocks along the P2 fault, the main ore-hosting fault of the McArthur River deposit. It compares the APS minerals along the P2 fault with those outside the fault, examined in this study, and those from other deposits of the Athabasca Basin and from other Paleo- to Mesoproterozoic basins worldwide. APS minerals are common along the P2 fault but rare outside of the P2 fault zone in the basement and along the unconformity between the Athabasca sandstones and the basement. The APS minerals along the P2 fault occur with sudoite (± illite, magnesiofoitite) and are zoned with Sr-, Ca-, and S-rich cores (solid solution between svanbergite, crandallite, and goyazite) and LREE- and P-rich rims (close to florencite composition). APS minerals in the Bleached Zone (altered rocks along the unconformity consisting predominantly of kaolin and illite) are Sr-, Ca-, and S-rich (high svanbergite component) and occur with kaolin. APS minerals in the Red-Green Zone (mingled red hematitic and green chloritic basement rocks below the Bleached Zone) occur with sudoite and clinochlore. They contain relict cores of LREE- and As-rich arsenoflorencite-(Ce) and rims of svanbergite-goyazite-crandallite solid solution. The occurrence of svanbergite-crandallite-goyazite along the unconformity suggests their formation by relatively oxidizing fluids during diagenesis of the overlying sandstones. The relict cores of arsenoflorencite-(Ce) in the Red-Green Zone are interpreted to be the product of paleo-weathering before the deposition of the Athabasca sandstones. Florencitic APS minerals are found along the entire studied strike length (7 km) of the P2 fault, including the ore zone and non-mineralized areas, but are absent outside the fault zone. The florencitic APS minerals contain low SO 4 2− in the ore zone, suggesting relatively reducing conditions during their crystallization. Zoned APS minerals (with svanbergitic cores and florencitic rims) proximal to ore contain elevated U (up to 16 ppm). These features suggest that diagenetic, oxidizing, and uranium-bearing fluids traveled along the P2 fault and became relatively reduced, especially within the ore zone. It also suggests florencitic APS minerals are contemporaneous with uranium mineralization. The restricted occurrence of florencitic APS mineral along the P2 fault in the basement suggests their use in identifying fertile basement structures associated with uranium mineralization.

Radiation-induced defects in quartz. IV. Thermal properties and implications

The thermal stabilities and decay kinetics of three peroxy radicals (Centers #1, B and B′) and three other radiation-induced defects (#3, C′ and E1′) in natural quartz from the high-grade McArthur River uranium deposit (Athabasca basin, Canada) have been investigated by isochronal and isothermal annealing experiments and electron paramagnetic resonance (EPR) spectroscopy. Single-crystal EPR spectra of isochronally (2 h) annealed quartz show that these centers all grow in intensity to 280°C and then decay with further increase in temperature, but their disappearance temperatures differ markedly and depend on the initial concentrations (e.g., Center #1 in a dark smoky quartz is annealed out at 380°C, B and B′ at 420°C and #3 and C′ at 580°C). The isothermal decay processes of these centers are all of the second order type. The calculated activation energies for the peroxy radicals [#1 and B + B′ at 0.36 (9) and 0.83 (8) eV, respectively] are smaller than those of Centers #3, C′ and E1′ [1.09 (8), 1.24 (8) and 1.45 (7) eV, respectively]. Gamma-ray irradiations of thermally bleached quartz restore a fraction of the peroxy radicals, suggesting that their diamagnetic precursors are stable up to at least 800°C. The unusual decay characteristics of “peroxy radicals” in quartz reported in the literature are shown to most likely arise from multiple radiation-induced defects. These results have implications for not only applications of peroxy radicals in quartz for EPR dating but also better understanding of thermoluminescence and cathodoluminescence spectra of this mineral.

Three-dimensional inversion of magnetotelluric data for mineral exploration: An example from the McArthur River uranium deposit, Saskatchewan, Canada

Shallow exploration targets are becoming scarce, meaning interest is turning towards deeper targets. The magnetotelluric method has the necessary depth capability, unlike many of the controlled-source electromagnetic prospecting techniques traditionally used. The geological setting of ore deposits is usually complex, requiring three-dimensional Earth models for their representation. An example of the applicability of three-dimensional inversion of magnetotelluric data to mineral exploration is presented here. Inversions of an audio-magnetotelluric data-set from the McArthur River uranium mine in the Athabasca Basin were carried out. A sub-set comprising data from eleven frequencies distributed over almost three decades was inverted. The form of the data used in the inversion was impedance. All four elements of the tensor were included. No decompositions of the data were done, nor rotation to a preferred strike direction, nor correction for static shifts. The inversions were successful: the observations were adequately reproduced and the main features in the conductivity model corresponded to known geological features. These included the graphitic basement fault along which the McArthur River uranium deposit is located.

NATURAL RADIATION-INDUCED DAMAGE IN QUARTZ. III. A NEW OZONIDE RADICAL IN DRUSY QUARTZ FROM THE ATHABASCA BASIN, SASKATCHEWAN

Quartz crystals collected in druses, including one annealed at 400°C, from a fracture in the hanging-wall sandstone at the McArthur River uranium deposit in the Paleoproterozoic Athabasca basin, northern Saskatchewan, have been investigated by single-crystal X-band electron paramagnetic resonance (EPR) spectroscopy at room temperature and 110 K. The single-crystal EPR spectra allow us to distinguish eight paramagnetic defects, including one new center and seven previously reported centers. These centers, most of which were discovered in artificially irradiated crystals and only a few had been suggested to occur in natural quartz by powder EPR, are herewith positively established for the first time as natural radiation-induced defects by single-crystal EPR. The new center is similar in principal g-factor values to the ozonide radical (O 3 − ) in various minerals and synthetic compounds, and is characterized by its g-maximum and g-minimum axes approximately parallel to two O–O edges of the SiO 4 quasi-tetrahedron in the quartz structure. This geometry is also compatible with an ozonide radical formed from a silicon vacancy. The observed linewidths of the new ozonide radical vary from 0.087 to 0.257 mT, which are attributed to unresolved site-splittings or unresolved hyperfine splittings. This new ozonide radical is distinct from a previously reported ozonide radical in citrine quartz, which is characterized by the presence of a small 27 Al hyperfine structure. The new ozonide radical is probably linked to a Si atom in the neighboring tetrahedron and hence represents a general case in quartz, whereas the previously reported ozonide radical linked to a neighboring Al atom is a special variant in Al-bearing quartz. The presence of natural radiation-induced defects in drusy quartz is attributable to the presence of uranium in mineralized assemblages nearby or late hydrothermal fluids.

NATURAL RADIATION-INDUCED DAMAGE IN QUARTZ. I. CORRELATIONS BETWEEN CATHODOLUMINENCE COLORS AND PARAMAGNETIC DEFECTS

Quartz grains from the McArthur River uranium deposit of the Athabasca Basin, Saskatchewan, are characterized by three distinct types of cathodoluminescence (CL): 1) halos surrounding U- and Th-bearing mineral inclusions, and 2) patches and 3) continuous rims along grain boundaries and fractures. These three types of CL have a constant width of ~35 to 45 m, consistent with the maximum depth of penetration of alpha particles, and therefore they record alpha-particle-induced radiation damage. Relative to the host grains, the radiation-damaged areas are characterized by pronounced but broad CL bands in the ultraviolet (~350 nm) and red (~620–650 nm) regions. Isochronal annealing experiments reveal that the ultraviolet CL persists to 500°C but is annealed out at 600°C, whereas the red CL persists to at least 800°C. Electron paramagnetic resonance (EPR) spectroscopy, including detailed measurements on saturation behavior and thermal properties, revealed six paramagnetic defects: one oxygen vacancy center (E1’), three silicon vacancy hole centers [O2 3– /H + (I), O2 3– /H + (II) and O2 3– /M + ] and two O2 – peroxy centers. Spectral simulations confi rm the presence of these centers. Moreover, EPR spectra of HF-treated samples show that the silicon vacancy-hole centers and the peroxy centers are concentrated in the radiation-damaged rims and fractures. Center E1’ appears to occur throughout the quartz grains and is annealed out at ~500°C; it thus cannot be responsible for the ultraviolet or red CL. The silicon vacancy-hole centers are all annealed out between 550° and 600°C, similar to the annealing temperature of the ultraviolet CL. The peroxy centers are the only paramagnetic defects stable above 600°C, corresponding to the preservation of red CL in radiation-damaged areas at high temperatures. Therefore, the silicon vacancy-hole centers and the peroxy centers are probably responsible for the characteristic ultraviolet and red CL, respectively, associated with radiation-damaged halos, patches and rims in quartz.

A high resolution gravity survey near McArthur River uranium deposit, Athabasca Basin, Canada

A high resolution gravity traverse (27.2 km-long, 50 m or 100 m station-spacing) across the unconformity-associated McArthur River uranium deposit, Athabasca Basin, Canada was conducted to investigate basement geology and locate alteration zones related to mineralization. In this geological environment, amplitudes of gravity anomalies relevant to uranium exploration (related to offsets of the basement unconformity, basement relief and alteration zones) are typically <1 mGal, but range up to 2 mGal. Alteration near the McArthur River deposit is commonly present as silicification of the basin sandstones, located above basement quartzite units. Many short to intermediate wavelength (250-1800 m) anomalies, amplitudes <0.5 mGal, are identified along the traverse profile, superposed on longer wavelength and larger amplitude (up to 6000 m, and 1.8 mGal, respectively) anomalies. Preliminary modeling of anomalies, constrained by magnetic data, drill-hole logs and density information, and reflection seismic data, indicates a variety of geological contributions. Short wavelength anomalies are attributed to changes in thickness of the glacial overburden, thickening in some places being linked to preferential erosion along an underlying fault. Long wavelength anomalies are explained by variations in basement density and offsets of the basement unconformity. Importantly, from an exploration perspective, most intermediate wavelength positive anomalies are explained by a combination of silicification within the Athabasca Sandstone, and associated basement relief produced by underlying basement quartzite units. Desilicification is also modeled, but limited. The gravity method, apparently, has been successful in identifying alteration zones that may be of significant interest for uranium exploration, because of their association with mineralizing fluids, and because patterns of silicification/desilicification are important for studies of fluid flow.