Formation Of Unconformity-related Uranium Deposits Research Articles

Coupling of thermal convection and basin-basement fluid mixing is critical for the formation of unconformity-related uranium deposits: Insights from reactive transport modeling

Thermal convection and fluid mixing are involved in the formation of many mineral deposits, but their relationships and roles in mineralization remain poorly understood because they are generally investigated separately in different studies. Here we present a case study combining thermal convection and fluid mixing to examine the critical factors that controlled the formation of high-grade, world-class unconformity-related uranium (URU) deposits in the Athabasca Basin (Canada). Reactive transport modeling of various scenarios with different degrees of involvement of thermal convection and fluid mixing shows that significant URU mineralization occurs at the intersection of a basement fault with the basin-basement unconformity only if thermal convection and basin-basement fluid mixing take place concurrently. If there is no thermal convection in the basin, only sparse U mineralization occurs along the unconformity. If insufficient amount of reducing fluid is provided from the basement along the fault, no significant U mineralization occurs either. Furthermore, no significant U mineralization occurs if the U concentration in the basinal fluid is low. We conclude that the exceptionally rich U endowment in the Athabasca Basin is the result of coupling of three critical factors: high-permeability sandstone favoring thermal convection in the basin, ample supply of reducing agents by basement fluids along reactivated basement faults, and abundant U-rich basinal fluid available in the basin sequences. Similar coupling of fluid processes may be essential for the formation of other types of mineral deposits.

The role of graphite in the formation of unconformity-related uranium deposits of the Athabasca Basin, Canada: A case study of Raman spectroscopy of graphite from the world-class Phoenix uranium deposit

Abstract The unconformity-related uranium (URU) deposits in the Proterozoic Athabasca Basin (Canada) represent the richest, and one of the most important, uranium endowments in the world. Most of the URU deposits are associated with pre-existing graphitic basement faults that were reactivated after the formation of the basin. These graphite-rich structures have been widely used as a vector for exploration, but the nature of the association of the URU deposits with graphitic basement faults has been debated for over four decades. Proposed roles of graphite include: (1) as a direct reducing agent to reduce U6+ to U4+ and precipitate uraninite; (2) as a precursor of hydrocarbons (mainly CH4) produced in situ or nearby and then used as a reducing agent for uraninite precipitation; (3) as a precursor of hydrocarbons produced at depth that were remobilized to the site of mineralization and acted as a reducing agent for uraninite precipitation; and (4) as a lubricant facilitating faulting and fluid flow that led to uranium mineralization. This paper uses the Phoenix uranium deposit in the southeastern Athabasca Basin as a case study to address these uncertainties. Petrographic studies indicate that there is no direct contact between graphite and uraninite at microscopic scales, and the content of graphite in the graphitic metapelite along the ore-controlling WS Shear Zone does not show a systematic change with the distance from the unconformity surface. Raman spectroscopic studies of graphite suggest that the degree of structural disorder of graphite, expressed by various parameters related to the D bands and G band ratios, does not change systematically with the distance from the unconformity surface either. The minor irregularities in these parameters near the unconformity are better explained by paleo-weathering related to the unconformity and/or diagenetic processes than by hydrothermal activity related to uranium mineralization. Based on these observations and interpretations, the role of graphite as an in situ reducing agent, either directly or as a provider of hydrocarbons, is discounted. It is proposed that hydrocarbons derived from graphite at depth, tapped by episodic reactivation or seismicity of the basement faults that was facilitated by graphite as a lubricant, were responsible for URU mineralization.

Interplay between thermal convection and compressional fault reactivation in the formation of unconformity-related uranium deposits

A number of large and high-grade unconformity-related uranium deposits formed in intracratonic Proterozoic basins, including the Athabasca Basin in Canada and the McArthur Basin in Australia. These deposits occur close to the unconformities between the Paleo- to Mesoproterozoic basinal sequences and their Archean to Paleoproterozoic metasedimentary basement, and are spatially associated with reactivated basement faults showing reverse offsets due to contractional deformation. Previous studies have suggested that fluid flow responsible for uranium mineralization in such basins may be related to thermal convection and tectonic deformation; however, the relative roles of these two driving forces, as well as their interrelationships, have not been explored in detail. In this study, two-dimensional numerical modeling of fluid flow in relation to coupled tectonic compression and heat transport was carried out to evaluate the relationships between deformation-driven flow and fluid convection, and their relative importance for uranium mineralization. The results indicate that tectonic compression at a relatively high strain rate (6.66 × 10−11 s−1) may instantaneously destroy pre-existing thermal convection developed in the sedimentary rocks in the basin, due to the rapid development of overpressure in the system. Convection may then reappear over time as deformation progresses and the overpressure wanes. By contrast, tectonic compression at a relatively low strain rate (6.66 × 10−13 s−1) does not affect pre-existing thermal convection, and deformation-driven fluid flow in the basement coexists with thermal convection in the basin. It is proposed that during an overall tectonically active period associated with a far-field tectonic event, compressional reactivation of basement faults creates permeability and causes fluid to flow toward dilatant zones accompanying individual seismic events, whereas during seismically quiet times, fluid flow driven by thermal convection dominates. The formation of the unconformity-related uranium deposits likely involves alternating relatively short-lived, deformation-driven fluid flow and relatively prolonged fluid convection events.

Structural Controls on Fluid Flow During Compressional Reactivation of Basement Faults: Insights from Numerical Modeling for the Formation of Unconformity-Related Uranium Deposits in the Athabasca Basin, Canada

The unconformity-related uranium deposits in the Athabasca basin, Canada, are generally associated with reactivated basement faults crosscutting the basin-basement unconformity. Two-dimensional (2-D) numerical modeling of coupled compression and fluid flow in a reverse fault system was carried out with varying dip angles, preexisting offsets, and degrees of deformation. The results indicate that the fluid flow pattern associated with structural reactivation is sensitive to the degree of bulk shortening. At a low bulk shortening stage, fluid is driven up along the fault zone into the sandstone in the basin, whereas at a relatively high bulk shortening stage, fluid tends to flow down into the fault zone and the basement. This is interpreted to indicate that the fluid pressure within the fault zone increases at the early stage of deformation due to pore volume decreases in relation to shortening, whereas the fluid pressure within the fault zone decreases at later stages of deformation due to dilation in relation to fracturing and reverse displacement. Variation of the dip angles and preexisting offsets of the faults shows little effect on the strain distribution and fluid flow patterns. The simulation results demonstrate that both unconformity contact and basement-hosted orebodies may be generated at different stages of deformation within the same fault system under a unified compressional stress regime.

Noble gases (Ar, Kr, Xe) and halogens (Cl, Br, I) in fluid inclusions from the Athabasca Basin (Canada): Implications for unconformity-related U deposits

The formation of unconformity-related uranium deposits in the Proterozoic Athabasca Basin (Canada) involved basin-scale circulation of U-bearing brines during high-grade diagenesis (150–200°C) at ∼1.6–1.5Ga. The UO2 ores occur both sides of the unconformity and are associated with extensive brecciation and illite–sudoite–dravite alteration. Quartz and dolomite cementing veins and breccias are associated with alteration and mineralisation and contain a fairly uniform population of fluid inclusions characterised by variable Na:Ca and salinities of 25–35wt.% salts and high U concentrations of up to 600ppm U. In order to further constrain the origin of these U-rich brines, we analysed the naturally occurring isotopes of Ar, Kr and Xe, together with halogens (Cl, Br and I), K, Ca and U in irradiated quartz and dolomite samples containing representative fluid inclusions. This was achieved by the noble gas method for halogen measurement (extended 40Ar–39Ar methodology) using a combination of noble gas extraction techniques.The fluid inclusions opened by crushing quartz and dolomite samples in vacuum have similar molar Br/Cl ratios of 5.8×10−3 to 10.4×10−3, and molar I/Cl ratios of 1.8×10−6 to 8.2×10−6. These compositions lie over the top half of the modern-day seawater evaporation trajectory, consistent with the fluids deriving the bulk of their salinity by subaerial evaporation of seawater, beyond the point of halite saturation. The I/Cl ratios are much lower than is typical of fluids that have interacted with I-rich organic matter present in many sedimentary basins or fluid inclusions found in Mississippi Valley type (MVT) Pb–Zn ore deposits. This is significant because provided the U-rich fluid inclusions are representative of the ore-stage fluids, the low I/Cl ratios of the fluid inclusions do not favour fluid interaction with organic matter (or hydrocarbons), as a major process for localising U mineralisation.The majority of samples contain fluid inclusions with age-corrected 40Ar/36Ar of between the modern atmospheric value of ∼300 and 450. These values are considered representative of the fluid's initial composition and are typical of upper crustal sedimentary formation waters. The fluid inclusions non-radiogenic 84Kr/36Ar and 129Xe/36Ar ratios are slightly enriched in 129Xe relative to air and the fluid inclusions are estimated to contain 0.5–17.3×10−10molg−136Ar which is up to twenty times the 36Ar concentration of air-saturated seawater. The data are interpreted to reflect acquisition of atmospheric noble gases from sedimentary rocks and suggest acquisition of radiogenic 40Ar within K-rich basement rocks, that would have been an important source of excess 40Ar, was limited by temperatures of less than 200°C.Taken together the halogen and noble gas composition of the U-bearing fluid inclusions are strongly controlled by subaerial evaporation and subsequent interaction with sedimentary rocks, showing that low temperature evaporitic brines dominated the mineralising system. Mineralisation is unlikely to have been triggered by fluid interaction with organic matter, or mixing with voluminous basement-derived fluids; however, the data do not completely preclude a role for volumetrically minor fluid or gas phases introduced by deep-seated basement faults preferentially located at the sites of mineralisation.

Effect of graphite zone in the formation of unconformity-related uranium deposits: Insights from reactive mass transport modeling

In this study, reactive mass transport modeling is conducted for evaluating the role of faulted graphite zones in the formation of unconformity-related uranium (URU) deposits. For this aim, two different reducing mechanisms are examined for the precipitation of uraninite in a typical URU deposit. In the first mechanism, precipitation of the uraninite involves methane as a reducing agent which is produced by the alteration of the graphite zone. The second reducing mechanism does not incorporate methane as a reducing agent, and oxygen is used for formulating the redox reaction of uraninite precipitation. Numerical simulations show the following results. Firstly, by employing either reducing mechanisms, uraninite can precipitate in the basement close to the unconformity interface. Secondly, uraninite can precipitate below the unconformity interface away from the faulted graphite zone even if methane is not involved as a reducing agent. Physiochemical parameters such as oxygen fugacity and temperature play a significant role in localization of uraninite. Localization of uraninite below the unconformity interface is related to the decrease of oxygen fugacity, generally resulting from the interaction of oxidized uranium-bearing fluids with reductants. Through the second reducing mechanism, uraninite cannot precipitate around the faulted graphite zone. In comparison with the precipitation by the first reducing mechanism, it takes longer time for uraninite to precipitate through the second mechanism, and the volume fraction of uraninite precipitated by the second mechanism is lower than that by the first mechanism. Finally, faulted graphite zone has a major role in providing the pathway for transporting the fluid. The uranium bearing brines flow into the faulted graphite zone and interact with the basement lithology. Also, the basal fluids use faulted graphite zone as a conduit to mix with the basinal fluids. Maximum fluid flow rate happens along the faulted graphite zone because of the high permeability of this zone in comparison with that of other stratigraphic units present in the model.

Magmatic and Metamorphic Uraninite Mineralization in the Western Margin of the Trans-Hudson Orogen (Saskatchewan, Canada): A Uranium Source for Unconformity-Related Uranium Deposits?

Abstract The genetic model for the giant unconformity-related uranium deposits of the Athabasca Basin is still being debated, with one of the main issues being the source of the uranium concentrated by Mesoproterozoic era (ca. 1.60–1.00 Ga) diagenetic-hydrothermal events at the interface between the Athabasca Basin and the underlying Archean/Paleoproterozoic basement rocks. Currently, accessory minerals like monazite, zircon, and/or apatite from the sedimentary basin and basement rocks are proposed as the primary uranium source for these high-grade uranium deposits. Numerous occurrences of U mineralization of Hudsonian age have been documented for decades all around the Athabasca Basin; however, so far these have not been regarded as viable U sources. Here, a systematic and detailed study of two areas of basement rocks near the eastern part of the Athabasca Basin is presented (i.e., the Way Lake property, lying outside the current margin of the basin, and the Moore Lakes property, currently covered by the basin). This study highlights the significant and widespread occurrence of Hudsonian (ca. 1.81–1.76 Ga) uranium oxide (UO2) mineralization in these zones. Two types of mineralization are identified and documented here: magmatic uranium oxides related to granitic pegmatites and leucogranites, which are more common, and high-temperature, vein-hosted uranium oxides, which have the highest grades. The two types were formed during the peak (1.82–1.81 Ga) and/or postthermal peak (1.81–1.72 Ga) events related to the evolution of the Trans-Hudson orogeny. The magmatic uranium oxides formed by partial melting of Wollaston Group metasedimentary rocks. The origin of the vein-type occurrences is unclear, but their high thorium and rare earth element (REE) contents suggest a high-temperature process associated with Ca and/or Na metasomatism. The uranium oxides are associated with other U-, Th-, and REE-bearing accessory minerals like U-rich thorite, thorite, zircon, and/or monazite, adding to the exceptional U contents (100–2,460 ppm) of these UO2-bearing rocks (up to 200 times more primarily enriched in U than other basement or basin rock types). A 3-D model of a 1,300 × 630 × 200-m basement zone from the Way Lake property indicates that uraninite-bearing granitic pegmatites and leucogranites represent 7% of the total volume of crystalline rock. Within this rock volume are approximately 8,121 (assuming a mean U content of 250 ppm) to 16,242 (assuming a mean U content of 500 ppm) tonnes U. The U tonnage of this limited rock volume, contained mainly by the Hudsonian-age UO2, corresponds to between 4% (for McArthur River) and 103% (for Rabbit Lake) of the U tonnage of known unconformity-related U deposits of the basin. Some of the studied rock samples, even macroscopically fresh and located far away from any known unconformity-related U deposit, present clear evidence of alteration, including clay minerals, aluminophosphate-sulfate (APS) minerals, and UO2 dissolution, indicating the percolation of the brines associated with the formation of unconformity-related uranium deposits when the basin was far more geographically extensive. Due to geologic similarities between the studied zones and the basement domains from the eastern part of the Athabasca Basin, i.e., the Hearne province, it is proposed that these domains hosted widespread Hudsonian-age uranium oxide protores. These protores provided easily leachable uranium for the metal enrichment of basinal brines during their percolation within the basement and the formation of the unconformity-related U deposits. These observations bring new insight to the debate about the genetic model of unconformity-related U deposits, and reinforce the metal source potential of the basement compared to that of the sedimentary basin.

Solute transport across basement/cover interfaces by buoyancy-driven thermohaline convection: Implications for the formation of unconformity-related uranium deposits

Previous studies have suggested that buoyancy-driven convection could only have developed in thick sandstone aquifers within Proterozoic intracratonic sedimentary basins that host unconformity-related uranium deposits. On the other hand, oxygen and hydrogen isotopic and fluid inclusion studies of quartz and carbonate veins have shown that basinal brines have interacted with basement rocks and basement-derived fluids pervasively. Finite element modeling was conducted to investigate the potential mechanisms driving fluid interaction across a basement/cover unconformity. We firstly constructed a simplified conceptual model by integrating the known features shared by the Athabasca, Thelon and Kombolgie basins and their Phanerozoic counterparts. Based on this conceptual model, various numerical scenarios were designed to examine buoyancy-driven (heat and solute transport are coupled) fluid flow patterns and the corresponding solute transport. The results show that thermohaline convection may have penetrated into the basement for up to 1 to 2 km below the unconformity, when typical hydrological parameters for these Proterozoic hydrogeological units were used. Fluid flow velocities in the sandstone sequence were several orders of magnitude larger than those in the basement. If a uranium source (a pore fluid with 500 mg/l uranium) was assumed to be located in the center of the basin below the unconformity, uranium was able to gradually spread into the sandstone aquifer through thermohaline convection without considering any contribution from fluid-rock interaction. The uranium concentration of basinal fluids above the uranium source approached 15 and 24 mg/l after 1 and 5 m.y. of modeling time, respectively. If the uranium source was initially located at the center of the aquifer, a uranium plume developed and percolated down to 2 km below the unconformity at 5 m.y. The location of the uranium source also affects the solute transport efficiency. A uranium source located around the sloping basal unconformity, either in the basin fill or basement, close to the basin margin, led to a wider uranium plume than if it was located near the center of the basin. Given appropriate hydrological conditions, thermohaline convection could have caused widespread interaction of basinal brines with basement rocks or basement-derived fluids in Proterozoic basins where unconformity-related uranium deposits have developed, and that enough uranium could have been leached from the uranium-rich basement to form large, high-grade unconformity-related uranium deposits.

Tectonic Deformation and Fluid Flow: Implications for the Formation of Unconformity-Related Uranium Deposits

Numerical experiments were carried out to investigate the fluid flow in typical Proterozoic basins that host unconformity-related uranium deposits. We have constructed a simplified conceptual model by integrating important features shared by the Athabasca, Thelon, and Kombolgie Basins. Based on this conceptual model, various numerical scenarios were designed to explore the interaction between fluid flow, heat transport, topographic relief, and tectonic deformation. Our modeling suggests that thermally driven free convection may develop in the thick sandstone sequence at a geothermal gradient of between 25° and 35°C/km during periods of tectonic quiescence, with a maximum velocity of about 1.0 m/yr. However, reactivation of preexisting basement structures and the generation of new faults suppressed free convection and led to deformation-dominated fluid flow or mixed convection, depending on strain rates. During compressive deformation, reduced brines in the basement may flow out along fractured zones and encounter uranium-bearing fluids in the clastic sequence to form sandstone-hosted deposits. In contrast, basement-hosted deposits are likely to form during extension, when oxidized basinal brines flow into faulted structures to interact with reduced minerals or fluids in the basement. Maximum flow rates are estimated to be up to ~7 and ~2.5 m/yr in the relatively high permeability fracture zone for compressive and extensional deformation, respectively, given a strain rate of 10 −13 s −1 and a geothermal gradient of 30°C/km. In addition, the rate of pressure accumulation and dissipation is different in various geologic units depending on their hydrological properties and strain rates.

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.

Hydrothermal alteration of monazite in the Precambrian crystalline basement of the Athabasca Basin (Saskatchewan, Canada): implications for the formation of unconformity-related uranium deposits

Microanalytical studies of basement rocks below the Athabasca sandstone basin indicate that monazite is the dominant uranium-bearing mineral in the study area. Drill core samples of hydrothermally altered basement show that monazite is commonly altered to a Th–silicate phase, and uranium has been significantly mobilized. On average, 75% of the uranium bound to monazite is leached out during monazite alteration. In contrast, no substantial mobilization of uranium from detrital minerals (e.g. zircon) has yet been observed in the Athabasca sandstones. It is suggested that hydrothermal alteration of granitic rocks (especially potassic pegmatoids and potassic orthogneisses) of the sub-Athabasca basement, represents the most important uranium source for the unconformity-type deposits.