Unconformity-type Uranium Deposits Research Articles

Comparison of diagenetic fluids in the Proterozoic Thelon and Athabasca Basins, Canada: implications for protracted fluid histories in stable intracratonic basins

The Paleoproterozoic Thelon Basin, located on the border between Nunavut and the Northwest Territories of Canada, is a contemporaneous analog of the uranium-rich Paleoproterozoic Athabasca Basin in Canada. Early diagenesis resulted in precipitation of extensive hematite on the surfaces of detrital quartz grains throughout the Thelon Formation and minor hydroxy-phosphate in veins locally. Continued diagenesis then resulted in syntaxial quartz cementation of detrital quartz at 130°C from fluids having ca. 17 wt.% equivalent NaCl, similar to the Athabasca Basin. Cementation of this type is most pronounced in fine-grained sequences in the Thelon Basin. A period of extensive desilicification during continued burial was followed by formation, at ca. 200°C, of peak-diagenetic illite having Ar–Ar ages of ca. 1400–1690 Ma in the Thelon Formation. This illite was associated with fluids with δ18O and δD values of ca. 6‰ and –50‰, respectively, similar to those during peak diagenesis of the Athabasca Basin. Although the timing, salinity, and isotopic composition of the peak-diagenetic fluids in the Thelon and Athabasca Basins are similar, the peak-diagenetic mineral assemblage in the Athabasca Formation is dickite and illite, with minor dravite and goyasite rather than simply illite. Consequently, the fluids at peak diagenesis, which in the Athabasca Basin are synchronous with formation of world-class unconformity-type uranium deposits, had different compositions in each basin. Post-peak diagenesis in the Thelon Basin was quite distinct from that in the Athabasca Basin in that illite was replaced in the central portion of the basin by K-feldspar and then sudoite, which crystallized from saline brines at ca. 1000 Ma and 100°C. Evidence for later infiltration of these brines is absent in the Athabasca Basin, although uranium mobilization at ca. 900 Ma from fluids having the same characteristics as those at peak diagenesis was pronounced in the Athabasca Basin. Recent incursion of meteoric waters along reactivated structures into the Athabasca Basin has variably affected hydrous and uraniferous minerals, but evidence for this is lacking in the Thelon Basin. The Thelon Basin reflects less intensive fluid–rock interaction in its early history than that recorded in the basal units of the Athabasca Basin.

O and Pb isotopic analyses of uranium minerals by ion microprobe and U–Pb ages from the Cigar Lake deposit

We apply a rapid and accurate in situ technique to make U–Pb isotopic measurements of complexly intergrown uranium minerals and oxygen isotopic analyes of uraninite from the unconformity-type Cigar Lake uranium deposit. Secondary uranium minerals intergrown with uraninite, such as coffinite, USiO 4· nH 2O and calciouranoite, CaU 2O 7·5H 2O, were identified by high-resolution transmission electron microscopy (HRTEM). In situ U–Pb results from three stages of uraninite and coffinite define well-correlated arrays on concordia with upper intercepts of 1461±47, 1176±9, and 876±14 Ma (±1 σ). These ages are interpreted as the minimum ages of mineralization correlate with the timing of clay mineral alteration (∼1477 Ma) associated with these unconformity-type uranium deposits, the ages of magnetization events at 1600–1450 and 900 Ma from the Athabasca Basin, and the Grenvillian Orogeny at ∼1100 Ma. In situ U–Pb isotopic analyses of uraninite and coffinite can document the Pb*/U heterogeneities that occur on the scale of 15–30 μm, thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits. The high spatial resolution and precision of the ion microprobe allow us to measure δ 18O values of 20–100 μm unaltered portions of uraninites from Cigar Lake. The range of δ 18O values (−33.9 to −20.4‰) are among the lowest reported for unconformity-type deposits, confirming that conventional fluorination analyses of material sampled at the mm-scale are insufficient to avoid contamination from isotopically heavier coffinite and calciouranoite.

Application of composite glacial boulder geochemistry to exploration for unconformity-type uranium deposits in the Athabasca Basin, Saskatchewan, Canada

Abstract Sampling glacially transported boulders is effective for mapping subcrop clay alteration patterns associated with deeply-buried unconformity-type uranium deposits in the Athabasca Basin of northern Saskatchewan, Canada. The technique works well because the subcrop alteration haloes show significant geochemical contrast with background, because the glacial deposits are consistently distributed and rich in boulders, and the altered boulders are typically well-represented in the boulder population. Boulder sampling is preferred over some other techniques because it is rapid and inexpensive, and the use of a lithological sampling medium provides a direct measure of subcrop clay-mineralogy, in an area where outcrop exposure is very restricted. A large composite boulder sample data set, comprising c. 20 000 samples from within the eastern part of the Athabasca Basin, has been compiled. Sampling and analytical techniques are descirbed, and results are presented at both regional and semi-regional scales. The distributions show clear correlations with both basin-scale features, such as stratigraphical variation and regional alteration patterns, and with deposit-related features, such as hydrothermal illite, dravite and chlorite alteration. Composite boulder lithogeochemistry may be equally effective for mineral exploration in other glaciated regions with restricted bedrock exposure.

Low temperature oxygen isotopic fractionation in the uraninite–UO 3–CO 2–H 2O system

Oxygen-isotope fractionation factors for uraninite–water and UO 3–water were determined from a series of uraninite–CO 2 and UO 3–CO 2 exchange experiments between 100 and 300°C. The measured fractionation factors are similar to the theoretical fractionation factors of Hattori and Halas (1982) at high temperatures and Zheng (1991) at low temperatures and are nonlinear over the temperature range examined. Regression through these curve gives 1000 ln α UO 2–H 2 O = 16.58 (10 6/ T 2) − 77.52 (10 3/ T) + 77.48, 1000 ln α UO 3–H 2 O = −2.21 (10 6/ T 2) + 25.06 (10 3/ T) − 45.90 with 2σ errors of ±2.5‰. Diffusion coefficients of oxygen with temperature for uraninite and UO 3 can be represented by D (uraninite) = 3.1 × 10 −10 exp (−63981/ RT) and D (UO 3) = 1.3 × 10 −15 exp (−21,197/ RT), where the activation energies for uraninite and UO 3 are 64.0 ± 0.2 kcal/mole and 21.2 ± 0.2 kcal/mol, respectively. Exceptionally low δ 18O values of natural uraninites (i.e., −32‰ to −19.5‰) from unconformity-type uranium deposits in Saskatchewan, in conjunction with theoretical and experimental uraninite–water and UO 3–water fractionation factors, suggest that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals. The low δ 18O values have been interpreted as having resulted from the low temperature recrystallization of primary uranium mineralization in the presence of relatively modern meteoric fluids having δ 18O values of ca. −18‰, despite petrographic and U–Pb isotope data that indicate limited alteration. Therefore, it appears that uranium minerals can exchange oxygen isotopes with later fluids, with only limited actinide remobilization and minor disturbances to their original chemical compositions and textures.

A Rapid In Situ Method for Determining the Ages of Uranium Oxide Minerals: Evolution of the Cigar Lake Deposit, Athabasca Basin

We present a rapid and accurate technique for making in situ U-Pb isotopic measurements of uranium oxide minerals that utilizes both electron and ion microprobes. U and Pb concentrations are determined using an electron microprobe, whereas the isotopic composition of Pb for the same area is measured using a high-resolution ion microprobe. The advantages of this approach are: mineral separation and chemical digestion are unnecessary; homogenous uranium oxide standards, which are difficult to obtain, are not required; and precise and accurate U-Pb ages on ∼10 μm spots can be obtained in a matter of hours. We have applied our method to study the distribution of U-Pb ages in complexly intergrown uranium oxides from the unconformity-type Cigar Lake uranium deposit, Saskatchewan, Canada. In situ U-Pb results from early formed uraninite define a well-correlated array on concordia with upper and lower intercepts of 1467 ± 63 Ma and 443 ± 96 Ma (±lσ), respectively. The 1467 Ma age is interpreted as the minimum age of mineralization and is consistent with the age of clay-mineral alteration (∼1477 Ma) and magnetization of diagenetic hematite (1650 to 1450 Ma) that is associated with these unconformity-type uranium deposits and early diagenesis of the Athabasca Basin sediments. In situ U-Pb isotopic analyses of uraninite and coffinite can document the Pb∗/U heterogeneities that can occur on a scale of 15 to 30 μm, thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits.

Micro-structures associated with uraninite alteration

Primary uraninite from the Canadian Proterozoic unconformity-type uranium deposits have exceptionally low, but variable, δ 18O values (−32 to −15‰). Although these uranium deposits have been studied extensively, the oxygen isotope systematics in uraninite from these deposits are poorly understood. X-ray powder-diffraction patterns of uraninite with both low and high δ 18O values show that these uraninite samples are consistent with a composition of UO 2 and are texturally similar. Micro-diffraction and lattice images obtained using high-resolution transmission electron microscopy (HRTEM) of uraninite with the lowest δ 18O values show that this uraninite is well crystallized and essentially defect-free. Energy dispersive X-ray spectroscopy of well-crystallized uraninite indicates relatively high Pb contents but low Si and Ca contents. In contrast, micro-diffraction and lattice images of uraninite with δ 18O values near −18‰ show that this uraninite is polycrystalline, with micro-diffraction patterns that often show both streaking and concentric patterns. High-resolution images reveal sub-grain formation and rotation, formation of edge dislocations, low-angle grain-boundaries, and bent lattice-fringes. The uraninite is also characterized by relatively high Si and Ca contents and variable Pb contents. This indicates that incipient alteration occurs on a micro-scale as revealed by HRTEM. The sub-grains in this uraninite were preserved either because the annealing process in uraninite was retarded by the Si and Ca impurities, or they were formed during a late alteration event and the uraninite has had little time to anneal.

Geochemical modeling of the formation of an unconformity-type uranium deposit

Petrologic studies of alteration associated with uranium mineralization in unconformity-type uranium deposits have established a detailed paragenetic sequence of the preore and synore alteration. In addition, fluid inclusion studies have determined the presence of two distinct fluids, and thermodynamic analysis of the alteration assemblages has defined their basic characteristics. These fluids were probably basinal brines derived from the overlying sandstones and have been distinguished by their different salinities.Fluids responsible for alteration and ore formation were Na-Ca-Cl brines. Speciation calculations suggest that the hydrothermal fluids responsible for the formation of unconformity-type uranium deposits were distinguished by their high oxygen fugacity and relatively low pH. Uranium was transported mainly as uranyl chloride complexes.Reconstruction of the reactions associated with the formation of the deposits, employing chemical mass transfer calculations, helps unravel the geochemical processes operating during their genesis. Reaction path calculations indicate that Fe present in aluminosilicates in the host rocks is the principal reductant of oxidized uranium present in the ore-forming fluids. The use of variable host-rock lithologies results in the reproduction of mineralogical features unique to each deposit, along with the characteristics common to all unconformity-related uranium deposits. The calculations show that the observed paragenesis is a function not only of the fluid chemistry but also of the host-rock lithologies. The modeling indicates that although an orebody can be produced with or without graphite as a reactant, its presence leads to a better agreement with the observed alteration paragenesis.

Pre-ore hydrothermal alteration in an unconformity-type uranium deposit

Hydrothermal alteration associated with the formation of unconformity-type uranium deposits is manifested mainly by intense chloritic replacement of metamorphic mineral assemblages. At the Koongarra deposit (northern Australia), three stages of pre-ore alteration were identified in chloritized metamorphic muscovite using optical microscopy, the electron microprobe, and transmission and analytical electron microscopy (TEM and AEM). During the first stage metamorphic muscovite was altered to a mica with very low interlayer occupancy (K content as low as 0.35 compared to 1 for ideal muscovite), high octahedral cation content (up to 2.3 whereas ideal muscovite has 2) and high Si content. High resolution TEM images show that these changes in composition were accompanied by frequent layer terminations in contrast to the metamorphic muscovite, whereas selected area electron diffraction patterns indicate that the altered muscovite remained a two-layer polytype. The alteration of muscovite appears more extensive in the outer alteration halo and has been almost completely eradicated in the ore zone where chloritization is more complete. During the second stage of the pre-ore alteration the altered muscovite was replaced by Mg-rich serpentine (lizardite) and clinochlore. High resolution TEM images show two 10 A mica layers coalescing to one 14 A chlorite layer. This type of coalescence appears to involve transformation of one dioctahedral layer of the mica structure to one trioctahedral brucite layer in the chlorite structure through dissolution of the tetrahedral sheets and cation influx in the octahedral site. The final stage of the pre-ore alteration resulted in replacement of lizardite and clinochlore by Fe-rich chlorite. Comparison of the alteration observed in metamorphic muscovite with the alteration observed in metamorphic biotite and garnet shows clearly that chloritization of these phases was also a pre-ore alteration event. Pre-ore alteration was principally characterized by the evolution of chlorite from a Mg-rich phase to an Ferich one.

Hydrothermal alteration and the chemistry of ore-forming fluids in an unconformity-type uranium deposit

Compositions of hydrothermal chlorite and fine-grained white mica from the inner and outer alteration halos in the Koongarra U deposit were analyzed by electron microprobe and analytical electron microscopy. Analyses show that although chlorite and white mica compositions vary considerably outside the main ore zone, they are uniform inside the ore zone. Ore-zone chlorite has a ratio of Fe/(Fe + Mg) of 0.25 and low octahedral occupancy (average 5.5 per formula unit), which may represent a mixture of diand trioctahedral chlorite. White mica has a typical K + Na atomic content of 0.85 per formula unit. These compositions were used to calculate the activity ratios aFe+2/aH+2, aMg+2/aH+2, aK+/aH+, and aNa+/aH+ for the hydrothermal fluids associated with deposition of uraninite. Hydrothermal apatite analyses in conjunction with salinities suggested from fluid inclusion studies were used to calculate the pH of the fluids during the pre-ore alteration. The calculated pH values range from 4.8 to 6.0. Finally, the coexistence of chlorite with quartz and hematite was used to calculate oxygen fugacities. The calculated values are about 2 log units higher than for the hematite-magnetite buffer at 200°C. Consequently, the oxidation state of the fluid lay in the hematite field and U was probably transported as uranyl complexes.

Chemical factors controlling the solubility of uraninite and their significance in the genesis of unconformity-related uranium deposits

A quantitative evaluation of the solubility of uraninite (UO2) in aqueous solutions under hydrothermal conditions was made using previously reported thermodynamic data, so as to inquire into the controlling factors for Canadian unconformity-type ore mineralization as observed in the Athabasca uranium field. The results of solubility calculations suggest that uranyl carbonate complexes, such as UO2CO3o, UO2(CO3)22-and UO2(CO3)34-, predominate under relatively oxidizing and slightly acidic-alkaline conditions and that the uranyl chloride complex, UO2Cl+ is dominant under acidic conditions. These features are predicted at temperatures up to 200 °C over reasonable ranges of CO2 pressure (Pco2) and salinity. Consequently, the physico-chemical parameters, such as oxygen activity (ao2), and pH are regarded as the most important factors controlling uraninite solubility. Judging from the paragenetic sequences observed in most unconformity-type uranium deposits in the Athabasca district, appreciable decreases in the above variables are postulated to have occurred in the stage of principal uranium deposition. Such changes would be due to fluid-mixing phenomenon accompanied by the ‘diagenetic-hydrothermal’ activity (Hoeve and Quirt 1987).

The Cigar Lake uranium deposit, Saskatchewan, Canada

The Cigar Lake uranium deposit is exceptional in terms of both tonnage and grade. Geological reserves are estimated at 154 600 t of uranium with an average grade of 7.9%. The richest zone, called the pod, contains 113 000 t of uranium at 12.2%. The surface projection of the orebody would cover an area equivalent to the Champs ElysCes. The Cigar Lake uranium deposit was discovered in May 1981 by Cogema Canada Limited, acting as operator of the Waterbury Lake Joint Venture. The discovery hole was the 13th and last hole of the 1981 drilling campaign. It is interesting to note that the first showing of uranium in the Athabasca Basin was discovered about 40 years ago. However, progress in the development of exploration models was slow and it was not until 1968 that a first uranium deposit, Rabbit Lake, was finally located at the eastern margin of the Athabasca Basin. This discovery was followed the same year by the Cluff Lake deposits in the Carswell structure and since then by several other discoveries. Cogema has encouraged the scientific study of the unconformity-type uranium deposits. Initial studies were compiled in the 1985 Geological Association of Canada Special Paper entitled f i e Carswell structure uranium deposits, Saskatchewan. Similar support was given for the study of the Cigar Lake deposit in both France and Canada. We now have eight manuscripts to present dealing with the main petrological, mineralogical, and geochemical aspects of this deposit. We would like to thank H. Baadsgaard, M. Bernat, G.N. Breit, M. Cuney, Ph. Holliger, T.K. Kyser, J. Leventhal, K.R. Ludwig, C. Hallenstein, A. Meunier, V. Ruzicka, J. Smellie, L. Turpin, B. Velde, E. von Pechmann, and I. Wendt for having reviewed these manuscripts. The contribution of John Clague, former Editor of the journal, is also greatly acknowledged. We also thank FrCdCric Tona, Mineral Exploration Manager at Cogema, and Michel Poissonet, President of Cogema Canada, for their encouragement in having these results published, as well as all the geologists (H. Knipping, P. Caumartin, J. Learn, S. Lavoie, M. Fowler, and J.P. Fouques) whose work provided a solid geological framework. Their assistance and guidance in the choice of study sample material are appreciated. Finally, we would like to thank our joint venture partners: Cameco, Idemitsu, and Kepco for supporting the publication of these manuscripts.

Retrograde alteration of clay minerals in uranium deposits: Radiation catalyzed or simply low-temperature exchange?

Stable and radiogenic isotopic compositions have been used to trace the origin and age of fluids associated with the formation and alteration of ore deposits such as the unconformity-type uranium deposits in the Athabasca Basin, Saskatchewan, Canada. Since the formation of the U deposits at the unconformity between Aphebian basement rocks and the overlying Athabasca sandstones at ∼ 1200 Ma, the Athabasca Basin has expeienced episodic influxes of meteoric water that permeated large fault structures rooted in the basement graphitic metapelites and gneisses. Because these structures are associated with the initial formation of the U deposits, alteration due to late meteoric water influx is often prominant near the deposits. The development of late-stage kaolinite, lowering of δD-values in all clay minerals in fractures near the ore zones, loss of K2O and decreasing KAr ages with increasing water contents in illite are indicative of retrograde alteration of clay minerals due to influx of late-stage meteoric water in the Athabasca Basin. Recent proposals that radiation damage and subsequent retrograde alteration of clay minerals near the U deposits have occurred are not consistent with either the data from the McArthur River area in Saskatchewan where kaolinite having low δD-values formed with small quantities of remobilized U at ∼ 400 Ma, or from data throughout the basin where clay minerals devoid of U mineralization also record this retrograde alterations. The results from McArthur River, in conjunction with data from a number of unconformity-type U deposits indicate that radiation does not promote retrograde alteration but do substantiate the simpler process of retrograde alteration of clay minerals in these deposits by influxes of late, low-temperature meteoric water along reactivated shear zones.

Stable isotope constraints on the role of graphite in the genesis of unconformity-type uranium deposits

The Key Lake unconformity-type uranium deposit occurs in a shear zone where it intersects the unconformity between Archean and Aphebian gneisses and the overlying Proterozoic Athabasca Group sandstones. The roots of the Key Lake and many other unconformity-type uranium deposits in the Athabasca basin are close to gneisses rich in graphite and most deposits have small amounts of carbonaceous materials (bitumen and hydrocarbon buttons) within and around altered basement and sandstone ore zones. In many Athabasca uranium deposits, hydrothermal fluids have destroyed graphite disseminated in the strongly altered gneisses in the vicinity of the deposits, prompting some to suggest that graphite was converted to CH4, which reduced and precipitated the uranium and partially condensed to form bitumen. The δ13C values of graphite collected from unaltered and altered gneisses around the Key Lake deposit have a limited range (−25 ± 5) and are not a function of distance from the mineralization or the intensity of alteration or deformation. The uniformity of these δ13C values suggests that the destruction of graphite was due predominantly to oxidation by basinal fluids from the sandstone and that the graphite near the deposits did not react to form substantial amounts of 12C-rich phases such as CH4. Most of the bitumen samples, which have higher H/C ratios than the graphite, have δ13C values identical to those of the graphite (−25 ± 5). The similarity in the isotopic compositions of carbon in the bitumen and in the graphite indicates that the bitumen formed from degradation of graphite as a result of reactions with no significant isotopic fractionations, such as ones involving radiolysis of graphite. The hydrocarbon buttons and a few samples of bitumen have petrographic relations and 13C/12C ratios (δ13C values less than −30) that are indicative of reduction of graphite by H2 produced from water by radiolysis. Graphite in these deposits did not play a central role as a reducing agent for uranium, rather it represents a critical structural factor by providing shear zones along which fluid flow can be focussed.

Nd-Sr isotope constraints on the age and origin of unconformity-type uranium deposits in the Alligator Rivers uranium field, Northern Territory, Australia

The results of an Sm-Nd and Rb-Sr isotope study on the large and rich unconformity-type uranium-(gold-platinum-group elements) deposits in northern Australia indicate identical Sm-Nd ages for primary uranium mineralization at Nabarlek (1,616 + or - 50 m.y.) and Jabiluka II (1,614 + or - 132 m.y.), in excellent agreement with new and previously published Rb-Sr data on U ores and altered host rocks. In contrast, the Sm-Nd ages are older than published U-Pb ages which range from 900 to 1,437 m.y. Sm-Nd data on uraninites from Koongarra are strongly scattered, but the geologic setting and Nd model ages for the least disturbed samples suggest an age between 1,600 to 1,650 m.y., identical to the age of primary mineralization at Nabarlek and Jabiluka II. The narrow range of Sm-Nd ages for these three deposits (1,600-1,650 m.y.) indicates that mineralization took place soon after the deposition of unconformably overlying sandstones and volcanics of the 1,650-m.y.-old Kombolgie Formation.Relatively high initial epsilon Nd values and low 87 Sr/ 86 Sr ratios in the ores indicate that ore fluids derived their metal content from two main sources, a preunconformity source comprising the local wall rocks (lower Proterozoic metasediments and amphibolites) and a postunconformity source including the volcano-sedimentary rocks of the Kombolgie Formation which form part of the evaporite-bearing McArthur Group. At Nabarlek, the Kombolgie Formation isotopic signature is obscured by the intensive wall-rock alteration involving isotopically similar amphibolites and dolerites, but mass balance calculations indicate major contributions of U and heavy rare earth elements from sources outside the alteration zone. The most likely external metal sources are again the postunconformity volcanics and sandstones. In contrast, isotopically more evolved sources such as U-rich late Archean basement gneisses and post-tectonic high-level granites are less probable metal sources on the basis of their much lower epsilon Nd and higher 87 Sr/ 86 Sr values, suggesting that the ore-forming hydrothermal activity did not penetrate the metamorphic basement except in the mineralized shear zones.The new data lend support to models of ore deposition that involve reaction of hot, oxidized, U-bearing formation-meteoric waters, whose source is the postunconformity rocks (e.g., the Kombolgie Formation), with reducing metasediments of the lower Proterozoic basement. Highly altered volcanics within the Kombolgie Formation are favored as major U sources.The origin of the mineralization at Ranger may have involved different processes. Recent U-Pb data indicate a 1,737-m.y. age for mineralization at this deposit, significantly earlier than that of the other deposits and, most importantly, earlier than that of deposition of the Kombolgie Formation. The Sm-Nd data for Ranger uraninites do not form an isochron but indicate a possible age range from 1,600 to [asymp] 1,750 m.y. Data resolution is insufficient to allow reliable distinction between a pre- or post-Kombolgie age. However, Nd-Sr initial ratios for Ranger point to unreasonably high epsilon Nd and high 87 Sr/ 86 Sr value sources when calculated for any age younger than 1,650 m.y., the age of Kombolgie sedimentation and basin formation. In contrast, uraninite Nd-Sr initial ratios at 1,737 m.y. are easily explained as mixtures involving Nd and Sr leached from various lower Proterozoic and late Archean sources. The data are thus more compatible with a pre-Kombolgie origin for uranium mineralization at Ranger.

The age and origin of unconformity-type uranium deposits, Sm-Nd and Rb-Sr isotopic evidence

The age and origin of unconformity-type uranium deposits, Sm-Nd and Rb-Sr isotopic evidence

Near-surface lithogeochemical halo as an aid to discovery of deeply buried unconformity-type uranium deposits, Athabasca Basin, Canada

Unconformity-type U deposits are being found to depths of more than 400 m where graphitic rocks underlie the Proterozoic sandstone unconformity. Exploration to date involves drilling weak electromagnetic conductors that reflect the graphitic basement. The deposits are believed to have formed through circulation of heated formation waters which discharged essentially vertically above the deposits. Geochemically detectable zones of alteration are reported for cross-sections through the Deilmann, Midwest, and McClean deposits. Information on the areal dimensions of near-surface haloes was required to justify the drilling of grids of shallow holes along conductors or favourable structures as a cost-effective exploration alternative to deep holes, and to define grid spacings. Orientation surveys were undertaken by sampling the uppermost 10 m of sandstone in a selection of holes from the Cigar Lake, Dawn Lake, and Wolf Lake areas where the deposits are, respectively, 400–430 m, 100–160 m, and 180–210 m below surface. The first two deposits have published reserves of 385 and 30–40 million pounds U 3O 8, respectively, while Wolf Lake is much smaller. Well-developed anomalies lie vertically above the richest portion of the Cigar Lake deposit in an area 150 × 600 m and a weak anomaly in many elements continues over the 1800-m length that was sampled. Total and leachable U are 13 to 8 times background with maximum values of 53 and 26 ppm, respectively. Anomalies of 14 to 8 times background are also present for V, Na 2O and Zn, and 5 to 3 times background for Sr, Y, Pb, Th, As, and P. Of the four Dawn Lake mineralized zones, only the 11B Zone was studied, with partial coverage of the 11A Zone. Anomalies 50–100 m by 250–600 m in size are often displaced up to 100 m northeast of the 11B Zone. B, MgO, Pb and both total and leachable U anomalies are 16 to 8 times background with Y, V and As 7 to 5 times background. By contrast, at the Wolf Lake deposit, only irregular variations occur for most of these elements. The unusual patterns may reflect post-alteration tectonism of the sandstone overlying the deposit as regional thrust faulting occurs nearby. Detection of near-surface lithogeochemical haloes using grids of shallow holes would facilitate discovery of deep deposits and reduce expensive, deep drilling.

Uranium in early Proterozoic phosphate-rich metasedimentary rocks of east-central Minnesota

Exploration for unconformity-type uranium deposits in the late 1970s in east-central Minnesota led to the discovery of several uranium-bearing phosphorite occurrences in rocks of early Proterozoic age. In this report the authors use the term phosphorite for a rock or specimen that contains substantial sedimentary apatite (Altschuler et al., 1958). The deposits in Minnesota are especially interesting because of their high uranium content but low metamorphic grade. These occurrences characteristically contain 0.025 to 0.085 percent U and locally as much as 0.157 percent U (Ullmer, 1981), whereas typical primary marine phosphorites have uranium contents of 0.005 to 0.02 percent U (Altschuler et al., 1958). The presence of uranium in a marine phosphorite generally is explained by either the replacement of calcium in the apatite crystal structure or the adsorption of uranium in admixed organic matter and cryptocrystalline apatite. In east-central Minnesota the uranium is closely associated with the finely crystalline apatite, but the uranium has also been involved in several episodes of remobilization and redeposition. Thus, even though the phosphorite deposits are an interesting geologic phenomenon in themselves, they also are important as a possible source for epigenetic uranium deposits that may occur in the area.

The results of exploration for unconformity-type uranium deposits in east-central Minnesota

East-central Minnesota is one of the few areas in the United States with geologic characteristics grossly similar to provinces in Canada and Australia that host high-grade, large tonnage, unconformity-type uranium deposits. These deposits constitute about 25 percent of the world's reasonably assured reserves of uranium. They commonly carry grades ranging from 0.2 to greater than 5 percent U/sub 3/O/sub 8/. In addition, the uranium is often accompanied by economic quantities of nickel, cobalt, gold, and silver. Their large size and high grade make these deposits particularly attractive exploration targets in the typically unstable uranium market of recent years. This report is a synthesis of much of the information collected by many geologists who explored the region and from my own experience in the area. Much of the exploration data and drill core from various private companies, now stored with the Minnesota Department of Natural Resources, was available for this review.

Sm-Nd direct dating of the Collins Bay hydrothermal uranium deposit, Saskatchewan

Primary pitchblende mineralization from the Collins Bay unconformity-type uranium deposit in northern Saskatchewan has been dated using the Sm-Nd method. The mineral isochron age of 1281 +/- 80 Ma is in good agreement with other geologic and geochronological data, and demonstrates the potential of the Sm-Nd technique for the direct dating of hydrothermal mineral deposits. 23 references, 5 figures, 7 tables.

Geochemical exploration for unconformity-type uranium deposits in permafrost terrain, Hornby bay basin, Northwest territories, Canada

The Hornby Bay sandstone basin of the Northwest Territories represents one of three areas in Canada in which the geological environment is favourable for the occurrence of unconformity-type uranium deposits. The project area lies within the zone of continuous permafrost and is associated with relatively thin and locally derived tills and abundant outcrops. Topographic relief is normally in the order of 100 m but may locally exceed 300 m/km in more rugged areas. Geochemical case histories for soil and lake-sediment surveys illustrate: identification of regional anomaly, anomaly follow-up, and detailed investigations. Success in locating uranium occurrences using geochemical surveys is high. Orientation studies trace anomalous dispersion trains 10 to 200 m downslope of recognized sources. Anomaly generation is favoured by active hydromorphic processes in geochemically homogeneous overburden containing an average background of 0.5 ppm uranium (using a 4 N nitric acid leach). Anomaly contrasts are typically 2 X to 3 X this value, and anomalous values exceeding 1.5 ppm uranium are highly reproducible. Copper and radium anomalies act as pathfinders for uranium. Lead is present in anomalous concentrations in specimens from the uranium occurrences but lead enrichment in overburden is not sufficiently great to permit its use as a pathfinder element. Preliminary work based on radon and 210Po has not led to identification of anomalies other than those recognized by more routine geochemical surveys. Association of geochemical anomalies with faults within the sandstone basin constitutes one of four conditions elevating an area to drill target status. Methods for drill testing of geochemical soil anomalies depend on the relationship between the geochemical anomaly and an identifiable fault zone.