Athabasca Group Research Articles

Architecture of pre-vegetation sandy-braided perennial and ephemeral river deposits in the Paleoproterozoic Athabasca Group, northern Saskatchewan, Canada as indicators of Precambrian fluvial style

The Late Paleoproterozoic Athabasca Basin contains more than 1.5 km of predominantly sandy strata, most of which are of braided fluvial origin. In the eastern part of the basin, at McClean Lake, sandstones and minor conglomerates of the Read Formation at the base of the succession are preserved within a steep-walled valley system. They consist predominantly of meter-scale sheet elements, characterized by massive and flat-laminated fine- to very coarse-grained sandstone with minor discontinuous cobble and boulder conglomerate along lower set boundaries. These are interpreted as sheet-flood and stream-flood deposits of a terminal dry-land system, deposited in an arid climatic setting. Analysis of closely spaced drill-core indicates that late stage run-off was confined to small shallow channels, typically only a few tens of meters wide and a few meters deep. Overlying strata of the Bird Member of the Manitou Falls Formation are predominantly coarse- to very coarse-grained sandstones with abundant small-scale cross-stratification and minor granule and small pebble conglomerate. At McClean Lake, these appear to have been deposited as sheet-elements by semi-ephemeral to perennial braided rivers under more humid conditions. At Key Lake, 160 km to the southwest, architectural analysis of a 184 m wide section indicates that at least part of the Bird Member was deposited in deeper, sandy-braided rivers characterized by seasonally varied flow. The closest modern analogue is the South Saskatchewan River, in which large simple flow-transverse bedforms become exposed and dissected during falling stage, and act as nuclei for sand-flat development. The presence of numerous low-relief 4th order surfaces suggests continued reworking of bar-tops during rising stage. The incremental character of downstream accretionary elements suggests periodic migration of barforms during peak-flood stages, separated by periods of low flow. This indicates similarities with modern perennial braided systems, and is counter to the idea that all pre-Devonian fluvial systems should consist of stacked sheets formed by individual flood events. Thin gravel layers accumulated as lags on fourth order surfaces, with discontinuous mud layers suggesting deposition within temporary ponds in channel thalwegs after major floods. Strata in overlying units indicate a return to semi-perennial conditions.

Mineralogy and Geochemistry of the Host-Rock Alterations Associated with the Shea Creek Unconformity-Type Uranium Deposits (Athabasca Basin, Saskatchewan, Canada). Part 2. Regional-Scale Spatial Distribution of the Athabasca Group Sandstone Matrix Minerals

AbstractThe spatial distribution of the dominant matrix minerals present in the middle-Proterozoic Athabasca Group sandstone (kaolin, illite, sudoite, dravite, hematite) was studied at a regional scale in the Shea Creek region (Saskatchewan, Canada), in which two epigenetic unconformity-type uranium deposits have been discovered. 3D models of matrix mineral distribution were derived from normative mineral calculations and 3D interpolation using whole-rock geochemical analyses of sandstone samples collected from both mineralized and barren areas. The calculations were constrained by information obtained from petrographic and crystal-chemical clay mineralogical studies on representative samples. The 3D mineral distribution models were compared to the lithostratigraphy and structural features of the Athabasca Group sandstone to ascertain the source and mobility of the main elements involved in the sandstone host-rock alteration processes related to the U mineralization. The distribution of Al is conformable with the lithostratigraphy throughout the studied area, regardless of proximity to basement-rooted structures and U ore bodies. The distribution of illite displays similar features, but the intensity of the illitization of kaolin decreases with increasing distance from the structures and U ore bodies. Hematite bleaching and neoformation of sudoite and dravite were restricted to the vicinity of the fault zones above the U ore bodies. The spatial configurations of the mineral anomalies show that syn-ore fluids flowed from the basement towards the sandstone cover via the fault zones, as described in current metallogenic models. Although Al remained immobile (mass transfer), the anomalous K, B and Mg present in the host-rock alteration haloes were probably imported from the basement rocks (mass transport). Unlike B and Mg, K migrated laterally at least several kilometers from the basement-rooted faults. The mineral distribution models were used to quantify the volume of altered sandstone (10−2−10−1 km3) and the amounts of K, Mg and B which were imported to the alteration haloes above the Shea Creek U ore bodies: 186,000 t of K, 66,000 t of Mg, and 11,000 t of B above the Anne ore body, and 24,000 t of K, 185,000 t of Mg, and a similar 11,000 t of B above the Colette ore body.

Diagenetic fluorapatite and aluminum phosphate–sulphate in the Paleoproterozoic Thelon Formation and Hornby Bay Group, northwestern Canadian Shield

In the northwestern part of the Canadian Shield, fluorapatite and a rare-earth element-bearing hydrated aluminum phosphate–sulphate mineral (APS) occur as cements in continental successions near the base of the Paleoproterozoic Thelon Formation (Thelon Basin) and Hornby Bay Group (Hornby Bay Basin). These minerals occupy interstitial sites, form euhedral crystals, display micro-scale zonation, make up part of an unmetamorphosed paragenetic assemblage, and are distributed in correlative units across thousands of square kilometres, suggesting a diagenetic origin. Stratigraphy, geochronology, and other lines of evidence suggest that the Thelon Formation and Hornby Bay Group containing these phosphatic cements, as well as the Ellice Formation and Athabasca Group, are correlative and may have been originally interconnected. The evidence suggests that the basal Thelon Formation and the Hornby Bay Group underwent similar, and approximately coeval, diagenetic mineral paragenesis. Furthermore, the diagenetic fluids in these different locations must have been remarkably similar, especially those that produced the delicate APS mineral. Compared to phosphatic cements in the Hornby Bay and Thelon basins, unmineralized sandstone in the Athabasca Basin contains "crandallite group" and fluorapatite cements higher in the basin fill sequence (Wolverine Point Formation) in tuffaceous sandstone and as relatively early cement in the paragenetic sequence.

Thermodynamic constraints on the mineralogical and fluid composition evolution in a clastic sedimentary basin: the Athabasca Basin (Saskatchewan, Canada)

To evaluate the critical parameters controlling the mineralogical evolution of a dominantly clastic sedimentary basin, the Athabasca Basin (Saskatchewan, Canada), activity diagrams have been constructed with respect to the mineralogy, thermo-barometry, and fluid composition documented from previous studies. Relative to standard diagrams, which exist for ideal clay minerals, the present diagrams are the first to include all of the significant minerals present in the sediments, accounting specifically for the occurrence of dravite, sudoite, and two types of illite having identical chemical compositions, but different crystal morphologies and structures. The occurrence of the different mineralogical assemblages is mainly controlled by the K + /H + and Mg 2+ /(H + ) 2 activity ratios, and the silica and B(OH) 3 activities in addition to temperature and pressure. Variations in the activities of the major brine components (Cl - , Na + , Ca 2+ ) and the oxygen fugacity (above the hematite-magnetite buffer) have little effect on the mineral stabilities. The mineralogical evolution of the Athabasca Group sandstone results from mass transport processes involving a basement-derived fluid, richer in K, Mg, B, and/or pH, and possibly undersaturated in silica. The zoned distribution of the minerals in the uranium-mineralized Shea Creek area can be explained by a progressive interaction of the basement-derived fluid with the diagenetic brine and minerals of the Athabasca Group sandstone. During peak diagenesis, the brine in the basal Athabasca Group was buffered by the assemblage I Mc illite-kaolinite. During the syn-ore stage, the estimated values of log(K + /H + ), log(Mg 2+ /(H + ) 2 ) and B(OH) 3 activity decrease from 3.9, 6.4, and 0.005 in breccia zones immediately above the orebody, down to 2.7-3.6, 4.7-6. 1, and 0.004 in the inner alteration halo, and down to 2.7, 4.9, and 0.003 in the outer alteration halo, respectively.

Physical rock properties from the Athabasca Group: designing geophysical exploration models for unconformity uranium deposits

Multi-parameter borehole geophysical measurements were conducted in 11 boreholes in the eastern and western Athabasca basin as part of the joint Geological Survey of Canada (GSC)/industry exploration technology initiative called EXTECH IV to determine the geophysical characteristics of uranium deposits and their host rocks. The multi-parameter measurements included natural γ-ray spectrometry, resistivity, density and compressional wave velocity (Vp). Laboratory physical rock property measurements including resistivity, density, Vp and porosity were also made on several samples selected from the sandstone and basement rocks in the eastern Athabasca basin. These in situ physical rock property data were correlated with laboratory measurements and drill-core geological data. The Athabasca sandstones overlying the uranium deposits have a relatively uniform bulk mineralogical composition but the resistivity, density and Vp vary as a function of porosity and pore fluid conductivity. Physical properties of basement quartzites and gneisses are only slightly different from the overlying silicified sandstones, but graphitic zones are dramatically different in conductivity. Variations in acoustic properties (density, Vp and acoustic impedance) in the sandstone depend on the degree of silicification. Highly silicified zones have higher acoustic impedance than the unsilicified sandstone units and therefore seismic reflectivity in the sandstone is a function of silicification. The basement contact is recorded as a velocity gradient; the acoustic impedance of the sandstone increases downward to the contact. The physical rock property data indicate that the sandstone–basement contact can be imaged by electrical and seismic methods. High-resolution gravity measurements may also provide basement structural information. Alteration zones as vectors to uranium ore may also be mapped via significant variations in conductivity, acoustic impedance and density between altered and unaltered formations.

Mineral chemistry and oxygen isotopic analyses of uraninite, pitchblende and uranium alteration minerals from the Cigar Lake deposit, Saskatchewan, Canada

The Cigar Lake unconformity-type U deposit is one of the largest and highest grade U deposits in the Proterozoic Athabasca Basin, northern Saskatchewan, Canada. Cigar Lake has recently been the focus of an international, 3-a, collaborative program in which this U deposit was studied as a natural analogue for a spent nuclear fuel repository. The deposit is located near the eastern margin of the Athabasca Basin, 430 m below the surface, at the intersection between Hudsonian-age faults and the unconformity between Athabasca group sandstones and Aphebian metasediments. Three stages of U mineralization have been identified based on cross-cutting relationships and textures observed in thin section and back-scattered electron (BSE) images, O isotope data and chemical compositions. All stages of U mineralization have been variably altered to Ca-rich, U-hydrate minerals or uranyl oxide hydrate minerals and coffinite. U Pb chemical ages of the 3 stages of U mineralization from Cigar Lake coincide with the 3 major fluid events that precipitated clay and silicate minerals at 1500 Ma, 950 Ma, and 300 Ma, throughout the entire Athabasca Basin. Stage 1 and 2 uraninite and pitchblende have the lowest δ 18O values that range from −30.1 ‰ to −15.2‰; whereas, stage 3 uraninite has δ 18O values ranging from −10.0‰ to −3.4‰. Uranyl oxide hydrate minerals have δ 18O values that range from −11.3‰ to −8.2‰; whereas, uranyl minerals have much higher δ 18O values. Based on U Pb chemical ages, δ 18O values, and petrographic relationships of U alteration minerals associated with primary U mineralization, the Cigar Lake U ore is similar to U ore from other unconformity-type U deposits in the Athabasca Basin. Therefore, the Cigar Lake ore deposit, although surrounded by clay and sandstone barriers, has been effected by the same fluid events that have altered other unconformity-type U deposits in the Athabasca Basin. The 3 stages of ore formation and associated alteration minerals permit the detailed study of fluids responsible for U deposition and alteration. This information provides the necessary context for the evaluation of the Cigar Lake deposit as a “natural analogue” for the disposal of spent nuclear fuel in underground vaults in rocks of the Canadian Shield.

Geological environment of the Cigar Lake uranium deposit

The Cigar Lake uranium deposit occurs within the Athabasca Basin of northern Saskatchewan, Canada. Like other major uranium deposits of the basin, it is located at the unconformity separating Helikian sandstones of the Athabasca Group from Aphebian metasediments and plutonic rocks of the Wollaston Group. The Athabasca Group was deposited in an intra-continental sedimentary basin that was filled by fluviatile terrestrial quartz sandstones and conglomerates. The group appears undeformed and its actual maximum thickness is about 1500 m. On the eastern side of the basin, the detrital units correspond to the Manitou Falls Formations where most of the uranium deposits are located. The Lower Pelitic unit of the Wollaston Group, which lies directly on the Archean basement, is considered to be the most favourable horizon for uranium mineralization. During the Hudsonian orogeny (1800–1900 Ma), the group underwent polyphase deformation and upper amphibolite facies metamorphism. The Hudsonian orogeny was followed by a long period of erosion and weathering and the development of a paleoweathering profile.On the Waterbury Lake property, the Manitou Falls Formation is 250–500 m thick and corresponds to units MFd, MFc, and MFb. The conglomeratic MFb unit hosts the Cigar Lake deposit. However, the basal conglomerate is absent at the deposit, wedging out against an east–west, 20 m high, pre-Athabasca basement ridge, on top of which is located the orebody.Two major lithostructural domains are present in the metamorphic basement of the property: (1) a southern area composed mainly of pelitic metasediments (Wollaston Domain) and (2) a northern area with large lensoid granitic domes (Mudjatik Domain). The Cigar Lake east–west pelitic basin, which contains the deposit, is located in the transitional zone between the two domains. The metamorphic basement rocks in the basin consist mainly of graphitic metapelitic gneisses and calcsilicate gneisses, which are inferred to be part of the Lower Pelitic unit. Graphite- and pyrite-rich "augen gneisses," an unusual facies within the graphitic metapelitic gneisses, occur primarily below the Cigar Lake orebody.The mineralogy and geochemistry of the graphitic metapelitic gneisses suggest that they were originally shales. The abundance of magnesium in the intercalated carbonates layers indicates an evaporitic origin.The structural framework is dominated by large northeast–southwest lineaments and wide east–west mylonitic corridors. These mylonites, which contain the augen gneisses, are considered to be the most favourable features for the concentration of uranium mineralization.Despite the presence of the orebody, large areas of the Waterbury Lake property remain totally unexplored and open for new discoveries.

The age of unconformity-related uranium mineralization in the Athabasca Basin, northern Saskatchewan

Age data are presented for major Athabasca Basin uranium deposits at Cigar Lake, Cluff Lake, Collins Bay, Dawn Lake, Eagle Point, McArthur River, Midwest, and Rabbit Lake, as well as for several minor or undeveloped deposits, including Hughes Lake and Nisto. The best constrained data indicate that almost all the deposits formed in a restricted time interval between about 1330 and 1380 Ma. This range of ages is believed to be real and not the result of uncertainties in the calculation of ages based on discordant data. The one major exception is the recently discovered NiAs-free deposit at McArthur River, for which a well-determined age of 1514 ± 18 Ma (2σ) has been obtained. Even this deposit yields an age in the1330–1380 Ma range for some material. Periods of reworking–redeposition occurred at ~1280, ~1000, ~575, and ~225 Ma. These may be basin-wide, affecting to some degree all the deposits that we have studied. Other times of redeposition are less well determined, but may be present as well. No ages that approach the ~1700 Ma age of the Athabasca Group have been found to date for unconformity-related deposits, and the Athabasca Basin mineralization is unrelated to the ~1750 Ma pitchblende vein deposits in the Beaverlodge Lake area.

Mineralogy and U/Pb, Pb/Pb, and Sm/Nd geochronology of the Key Lake uranium deposit, Athabasca Basin, Saskatchewan, Canada

The Key Lake deposit is one of several large, high-grade, unconformity-related uranium deposits located at the eastern margin of the Athabasca Basin in northern Saskatchewan, Canada. The deposit consists of the Gaertner orebody, now mined out, and the Deilmann orebody, which is presently being mined. In the past, radiometric dating efforts yielded an age of oldest ore-forming event of 1250 ± 34 Ma at the Gaertner orebody and 1350 ± 4 Ma at the Deilmann orebody. This unlikely age difference called for further investigation. Innovative preparation techniques were used to separate the paragenetically oldest U mineral, an anisotropic uraninite. Ore microscopy and U/Pb isotopic data show that the oldest event of uranium emplacement occurred simultaneously at the two orebodies, at 1421 ± 49 Ma. The primary ore-forming phase was followed by younger generations of U mineralization and periods of remobilization. Sm/Nd data of Key Lake uraninite form an isochron corresponding to an age of 1215 Ma. This is interpreted as the age of a uranium remobilization or a new mineralizing event. The lead found in the Athabasca Group above the Deilmann deposit and in galena appears to be a mixture of a common lead and radiogenic lead mobilized from the orebody over a time span of at least 1000 Ma.

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.

Stable isotope geochemistry of alteration associated with the Key Lake uranium deposit, Canada

The Key Lake uranium deposit occurs within a shear zone that intersects the unconformity between early Proterozoic basement gneiss and overlying middle Proterozoic Athabasca Group sandstone. Uranium is concentrated near the unconformity and is associated with intense illite, chlorite, and kaolinite alteration. The delta D and delta 18 O values of the clay minerals suggest that several different fluids have affected the ores and surrounding host rocks. Alteration of the basement gneisses associated with the uranium mineralization consists mainly of Mg chlorite that equilibrated about 200 degrees C with a fluid having a delta D value near -20 per mil and a delta 18 O value near +5 per mil. An illite-rich sandstone alteration halo, preserved only locally above the deposit, equilibrated about 200 degrees C with a basinal brine having a delta D value near -65 per mil and a delta 18 O value near +5 per mil. The uranium was concentrated near the unconformity by the mixing of these two distinct fluids. The delta 18 O values of coexisting quartz and illite sampled from the shear zone hosting the uranium mineralization yield equilibration temperatures of 190 degrees to 230 degrees C, which is postulated to be the temperature of the ore-forming event.Fracturing in the sandstone above the deposit controlled late, low-temperature (50 degrees C) kaolinitization of preexisting hydrothermal illite and chlorite by meteoric waters having delta D values of -50 + or - 30 per mil and delta 18 O values of -7 + or - 3 per mil. The youngest event to affect the Key Lake deposit is related to the influx of modern meteoric waters that have lowered the delta D values but not greatly affected the delta 18 O values of some of the clay minerals associated with the shear and breccia zones. This latest event has recrystallized much of the hydrothermal Mg chlorite associated with the uranium to produce an Fe-Mg chlorite with delta D values as low as -170 per mil.

Unusual organic matter associated with uranium from the Claude Deposit, Cluff Lake, Canada

Blebs and pods of organic matter (0.2-7.0 cm in diam) are associated with uranium mineralization in all the major deposits (D, N-R-F, Claude, OP, and Dominique-Peter zones) of the Cluff Lake area in northwestern Saskatchewan. These deposits are hosted in strongly altered quartzofeldspathic, graphitic pelitic, and granitoid gneisses of suspected Archcan age and locally, as in the D zone, in the basal section of the unmetamorphosed middle Proterozoic Athabasca Group sediments, which comprise conglomerate, sandstone, and siltstone. Samples of the organic matter from the Claude deposit contain up to 12 percent uranium, principally as small uraninite grains (10-40 mu m in diam) and minor coffinite. Associated mineral grains include pyrite, marcasite, chalcopyrite, galena, molybdenite, jordisite, gersdorffite, and altaite.The uranium and its daughters have caused massive radiation damage that is expressed as complexly zoned halos approximately 50 mu m wide in the surrounding organic material. Stable isotope determinations of the bulk organic matter give a delta 13 C value of -44 per mil vs. PDB, whereas occluded gases CH 4 and CO 2 , produced by irradiation, show delta 13 C values of -48 and -32 per mil vs. PDB, respectively. Pyrolysis-gas chromatography-mass spectrometry of the sample yields mainly one- and fused-ring aromatic molecules from the organic matter that are out of range of the alpha particles from the uraninite grains.The presence of possible flow structures in the organic matter and the angular nature of the mineral inclusions suggest that most of the minerals were broken from vein walls and mixed into the organic matter as it filled the veins. Thus, their presence in the organic matter is the result of mechanical processes that occurred after the minerals were originally precipitated.

Crandallite group minerals in the Helikian Athabasca Group in Alberta, Canada

Members of the crandallite group of aluminous hydroxy phosphates are present in trace amounts in every formation of the Athabasca Group in Alberta. The minerals of the group present in Alberta form a solid-solution series with end members goyazite (SrAl3(PO4)OH5), crandallite (CaAl3(PO4)OH5), and gorceixite (BaAl3(PO4)OH5). These minerals are present as cubes and subhedral grains, 2–20 μm across, in isolation or in clusters interstitially in the Athabasca Group sandstones, siltstones, and tuffs. Petrographic study indicates an authigenic origin for the crandallite-group minerals in the Athabasca Group. Their presence, locally, beneath quartz overgrowths and early diagenetic fluorapatite suggests formation very early in the post-depositional history of the rock. The presence of the crandallites within the regolith beneath the Athabasca Group and within a metamorphic rock fragment incorporated into the sandstone suggests more than one origin for the minerals and possibly a detrital component.

Sr isotopic study of Helikian sediment and diabase dikes in the Athabasca Basin, northern Saskatchewan

The Athabasca Group overlies 1.8 Ga or older basement of the Churchill Province and predates uranium mineralization at 1.3 Ga. Cross-cutting diabase dikes have been dated by Rb–Sr mineral isochrons at 1.31 ± 0.07 Ga at Diabase Peninsula, Cree Lake, and 1.16 ± 0.04 Ga at Lazy Edward Bay, Cree Lake. Initial 87Sr/86Sr ratios for these dikes are 0.7040 ± 0.0005 and 0.7036 ± 0.002, respectively. Clastic sedimentary rocks of the Athabasca Group normally show limited variation in Rb/Sr and variable initial 87Sr/86Sr. Sedimentary rocks in two areas near the northern margin of the Athabasca Basin scatter about isochrons of 2.6 Ga, reflecting Archean provenance of detrital material and lack of Sr isotopic homogenization since deposition. Sediment of the tuffaceous Wolverine Point Formation, near Rumpel Lake, is much more variable in Rb/Sr and enriched in Rb relative to Sr and defines a 1.43 ± 0.03 Ga isochron with a 0.7057 ± 0.0005 initial ratio. This is possibly the age of deposition of the Athabasca Group.

Geochemical zoning around the McClean uranium deposits, Saskatchewan, Canada

Some 3400 drill core samples of the Athabasca Group and underlying regolith have been obtained by sampling 8-m sections from 55 diamond drill holes in the McClean Lake Area, Saskatchewan. Forty eight holes are from sections over six areas known to contain uranium mineralization and 7 are from unmineralized areas. The uranium zones are covered by 30 to 180 m of Athabasca sandstones and conglomerates. The uranium mineralization of the McClean deposits can be described as belonging to two different facies; a more reduced sulphide-arsenide facies and a more oxidized hematite-“bleached” facies, superimposed on any one of three host rocks. The sulphide-arsenide facies is probably older and, except in one deposit, it forms a patchy distribution of relicts. The principal pattern is an upwards increase in sulphur/arsenic ratio. The hematite-“bleached” facies appears to be younger and may represent a reworking of the earlier sulphide-arsenide facies mineralization under more oxidizing conditions. The trace metals can be grouped according to their redox behaviour. Vanadium, Mo and U, occurring as oxides, form one group while Ni, Co, Zn, Cu and As, occurring as sulphides and/or arsenides, form intermediate and most reduced groups, respectively. Iron is concentrated in the most oxidized facies as hematite or goethite and in the most reduced facies as pyritebravoite, siderite and chlorite. Manganese follows the distribution of siderite. The ratio of oxidized to reduced minerals can be represented by the ratio of U/Ni. This ratio can be used to estimate the variation of redox potential in the deposit at the time of deposition or alteration. The principal pattern is one of systematic increase in U/Ni ratio or oxidation potential both upward and to the northeast in each of the hematite and bleached facies. A generalized Eh-pH diagram is used to qualitatively describe the significance of each mineral facies. The U/Ni ratio of the transition between the hematite and “bleached” facies increases upwards. The phase diagram suggests that a possible cause is an upward decrease in pH and increase in Eh. The overall distribution is compatible with the origin of the deposits due to reduction at the interface between a plume of reductant emanating from fractures in the sub-Athabasca basement and an oxidizing aquifer moving southwestward in the sandstone. This model does not accurately predict the distribution of elements among the seven McClean Lake pods. However, it explains the overall pattern. Local variations in metal ratios in solution either in time or reflecting local provenance could be adduced to explain local deviations from the overall pattern. Clay alteration, apparently largely contemporaneous with the earliest mineralization, is largely illitic but with sporadic occurrences of xenotime and goyazite. Thus K and to some extent P, Y and light REE's are also enriched to some degree in the deposits. Uranium analysis of the drill core shows that there is little movement of U into the overlying sandstones from basement rocks and regolith that contain no uranium deposits. Uranium in the Athabasca sandstone from these areas averages less than 1 ppm. However, where uranium zones have been found in the basement rocks, regolith and lower Athabasca sandstone, U values greater than 2–3 ppm consistently occur in the overlying sandstones at or near surface. There appear to be two types of U anomalies, those that flank the deposits and those that directly overlie them. Both types exhibit vertical and lateral continuity. They are associated with vertical to steeply dipping fracture systems that traverse the Athabasca sandstone from basement to surface. The fractures are thought to be related to diagenetic processes from compaction of the sedimentary rocks over basement irregularities as well as tectonic processes (i.e. re-adjustment along old basement structures). These results suggest that target areas containing deeply buried uranium deposits could be defined by U analyses of the Athabasca sandstone from quite widely spaced holes of limited depth. This conclusion can be compared to XRD and chemical studies of clay mineral distribution within the Athabasca sandstone about the deposits.

The application of lithogeochemistry in the search for unconformity-type uranium deposits, Northern Saskatchewan, Canada

Lithogeochemical-mineralogical haloes around unconformity-type uranium deposits in northern Saskatchewan can expand the size of drill targets up to fifteen times. The deposits are located at or near the unconformity between Aphebian metamorphosed basement rocks in close proximity to Archean granitoids and overlying unmetamorphosed sandstones of the Helikian Athabasca Group. Deposits studied include Key Lake (Deilmann), Midwest Lake and Eagle Point. Unconformity-type deposits are associated with broad alteration haloes in the overlying sandstones and more restricted haloes in the basement rocks. The haloes in the sandstone are localized around steeply dipping fault structures and are characterized by zones of intense leaching of specular hematite and detrital heavy mineral layers and by changes in chemistry related to clay mineral alteration and tourmalinization. Clay mineral alteration haloes are common in the sandstone and basement host rocks. Interstitial clays consist mainly of kaolinite and illite with lesser amounts of chlorite. The ratio K 2O/AI 2O 3 proved useful in delineating illite-kaolinite patterns in the sandstone and in the uppermost portions of the paleoweathering profile in basement rocks. The Midwest deposit is characterized by a broad bell-shaped zone (500 m across strike) of high K 2O/AI 2O 3 ratios (>0.18) in which illite is the dominant clay mineral. The sandstone above the Deilmann deposit, on the other hand, is characterized by silicification and kaolinitization with low K 2O/AI 2O 3 ratios (<0.04). This kaolinite cap overprints a preexisting illite zone. Anomalously high boron values are characteristic of the three deposits considered in this study. Boron anomalies are similar in extent to the anomalous clay mineral alteration haloes. Altered sandstones commonly contain aggregates of radiating magnesium-rich dravite needles within the clay matrix. The interpretation of boron patterns is problema tical however, mainly because of the detrital tourmaline component in sandstones and metamorphic tourmalines present in the Aphebian metasediments. Trace elements such as U, Ni, As and Co are generally of limited use in expanding targets in sandstone because their haloes are restricted to a few tens of metres horizon-anomalies along the steeply dipping zones of mineralization related to fault structures. Uranium (>3 ppm) does form an anomaly more than 200 metres laterally across the Midwest deposit. At the Deilmano deposit anomalous uranium dispersion is restricted to within a few metres of high grade ore. In the basement rocks, the various layers of the paleoweathering profile are geochemically overprinted up to 250 m from mineralization. Bleaching related to illitization and chloritization is associated with enrichment in K 2O, MgO, B, S, U, As, Ni and P 2O 3. The ratio Fe 2O 3/MgO is useful in delineating chloritization in the upper portion of the paleo weathering profile. Deposits with large root extensions in the Aphebian metasediments such as Eagle Point, have intense dravite-chlorite-illite alteration zones which are restricted to within a few metres of mineralization across strike. ‘Quartzite” units are alteration related. The complexity of the basement lithology inhibits the use of individual elements as alteration guides other than in the intensely altered zone. The application of multivariant techniques, element ratios and clay mineralogical work prove useful in identifying the mineralogical changes at Eagle Point.