Mineralogy and UIPb, PbIPb, and SmINd geochronology of the Key Lake uranium deposit, Athabasca Basin, Saskatchewan, Canada C. CARL^ Bundesanstalt fir Geowissenschaften und Rohstoffe, P. 0. Box 510153, D(W)3000 Hannover 51, Gennany E. VON PECHMANN~ Uranerzbergbau-GmbH, Kolner Strasse 38-44, D(W)5047 Wesseling, G e m n y A. HOHNDORF Bundesanstalt &r Geowissenschaften und Rohstoffe, P. 0. Box 510153, D(W)3000 Hannover 51, Germany AND G. RUHRMANN CAMECO, 2121 - 11th Street West, Saskatoon, Sask., Canada S7M 153 Received May 14, 1991 Revision accepted December 12, 1991 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 Deilrnann 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 f 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 f 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. Le gite du lac Key est un parmi les nombreux grands et riches dCp6ts d'uranium, qui sont discordants et localisCs sur la marge orientale du bassin d'Athabasca, dans le nord de la Saskatchewan, Canada. Le gite est form6 de deux corps minCra- lists, celui de Gaertner actuellement CpuisC, et celui de Deilmann prksentement exploitt. Dans le pass&,les essais de datation radiomCtrique ont fourni un Bge de 1250 + 34 Ma pour le corps minCralisC de Gaertner, et de 1350 f 4 Ma pour le corps minCralisC de Deilmann, ces Bges datent le plus ancien Cvknement de minkralisation. Cette difference d'Bges inattendue a justifiC une Ctude plus approfondie. De nouvelles techniques de prtparation innovatrices ont Ctt utilisCes pour stparer le plus vieux minCral d'U de la paragenkse, une uraninite anisotropique. L'Ctude minkragraphique et les donnCes isotopiques U/Pb rkvklent que le plus ancien Cvknement de minkralisation en U est contemporain dans les deux gites, avec un Bge de 1421 + 49 Ma. La phase primaire de formation du gite fut suivie de d'autres gCnCrations de minkralisation en U plus jeunes et de pCriodes de remobilisation. Les donntes de Sm/Nd de l'uraninite du lac Key forment une isochrone qui fournit un Bge de 1215 Ma. Lequel est interprCt6 comme Ctant l'lge de la remobilisation de l'uranium ou d'un nouvel Cvknement de minkralisa- tion. Le plomb trouvC dans le Groupe d'Athabasca sus-jacent au gite de Deilmann et dans la galhe apparait comrne un mClange de plomb commun et de plomb radiogtnique mobilist a partir du corps minCralisC, sur une pCriode de temps d'au moins 1000 Ma. [Traduit par la rkdaction] Can. J. Earth Sci. 29, 879-895 (1992) Introduction Since the discovery of the Key Lake uranium deposit in 1975, several techniques have been employed to determine the age of formation of its ore minerals. Wendt et al. (1978) reported an age for the uranium ore of 1270 Ma. Hijhndorf et al. (1985~) dated uraninite and coffinite from the south- western part of the deposit (the Gaertner orebody); they indi- cated three geological events, at 1250, 900, and 300 Ma, but these ages were calculated from three-point discordias only. Trocki et al. (1984) analyzed nine core samples from below the unconformity in the northeastern zone (the Deilmann ore- body); four of the nine whole-rock samples investigated define 'Present address: Technologieberatungsstelle beim DGB Nieder- sachsen, Dreyerstrasse 6, D(W)3000 Hannover 1, Germany. 'Present address: ABB Asea Brown Boveri AG, Corporate Research Centre, Eppelheimer Strasse 82, D(W)6900 Heidelberg 1, Germany. hlnted in Canada I Imprimt au Canada a discordia that intersects the concordia at 1350 and 300 Ma, respectively. The unsatisfactory nature of these discordant ages and the differences of the formation age of both orebodies stimulated the Federal Institute of Geoscience and National Resources in Hannover, Germany, to carry out a 3 year program in cooper- ation with Uranerzbergbau-GmbH in Bonn - Wesseling (Ger- many), Uranerz Exploration and Mining Ltd. in Saskatoon (Canada), and Key Lake Mining Cooperation (now CAMECO) in Saskatoon (Canada). The aim of this program was to obtain a more detailed picture of the ages of formation of ore and host rocks of selected unconformity-related uranium deposits in the Athabasca Basin. In addition to conventional U/Pb age determination of U minerals, we applied the SmINd method based on the follow- ing studies and considerations. Extensive investigations of rare-earth elements (REE) of uranium deposits of various origins (Fryer and Taylor 1987; Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Monash University on 03/12/13 For personal use only.
Transcript
Mineralogy and UIPb, PbIPb, and SmINd geochronology of the Key Lake uranium deposit, Athabasca Basin, Saskatchewan, Canada
C. CARL^ Bundesanstalt f ir Geowissenschaften und Rohstoffe, P. 0. Box 510153, D(W)3000 Hannover 51, Gennany
E. VON PECHMANN~ Uranerzbergbau-GmbH, Kolner Strasse 38-44, D(W)5047 Wesseling, G e m n y
A. HOHNDORF Bundesanstalt &r Geowissenschaften und Rohstoffe, P. 0. Box 510153, D(W)3000 Hannover 51, Germany
AND
G. RUHRMANN CAMECO, 2121 - 11th Street West, Saskatoon, Sask., Canada S7M 153
Received May 14, 1991 Revision accepted December 12, 1991
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 Deilrnann 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 f 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 f 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.
Le gite du lac Key est un parmi les nombreux grands et riches dCp6ts d'uranium, qui sont discordants et localisCs sur la marge orientale du bassin d'Athabasca, dans le nord de la Saskatchewan, Canada. Le gite est form6 de deux corps minCra- lists, celui de Gaertner actuellement CpuisC, et celui de Deilmann prksentement exploitt. Dans le pass&, les essais de datation radiomCtrique ont fourni un Bge de 1250 + 34 Ma pour le corps minCralisC de Gaertner, et de 1350 f 4 Ma pour le corps minCralisC de Deilmann, ces Bges datent le plus ancien Cvknement de minkralisation. Cette difference d'Bges inattendue a justifiC une Ctude plus approfondie. De nouvelles techniques de prtparation innovatrices ont Ctt utilisCes pour stparer le plus vieux minCral d'U de la paragenkse, une uraninite anisotropique. L'Ctude minkragraphique et les donnCes isotopiques U/Pb rkvklent que le plus ancien Cvknement de minkralisation en U est contemporain dans les deux gites, avec un Bge de 1421 + 49 Ma. La phase primaire de formation du gite fut suivie de d'autres gCnCrations de minkralisation en U plus jeunes et de pCriodes de remobilisation. Les donntes de Sm/Nd de l'uraninite du lac Key forment une isochrone qui fournit un Bge de 1215 Ma. Lequel est interprCt6 comme Ctant l'lge de la remobilisation de l'uranium ou d'un nouvel Cvknement de minkralisa- tion. Le plomb trouvC dans le Groupe d'Athabasca sus-jacent au gite de Deilmann et dans la g a l h e apparait comrne un mClange de plomb commun et de plomb radiogtnique mobilist a partir du corps minCralisC, sur une pCriode de temps d'au moins 1000 Ma.
[Traduit par la rkdaction] Can. J. Earth Sci. 29, 879-895 (1992)
Introduction Since the discovery of the Key Lake uranium deposit in
1975, several techniques have been employed to determine the age of formation of its ore minerals. Wendt et al. (1978) reported an age for the uranium ore of 1270 Ma. Hijhndorf et al. (1985~) dated uraninite and coffinite from the south- western part of the deposit (the Gaertner orebody); they indi- cated three geological events, at 1250, 900, and 300 Ma, but these ages were calculated from three-point discordias only. Trocki et al. (1984) analyzed nine core samples from below the unconformity in the northeastern zone (the Deilmann ore- body); four of the nine whole-rock samples investigated define
'Present address: ABB Asea Brown Boveri AG, Corporate Research Centre, Eppelheimer Strasse 82, D(W)6900 Heidelberg 1, Germany. hlnted in Canada I Imprimt au Canada
a discordia that intersects the concordia at 1350 and 300 Ma, respectively.
The unsatisfactory nature of these discordant ages and the differences of the formation age of both orebodies stimulated the Federal Institute of Geoscience and National Resources in Hannover, Germany, to carry out a 3 year program in cooper- ation with Uranerzbergbau-GmbH in Bonn - Wesseling (Ger- many), Uranerz Exploration and Mining Ltd. in Saskatoon (Canada), and Key Lake Mining Cooperation (now CAMECO) in Saskatoon (Canada). The aim of this program was to obtain a more detailed picture of the ages of formation of ore and host rocks of selected unconformity-related uranium deposits in the Athabasca Basin.
In addition to conventional U/Pb age determination of U minerals, we applied the SmINd method based on the follow- ing studies and considerations.
Extensive investigations of rare-earth elements (REE) of uranium deposits of various origins (Fryer and Taylor 1987;
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880 CAN. J. EARTH SCI. VOL. 29, 1992
McLennan and Taylor 1979, 1980) have shown that uraninites are REE rich, and the amount of incorporated REE apparently depends on its mode of formation (e.g., pegmatitic uraninite may contain over 1.5 wt. % REE, whereas uraninite from the unconformity-related deposits contains up to 0.2 wt. % REE). McLennan and Taylor (1979, 1980) suggested that in the unconformity-type deposits, U and the REE were transported together in a hydrothermal fluid as a soluble carbonate complex.
The SmINd age dating method is based on the a decay of 14'Sm to 143Nd. However, 147Sm is not the only parent of 143Nd: 238U decays by spontaneous fission with a half-life of 1 x 1016 a, to several fission products; among them are 143Nd and lMNd, with yields of 7.1 and 6.2%, respectively. The ratio of 143Nd/1MNd from spontaneous fission of 238U is about 1.145; this ratio may be elevated in old samples with very large UINd ratios. However, for 1200 Ma old U minerals and a 238U/144Nd atomic ratio of less than 10000, the increase of the 143Nd/1MNd ratio is within the analytical l a error of 0.05%,.
This shows that uraninite, in general, is suitable for SmINd isotope investigations. It is, therefore, applied as a supple- mentary geochronological investigation method for the Key Lake deposit.
This paper deals exclusively with the results from the UIPb, SmINd, and Pb/Pb dating of ore from the Key Lake deposit; related host-rock data have been presented by Hohndorf et al. (1989~) and Strnad et al. (1992).
General geology Crystalline basement
The Key Lake deposit is located near the western margin of the Wollaston Domain (Fig. I), which is part of the Cree Lake Mobile Zone of the Trans-Hudson Orogen (Sibbald and Quirt 1987).
The basement rocks of the Wollaston Domain in the Key Lake area consist of pink granitoid gneisses (quartz - feldspar - biotite - hornblende gneiss) dated at 2600 Ma by whole-rock RbISr and SmINd methods (Hohndorf et al. 1989a; Strnad et al. 1992). They are overlain by a series of Early Proterozoic grey gneissic granitoid rocks and quartz -biotite - cordierite - feldspar gneisses, which carry accessory sillimanite, apatite, zircon, allanite, pyrite, chalcopyrite, and molybdenite. Graphite is present in distinct units either in disseminated form or in millimetre-thick discontinuous layers. Garnets are abun- dant in beds of varying thickness. Meta-arkoses, calc-silicates, arnphibolites, quartzites, and mylonites are less abundant rock types within the development drilling area of the orebodies.
These rocks are part of a sediment sequence that was deposited in a marine -terrestrial environment. They were subjected to the Hudsonian orogeny (1730 Ma, K/Ar on bio- tites and hornblendes; Hohndorf et al. 1989a), which resulted in granitoid-cored antiforms (Money 1968; Money et al. 1970). Later phases of folding resulted in doubly plunging, northeasterly trending domes. The Key Lake deposit is located at the northwestern flank of such a granitoid-cored anticline (Fig. 2). The metamorphic grade ranges from the upper amphibolite facies of the Abukuma type (Winkler 1967; high- grade metamorphism, Winkler 1976) to granulite facies.
A phase of peneplanation and laterite-like weathering fol- lowed the Hudsonian orogeny. The palaeoweathering profile is overprinted by a post-Athabasca ''diagenetic - hydrothermal"
process (Hoeve and Sibbald 1978; MacDonald 1985), which extends to the metamorphic basement in places down to a depth of more than 100 m, forming a "regolith" assemblage. The alteration profiles have mineralogical characteristics that depend on the original rock composition (MacDonald 1985). The profile in quartz -biotite - feldspar gneisses, the most common rock type in the Key Lake area, immediately under- neath the unconformity, consists of a bleached zone that is characterized by kaolinitization, chloritization, and concomi- tant quartz corrosion. The bleaching overprints a sericitic haematite zone. The haematite where preserved is localized along cleavage planes of sericitized biotite. Chlorite (di-trioct- ahedral sudoite; Hoeve 1982) is subordinately present in this palaeoregolith.
The haematite zone is underlain by a green zone that is com- posed mainly of sericite and trioctahedral chlorite (Hoeve 1982). A transitional red-green unit consists of alternating bands of the two zones.
Athabasca Group Sediments of the Middle Proterozoic Athabasca Group were
deposited after the weathering episode. In the Key Lake area, the Athabasca Group is represented by the lower fluviatile Manitou Falls Formation (Ramaekers 1979, 1990), consisting of quartzites containing a few heavy mineral layers. Lithic components are locally present close to the base of the forma- tion. Volcanic material, observed in other formations of the Athabasca Group, is absent from this unit. Syntaxial quartz, clay minerals, and, rarely, carbonates fill the interstitial space. Haematite, which gives the Athabasca Group its purple colour, may be of detrital origin (Ramaekers and Dunn 1977) or it may belong to early diagenetic stages (Hoeve and Quirt 1984). Haematite coating quartz overgrowths points to a late diagenetic or alteration-related origin.
The clay minerals in the matrix of the Manitou Falls Forma- tion generally consist of both sericite and kaolinite, in equal parts. Kaolinite, however, is the predominant clay mineral in silt layers and in structural zones (Hoeve et al. 1981).
Diabase dikes and sills intruded the Athabasca Group during the period from 1350 to 1000 Ma (Bell 1981; Armstrong and Ramaekers 1985). Kirchner et al. (1980) and Hoeve and Quirt (1984) attempted to correlate these intrusions with mineraliza- tion phases.
U/Pb age dating of apatite from the Wolverine Point Forma- tion, which is younger than the Manitou Falls Formation, shows that the deposition of the Athabasca Group terminated ca. 1650- 1700 Ma ago (Curnming et al. 1987).
Ore geology and types The Gaertner and Deilmann orebodies are located in an east-
northeast-trending fault zone several kilometres in length; this fault and associated cross faults intersect the unconformity between the Middle Proterozoic Athabasca Group and the Early Proterozoic (Aphebian) metasediments (Fig. 2).
Based on the ore grades and the geometry of the orebodies two main types of bedrock-hosted ore are distinguished: mas- sive U -Ni aggregates and veins and disseminated impregna- tions of both Athabasca Group and basement rocks. A third ore type consists of eroded ore cobbles deposited in till and esker material.
The bulk of the ore in the Gaertner orebody occurs as dis- continuous massive, high-grade aggregates replacing mostly Athabasca Group rocks. They are generally located immedi-
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CARL ET AL.
NORTHWEST TERRITORIES
Edge of Phanerozoic cover - Major fault or shear zone - .-- Lithostructural domain boundary
$( Uranium deposit (epigenetic )
0 Gold deposits
Gold-copper-zinc deposits
FIG. 1. Generalized geological map of the Athabasca Basin in Saskatchewan and Alberta (after Sibbald 1987, Fig. 2).
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CAN. J. EARTH SCI. VOL. 29, 1992
/ + + + + + + + + + + + + ++++/
+ + + + +
0 I 2 3 km C I 1 I
OREBODY r i GRANlTOlD ROCK
r EDGE OF THE ATHABASCABASY - - METASEDlMENTS
FIG. 2. Generalized geological map of the Key Lake mining area (after Ruhrmann 1987, Fig. 2).
ately above the Early -Middle Proterozoic unconformity . Ore shoots extend from this high-grade core of the orebody as veins into both Athabasca Group rocks and, to a lesser extent, along gneiss-hosted fractures.
The Deilmann orebody consists of four vertical or steep north-plunging, basement-hosted "pipes" that are joined along the unconformity. One of these basement zones is almost monometallic: it carries almost exclusively uranium, with no other metals.
Disseminated ore is present in basement rocks adjacent to mineralized fractures, in sandstone relics within locally strongly clay-altered structures, and in a dark grey sandstone that forms a halo around the veins and aggregate ore.
Strong post-ore alteration affected the mineralized zones and their primary haloes, as well as structurally disturbed barren rocks near the orebodies.
Ore mineral parageneses
The paragenetically oldest ore mineral at Key Lake is a uraninite showing distinct optical anisotropy (von Pechmann et al. 1991). It displays both radially textured botryoidal and euhedral/subhedral cubic forms. It averages 86.06% U02, 10.93% PbO, 1.35% CaO, 0.12% Ti02, 0.14% MnO, 0.35% FeO, 0.40% Si02, and 0.42% Y2O3; Th is basically absent. REE assay 0-0.08% La203, 0.12% Ce203, 0.04% Pr203, 0.04% Nd203, and 0.07% Yb2O3. The Vickers micro- hardness of this uraninite averages 815; the spectral reflec- tance in green light (546 nm) ranges crom 15.1 to 15.2% ; the lattice constant, ao, is 5.475(4) A (1 8, = 0.1 nm) (von Pechmann et al. 1991).
The anisotropic uraninite forms coherent aggregates up to several millimetres in size, which can easily be polished and
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FIG. 3. Plain (anisotropic) uraninite (medium grey), representing the oldest ore mineralization phase at Key Lake; it is replaced to a minor extent by darker grey reflecting uraniferous phases, which are associated with sulphides (white, mostly galena). Key Lake, Gaertner open pit, level 415 m; photomicrograph, polished section CN 12139; 20 x air; 1 nicol.
show even planes (Fig. 3). The aggregates are commonly slightly to moderately cracked, and later phases of uranium oxides and silicates (coffinite) replace the oldest uraninite along these cracks and its rims (Figs. 3, 4). These younger uraniferous mineral phases are associated with locally abun- dant crystals of radiogenic galena, whereas the oldest uraninite is free from inclusions (Fig. 3).
The anisotropic uraninite also predates rammelsbergite (NiAs2), maucherite (Ni, ,As8), nickeline (NiAs) , gersdorffite ((Ni,Co,Fe)AsS), breithauptite (NiSb), vaesite (NiS2), mil- lerite (NiS), bravoite ((Fe,Ni,Co)S2), pyrite (FeS2), marca- site (FeS2), galena (PbS) , clausthalite (PbSe) , sphalerite (ZnS) , chalcopyrite (CuFeS2), covellite (CuS), safflorite (CoAs2), bismuthinite (Bi2S3), hauchecornite ((Ni,c~,Fe)~(Bi,As,Sb)~- S8), jordisite (MoS2), haematite (Fe203), and limonite (FeO(0H)) (von Pechmann 1985; Ruhrmann and von Pech- mann 1989).
Gangue minerals are mainly the Mg chlorites sudoite and clinochlore IIb, kaolinite, anatase-rutile, and to a lesser extent siderite, dolomite, magnesite, quartz, and swelling (smectite-like) clay mineral varieties (von Pechmann 1985).
Mineralogical preparation
The main objective of the present study was to determine the age of the oldest mineralization phase and, if possible, the age of subsequent mineral phases.
To date, only the oldest, anisotropic uraninite and a few coherent areas of coffinitic character were observed to form zones large enough for the microscope-aided preparation used. As will be shown below, none of the isotopic data from coffi- nitic zones enabled us to construct a meaningful discordia. Hence, our efforts were concentrated on the separation of the oldest uraninite.
Prior to the present study, Wendt et al. (1978) and Hohndorf et al. (1985~) published data from uraninite concentrates recovered by mineral separation in heavy liquids and by partial dissolution of possibly present coffinite by hydrochloric acid (primary ages obtained: 1270 and 1255 Ma, respectively).
The mineralogical preparation of sample material for this study involved increasingly improved methods. First, a metal needle was used to scrape and remove approximately 15 pg material from U oxide aggregates under the microscope (15 samples). The resulting isotopic UIPb pairs showed an age of 1255 12 Ma for the Gaertner orebody and 1320 28 Ma for the Deilmann orebodv.
However, the age difference between the orebodies remained unexplained and the discordiae continued to be unsatisfactory. The discrepancies pointed to a migration of radiogenic daughter-products into the immediate vicinity of the source grains. (Ludwig et al. (1987) analyzed about 50 highly urani- ferous whole-rock samples from Jabiluka in the East Alligator River Uranium Field (Northern Territory, Australia) and used
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8 84 CAN. J. EARTH SCI. VOL. 29, 1992
FIG. 4. Plain (anisotropic) uraninite, representing the oldest ore mineral phase (light to medium grey). Younger uranium minerals (dark grey), associated mainly with galena (white), have formed along fractures. The irregular large area (right side of photograph) represents a scrape mark of a steel needle. The seven black holes were cut with a diamond-armoured microdrill (designed by Medenbach 1986). The material from the drill sites produced a discordia showing a UIPb age of 1421 + 49 Ma. Key Lake, Deilmann orebody, ddh 764, 86.87 m depth; photo- micrograph, polished section CN 10098; 5 x air; 1 nicol.
the above consideration of micromigration after they had gained a linear array of highly discordant sample points point- ing to a formation of the Jabiluka ore about 1440 Ma ago.)
We then dated macroscopically scraped uraniferous mineral aggregates from 49 specimens that were subjected to X-ray diffractometry (XRD) to achieve mineralogical control. The "ages" of this material ranged from 1240 to 1460 Ma. It was then recognized that the material with the purest XRD compo- sition of highly crystalline uraninite and concomitantly with the largest amount of microscopically determined anisotropic uraninite grouped nearest to the concordia and approximated a linear array. Also, a few samples, containing abundant amounts of anisotropic uraninite, produced a possibly mean- ingful discordia, although the individual sample points were very discordant.
These promising samples were selected for further investi- gation. &re zones of a&otropic uraninite were removed from the respective polished sections under the microscope with a microdrill designed by Verschure (1978) and modified by Medenbach (1986). The device consists of a diamond- armoured needle spinning at 12 000 rpm which is carefully lowered onto the polished section. The section turns in a slow circular motion, driven by a motor attached to the stage, caus-
ing the needle to carve out a circular slab 0.08-0.13 mm in diameter. At a thickness of 0.04 mm, 3 pg of material can be cut and removed from the section, under a stereo lens. The first set of slabs was produced using a diamond-tipped needle (Figs. 5, 6), and a second set was drilled more precisely with a finer diamond-chip-coated needle tip (Fig. 4). The U/Pb iso- topic analysis of these, almost pure (at least on the section plane) uraninite slabs resulted in a closer alignment of all respective sample points to the concordia (Figs. 7, 8). The results are discussed below in detail.
Chemical analytical procedure
Both U/Pb and SmINd methods are well known (Krogh 1973; Richard et al. 1976) and do not need to be described here in detail, but some discussion of the problems caused by the extremely small amounts of sample material is warranted.
U/Pb The samples were dissolved in purified concentrated nitric
acid and evaporated to dryness. The residue was taken up in an HC1-HBr solution and split into two fractions for isotope composition (ic) and isotope dilution (id) analysis. A mixture of 235U and 208Pb solution with a ratio of 5:l was added as
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FIG. 5. Plain (anisotropic) uraninite (medium grey), replaced by darker grey reflecting uraniferous phases associated with sulphides consisting mostly of galena (white). The diamond needle (black, out of focus in the bottom left corner) has engraved a circular cut in uraninite of the oldest generation. No other minerals are affected by the sampling process. Drill dust appears black along the cut. Key Lake, Gaertner open pit, level 415 m; photomicrograph, polished section CN 12139; 10x water; 1 nicol.
spike to the latter fraction. U and Pb were separated from all other elements by conventional ion-exchange techniques (Krogh 1973). The isotope analysis was carried out on a MAT 261 thermal ionization mass spectrometer.
The total blank is about 100 pg 206Pb, and the amount of radiogenic 206Pb of the sample is 500 ng, assuming 3 pg pure uraninite and an age of about 1400 Ma. Therefore, the blank is also insignificant for those small (approx. 3 pg) samples pre- pared with Medenbach's (1986) microdrilling device. Because
I of the small amount of Pb, the radiogenic 207Pb/206Pb ratio is influenced by contamination with common lead; however,
I since most of the Pb is radiogenic, the correction of the ratio 207Pb/206Pb is not sensitive to the choice of the common lead.
I To simplify the calculation we corrected the measured Pb-isotope ratios of all samples using a 1400 Ma common lead after Stacey and Kramers (1977); this results in the same radiogenic Pb-isotope ratios, within the la error of I%,, as a double correction (first with contamination lead, and then with a 1400 Ma common lead).
Sm/Nd About 5 mg of uraninite was dissolved in purified concen-
trated HN03 -HCl, and a 2.5: 1 mixture of 147Sm and 148Nd solution was added as spike. After decomposition and separa-
tion by anion exchange of the main element, U, the 2.5 N HC1 sample solutions were loaded onto a 10 cm3 column contain- ing the cation exchange resin AG 50 W-X12, 200-400 mesh. All major elements were then eluated with 100 mL 2.5 N HCI. REE and Ba were subsequently eluated with 6.1 N HC1. The solution was evaporated to dryness, redissolved in 200 pL 0.18 N HC1, and loaded onto a second column filled with a mixture of HDEHP (di(2-ethylhexyl)orthophosphoric acid) with teflon powder. Nd was eluated with 0.18 N HC1 and Sm with 0.5 N HC1. This procedure represents a modified version of that originally designed by Richard et al. (1976).
The total blank for the chemical procedure was negligible with about 50 pg 144Nd and 20 pg 147Sm, although the former was high in relation to lUNd contributed by spontaneous fis- sion of 238U. The influence was nevertheless minor, since the 143Nd/1UNd ratio of the blank was similar to that of the sam- ple and different from the ratio resulting from spontaneous fission.
Both elements were loaded separately onto a double rhenium filament assemblage with water. The isotopic composition was measured with a MAT 261 thermal ionization mass spectro- meter equipped with five collectors.
The reproducibility calculated from the standard deviations of replicate analyses was *0.005% for the 143Nd/1UNd iso-
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FIG. 6. Same as Fig. 5 . The cut is complete. The small area of uraninite in the centre of the engraved cut is ready to be removed. Key Lake, Gaertner open pit, level 415 m, photomicrograph, polished section CN 12139; 10X water; 1 nicol.
tope ratio and f 0.5% for the 147Sm1144Nd ratio at the 66% confidence level. The isotope composition of the La Jolla standard was measured as 0.51 1866 -t 0.000016 (standard deviation). The Nd isotopic ratios were normalized to 146Nd/144Nd = 0.7219.
Results of investigations U/Pb data
A plot of all 80 analyzed samples on the concordia diagram (after Tera and Wasserburg 1972) illustrates that it is not pos- sible to calculate a reasonable discordia (Fig. 7).
The measured UIPb, PbIPb, and radiogenic isotope ratios of the 25 samples consisting of anisotropic uraninite used in the construction of a discordia are listed in Table 1. The data for the other 48 samples, which could not be used for such a con- struction, are not listed, although the sample points are shown in Fig. 7.
Table 1 contains the analytical data for an additional seven samples that were not used in the construction of the discordia, even though they seemed to consist predominantly of aniso- tropic uraninite: samples CN 121 13a, CN 121 14, and CN 12139 were excluded because they were prepared in such a way that a mixture of various uraninite generations could not be precluded; samples CN 1009812 and CN 12109 were excluded because their 238U/206Pb ratios of 25 and 66, respectively, are 5 - 13 times higher than those of the 25 samples used for the
discordia, which indicates a strong contamination of the anisotropic uraninite concentrate with material from other uraniferous compounds; and, finally, samples CN 121 12 and CN 12127 were not used because, had they been included, they would have increased the CHI factor, the square root of the MSWD (mean square of weighted deviates), of the discor- dia from 16.8 to over 30.
The radiogenic ratios were calculated using a 1400 Ma old common lead according to Stacey and Kramers (1977). Since the amount of common lead is extremely small, as in most U ores, the calculated radiogenic isotope ratios are not sensi- tive to the choice of the common lead. Due to the unweighable amounts of sample material used here, no figures can be given for the U and Pb content in the uraniferous material. However, on the basis of the mineralogy of the separated samples we estimate that the U concentration is in the range of 50% (coffinite-rich samples) to 80% (uraninite-rich samples).
The data of those samples containing pure or nearly pure anisotropic uraninite scatter along a line that intersects the concordia at 1421 f 49 and 671 + 67 Ma (24 , respectively (Figs. 8, 9). The upper intersection is interpreted as the best estimate of the age of the earliest formation of Key Lake uranium ore. The lower intercept does not have any time sig- nificance (cf. below). The MSWD of the regression line is 282 (CHI = 16.8), indicating that the scatter of the sample points is significantly larger than the analytical errors.
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CARL ET AL.
Deilmann orebody Basement . Sandstone
Gaertner orebody 0 Basement
Sandstone + Coffinite
BOO 0
FIG. 7. Tera-Wasserburg diagram of U/Pb data of all samples analyzed from the Key Lake uranium orebodies. Subscript r indicates radiogenic.
FIG. 8. Tera-Wasserburg diagram showing the U/Pb data points of macroscopically prepared uraninite-rich samples. The tie lines point to the respective samples prepared under the ore microscope with a microdrill designed by Medenbach (1986) (m, a). Subscript r indicates radiogenic.
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CAN. J. EARTH SCI. VOL. 29, 1992
TABLE 1. Results of UlPb analyses of anisotropic uraninite at Key Lake
Rock 238U/206pb 207pbb1206pbb 2 3 8 ~ 1 2 0 6 p b b
"S, sandstone-hosted ore; B, basement-hosted ore. bRadiogenic Pb. 'Samples not used for construction of discordia because they were macroscopically prepared from hand specimen. dSample not used for construction of discordia because the Z3sU/Z06Pb ratio indicates a mixture of several uraninite
phases. 'Sample not used for construction of discordia because the CHI factor of the discordia would be almost doubled when
using these data.
The scatter of the sample points cannot be explained by a simple episodic lead loss model, by a mixture of two genera- tions of uranium minerals, or by continuous lead diffusion. The U-Pb systems are apparently disturbed by more than one of these processes, and despite our careful preparation of the anisotropic uraninite we cannot exclude the possibility that the samples used for the construction of the discordia (Fig. 9) may contain minor remnants of other uranium phases. Lead loss as well as later addition of uranium and the "contamination" by younger uranium phases would tend to displace the sample points towards higher UIPb ratios, thus lowering the upper intersection on the concordia. We therefore conclude that the age of the earliest formation of the Key Lake ore cannot be younger than the date of the upper intersection of the regres- sion line calculated for the 25 samples of anisotropic uraninite.
It is concluded from the above considerations that the lower intersection of the regression line should not have geological significance if the U-Pb systems have been disturbed by mul- tiple processes.
The 49 data points from the second sample set were not suitable for UIPb age determination because of their wide and irregular scatter on the concordia diagram (Fig. 7). These macroscopically prepared samples most likely consist of mix- tures of several U mineral phases of different ages. Further- more, each U mineral phase may have experienced lead loss at different times.
It also was not possible to construct any discordia from the coffinite-rich samples (cf. Fig. 7).
Sm/Nd isotopic data Eleven uraninite samples from the Deilmann orebody of the
Key Lake deposit were analyzed for their SmINd isotopic compositions. Some of these samples were scraped from hand specimens, so probably a mixture of uranium oxide genera- tions has been analyzed.
Nine of these 11 samples define a straight line correspond- ing to an age of 1215 Ma (Fig. 10). Two samples do not fit the straight line and were omitted from the age calculation.
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CARL ET AL
1 1 5 0 0 1 Deilmann orebody
, T, - I n - , r z ..- Sandstone 1-C I J IVIU
0,09 -I \hJ ~ S W D = ~ E I ~ Basement Gaertner orebody
Sandstone
I 0 Basement
/ Calculation of the TW discordia without these samples
FIG. 9. Tera- Wasserburg (TW) diagram of UIPb data showing the discordia of the anisotropic uraninite samples from the Key Lake uranium orebodies; the arrows point to those samples that were not used in the construction of the discordia. Subscript r indicates radiogenic.
Their 14Nd contents are much lower (3 and 10 ppm) than TABLE 2. SmINd isotope data for anisotropic uraninites of the Key those of the other nine samples (24 - 182 ppm; Table 2). The Lake orebodies date could be lowered within the l a error only if a 143Nd/14Nd correction were applied with respect to the spon- 144Nd I4'Sm
taneous fission Nd. Sample no. I4'sm/ 1 4 4 ~ d '43Sm/ ' 4 4 ~ d (pprn) (pprn)
The Sm/Nd date of 1215 Ma does not agree with the U/Pb CN 6407 0.46989 0.514389 70.3 33.7 age of 1421 Ma, but it indicates that there was a phase of either CN 6408 0.62805 0.515873 52.1 33.4 U emplacement or U remobilization ca. 1215 Ma ago. CN 6409 0.64268 0.515685 44.4 69.5
I ~ h ~ ~ r e ~ a r a t i o n technique used for the Sm/Nd isotope anal- CN 12113a 0.65643 0.515665 39.3 26.4 yses is different from the far more refined method that was CN 12113b 0.61872 0.515370 24.7 10.4
1 finally used for the U/Pb analyses. Nevertheless, the linear CN 12114a 0.83601 0.5 16597 3.2 2.7 array of Sm/Nd isotope data for 9 of the 11 samples suggests CN 12114b 0.50329 0.514842 67.7 34.8
that this isochron has geological significance. CN 12135a 0.29600 0.512891 182.7 320.7 1 CN 12135b 0.53190 0.513998 10.4 5.6
Pb data of the Athabasca Group The Pb isotopic compositions of four lead-rich samples
(D-482-5, D-482-14, D-490-31, D-498-16B) and of three galena samples (CN 11941, CN 12354, CN 12364) from the Athabasca Group above the Deilmann orebody were analyzed (Table 3). The "corrected values" of the 206Pb/204Pb and 207Pb/204Pb ratios were calculated from the measured ratios by subtraction of the radiogenic lead produced in situ by the uranium present in the samples since 1700 Ma, the time of deposition of the Athabasca Sandstone.
Figure 11 shows the measured and corrected data in the 207Pb/204Pb versus 206Pb/204Pb diagram. The linear array is interpreted as a mixing line of a common lead component with a radiogenic lead. The slope gives the 207Pb/206Pb ratio of the radiogenic lead. The correction necessary for radiogenic lead produced in situ displaces the sample points essentially along the mixing line. Its slope, therefore, remains unchanged.
The data from sandstone samples alone result in a slope of the regression line of 0.1007 f 0.0031 (la). The slope changes insignificantly to 0.0994 f 0.0028 (la) by including the data from the galena samples.
The Pb mixing line does not intersect the Stacey -Kramers model line (Fig. 11). Assuming an evolution of the common lead component according to the Stacey -Kramers model until the commencement of deposition of the Athabasca Group at 1700 Ma, a p2-value (238U/204Pb ratio during the second stage) of about 10.06 is required for the growth curve to inter- sect with the Pb mixing line.
In the Tera-Wasserburg diagram (Fig. 12) the Athabasca Group samples lie to the left of the concordia line. This can
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890 CAN. J . EARTH SCI. VOL- 29. 1992
12114 b ) / 1 2 l 3 9 b
6407. 12139a T=1215 + I 5 Ma
/ a ( without 12114a,12135b)
FIG. 10. SmINd isochron of some anisotropic uraninite samples from the Key Lake uranium orebodies.
TABLE 3 . Pb isotopic data of samples from the Athabasca Group and galena from above the Deilmann orebody
"An analytical lo error of &0.2% was estimated on the basis of replicate analyses. bThe corrected values were obtained by subtracting the radiogenic lead, produced in situ since the beginning of the
sedimentation of the Athabasca Group ca. 1700 Ma, from the measured isotopic pairs.
be explained either by loss of uranium or by gain of radiogenic lead. The latter seems to be more realistic in view of the obvi- ous lead loss suffered by most of the sampled uranium ore.
Because the samples contain a large amount of common lead, the calculated radiogenic 207Pb/206Pb ratios are strongly dependent on the assumed isotopic composition of the com- mon lead. In the present case we used the composition taken from the intersection of the lead mixing line with the 1700 Ma isochron of the Stacey -Kramers model (Fig. 11).
The reference line drawn in Fig. 12 connects the 207Pb/206Pb ratio of 0.0994 on the ordinate (belonging to the radiogenic lead component and calculated from the mixing line) with the composition of a 1400 Ma old concordant uranium mineral. This reference line corresponds to a lead loss discordia with intersections at 1400 and 350 Ma, or, in other words, the radiogenic lead found in the Athabasca Sand- stone samples has the same composition as the radiogenic lead
evolved in the orebody between 1400 and 350 Ma. Because the lead mobilization probably was more or less
continuous, the present-day radiogenic lead has to be regarded as an average of a long period of accumulation that lasted at least until 350 Ma.
Discussion and conclusions Previously published Key Lake age dates may not be reli-
able because of the possible haphazard arrangement of a few data points and indiscriminate analysis of bulk material, such as mineral concentrates (Wendt et al. 1978; Hijhndorf et al. 1985a) and whole rock samples (Trocki et al. 1984).
A microscopically controlled sampling technique (Meden- bach 1986) allowed the highly selective separation of the paragenetically earliest uraninite for radiometric UIPb and SmINd age dating. A U/Pb discordia based on 25 data points
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FIG. 11. 207Pb/204Pb VS. 206Pb/204~b diagram of the samples from Athabasca Group rocks above the Deilmann orebody. 0, sandstone; 0, galena; +, sandstone samples corrected for in situ produced radiogenic lead since 1700 Ma.
,,,,\\ Age ot primary \ uranium mineral izat ion
FIG. 12. Tera-Wasserburg diagram for the galena and the lead-rich samples from the Athabasca Group from above the Deilmann orebody. Subscript r indicates radiogenic.
from the selected uraninite intersects the Tera and Wasserburg (1972) concordia at 1421 f 49 and at 671 f 67 Ma. The lower intercept is interpreted as a result of multiple distur- bances of the U -Pb system and probably does not have a geo- logical meaning.
The main advantage of this new discordia is that it consists of data points derived from a single (i.e., the oldest) uraninite generation: data points from macroscopically prepared sam- ples, very rich in the oldest uraninite, have not been used for
the construction of the discordia, even though they possibly fit the discordia (cf. Fig. 9).
The geology of the Key Lake orebodies, their ore mineral- ogy, and the new isotope data point to a uniform age of the paragenetically oldest uraninite, regardless of its affiliation with either orebody, its location within the orebody, or the type of host rock.
The U/Pb age of uraninite presented here is similar to a preliminary U/Pb mineralization age of 1400 f 25 Ma for the
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892 CAN. J. EARTH SCI. VOL. 29, 1992
TABLE 4. Compilation of dates of ore minerals in unconformity-related uranium deposits in the Athabasca Basin, Saskatchewan (ABUP-S) and Alberta (ABUP-A) (both Canada), the Thelon Basin (TB), Northwest Territories, Canada, and the Rum Jungle Uranium Field (RJUF) and
the East Alligator River Uranium Field (EARUF), both of the Pine Creek Geosyncline, Northern Territory, Australia
UIPb agec (Ma) PbIPb
model age SmINd age Deposit District Referencea Mineral datedb ui li (Ma) (Ma)
Deposit District Referencea Mineral datedb ui li (Ma) (Ma)
Devil's Elbow EARUF 24 Uraninite 795 f 8 27+11 Maurice Bay ABUP-S 25 Uraninite 750 +26 1196+24
"1, von Pechmann and Carl (1990); 2 , Hills (1973) and Hills and Richards (1976); 3, Ludwig et al. (1985); 4 , Ludwig et al. (1987); 5 , Maas (1989); 6 , Cumrning and Krstic (1991); 7 , von Pechmann and Carl ( 1 9 9 1 ~ ) ; 8, von Pechmann and Carl (1991~); 9 , Gulson and Mizon (1980); 10, This study and Carl et al. (1990); 1 1, Trocki et al. (1984); 12, Hohndorf et al. ( 1 9 8 5 ~ ) ; 13, Davidson and Gandhi (1989); 14, Eldorado Resources Limited (1987) and Andrade (1989); 15, Hohndorf et al. (19896) and von Pechmann and Carl (1991b); 16, Philippe and Lancelot (1988); 17, Persaud (1987); 18, Baadsgaard et al. (1984) and Worden et al. (1985); 19, Cumming et al. (1984); 20, Bell (1985); 21, Cumming and Rimsaite (1979); 22, F ~ y e r and Taylor (1984); 23, Richards (1963); 24, von Pechmann (1992); 25, Hohndorf et al. (1985b).
b ~ r , whole-rock powder. "ui, upper intercept on concordia; li, lower intercept on concordia.
Eagle Point deposit at Wollaston Lake in northern Saskatche- wan (Andrade 1989) and also to the age of 1401 f 18 Ma on anisotropic uraninite from the P2 North orebody at McArthur River in northern Saskatchewan (von Pechmann and Carl 1991~). These ages are 40-90 Ma older than most other ages from unconformity-related uranium deposits (including the P2 North orebody) in the Athabasca Basin Uranium Province (Cumrning and Krstic 1991), with the exception of the 1521 f 8 Ma age from McArthur River ore (Curnming and Krstic 1991).
An examination of Fig. 9 suggests that it is possible to calcu- late discordias that have slightly flatter slopes than ours. G. L. Cumming (personal communication, 1991) offered two such solutions, one showing an upper intersection on the concordia at 1324 f 13 Ma, the other at 1288 f 31 Ma. Such construc- tions seem quite reasonable from a mathematical standpoint; however, based on our microscopy observations, electron microscopy - electron microprobe work, and X-ray diffrac- tometry of optically anisotropic uraninite from Key Lake (von Pechmann and Hiirter 1988; von Pechmann et al. 1991), we were unable to separate this homogeneous mineral phase into groups that might warrant different discordias. Therefore, we treated the set of 25 samples as a whole, producing a dis- cordia with the admittedly large CHI factor of 16.8 (the alter- native but less reliable discordia, with a CHI factor of 30, which is produced by adding samples CN 121 12 and CN 12127 to the calculation, intersects the concordia at 1348 f 2 and 580 f 2.9 Ma).
Of relevance to this discussion is the work of Ludwig et al. (1987) on the UIPb age of uranium ore from Jabiluka, East Alligator River Uranium Field, Northern Territory, Australia. There, a similar, more or less linear array of isotopic data pairs (of macroscopically concentrated ore samples) yielded a primary age of 1437 f 40 Ma and a secondary intersection at around 400 - 600 Ma (Table 4).
The UIPb age obtained for the oldest (anisotropic) uraninite generation at Eagle Point (Fig. l) , also produced by the highly selective separation techniques described above, is 1242 f 2 Ma (Carl et al. 1990). This is in accord with the SmINd ages of 1267 f 27 Ma at Eagle Point from macroscopically pre- pared samples (Carl et al. 1990), 1281 f 80 Ma for the nearby Collins Bay B-Zone (Fryer and Taylor 1984), and 1215 f 15 Ma at Key Lake illustrated in Fig. 10.
The number of SmINd dates in this age range points to a regional event that has affected the uranium deposits in the eastern Athabasca Basin. This is supported by the well-defined SmINd isochron constructed from the Key Lake samples.
Age dates on the geologically similar uranium deposits in the Pine Creek Geosyncline in the Northern Territory of Australia resemble those reported here (Table 4). SmINd ages either predate (Jabiluka, Nabarlek) or match (Ranger) the UIPb dates. However, a SmINd isochron of Koongarra uraninite indicates an age of about 420 Ma, which is probably a recrystallization event (Maas 1989).
The discrepancies between the UIPb and SmINd isotopic data in the Australian and Canadian ore provinces are not yet fully understood and point out the need for further careful analyses. This paper is a contribution towards solving some of these problems. We are aware that more sophisticated analyti- cal methods, such as ion probe mass spectrometry, would help (Holliger 1988), because the problem of contamination of a single mineral phase (here the anisotropic uraninite) could be avoided.
At any rate, the present isotopic analyses confirm that UIPb ages of uraninites from unconformity-related deposits in the Athabasca Basin are younger than those of the albitite-hosted deposits of the Beaverlodge area in northern Saskatchewan, dated at 2200 -2270 Ma and 1890 f 150 Ma (Koeppel 1968), and from Way Lake (some 30 km southeast of Key Lake), which yielded an age of 1785 f 10 Ma (Hohndorf et al. 1989b). They are also younger than UIPb ages of synmeta- morphic uraninite enrichments in Aphebian metasediments in northern Saskatchewan dated at 1803 f 8 Ma (von Pechmann et al. 1984) and ca. 1800 Ma (Williams-Jones and Sawiuk 1985). However, at Key Lake there is no isotopic evidence for the presence of protore uraninites in this age range.
Acknowledgments We should like to thank the management of Uranerz-
bergbau-GmbH (BonnIWesseling), Uranerz Exploration and Mining Ltd., and CAMECO Corporation (Saskatoon) for their kind permission to publish this paper. We are grateful to Dr. F. Bianconi (Uranerzbergbau-GmbH, now with Union Rheinbraun Umwelttechnik-GmbH Cottbus) for his steady support and to many colleagues of Uranerzbergbau-GmbH and of the Bundesanstalt f i r Geowissenschaften und Rohstoffe in Hannover for their contribution to this project. In particular we are grateful to Mrs. M. Bockrath and Mr. H. Schyroki for laboratory assistance, to Messrs. H. Fahnenstich, Ch. Fritz, and G. Moller for preparation of polished sections, photo- micrographs, and drawings, and to Mrs. M. Hoffrnann for typing the manuscript. Last but not least, we are indebted to the journal referees Drs. G. L. Cumming and B. J. Fryer, and
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894 CAN. I. EARTH SCI. VOL. 29, 1992
to the journal editor, Dr. J. J. Clague, for their very construc- tive and helpful criticism.
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