Uraninite-bearing Granitic Pegmatite, Moore Lakes, Saskatchewan: Petrology and U-Th-Pb Chemical Ages 1 1 1 3 Irvine R. Annesley , Catherine Madore , Richard T Kusmirski , and Tom Bonli Anm:slcv. l.R .. Madore. C .. Kusmirski, R.T.. and Bonli. T. (2000): lJraninite-bearing granitic pegmat1tc, Moore Lakes, . Saskatcl1ewan: Petrology and l!-Th-Pb chcmic,tl ages: in Summary of Investigations 2000, Volume 2. Saskatchewan Gcolog1cal Survey. Sask. Energy Mines, Misc. Rep. 2000-4.2. attribute their genesis to some variation of the Abstract diagenctic-hydrothermal model proposed by Hoeve This paper documents the.fir.1·t results ofa detailed and Sibbald ( 1978), which invokes the mixing of study of uraninite-bearing granitic pegmatite, which highly saline oxidized basinal fluids with variably occurs near unconj(Jrmity-lJpe uranium mineralizalion reduced basement fluids. More recent research shows in the Moore Lakes area in the southeastern part ofthe that the transport and precipitation of uranium is Athabasca Basin. Drilling has revealed that wanitic controlled by basin paleohydrology, basement pegmatites comprise <5 to 10% ofthe basement topography, large-scale reactivated basement cornplex with hallestimated being radioactive. structures, fluid flow, heat flow, and physiochemical Relatively fresh, radioactive granitic pegmatite, the traps (Hoeve and Quirt, 1984, 1987; Wilson and Kyser, subject of this investigation, was intersected in drill 1987; Kotzcr and Kyser, l 991, 1992. 1995; Fayek and hole ML00-08. The pegmatite is JO m thick and occurs Kyser, 1997; Quirt. 1997). There are several 55 m he/ow the unconformity with sandstones olthe hypotheses as to the source of the uranium. Pagel et al. overlving Athabasca Group. ( 1980) and Dah lkamp ( 1993 ). amongst others, speculated that uranium is derived from the basement. The pegmatite is/ine to coarse grainecl, sheared and The regolith in the Archean/Palcoproterozoic basement foliated, and essentially unaltered It is composed has been proposed by Pagel ( 1991) as a major source mainly olquartz. greyfeldspar. and biotite. with of uranium. In contrast, Hoeve and Sibbald ( 1978) subordinate amounts olapatite, ::ircon, uraninite, and suggested that uranium was derived from sandstones of ilmenite. Uraninite and U-rich ::irr.:on are the dominant the Athabasca Group. Recent work by Fayek and uranium-hearing accessory minerals. Some uraninite Kyser ( 1997) favored this option and suggested that grains are zoned. SEM and mtcroprobe results showed most of the uranium was leached from detrital zircons that uraninite alteratwn is greatest along grain in the sandstones of the Athabasca Group. More recent boundaries andfractures, ivith significant U loss and research by Annesley and Madore ( I 999c) in the Ph gain/loss. southeastern part of the Athabasca Basin and by Hecht and Cuney (2000, in press) in the western part of the The pristine parts olthe uraninite grains are composed Athabasca Basin advocated that granitic rocks (i.e. mostly of U (66.-12 ~UOJ U/)8 ~72.40 wt%), Th (3.24 monazite-bearing leucogranites, granitic pegmatites, -5,ThO, -:;,884 wt %), and Ph (12. 92 -:;,PbO and potassic orthogneiss) were the major source of -5,/9 IJ wt%). U-Th-Ph chemical dating of the uranium. uraninite grains vielded a crysta//i::.ation age of?. 1772 ±88 Ma and other age clusters of 1429 (O I 649 Ma and This paper documents the first results ofa detailed //69 to 1233 Ma, which imply that the uraninite grains study ofuraninite-bearing granitic pegmatite, which started experiencing disturbances of their U-Th-Pb occurs near unconformity-type uranium mineralization isotopic system. We correlate these ages of isotopic in the Moore Lakes area (The Northern Miner, 2000). disturbance to the Stage 1 and Stage 2 U The aim of this work is: I) to characterize mineralization events documentedfor other petrographically and geochemically the uraninite­ unconjl1rmity-(vpe uranium deposits in the Athabasca bearing granitic pegmatite; 2) to characterize the nature Basin. Post-Athabasca alteration <if these pegmatites of the uranium-bearing accessory minerals: 3) to may have provided some U.for unconji1rmity-type document their behavior during hydrothermal uranium mineralization in the A-1oore Lakes area. alteration; 4) to date the age of the granitic pegmatites and the hydrothcnnal events that affected them, using I . Introduction U-Th-Pb geochemistry of uraninitcs; 5) to determine the crystallization temperature of the pegmatite using Unconformity-type uranium deposits of the Athabasca biotite-apatite gcothermometry: and 6) to compare Basin in northern Saskatchewan are the world's highest these results with similar data collected from outcrop grade uranium resources. Most of these deposits have and from core beneath the eastern Athabasca Basin common characteristics. Many researchers today (Annesley, 1989, 1990: Anncslcy and Madore, 1988. I Saskatchewan Research Cnuncil. 15 Innovation Blvd. Saskatoon. SK S7'-J 2X8 'JNR lksourcc, Inc. Suite 921. 4 70 (iranvilk Street. Vancouver. BC V6C I VS ' lkpartrncnt of<.icolngical Sciences. University of Saskatchewan. 114 Science Place Saskatoon. SK S7N 51-:2 Saskatchewan Geoloiical Survey ]()/ 1989a, 1989b, 1990a 1990b, 1991 a, 1991b, 1999: 1977. 1980; Sibbald, 1983: Gilboy, 1983; Anncsley Annesley er al., 1996, I 997a, 2000; Madore et al., and Madore, 1991 a: Annesley et al. 1996. J 997b; Tran 2000). and Yeo, 1997; Tran et al. , 1998. 1999, 2000: Madore et al.. I 999a; Orrell el al., 1999). Five main defomiarion events (Table 1) are recognized. 2. Geological Setting Metamorph ic g rade varies from amphibo li te to granulitc facies under hig h T/medium to low P The Moore Lakes area is located in the southeastern conditions. Metamorphic mineral assemblages record part of the Athabasca Basin (Figure IA), two main phases of mineral growth, coeval w ith 0 1 and approximately 40 km northeast of the Key Lake mine D2 defo rmation and followed by various retrograde and 35 km southeast of the McArthur River urani um events during 0 1, D~, and D,. Peak metamorphism (M!) deposit. Basement comprises rocks of the Woll aston Domain Tahle I - Summary of Paleoproterozoic deformatio 11 mu/ (Lewry and Sibbald, 1977; Gi lboy, 1983; Lewry et al. , metamorphic events f or the basement to the eastem 1985; Hoffman, 1990; Lewry and Collerson, 1990). Athabasca B11si,r (after Pon ella and Am1es/ey. this volume). which is subdivided into two subdomains. separated by ··--- a linear break. o n the basis of aeromagnetic data. These Age Thcrmotcctonic subdomain s are characterized by rocks of high (Ma) lk formation Metamorphism Stage magnetic total fi eld and those of low total fi eld. 1860-1 835 D, M , Early co ll isio nal respectively. 1835- 1820 D: M2 Collisional 1820- 1805 late D: Mz Oblique Four main groups of basement rocks are distinguished: w llisional I) Archean o rthogneisses and subordinate rocks, 2) 1805- 1795 early D1 M., Oblique high-grade Paleoproterozoic Wollaston Group collisional metascdiments, 3) deformed calc-alkalinc granitoids and subordinate gabbroids, and 4) pcraluminous 1795-1 775 late D3 MJ I.ate ohli4uc granitoids of different petrochemical types. co llis10 11 al 1775-1 760 0 4 M4 Pust-co llisional These rocks have been subjected to polyphase 1760-1 720 D, I .ate post- defo rmation and metamorphism (Lewry and Sibbald. co llisional 110° 102" I '. Athabasca Basin c::..., "<' . ,2 o~(Q. Superior Stud)· l\ra . , Craton HR H-."'-"' Lah' K~ -\. ~ . HlO TrAA, HIJJ1,tm f ~>~r:n SJIZ S 1i p,.:n or llow,J:uv Z()nc ~ ,·, 1 .oi,b,.•r'fl(II' ~hc:lr l~m~ _..,., 'i"/ ~-allc: F.oll~ ~lieu Zorw1 ~ :1 ! Sluff:~"N'l·W<u Thnut 11n 2to l m A Figure I - A) Location of the .<;tut~~· area i11 the southeastem pan of the Athafuu ca Basil! mul B) locatitm ofth e drill holes in the study 11re11. 202 Summar,· offm·e.1·1i,.:111 wm ]l)!HI. 1" o/ume :! was synchronous with calc-a lkaline plutonism at 1820 and essentially unaltered. It is composed mainly of to 1800 Ma. Pcgmatites o f S-type affinity yield ages of quartz, g rey feldspar, and biotite, with subordinate l 820 to 1800 Ma (Annesley et al. , 1997b). amounts of apatite (5 to 7 modal %), zircon (2 to 5 Decompression, upli ft, and cooling took place during modal %), uraninite (trace to 2 modal %), and ilmenite D, transpression under amphibo lite-facies co nditio ns. (Figures 2, 3, 4, 5, and 6). Other accessory m inerals and the timing of th is defonnatio n is constrained by include pyrite. Radioactivity reaches up to 1000 cps. monazite ages of 1806 to 1790 M a and titanite age of 1800 to l 775 Ma. These deformation and metamorphic Quartz grains, which are partly recrystallized, fonn events are related to the thcrmotectonic evolution of polycrystall ine aggregates elongated parallel to the the Trans-Hudson Orogen (Lewry and Sibbald, 1980; fol iation cleavage. Quartz grains range from 0.20 to Lewry, 1987; Bickford et al., 1990; Lewry and 4.00 mm in width and up to 6.00 mm in length. K­ Collcrson, 1990; C lowes et al., 1999; and references feldspar grains are variably recrystallized and are therein). intergrown with the quartz grains. Their grain size varies from 0.45 to 4.30 mm in width and up to Inliers of Archean/Paleoproterozoic rocks occur in the 5.30 mm in length. Biotite fl akes form massive clusters study area as part of the Moore Lakes Complex. The along the foliation planes. The fl akes range from 0.20 Moore Lakes Complex comprises A rchean to 1.00 mm in width and up to 4.60 mm in length. orthogncisses. Wo llaston Group metasediments, and They exhibit a pale brown to dark brown pleochroism. extensive diabase intrusions. Geological fe atures o f the Apatite grains, which arc distributed within biotitc-rich complex were mapped and documented by Forsythe clusters, are cuhedral and blocky to stumpy prismatic ( 1980).