Lac Cinquante Uranium Deposit, Western Churchill Province, Nunavut, Canada

Abstract Lac Cinquante is a mineralogically simple, vein-hosted uranium deposit in Archean basement rocks (Angikuni greenstone belt) that originally were unconformably overlain by Proterozoic sedimentary and volcanic rocks of the Baker Lake Group (BLG). Basement rocks are mainly pillowed and massive tholeiitic lavas (N-MORB) with interbedded tuff; overlying sedimentary rocks comprise a basal talus/fault breccia that grades upward into fluvial sedimentary rocks and subaerial, trachytic volcanic rocks; granite and porphyry dikes (Hudson and/or Nueltin granitoids) intrude the basement rocks. A typical section through the Main Zone comprises 1) laminated ‘tuff’, 2) altered (carbonate-hematite-chlorite) schistose ‘tuff’, 3) a pitchblende-bearing, breccia-vein, 4) altered ‘tuff’, and 5) laminated ‘tuff’. The vein comprises brecciated chlorite, hematite, and pitchblende with carbonate cementing the breccia. Graphitic ‘tuff’ with sulfide-stringer veinlets locally overlies or underlies the vein. Pitchblende also occurs in the altered rocks and in gash veins that cut basement rocks. U-Pb ages on pitchblende fall into two groups: one at 1828 ± 29 Ma reflects primary uranium mineralization, and a second at 1437 ± 31 Ma represents resetting. Pb-Pb chemical ages show other resetting events at 1260–1321Ma, 895 Ma, 741–813 Ma, and 660–706 Ma. One possible genetic model for the Lac Cinquante deposit involves leaching of uranium from granitoids and the BLG by basinal brines that migrated to depositional sites where graphitic rocks acted as a reductant. A second model requires that uranium was sole-sourced from the BLG and that granitoid rocks provided heat to drive the hydrothermal system, which leached and ultimately precipitated uranium. The deposit has characteristics similar to vein-type uranium deposits in the Beaverlodge District of Saskatchewan.
Keywords: NW Hearne, Archean basement, Baker Lake Group, Angikuni subbasin, vein-hosted uranium, graphite
Résumé Le dépôt du Lac Cinquante est un gîte filonien d’uranium minéralogiquement simple, encaissé dans des roches du socle archéen (ceinture de roches vertes d’Angikuni) qui ont initialement été recouvertes en discordance par des roches volcaniques et sédimentaires d’âge Protérozoïque du Groupe de Baker Lake (GBL). Les roches du socle sont principalement des laves tholéiitiques (N-MORB) massives et coussinées avec des interlits de tuf; les roches sédimentaires sus-jacentes comprennent une brèche de talus ou de faille qui fait verticalement transition avec des roches sédimentaires fluviatiles et des roches volcaniques subaériennes trachytiques; les roches du socle sont recoupées par des dykes de granite et de porphyre (granitoïdes de Hudson et / ou de Nueltin). Une section typique à travers la zone principale comprend: 1) un ‘tuf’ laminaire, 2) un ‘tuf’ schisteux altéré (carbonate-chlorite-hématite), 3) un filon bréchique de pechblende, 4) un ‘tuf’ altéré, et 5 ) un ‘tuf’ laminaire. La veine consiste en chlorite bréchique, en hématite et en pechblende avec une brèche à ciment de carbonate. Un « tuf » graphiteux avec des veinules de sulfures est localement noté au dessus ou au dessous de la veine. La pechblende est également présente dans les roches altérées et dans les veines de tension qui recoupent les roches du socle. Les âges U-Pb retournés par la pechblende se répartissent en deux groupes: le premier à 1828 ± 29 Ma reflète la mise en place de la minéralisation d'uranium primaire, et le second à 1437 ± 31 Ma correspond une remise à zéro. Les âges Pb-Pb chimiques révèlent d'autres événements de remise à zéro à 1260–1321 Ma, 895 Ma, 741– 813 Ma et 660–706 Ma. Un modèle génétique possible pour le gîte du Lac Cinquante serait que la lixiviation de l'uranium contenu dans les granites et le GBL par des saumures bassinales qui auraient par la suite migré vers des sites favorables au dépôt de la minéralisation et où les roches graphitiques auraient fourni un environnement réducteur. Un deuxième modèle possible serait que l'uranium ait été exclusivement fourni par le GBL et que les roches granitiques aient fourni la chaleur requise au fonctionnement du système hydrothermal, lequel aurait contrôlé le lessivage et la précipitation de l’uranium. Les caractéristiques de ce gîte sont similaires à celles des gisements d'uranium filoniens du district de Beaverlodge en Saskatchewan.
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Summary: We use the results of new bedrock mapping, combined with geochemical point data and airborne radiometric data (in particular equivalent uranium concentrations) to help determine which geological units in the Paleoproterozoic Great Bear magmatic zone (GBmz) contain elevated uranium. The data collectively indicate that the highest primary uranium concentrations are in equigranular granites and subvolcanic/volcanic porphyries. Notably, these rocks host the majority of known uranium-bearing...
Publication: Exploration & Mining Geology
Author(s): LUKE OOTES, JEFF HARRIS, VALERIE A. JACKSON, BRONWYN AZAR, AND LOUISE CORRIVEAU
Keywords: Wopmay orogen, Great Bear magmatic zone, bedrock geochemistry, IOCG, unconformityrelated uranium
Issue: 1
Volume: 21
Year: 2013
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Summary: A 10-cm thick clay-rich layer near the top of the Manitou Falls Formation of the Athabasca Group is unusual in its breccia texture, alteration, and detrital mineral composition relative to the adjacent overlying and underlying sedimentary beds. This layer is composed of angular quartz grains set in an illite >> kaolinite + dickite matrix. Deformed clay-rich fragments within the layer have very similar mineral assemblages. The presence of euhedral accessory minerals including Ti-oxides and...
Publication: Exploration & Mining Geology
Author(s): JEANNE B. PERCIVAL, SEAN A. BOSMAN, ERIC G. POTTER, PAUL RAMAEKERS, KATHERINE E. VENANCE, PAT A. HUNT, WILLIAM DAVIS, AND CHARLES W. JEFFERSON
Keywords: microbreccia, hydrothermal alteration, hydraulic fracturing, uranium mineralization
Issue: 1
Volume: 21
Year: 2013
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Summary: The nature and distribution of radiation-induced defects (RIDs) in quartz from the Maw Zone, a yttrium- and rare-earth-element-enriched sandstone breccia complex exhibiting intense hydrothermal alteration, have been investigated by electron paramagnetic resonance (EPR) spectroscopy. EPR spectra show that detrital quartz in sandstone, from five diamond drill holes intersecting the Maw Zone and one above the crest of the so-called Quartzite Ridge, contain only background-level RIDs, indicating...
Publication: Exploration & Mining Geology
Author(s): YUANMING PAN, GARY YEO, BRETT ROGERS, CHRISTINE AUSTMAN, AND BAOQUAN HU
Keywords: quartz EPR, radiation-induced defects, hydrothermal alteration, uranium exploration, Athabasca Basin
Issue: 1
Volume: 21
Year: 2013
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Summary: The use of CR-39 plastic polymer as a solid state nuclear track detector in the textural analysis of radioactive minerals in geological samples was first described by I.R Basham in 1981. CR-39 autoradiographs provide a detailed, high-resolution image of the in situ distribution of the radioactive minerals within geological samples on both the macroscopic and microscopic scales. This technique is an inexpensive and effective means of obtaining detailed textural information that provides...
Publication: Exploration & Mining Geology
Author(s): G.W. SPARKES
Keywords: uranium, CR-39, autoradiograph, Labrador, uraninite
Issue: 1
Volume: 21
Year: 2013
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Summary: The U-Th-REE mineralization at Fraser Lakes Zone B is hosted by granitic pegmatites and leucogranites, which lie along the deformed contact between Paleoproterozoic metasedimentary gneiss of the Wollaston Group and Archean orthogneiss, approximately 25 km from the southeastern edge of the Athabasca Basin. The pegmatites/leucogranites are subcordant to concordant with the regional foliation and are concentrated within a northeast-plunging antiformal fold nose, the study area, which lies west...
Publication: Exploration & Mining Geology
Author(s): CHRISTINE L. MCKECHNIE, IRVINE R. ANNESLEY , AND KEVIN M. ANSDELL
Keywords: U-rich pegmatites, Th-REE-rich pegmatites, Fraser Lakes Zone B, Wollaston Domain, Athabasca Basin, U metallogeny
Issue: 1
Volume: 21
Year: 2013
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Summary: A new method for mapping faults within basement rocks underlying the Thelon Formation and glacial overburden was developed and tested in the Aberdeen Sub-basin. This method utilizes newly acquired aeromagnetic data, the Blakely algorithm for defining magnetic source edges, a calculated dip-direction map, a digital elevation model (DEM) derived from the Canadian Digital Elevation Database, and the positions of identified, inferred and newly mapped faults that are within and adjacent to this...
Publication: Exploration & Mining Geology
Author(s): VICKI TSCHIRHART, BILL MORRIS, AND CHARLES JEFFERSON
Keywords: faults, uranium, Thelon Basin, source edge detection, lineament analysis
Issue: 1
Volume: 21
Year: 2013
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Summary: The Dragon Lake Zone (uranium prospect) comprises a N160º trending vein that is approximately 110 m long, up to 5 m wide and from 1 to 40 m high. The zone is mostly within altered sandstone of the Fair Point Formation, the basal unit of the Paleoproterozoic Athabasca Group, but it also extends downward (up to 3.5 m) into basement rocks that occupy the northerly trending Maybelle River Shear Zone (MRSZ). This subvertical shear zone separates graphitic paragneiss and weakly deformed granitoid...
Publication: Exploration & Mining Geology
Author(s): K. WHEATLEY AND C. CUTTS
Keywords: uranium, unconformity, Alberta, Maybelle River, Dragon Lake, Athabasca
Issue: 1
Volume: 21
Year: 2013
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