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Athabasca Basin - Centennial |
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Saskatchewan, Canada |
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U
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Super Porphyry Cu and Au
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The Centennial unconformity style uranium deposit is located in the south-central Athabasca Basin In Saskatchewan, Canada, ~170 km west and 110 km NW of the Cigar Lake and Key Lake deposits respectively, and ~600 km north of Saskatoon.
The Athabasca Basin straddles both the Rae and Hearne Archaean Provinces and section of the Palaeoproterozoic Trans-Hudson Mobile Belt which separates the Hearne and the Superior Archaean Province to the east. Like other major uranium deposits of the basin, Centennial is located at the unconformity separating late Palaeo- to Mesoproterozoic sandstones of the Athabasca Group from unconformably underlying metamorphosed Palaeoproterozoic to Neoarchaean basement rocks. The bulk of the major deposits of the basin occur along a NE-SW structural trend following the north-western margin of the Wollaston Domain, a north-east trending zone of more intense metamorphism on the north-western margin of the Trans-Hudson Mobile Belt, and the southeastern margin of the Hearne Province.
In contrast, Centennial is underlain by a prominent belt of Palaeoproterozoic to Neoarchaean-aged mylonitised rocks of the Snowbird Tectonic Zone, a major "crustal-scale" structural discontinuity that forms the boundary between the Hearne Province (Virgin River Domain) to the east and the Rae Province (Lloyd Domain) to the west. This structure is some 140 km to the NW of, and parallel to, the trend of the deposits on the margin of the Wollaston Domain.
Where the Snowbird Tectonic Zone is exposed at the southern edge of the Athabasca basin, it occurs as a 5 to 7 km wide zone of highly strained cataclasites and mylonitised rocks known as the Virgin River Shear Zone (VRSZ; Lewry and Sibbald 1980; Card et al., 2008). The VRSZ is interpreted (Card et al., 2008) to represent a suture between the Virgin River and Lloyd domains, which entrained a narrow package of lower metamorphic grade (lower to middle amphibolite) pelite and psammopelitic schists termed the Virgin Schist Group (Johnson, 1968). The two domains are interpreted to have had separate tectonometamorphic histories prior to being sutured (Card and Bosman, 2007; Card et al., 2008), with the Virgin River Domain containing upper amphibolite facies assemblages, while the Lloyd Domain has been subject to granulite facies conditions. The high metamorphic grade rocks are overprinted by amphibolite facies metamorphism along the VRSZ (Reid et al., 2010).
Post-Athabasca Group reactivation along mylonitised rocks of the Snowbird Tectonic Zone is manifested in the NE-SW oriented, NW-dipping, dextral-oblique, Dufferin Lake reverse fault. This structure has a strike length of more than 50 km and affects all rock types, with a >250 m west side up vertical displacement of the Athabasca Group unconformity (Reid et al., 2010).
The immediate sub-Athabasca Group basement at Centennial belongs to the Virgin River Domain, the westernmost section of the Cree Lake mobile zone of the Hearne Province, represented by metasedimentary and metavolcanic rocks, supracrustal schists and gneisses, felsic and mafic gneisses, and deformed Hudsonian-aged granitoids (Chandler, 1978).
The metamorphic grade of these rocks ranges from upper greenschist to lower amphibolite, with widespread retrograde greenschist facies. Intensive palaeoweathering to kaolinite in the upper portions of the profile and illite and chlorite in the lower portions, affected the basement rocks prior to deposition of Athabasca Group sediments. In locations distal to mineralisation, palaeoweathering is observed affecting granitoid rocks as pitting and dissolution of feldspar up to 100 m below the unconformity (Reid et al., 2010).
The Athabasca Basin was initially formed at ~1750 Ma as a series of three NE-SW oriented sub-basins (Armstrong and Ramaekers, 1985; Kyser et al., 2000). The basin was filled by up to 2 km of Palaeoproterozoic age clastic rocks derived from the rapid exhumation of the Trans-Hudson orogenic belt (Ramaekers, 1990). At Centennial, the Athabasca Group is 800 to 900 m thick and is entirely composed of the Manitou Falls Formation, which comprises four members, from the base upwards:
Manitou Falls-a member which is represented by interbedded, matrix-supported quartz pebble conglomerate and poorly sorted medium-grained sandstone;
Manitou Falls-b member which is only rarely found in the district, where it is dominated by medium-grained quartz sandstone with a substantial component of poorly sorted, clast-supported conglomeratic sandstone;
Manitou Falls-c member, a relatively clean, medium- to coarse-grained sandstone with thin, generally <2 cm, granule and pebble beds;
Manitou Falls-d member, which is also a relatively clean, medium- to coarse-grained sandstone, but is characterised by the presence of a significant
amount of clay intraclasts.
Regionally, the Athabasca Group sedimentary rocks and the basement complex are intruded by a series of 1 to several hundred metres wide, 1267±2 Ma (U-Pb; Le Cheminant and Heaman, 1989), NW-trending mafic dykes of the Mackenzie dyke swarm (Cumming and Krstic, 1992), exploiting tensional fractures associated with sinistral movement along Hudsonian faults (Hoeve and Sibbald, 1978; Sibbald and Quirt, 1987). Mafic dykes that post date the mineralisation at Centennial most likely belong to this swarm.
The Centennial deposit occurs at or near the Athabasca Group unconformity, is clearly fault-related, and is associated with large-scale zones of Athabasca Group uranium enrichment, silicification, desilicification, chloritisation, pyritisation, kaolinisation, dravitisation and brick-red hematisation. The deposit occurs as a 650 m long, NNE-SSW oriented zone, occurring at a depth of ~800 m, close to the unconformable contact between basement rocks and the overlying Athabasca Group (Jiricka and Witt, 2008). Drilling has also indicated that the deposit has minimum across-strike widths ranging from 10.0 to 52.5 m and intersection thicknesses ranging from 1.8 to 33.9 m (Jiricka and Witt, 2010). Mineralisation is situated ~300 m to the east of the interpreted location of the Dufferin Lake fault, where it straddles an unconformity high formed at the contact between highly strained, weakly mylonitic, metasedimentary and granitoid rocks of the basement. A steeply west-dipping zone of brittle deformation is developed at this contact, representing the reactivation of earlier ductile structures of the Snowbird Tectonic Zone/VRSZ under brittle conditions. Brittle faulting is best developed in the quartzite located in the hanging wall of the fault zone. The footwall and hanging wall to the fault zone comprise, granitoid and metasedimentary rocks respectively. This brittle deformation extends into the Athabasca Group as desilicified and clay-indurated breccias which sometimes contain minor perched uranium mineralisation (Reid and Ansdell, 2010).
In contrast to other Athabasca Basin deposits, the brittle structure with which Centennial mineralisation is associated, does not appear to have substantial post-Athabasca displacement (Jiricka and Witt, 2008), unlike the adjacent Dufferin Lake fault.
The Athabasca Group Manitou Falls Formation column above the deposit is variably silicified, desilicified, hematised, clay-indurated and commonly has a sooty grey appearance. The upper 200 to 250 m of the formation is strongly silicified by quartz overgrowths, while moderate to strong desilicification is observed in the lower parts. Basement rocks have been locally affected by a pervasive dark green chloritisation. Within 30 m of the west-dipping brittle fault zone associated with mineralisation, there is a zone of pale green alteration that can be observed overprinting the dark green chloritisation. This pale green alteration is in turn overprinted by a creamy yellow alteration and fracture-controlled hematite within 15 m of the mineralisation. Pale grey clay is observed filling fractures that cross-cut earlier alteration, and which are in turn cross-cut by dark green hairline fractures. Weakly graphitic metasedimentary lithologies of the hanging wall appear to extend into areas of uranium mineralisation, but graphite has not been observed within the strongly clay altered zone that surrounds the mineralisation (Reid et al., 2010).
The Centennial uranium mineralisation most commonly extends continuously from a few metres above the unconformity in the Athabasca Group, to more than 30 m into basement, although it may be locally perched up to 100 m above the unconformity. Mineralisation in the basement is generally associated with intensely clay altered and faulted pelite and quartzite, and is locally found adjacent to strongly chloritised granitoid. The primary mineralisation appears to occur as earthy to sub-metallic black blebs of uraninite intimately associated with brick red hematite alteration, while secondary uranium minerals are ubiquitous, commonly observed as white and yellow fractures and haloes around the black uraninite blebs. The abundance and variety of these secondary uranium minerals implies multiple fluid events have affected the primary mineralisation. Close to the unconformity, and within the Athabasca Group, Mackenzie swarm dolerite dykes are generally intensely clay altered, and is in some cases mineralised (up to 6500 cps), suggesting that a post-intrusion fluid has remobilised and precipitated uranium in or at the contact of the dolerite due to its reduced character (Reid et al., 2010).
Reserves and resources had not been estimated in 2012. Better intersections encountered include 34 m @ 8.8% U3O8; 28.7 m @ 10.27% U3O8, using a 1.0% grade cut-off (Jiricka and Witt, 2008; 2010).
The most recent source geological information used to prepare this decription was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Alexandre P, Kyser K, Jiricka D and Witt G, 2012 - Formation and Evolution of the Centennial Unconformity-Related Uranium Deposit in the South-Central Athabasca Basin, Canada : in Econ. Geol. v.107 pp. 385-400
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Reid K D, Ansdell K, Jiricka D, Witt G and Card C, 2014 - Regional Setting, Geology, and Paragenesis of the Centennial Unconformity-Related Uranium Deposit, Athabasca Basin, Saskatchewan, Canada : in Econ. Geol. v.109 pp. 539-566
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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