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Athabasca Basin - Cigar Lake

Saskatchewan, Canada

Main commodities: U
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The Cigar Lake unconformity hosted uranium deposit is located 40 km west of the eastern margin of the Athabasca Basin, near Waterbury Lake, around 60 km north-east of the McArthur River deposit and approximately 660 km north of Saskatoon in northern Saskatchewan, Canada.

The Cigar Lake uranium deposit occurs within the Athabasca Basin of Northern Saskatchewan, Canada. The Athabasca Basin straddles both the Rae and Hearne Archaean Provinces and section of the Paleoproterozoic Trans-Hudson Mobile Belt which separates the Hearne and the Superior Archaean Province to the east. Like other major uranium deposits of the basin, Cigar Lake is located at the unconformity separating late Paleo- to Mesoproterozoic sandstones of the Athabasca Group from early Paleoproterozoic metasedimentary gneisses and plutonic rocks of the Wollaston Domain and Archaean crystalline rocks. The Wollaston Domain is a north-east trending zone of more intense metamorphism on the north-western margin of the Trans-Hudson Mobile Belt. It is composed of interlayered biotite-cordierite gneiss, biotite gneiss±garnet, arkosic meta-conglomerate and meta-arkose, and minor hornblende-biotite rocks, amphibolite, meta-pelites, and calc-silicates with marble and graphitic meta-pelite units.

The Manitou Falls Formation, the basal unit of the overlying 1450 to 1630 Ma Athabasca Group, was deposited in an intra-continental sedimentary basin filled by fluviatile terrestrial quartz sandstones and conglomerates. The Athabasca Group is largely undeformed and has a maximum preserved thickness of about 1500 m in its centre. The sandstone units of the Manitou Falls Formation on the eastern side of the basin, and the early Paleoproterozoic metasedimentary gneisses that unconformably immediately underlie them, host most of the known uranium mineralisation. Superficial overburden, mostly glacial till, ranges from zero, up to a thickness of 50 m in the deposit area.

The Lower Pelitic unit of the Wollaston Group, which lies directly on the Archaean granite basement, is considered to be the most favourable unit for uranium mineralisation. During the 1.8 to1.9 Ga Hudsonian orogeny, this group underwent polyphase deformation and upper amphibolite facies metamorphism, with local greenschist facies retrograde metamorphism. The Hudsonian orogeny was followed by a long period of erosion and weathering and the development of a palaeo-weathering regolith profile that is preserved beneath the unconformity.

At Cigar Lake, the Manitou Falls Formation is 250 to 500 m thick at Cigar Lake. The sandstone hosted mineralisation of the deposit is hosted by conglomerates of the second unit of the formation, the underlying basal conglomerate having wedged out against a 20 m high, east-west trending basement ridge which is immediately below the orebody.

Two litho-structural domains have been defined in the metamorphic basement at Cigar Lake. as follows:
i). The Wollaston Domain - composed of mainly pelitic metasedimentary gneisses and located in the south of the deposit area;
ii). The Mudjatik Domain to the north, characterised by large lensoid granitic domes.

The east trending transitional zone between the two metamorphic basement domains passes immediately beneath the Cigar Lake deposit, and is mainly composed of graphitic metapelitic gneisses and calc-silicate gneisses of the 'Lower Pelitic' unit. The graphitic metapelitic gneisses include unusual graphite- and pyrite-rich 'augen gneisses' which are primarily localised below the orebodies. The mineralogy and geochemistry of the graphitic metapelitic gneisses suggest they were derived from carbonaceous shale protolith, while the abundance of magnesium in the intercalated carbonates layers implies an evaporitic origin.

The Cigar Lake mine area is dominated by large NE-SW trending lineaments and wide east-west trending mylonitic corridors. The contact between these basement mylonite zones, which contain the augen gneisses, and the unconformably overlying sandstones of the Athabasca Basin, are considered to be the most prospective sites for the concentration of uranium mineralisation, particularly where graphitic basement fault zones were locally reactivated as brittle faults subsequent to the deposition of the sandstone.

The Cigar Lake uranium deposit was entirely concealed at a depth between 410 and 450 m below surface at the unconformity between the early Paleoproterozoic Wollaston Group basement metasediments and the overlying late Paleo- to Mesoproterozoic Athabasca Sandstone. The ore deposit has a lateral and longitudinal continuity, occurring as a flat-lying lens approximately 1950 m long by 20 to 100 m wide and up to 16 m thick, averaging around 6 m. It has a crescent shape in cross section, closely reflecting the basement ridge topography of the unconformity.

The location of the orebody, which is principally hosted by the Athabasca Sandstone, is controlled by an east-west trending structure developed within the graphitic metapelites of the Wollaston Group. The ore and the controlling structure are surrounded by a strong halo of alteration in both the Athabasca Sandstone and basement rocks, characterised by extensive development of Mg-Al rich clay minerals (illite-chlorite). Within the sandstone, this alteration halo which is centred on the deposit is up to 300 m in both width and height above the unconformity, while in the basement rocks, it extends over a width of up to 200 m and as much as 100 m below the ore.

Three distinct mineralisation styles have been recognised at Cigar Lake, as follows:

i). High grade mineralisation, developed at the unconformity and is characterised by the occurrence of massive clays.   This style contains the bulk of the deposit's total uranium metal and is the only economically viable style of mineralisation.   It consists primarily of three dominant rock and mineral facies occurring in varying proportions, namely, a). quartz, b). clay (primarily chlorite with lesser illite) and c). metallic minerals (oxides, arsenides, sulphides).
   In the two higher grade eastern lenses, the ore contains approximately 50% clay matrix, 20% quartz and 30% metallic minerals by volume, and is overlain by a very weakly mineralised 1 to 5 m thick contiguous clay cap. In the lower grade western lens, the ore comprises approximately 20% clay, 60% quartz and 20% metallic minerals. There is good continuity and homogeneity of the mineralisation, particularly in the eastern part of the deposit, while a very sharp demarcation is observable between well mineralised and weakly mineralised rocks, both at the upper and particularly at the lower surface of the orebody. This is taken to imply that while both pre- and post-mineralisation faulting provided preferential pathways for uranium bearing groundwater and to some extent for re-mobilised mineralisation, the internal distribution of uranium in this ore style was primarily controlled by geochemical processes.
   Uranium occurs as oxide minerals uraninite and pitchblende, present in both the sooty form and as botryoidal and metallic masses. It occurs as disseminated grains in aggregates ranging in size from millimetres to decimetres, and as massive metallic lenses up to 1 m or more thick and inferred to be 1 to 3 m in diameter, floating in a matrix of sandstone and clay. Coffinite is estimated to form <3% of the total uranium mineralisation. The ore is variably green, red and black in colour. Uranium grades range up to 82% U3O8 (70% U) for single drill hole intersections, accompanied by variable, but uneconomic amounts of Ni, Cu, Co, Pb, Zn, Mo and As. These higher grade intervals contain massive pitchblende and/or massive arseno-sulphides. The primary age of this style of mineralisation has been estimated at 1.3 Ga.
ii). Fracture controlled, vein-like mineralisation higher in the sandstone, above the "unconformity" mineralisation, known as "perched" mineralisation.
iii). Fracture controlled, vein-like mineralisation in the basement rock mass, known as "basement" mineralisation below the "unconformity" ore. Both the "perched" and "basement" mineralisation, are believed to have been the result of faulting subsequent to the formation of the "unconformity" ore which has led to the development of vein-type mineralisation in the fault zone, above and below respectively. The resultant mineralised bodies are volumetrically a very small part of the total mineralised system and are currently (2007) of no economic significance.

Remaining reserves and resources, quoted by Cameco (2007) are:
    Proven + probable reserve - 0.497 Mt @ 20.7% U
3O8 = 103 000 t U3O8;
    Indicated resources - 0.061 Mt @ 4.9% U
3O8 = 3000 t U3O8;
    Inferred resource - 0.317 Mt @ 16.9% U
3O8 = 53 800 t U3O8.

This description is largely summarised from Mainville, et al., 2007, Cigar Lake Project Northern Saskatchewan, Canada, Technical Report available on-line from the Cameco website in 2007.

The most recent source geological information used to prepare this summary was dated: 2007.    
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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