Athabasca Basin - Millenium

Saskatchewan, Canada

Main commodities: U
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The Millennium unconformity-style uranium deposit is located in the Palaeoproterozoic Athabasca Basin, central Saskatchewan, Canada, 45 km from the basin's eastern margin, 35 km southwest of the McArthur River deposit, and 35 km north of the Key Lake mine.

Mineralisation is hosted in basement rocks at the transition zone from the Archaean and Palaeoproterozoic rocks of the Wollaston Domain to the Mudjatik Domain, both belonging to the Cree Lake Mobile zone of the Hearne province. The Wollaston Domain is a NE-trending fold and thrust belt, fault bounded to the east by the Peter Lake Domain and to the west by the Mudjatik Domain. Three main rock groups have been outlined, namely: i). 2800 to 2950 Ma Archaean granitic, granodioritic and tonalitic orthogneisses, and lesser metamorphic rocks; ii). unconformably overlying high-grade ~1920 Ma Palaeoproterozoic graphitic and non-graphitic pelitic, psammopelitic and psammitic gneisses, lesser meta-quartzite, calc-silicates and amphibolites, and rare BIF of the Wollaston Group; iii). deformed 1810 to 1840 Ma granitoids, minor gabbros and pegmatites.

The Mudjatik Domain is a NE-trending fold and thrust belt, fault-bounded to the east by the Wollaston Domain and to the west by the Virgin River Domain. It is composed of rocks that are compositionally similar to those of the Wollaston Domain, other than that it is dominated by granitoid gneisses derived in part from in situ migmatisation and anatexis, and contains a large volume of late, peraluminous granite.

The Wollaston-Mudjatik transition zone is a structurally complex zone composed of elongated Archaean granitoid domes (mega-boudins), thrust- and strike-slip structures, and related duplex structures, with the boundary being defined on the basis of lithological changes in the Archaean basement, lithofacies changes in the Wollaston Group, and changes in types of Hudsonian intrusive rocks. This boundary represents a major anastomosing NE-trending transcurrent shear zone associated with a thermotectonic front, and is defined as a transitional zone from thick-skin tectonics in the Wollaston Domain to thin-skin tectonics in the Mudjatik Domain. The rocks of both domains have been complexly deformed and metamorphosed to amphibolite facies by the Trans-Hudson orogen, with peak metamorphism at ~1800 to 1820 Ma. The subsequent rapid uplift initiated deposition of the Athabasca basin which has a maximum and minimum ages of 1750 Ma and 1620 to 1640 Ma respectively.

The Athabasca basin was formed as a series of three NE-SW–oriented subbasins, with the easternmost Cree subbasin hosting the majority of the known uranium deposits. Sedimentation culminated with deposition of the carbonaceous marine shale of the Douglas Formation at ca. 1540 Ma. In summary, the sediments of the Athabasca basin comprise sequences of Palaeo- and Mesoproterozoic polycyclic, mature fluvial to marine quartz sandstone, collectively referred to as the Athabasca Group, which were deposited in a near-shore shallow shelf environment. In the eastern part of the Athabasca basin this Group is made up exclusively of the Manitou Falls Formation which contains coarse to fine-grained, hematite-rich conglomerates.

The Manitou Falls Formation and underlying basement are cut by a series of 1267±2 Ma mafic dikes known as the McKenzie dyke swarms which range from 1 m to several hundred metres in width, controlled by tensional trends associated with sinsitral movement along the ancient Hudsonian faults.

The Millennium deposit is located within Palaeoproterozoic metasedimentary basement rocks of the Wollaston-Mudjatik transition zone at a depth ranging from 600 to more than 750 m below the surface, and from 100 to more than 150 m below the sandstone-basement unconformity. It is interpreted to lie along the NW-trending limb of a Z-type parasitic fold associated with the eastern limb of a larger, regional, north-plunging synformal, SSW closing fold. Mineralisation is essentially stratabound, and lies between two reverse faults. The footwall reverse fault (the Mother fault), is filled with preore quartz, while the hanging wall reverse fault is associated with a cordierite-graphite metapelitic gneiss unit closely associated with the mineralisation.

Basement rocks in the deposit area comprise graphitic metapelitic schists and gneisses with lesser meta-calc-pelitic rocks that have been intruded by numerous pegmatitic and granitic rocks. The basement is are overlain by the Palaeoproterozoic Athabasca Group which locally consists of 500 to 700 m of diagenetically altered members of the Manitou Falls Formation cut by fractures and faults. The host to mineralisation is the metapelitic gneisses which are overprinted by extensive illite alteration, interpreted to reflect a zonation from distal saussuritisation (plagioclase retrogression) and sericitisation, through a more proximal zone of chloritisation into a central zone of increasing illitic and dravite alteration. This contrasts with other major basement-hosted deposits which are characterised by a small outer illitic halo, a large inner chloritic halo and a core containing most of the uranium.

Uranium mineralisation at Millenium is predominantly of uraninite with minor amounts of coffinite and occurs in a variety of styles, including: i). massive foliation-controlled replacement; ii). uraninite matrix in breccias; iii). irregular fracture-controlled infillings and thin vein-type uraninite; iv). bleblike aggregates and v). thin discordant uraninite veinlets and rims around fragments or quartz veins. The dominant style is the massive replacement type while fracture infilling and vein-type are less well developed. Textures in the breccia-hosted mineralisation suggest a progression from simple fracture-controlled, through more complex hydraulic fracturing, and finally into corrosive solution breccias characterised by progressive rounding and embayment of clasts. The width and grade of the ore zones are variable with grades ranging between 1 and 4 percent U over 20 to 30 m.

Indicated resources in 2009 (Clouthier, et al., 2009) were 18 000 t U, at an average grade of 4.53% U3O8

The most recent source geological information used to prepare this summary was dated: 2009.    
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.

  References & Additional Information
   Selected References:
Cloutier J, Kyser K, Olivo GR and Alexandre P,   2010 - Contrasting Patterns of Alteration at the Wheeler River Area, Athabasca Basin, Saskatchewan, Canada: Insights into the Apparently Uranium-Barren Zone K Alteration System: in    Econ. Geol.   v105 pp 303-324
Cloutier J, Kyser K, Olivo GR, Alexandre P and Halaburda J,  2009 - Millennium Uranium Deposit, Athabasca Basin, Saskatchewan, Canada: An Atypical Basement-Hosted Unconformity-Related Uranium Deposit : in    Econ. Geol.   v104 pp. 815-840
Richard, A., Cathelineau, M., Boiron, M.C., Mercadier, J., Banks, D.A. and Cuney, M.,  2016 - Metal-rich fluid inclusions provide new insights into unconformity-related U deposits (Athabasca Basin and Basement, Canada): in    Mineralium Deposita   v.51 pp. 249-270

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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