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Athabasca Basin - Key Lake |
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Saskatchewan, Canada |
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U Ni
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Key Lake uranium-nickel deposit, comprising the Gaertner and Deilmann orebodies, lies on the south-eastern margin of the Athabasca Basin in 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. An anomalously large concentration of unconformity-style uranium depsoits are found in the basal section of the Athabasca Basin and the immediately underlying crystalline basement. With the exception of the deposits of the Carswell Structure, all of the large orebodies are in the western section of the basin, within a north-east trending strip coincident with the overlap of the Athabasca Basin and the underlying Wollaston Domain of the basement. 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. More specifically, the majority of the Athabasca Basin orebodies are hosted both within, and immediately above Paleoproterozoic (pre 1800 Ma) graphitic meta-pelites of this suite, underlying the late Paleo- to Mesoproterozoic (1.63 to 1.45 Ga) Athabasca Group sandstones with an angular unconformity.
At Key Lake, the sub-Athabasca basement comprises Archaean gneissic granitoid rocks flanked by early Paleoproterozoic metasedimentary rocks of the Wollaston Group. A strong NE- trending structural grain, defined by doubly plunging folds cored by Archaean gneiss, characterises these rocks on a regional scale and was imparted by NW-SE compression during the 1.88 to1.80 Ga Trans-Hudson orogeny.
The late Paleo- to Mesoproterozoic Athabasca Group comprises a sequence of mature, fluviatile quartzose sandstones and conglomerates which have red bed characteristics. The sandstones are primarily composed of quartz and clay with no remaining feldspar. At the base of the sequence, immediately above the unconformity, there are approximately 60 m of non-marine fluvial sandstone and conglomerate of the Manitou Falls Formation. Around 1000 m above its base is a marine unit, overlain again by further sandstones and another marine unit of organic shales and stromatolitic dolomites. The group is approximately 2200 m thick, although fluid inclusion data from minerals at its base suggests a depth of burial of as much as 4 to 5 km.
The Pre-Athabasca Basin basement surface in the Key Lake area is dominated by a NE-elongate palaeovalley which coincides with the weak, highly sheared and faulted graphitic units in the Key Lake Fault Zone which controls the mineralisation. Basal conglomeratic beds of the Manitou Falls Formation show stratigraphic onlap onto the erosion surface, indicating an early valley-fill within this area of subdued basement topography. The unconformity surface is marked by regolith development which resulted in a characteristic vertical zonation, with a lower green zone (chloritic), an upper red zone (hematitic and kaolinitic) and a narrow "bleached zone" very close to, or along, the unconformity surface (Harvey and Bethune, 2000).
Five phases of deformation have been defined in the Key Lake area, namely:
D1 produced the regionally penetrative S1 foliation oriented sub-parallel to the Archaean basement and early Paleoproterozoic cover contact, characterised by transposed compositional layering and rootless F1 isoclinal folds. Late-D1 mylonitisation takes the form of decimetre-scale, discontinuous high-strain zones, marked by protomylonitic to mylonitic fabrics superimposed on the earlier gneissic fabric. These zones are commonly concentrated in the graphitic units and are believed to have been related to early thrusting marking the Archaean-Paleoproterozoic contact.
D2 produced rare, steeply dipping, WSW-trending, tight to isoclinal folding of the S1 foliation.
D3 resulted in open to tight, NE-trending F3 folds that are responsible for the structural grain of the Wollaston Domain. Minor, moderately NE-plunging, Z-asymmetric folds of this set have been mapped on the NW limb of a regional F3 fold cored by the Zimmer Lake inlier.
D4 was responsible for open to very open NW- trending, subvertical folds.
D5 is expressed by two sets of faults that were initiated before deposition of the Athabasca Sandstone, comprising: a). An older brittle-ductile set which strikes ENE-WSW sub-parallel to the S1 foliation and are typically concentrated in highly sheared graphitic units to collectively constitute the 60 to 80 m wide Key Lake Fault Zone. Reactivation of these faults following the deposition of the Athabasca Sandstone is indicated by the reverse displacement of the unconformity surface by up to 12 to 15 m. One major reverse fault of this set is spatially associated with the mineralisation. b). A younger brittle set which comprises NW-striking discrete, steeply dipping faults that were only locally, and weakly, reactivated after deposition of the Athabasca sandstone (Harvey and Bethune, 2000).
The Key Lake deposit consists of two orebodies, namely: i). the Gaertner orebody was approximately 800 m long, 10 to 40 m wide, and as much as 50 m thick, and ii). the larger Deilmann orebody which was about 900 m long, 30 to 50 m wide and 90 m thick.
Both orebodies are localised in an ENE trending fault zone where it cuts the unconformity, and are mainly within the Wollaston Group, just NW of the Archaean Zimmer Lake inlier, and within around a hundred metres of the unconformity surface. The immediate hosts to ore are mylonitised metasediments within the fault zone. The Wollaston Group metasedimentary rocks hosting the orebodies include pelites and psammopelites with variable graphite content, along with lesser psammitic and calc-silicate-bearing sedimentary rocks and are part of the regionally extensive lower sedimentary sequence. The graphite content of psammopelitic to pelitic units is quite variable, ranging from 2 to 40%. Rare porphyroblasts of intensely altered garnet and cordierite are present within the hosts. The graphite-rich units are strongly foliated and show local development of protomylonitic to mylonitic fabrics. Two suites of pegmatite intrude the metasedimentary rocks: an early sub-concordant suite cuts the S1 foliation and appears to have concentrated within the highly sheared graphitic units. A second suite is more alkalic than the first and has suffered very little deformation. The basement rocks are unconformably overlain by Unit B of the Manitou Falls Formation of the Athabasca Group comprising medium-grained quartz sandstone, conglomerate and minor, discontinuous, beds of shale and silt.
The primary ore minerals are gersdorffite, bravoite and rammelsbergite which were formed in conjunction with Fe chlorite and with kaolinisation of mylonite in the fault zone. The uranium mineralisation has been altered to sooty pitchblende with coffinite rims. Niccolite replaces gersdorffite, but is in turn converted to maucherite. Accessory galena and sphalerite are also present. The primary mineralisation is dated at 1350 Ma.
This is overprinted by Several phases of hydrothermal alteration associated with initial mineralisation and remobilisation overprint the 'regolith weathering' assemblage at the unconformity. Chlorite and illite alteration which occurs within tens of metres of the main mineralised reverse fault is characterised by an inner core dominated by Fe-Mg chlorite and a peripheral zone of Mg-chlorite. In the Athabasca sandstone, extensive illitisation is overprinted by fault- related kaolinitisation and bleaching, including a zone of intensive bleaching within metres of the main fault zone.
Harvey and Bethune, (2000) propose the following sequence of mineralisation related events. During the Hudsonian orogeny, late- to post-D1 high-strain was heterogeneously distributed in graphite-rich units, proximal to the Archaean basement-Wollaston Group cover contact, locally forming metre-wide mylonite zones sub-parallel to S1. These high-strain zones were subsequently intruded at around 1812 Ma by sub-concordant uraniferous pegmatite dykes. The high-strain zones and the pegmatite sheets were folded about moderately NE-plunging axes, forming the open to tight F3 folds and then faulted by ENE-WSW striking ductile- to brittle-faults localising within relatively weak, previously sheared graphitic units. Exhumation and erosion during the late-Paleoproterozoic produced a regolith and palaeovalley formation, followed by deposition of the Athabasca Group. After deposition, the ENE-WSW trending fault set was reactivated at around 1350 Ma forming the steeply north-dipping reverse faults of the Key Lake Fault Zone, resulting in brecciation, and offset of the unconformity by up to 15 m. Faulting and brecciation facilitated transport of ore-bearing fluids with mineralisation concentrating along, and at the intersection of, the main fault and the Athabasca cover. The blossoming out of the orebody at the unconformity surface suggests that the topographic depression in the sub-Athabasca basement served as a fluid trap.
Reserves in 1985 were 80 000 tonnes of U3O8 at an average grade of 2.71% U3O8.
Remaining reserves in 2005 were 0.069 Mt @ 0.52% U3O8 containing 52 100 tonnes of U3O8.
The total size of the deposit has also been published as: 4.2 Mt @ 20 kg/t U (2.36% U3O8) = 82 875 tonnes U3O8, including (Ghandi, 2005),
Gaertner - 1.345 Mt @ 2.01% U3O8 = 27 100 t U3O8
Deilmann - 2.242 Mt @ 2.49% U3O8 = 55 790 t U3O8
For detail see the reference(s) listed below, and Harvey & Bethune, 2000 - Geology of the Key Lake Unconformity-type Uranium deposit, Deilmann pit, northern Saskatchewan; Canadian Society of Exploration Geophysicists, 2000 Conference, Abstracts.
The most recent source geological information used to prepare this decription was dated: 2005.
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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Dahlkamp F J, 1978 - Geologic appraisal of the Key Lake U-Ni deposits, northern Saskatchewan: in Econ. Geol. v73 pp 1430-1449
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Trocki L K 1984 - Ages of major Uranium mineralization and Lead loss in the Key Lake Uranium deposit, northern Saskatchewan, Canada: in Econ. Geol. v79 pp 1378-1386
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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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