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Athabasca Basin - Rabbit Lake |
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Saskatchewan, Canada |
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U
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Rabbit Lake unconformity hosted uranium deposit is located close to the eastern margin of the Athabasca Basin, approximately 700 km north of Saskatoon, in northern Saskatchewan, Canada.
Regional Setting
The Rabbit Lake orebody is located in early Paleoproterozoic metasedimentary rocks of the Wollaston litho-structural domain, a few tens of metres below the unconformity with the overlying late Paleo- to Mesoproterozoic Athabasca Basin sequence. 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. The Wollaston Domain is a north-east trending zone of more intense metamorphism on the north-western margin of the Trans-Hudson Mobile Belt, and comprises metasediments of an early Paleoproterozoic supracrustal succession overlying a remobilised 2.47 Ga and older granitoid Archaean basement, which forms the cores of doubly plunging domal inliers.
The Wollaston Domain metasediments, the Wollaston Group, include 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. These rocks yield metamorphic ages of from 1.88 to 1.57 Ga, reflecting the 1.88 to 1.80 Ga Hudsonian and younger orogenesis. The pink Archaean granitoid basement locally shows a possible palaeo-weathering profile, which becomes "bleached" close to the contact with the overlying Paleoproterozoic Wollaston Group meta-sediments. A dominantly pelitic unit, which is commonly graphitic and corresponds to a strong electromagnetic conductor, directly overlies or is in close proximity to the Archaean basement. A thin basal quartzite or meta-arkose unit may locally intervene. These two units are overlain by a monotonous sequence up to 2400 m thick of calcareous meta-arkoses (metamorphosed to foliated gneisses and in places to unfoliated albite/plagioclase granulites) which are interlayered with subordinate calc-silicate rocks (with scapolite layers) and possibly semi-pelitic metasediments, followed by massive carbonate, overlying meta-arkose, calcsilicates and local carbonate beds. This sequence is followed by an approximately 1400 m thick quartzite-amphibolite unit characterised above the basal well-layered meta-arkose-calc-silicate-carbonate sequence by pelitic gneisses with inter-digitated sillimanite meta-arkoses, calcareous meta-arkosic to semi-pelitic metasediments, amphibolites and quartzites. The pelitic gneisses are locally graphitic as are the calcareous metasediments. Apatite is abundant in certain calc-silicate layers within the calcareous metasediments.
The main Rabbit Lake deposit is localised at the contact between the Meta-arkosic and the overlying quartzite-amphibolite units.
The unconformably overlying late Paleo- to Mesoproterozoic (1.63 to 1.45 Ga) Athabasca Formation comprises fluviatile quartz sands, with minor conglomerate, mostly concentrated to wards the base of the sequence, shale and sub-greywacke, and locally reaches a thickness of at least 1750 m. The sequence is extensively hematised with a concentration of ferric oxide in the coarse basal units, which are also enriched in thorium (detrital monazite). Superimposed bleaching, particularly well developed in halos around siderite-filled fractures and small faults and in permeable zones, is widespread and points to late reduction processes of basin-wide significance. The Athabasca Formation is cut by 1230 to 938 Ma dolerite dykes and gabbro bodies that give rise to well-defined linear aeromagnetic anomalies within the Athabasca Basin. In the Rabbit Lake area, the basal Athabasca Formation is typically a deep purple (due to hematite grain coatings) conglomeratic unit with pebbles up to 15 cm in diameter and intercalated gritty, sandy and finely banded shaly layers.
The unconformity at the base of the Athabasca Formation is immediately underlain by a 15 to 50 m thick regolith of deeply weathered, strongly hematised basement rocks with the mineralogical and chemical characteristics similar to present-day tropical laterites. It is marked from top to bottom by a). a red, structureless, hematite-rich clay (kaolinite) zone (with a pervasively bleached upper few cm), which merges downwards into b). a red hematised zone, pseudomorphous after the parent rock, with illite becoming the dominant clay and increasing chlorite, underlain by a downward gradation into c). a green zone characterised by chloritisation and illite, which in turn grades into d). fresh rock.
At least four Hudsonian ductile deformation events are recorded in the pre-Athabasca Basin rocks of the Rabbit Lake area, namely: i). An early episode of mobilisation of the Archaean basement producing mantled gneiss doming and development of a concordant penetrative foliation in the overlying supracrustals; ii). A second deformation which resulted in the flattening of gneiss domes and folding of the S1 foliation into doubly plunging upright structures with north-easterly trending axial surfaces and a parallel penetrative foliation which in places obliterates and transposes S1; iii). At least two subsequent fold events, accompanied by low-grade retrogressive metamorphism, overprinting the low-pressure upper amphibolite facies metamorphism of the first two deformation events; iv). At least two generations of faults, a). earlier sinistral strike-slip structures which include part of the north-south-trending Tabbernor Lake system, which were probably Hudsonian fractures which were subsequently reactivated at the same time as the formation of b). the later are low-angle reverse faults such as the Rabbit Lake fault, which are post-Athabasca Basin structures.
Deposit Geology
In the mine area, the following lithological units have been identified:
i). Upper Gneiss, composed of interlayered, typically well banded, calc silicates and meta-arkoses with pegmatite segregations, which form the hanging wall of the orebody. Green, medium to coarse grained calc-silicate layers are predominantly composed of diopside, feldspar and biotite, with local major pink carbonate. Meta-arkose layers are pink and comprise varying feldspar, quartz, amphibole, diopside and biotite with minor pyrite, chalcopyrite, and less frequently, fluorite. Pink sub-concordant pegmatite-microgranite segregations are present throughout. Dark green fracture controlled chlorite and pervasive hematite are widespread.
ii). Massive meta-arkose, which is exposed through the centre of the open pit where it closely related to the high grade core of the orebody. It is composed of feldspar with subordinate quartz and local concentrations of black tourmaline and dark green chloritic streaks locally forming a crude layering in this otherwise massive rock. Where unaltered the meta-arkose is grey and medium grained. The unit is characteristically strongly and regularly fractured and cut by massive, grey quartz veins and lenses, as well as by later euhedral quartz veins. The contact with the Upper Gneisses is sharp, in contrast to that with the underlying plagioclasite, which is obscured by alteration.
iii). Massive plagioclase-rich calc-silicate rock (plagioclasite), in the footwall to the orebody is largely well altered in the mine area. Where unaltered it is a grey and composed of medium-grained granoblastic feldspar (albite), enclosing aggregates of porphyroblastic clino-pyroxene and clino-amphibole with minor quartz segregations and accessory sphene, apatite and iron oxides and minor pyrite and chalcopyrite. The plagioclasite typically includes medium- to fine-grained, weakly foliated to unfoliated biotite-feldspar-clinopyroxene meta-arkose pods, which give the rock a pseudo-conglomeratic appearance. Medium-grained, equigranular, massive dolomite, impure layered carbonate, and diopside-biotite calcsilicate rock are interlayered with plagioclasite in the west of the open pit, but are largely absent in the main zone of alteration and mineralisation. In the footwall of the Rabbit Lake fault the basement is composed of interlayered plagioclasite and calcareous meta-arkose.
iv). Biotite microgranite which is weakly foliated to unfoliated, predominantly medium grained and occurs as a distinct sub-concordant layer within the massive meta-arkose.
In the mine area, two post F2 folding episodes are recognised. A major F3 fold with a gently NE dipping axial surface transforms layering from generally ESE in the south of the open pit to generally NW northward across the axial trace of the structure. F4 folds, which plunge gently with steep SE dipping axial surfaces are superimposed with different senses on the limbs of the major F3 fold.
The main Rabbit Lake deposit lies on the north-westerly limb of a major second generation syncline in the upper block of the Rabbit Lake thrust fault which dips SSE at 30° which has a vertical offset of around 75 m. While the Athabasca Formation and underlying regolith are preserved below the thrust fault, they have been eroded from the upthrown block in the vicinity of the mine. The orebody occurs as a laterally flattened, doubly plunging pipe tilted to the west. On its northern margin it is truncated at depth by the gently south dipping Rabbit Lake fault, whereas in the south it lenses out above the fault at a depth of around 200 m.
Alteration & Mineralisation
The orebody comprises a high-grade core, in part representing initial mineralisation, which occupies a zone of repeated brecciation and alteration, which dips steeply and strikes north-northeast subparallel to compositional layering. This core is interpreted to represent an earlier mineralised fault zone, cut by the younger Rabbit Lake thrust. It is surrounded by a lower grade dispersion halo generated during several episodes of remobilisation and redeposition. The high grade core is of the order of 200 m long by 75 m wide, while the lower grade halo ore extends over an area of around 550 m, a width of up to 225 m and to a depth of as much as 200 m below the pre-mining surface.
Within the deposit, several episodes of mineralisation, alteration and remobilisation are separated by intermittent brecciation. Host-rock alteration which pervades and envelopes the orebody is typified by chlorite, dolomite, silicification and dravite tourmaline.
In general the brecciation and alteration is most intense in the median portions of the alteration envelope. In this zone, the original rock textures are completely obliterated, altered to red or pale green, coarse-grained, flaky un-oriented chlorite, cemented by some quartz and/or dolomite and dravite, which forms a matrix to un-supported, slickensided breccia fragments of dark green chloritised rock. Both the breccia fragments and matrix are fractured and cut by euhedral quartz veins. Silicification and dolomite locally form massive secondary quartzites and dolomites containing vughs and veins lined with euhedral quartz and/or dolomite. In the marginal parts of the alteration envelope, fine grained chlorite pseudomorphs the protoliths, which although not brecciated, are intensely fractured with numerous euhedral quartz veins.
Three major episodes of alteration make up this pattern, as follows:
i). The initial alteration associated with the high-grade core of the orebody where brecciated host rocks are altered to dark green chlorite, possibly also related to the fracture controlled chlorite in the hangingwall Upper Gneiss. This phase accompanied the initial mineralisation. Biotite-amphibole-pyroxene aggregates in the host rocks are completely altered to aggregates of green chlorite with some associated anatase after Fe-Ti oxides and titanite. The chlorite aggregates are cut by larger fractures sometimes filled with veins of the first stage of mineralisation.
ii). Red alteration, characterised by pervasive hematite staining which took place under oxidising conditions, is composed of magnesium-rich, almost colourless chlorite replacing feldspar and mafics, accompanied by minor epidote and relict quartz, apatite, and tourmaline (dravite). Commonly, dravite occurs as clusters of very fine grained, radiating spherulitic aggregates, while in brecciated rocks it may comprise up to 20 percent of the matrix. The fine hematite which imparts the red colour, pervades both breccia fragments and matrix. Red dolomite alteration is locally important, occurring as fracture fillings and diffuse impregnation zones which grade into massive secondary dolomite.
iii). Late pale green alteration, representing a reducing environment, is superimposed upon red alteration and in places is pervasive that only small relict red spots remaining. The mineralogical character of this zone is similar to the red altered rocks, with the absence of hematite, but with small amounts of pyrite, chalcopyrite, chalcocite and galena. In the median parts of the alteration zone, this style of alteration is locally accompanied by silicification.
The uranium mineralisation is wholly enclosed within the alteration envelope and comprises different generations and modes of pitchblende and coffinite occurrence, accompanied by secondary uranium minerals, uranium mineral coatings and impregnations. It is characterised by the presence of uranium and iron with major to trace barium, lead, titanium and copper, as well as small amounts of nickel, cobalt, zinc, manganese, arsenic and selenium.
Three phases of mineralisation and remobilisation are recognized:
i). The earliest mineralisation occurs as fracture and breccia fillings in pre-existing dark green chlorite altered hosts. Mineralised veins are commonly enveloped by cream or red hematite-stained wall-rock alteration selvedges up to a few cm wide in which the dark green chlorite and quartz are replaced by calcite. The veins predominantly contain two types of pitchblende with euhedral quartz and calcite, accompanied by adularia, chlorite, sulphides, coffinite, and hematite. Pitchblende1 has a high reflectivity, is relatively hard and forms white-grey colloform encrustations on wall-rock and breccia fragments. Pitchblende2 replaces pitchblende1, has a low reflectivity, is more massive, and is often associated with coffinite and intergrown with sulphides and arsenides, including galena, pyrite, arsenopyrite, chalcopyrite, bornite, chalcocite and covellite. Finely divided hematite outlines and impregnates quartz, adularia and chlorite and also forms thin crystallographically oriented screens in some calcites. Most calcite however, has inclusions of the same sulphides as associated with pitchblende2, indicating the hematite predates deposition of pitchblende2.
ii). The second phase of mineralisation accompanies the development of euhedral quartz veins that separate the red and pale green alteration episodes described above. Most of the veins mineralogically simple comprising dravite, euhedral quartz, hematite, and a one or more of dolomite rhombohedrons, calcite, siderite, goethite, pitchblende, coffinite, chlorite, psilomelane, kaolinite, sulphides, arsenides, native copper and glassy pitchlike "buttons" of amorphous carbon or hydrocarbon. Euhedral quartz veins are found throughout the alteration envelope, although few are mineralised with uranium and those that are, are restricted to the high-grade core of the orebody. Dravite, quartz and calcite/dolomite were deposited early, followed by hematite and then sulphides and uranium mineralisation, as well as the hydrocarbon buttons. Pitchblende and coffinite are intimately intergrown, and may be embedded in a calcite matrix, often forming concentric, rhythmically zoned aggregates, which include sulphides, Ni and Co minerals and clausthalite.
iii). The third stage of mineralisation occurs as impregnations of sooty pitchblende and coffinite along fractures and joints in pale green altered rock and appears to be a re-deposition rather than a discrete mineralisation episode. Where pale green alteration encroaches upon mineralised veins of stage i). and stage ii)., lustrous pitchblende is transformed into sooty pitchblende and coffinite, often accompanied by galena and is somewhat dispersed. Spherulitic and earthy hematite of the euhedral quartz veins is replaced by fine-grained pale green chlorite.
Mineralised veins of all three stages are subjected to subsequent oxidation as shown by the development of bright-coloured secondary uranium minerals.
In addition to the unconformity style ores at Rabbit Lake, uranium mineralisation also occurs in radioactive "granitic" pegmatites within the basal pelite unit and the pelitic gneisses that belong to the quartzite-amphibolite unit of the Wollaston Group. Those within the pelite unit belong to a well-defined uranium-molybdenum-bearing pegmatite suite present throughout north-western Manitoba and in northern Saskatchewan.
The Rabbit Lake deposit is now mined out. The total production amounted to (Jefferson, 2005):
5.84 Mt @ 0.32% U3O8 for 18 595 t U3O8.
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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Hoeve J and Sibbald I T, 1978 - On the genesis of Rabbit Lake and other unconformity-type uranium deposits in northern Saskatchewan, Canada: in Econ. Geol. v73 pp 1450-1473
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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
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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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