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Uranium Metallogenic Studies: Uraniferous Pegymtite Prospect Geology

by D. Tha:ias

Radioactive rocks which may be broadly described as pegmatites* are found throughout Archean and l\phebian age t erranes in northern Saskatchewan; some areas have long been recognized as containing anrnalous uranium concentrations approaching subecon::>mic proportions. 'Ihe earliest rep::>rted occurrences, at Olarlebois Lake and Middle Foster Lake, were investigated in the early 1950's. 1'bre recent discoveries of radioactive pegmatites have folla,.,ed uranium exploration programs in the last ten years, and sCJTe have been described by the author ('Ihomas , 1978, 1979) .

'Ihe majority of the rep::>rted occurrences in northern Saskatchewan occur in the northeasterly-trending Mudjatik and WOllaston domains (the Cree Lake zone of Lewry and Sibbald , 1978) . 'Ihe region is interpreted as a Hudsonian nobile belt (Lewry, et al, 1977) carprising an Archean granitoid basement and an overlying Aphebian supracrustal sue.cession.

Petrography

'Ihe principal uranium hosts investigated during this study have been described by previous ...orkers as Hudsonian pegmatites generated pr imarily within the Aphebian supracrustal succession and locally within granitoid gneisses largely of pres1.111ed Archean age. 'Ihe "pegmatites" generally vary in grain size from 1 rrrn to 30 nm in the majority of rocks studied, and only rare ly reach 25 cm in length. Absolute grain size is of lesser s ignificance in leading to use of the term pegmatite than relative grain s i ze , carposition and ' igneous- like ' texture when conpared to the country rock .

'Ihe granitoid pegnatites are corrposed of varying anounts of quartz, plagioclase, microcline and biotite, and accessory minerals include nuscovite, magnetite, uraninite, pyrite, pyrrhotite, tourmaline, zircon, sphene, monazite, apatite, sericite, saussurite, and rrolybdenite. Although the accessory minerals rarely exceed 1 per cent of the total modal CO!!pOSition, magnetite, uraninite, molybdenite and apatite may individually form up to 0.5 per cent of a

* Many of these rocks are only pegmatitic in a very broad sense; the term pegmatite i s preserved ho,.,ever in view of the extent of previous writing.

single pegmatite sarrple .

'Ille pegmatites which host uranium mineralization can be broadly grouped either as granitoid or mafic (>75 per cent mafics, Hyndman, 1972). 'Ihe granitoid pegnatites may be divided into three grou{:6 on the basis of their rrodal corrposition (as in the system of Streckheisen, 1976):

A. Tonalites , granodiorites, B-granites.

B. Granites, quartz syenites , alkali granites .

c. Quartz-rich granitoids, silexites (quartzolites) .

OUARTZ

.... CUP LAKE

• a..EVElANlJ ISLANO

~ PIPEWRENCH I.AKE • PLUTO BAY

• MIOOLE r osrER l 4K£

K-fflO~R

Fig . 1 - Modal plot of radioactive pegmati tes f rom t he Cree Lake Zone. Nomenclature based on Streckheisen (1976).

'Ihe distinction between the three groups is irrportant in that radioactivity from Group A is fourrl to be primarily due to uranium while thoriun am potassil.lll are primarily responsibl e for radioactivity in pegmatites of Group B. Padioactivity in Group C pegmatites is found to be corrrnonly associ ated with

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visible molybdenite.

'Ihe mafic pegmatites are typically massive and coarse-grained with anphibole arrl/or diopside crystals up to 20 cm in l ength. They are found as irregular pod- like bodies associated with calc-silicate/marble assemblages . Anphiboles are generally black to greenish-black hornblende , and pyroxenes pale green to greyish-green diopside. Associated with the dominant ma.fie c011p01ents are biotite, quartz, plagioclase and scapolite. Accessory minerals include sphene, apatite, pyrit e, chalcopyrite, uraninite, molybdenite and tourmaline.

Radioactive Canponents

Binocular and pol arizing micrscope studies have tentatively indentified subnetallic black, opaque minerals ranging in diameter from l ess than O. 2 5 nun to 3 . 0 mm as the primary source of radioactivity. 'Ihese opaque crvstals are usually ril1TllE!d or veined by an ea~thy yellaw to brown isotropic alteration material which in turn is locally rimned by a thin (0.0111111) film of anisotropic , weakly pleochroic , colourless to yellaw material. Radioluxographs indicate that these opaque grains and associated veining/mantling .. materials are the primary sources of rad1at1on in the samples.

(A) Primary Uranium Phases

On the basis of XRD analysis the black oi:eque grains appear to be l argely uraninite . 'Ihe mineral cx:curs as suall, typical ly anhedral to subhedral crystals, a lthough euhedral cubic grains are observed locally. In the granitoid pegmatites , uz:aninite is invariably found either within or in contact with the biotite crystals, whereas in the mafic pegnatites i t is present in arrphiboles, pyroxenes, or scapolite. 'fue absence of cleavage distortion in the host minerals su;igests that the uraninite is formed during a late syn to p::ist-nucleation phase. Undulose extinction haloes in biotite in contact with uraninite indicate lcx::al lattice danage resulting fran radiation. 'fue cx:currence of urani nite as the primary uraniun phase in pegnatites fran the Olarlebois Lake area is noted by other w::>rkers (1-B.v.dsley, 1950; 1'-brra, 1976) .

( B) Secondary Uraniun Phases

Psetrlarorphic pale greenish to yellONish earthy al t eration products o f uraninite are tentatively i dentified as minerals of t he gUTIJni te group. Fractured and altered uraninite grains t end to display the following features:*

* This sequence corresponds closely to that described by Frondel (1958) for gunmite alterat ion of uranini te.

a) A central core, which may be veined or ernbayed, consisting of a black to steel grey submetallic material assuned t o be uranini t e.

b) An inner rim of microcryst alline ye llONish- earthy material corrposed of gunmi.te group minerals.

c) An outer rim of microcrystalline straw yella,., to yellcw-bro..m material, possibly uranophane.

GllTunite entirely psetrlarorfhs uraninite. Microscopic and rnegascopic radiating fractures are CCimlOn around uraninite grains, particularly where urani nite is embedded in a ccrrpetent matrix such as quartz or scapolite. Fracturing is not generally obs erved where uraninite is hosted by biot ite or arrphibole.

Secondary urani un miner alization is also camon as yellcw to yello,;-green films or crusts on outcrop surfaces , or along fractures in pegmatites . In the Olarlebois Lake area this secorrlary uraniun phase has been identified by 1'-brra (1976) as urano:i;:hane.

(C ) Radioactive Accessory Mineral Phases

other local sources of radioactivity in the pegmatites are attributed to isolated grains of sphene, metamict zircon, ai:etite, and monazite . These minerals tenJ to occur in only trace arrotmts generally forming grains l ess than O .1 mn in diameter.

Origin of the Pegmatite Phases

'Ihe OO!lp:)Sitional mode plot of granitiod pegrati t es and their host rocks, as ~11 as d)served field relationships sug:iest that t he majority o f granitoid pegmatites have been derived by a conbination of partial mel ting and conplete or nearly ccmplete melting of the supracrustal rocks. M'.Jdal plots fall into three catagories (Fig . 2): 1. Neosanal, paleoscxnal and restite material

which fal l roughly on a straight line suggesting partial melting o f CDUntry rocks ( metatexis) .

2. Paleosomal and neoscmal material plotting roughly in the same part of t he diagr am indicating rrore CC11plete melting (diatexis) .

3. Late quartz rich fractions , possibly representing excess silica which was sei:erated from the melt by a volatil e i:hase .

'Ihe origin of the rrafic pegnatites is n:>t as well understood, but t hey are presently interpr eted as forming during metasorratic

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( o) Quartz

e Peqmotite ( neosome ) /\ 'II

/ \ + Quartz rich pegmotit e

• Peli le (poleosome)

.. Mafic schist ( restito)

I . \.

a) Meta-arkose type, which includes low grade disseminations of uraninite in the basal meta-arkoses of the Needle Falls Group (eg. D..lddridge Lake), an::! uraniferous pegmatites in rreta-arkoses of the stratigraphically higher Wollaston Gro~.

~. . \ ; ·· ~ -., ·· .... M\ i

( b)

( c)

/ \ .! , ·.

/ ~ \ ! \ /,

I · II \\ /

Feldspo~

I ' I \

L --~ ~- -- ··- ·---·....:,. Mof1cs

Fig . 2 - Mi neral composi tion of intermediat e bi ot i t e-pl agiocl ase gneisses and associa ted pegma t i tes (af ter Mehnert 1968) . (a) Part ial melting, (b ) more complet e melting, (c) quartz­rich phases.

transfonnation o f ca J.c-silicate an::1 dolanitic marbles during high grade metamorrtiism.

Classification of Radioactive Pegnatite Occurrences

The regional l ithologic and stratigraµ1ic control of radioactive pegnatites has been previously described (Lewry and Sibbald, 1978). Radioacti ve occurrences of presuned Afilebian-Hudsonian age in the Cree Lake Zone are classified according to the ir daninant lithologic hosts (Fig. 3):

1. Occurrences in the ranobilized basement infrastructure.

2. Occurrences in the Aphebian supracrustal r ocks :

b) Cale-silicate type, which inclooes low grade disseminations in calc-sili cate gneisses (eg. Burbidge Lake ) as well as uraninite in mafic pegmatite phases in calc-silicate marble assemblages.

c) Pelite-Pegmatite type, which i s charact erized by uraninite concentrations in anatectic pegmatites in pelitic and semipelitic assemblages, particularly in the Wollaston Group basal pelites directly overlying the granitoid basement gneisses over lTUCh of the Cree Lake zone.

La-I to moderate tenperature epigenetic pitchblende mineralization has not as yet been identified in the pre-Helikan basement rocks in the Cree Lake zone except in close proximity to the sub-Athabasca Group unconformity. '!he absence of structurally controlled pitchblende mineralization reflects the high terrperature ductile-plastic style of Hudsonian deformation and recrystallization which produced f ew suitable brittle structural traps (Lewry & Sibbald, 1978).

Source of Ur anium

'Ihe relatively high concentration of uranium in some lithological units and their anatectic f ractures , in contrast to the observed lack of structual conduits s uggest a pre-tectonic origin for the uranium. 'Ihe original concentrations may have formed syngene tically, or by pre-rretamorphic enrichment. Alternativel y at the base of the W:,llaston Group the uraniun may have derived from the Archean gr ani toid basement by outward migration in response t o dehydration during granuli t e facies metarrophorhism. In this connection, 'Ihomas (1979) found that a noticable increase i n uranium and thorium occurs t<Mards the margins of basement granite gneiss domes i n the Karin Lake and Pipewrench Lake areas.

'Ihe redistribution and concent ration of uranium in the pegmatite phases may be ascribed to the high mobility of ionic uranium under high grade metamorphic and anatectic conditions (D::lstral & capeari, 1978; He i er, 1973 ; He i e r & Adarrs 1965; Ermolaev, 1970). Uraniun will begin to migrate at the early or incipient stages of melting when the roe.ks are

t .•. ·

',<::'

...... ,~ . ., ... , •.

. . ' I

I •· I',\ I

I

l I

- 52 -

.. .. ~ ~ .• .• l ...... • ... .. we ,.rt-

/

. -" ~-

DISTRIBUTION of APHEBIAN- HUOSONIAN AGE

URANIUM OCCURRENCES

in the

CREE LAKE ZONE

OCCURRENCES m Ille REMC8UZEO BASEMENT INFRASTRUCTURE

• Rl>DIQo.CTI\/E PEGMATITEs ,no SHEARS " GRANITIC GNEISSES

OCCU'rRENCES ,n tl>e AfflEB!AN SUPRAClrUSTAL Sl/CfISSION Y ME'TA - ARl<Osf Tl'P(

• CAlC-S!LICATE TYPE

• + PfUTE I PEGMATITE TYPE , ~ occ:wr1t1e• . hto O< mo,e oet~,r~

...

F1g. 3 - Distribut ion and c lassif ica tion of Aphebia n-Hudson1an uranium occurrences in the Cree lake Zone of northern Saskatchewan .

- ,a•

57•

...

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penreated ~ intergranular fluids. !>s the rrelt fractions begin to crystallize the uranium will be redistri buted with the fixation of ionic uraniun to hydrous mineral pahses or~ precipitation as uranium-rich phases.

.• ? ",..J-'

- ' - :-...: '\ u- -:--- .... .

...:THAl:JASCA

r ORMATION

CHARLEBOIS LAKE

'1

I I I ., •• I I I

I I CUP<"LA~KE , ·~

', ,, , ~ · , IJ . DUOORIOGE LAKE

• • ... , ,·.,,~ •. , Cv ~---. . ·. - .. -t-t ~

I I,..

~!, Mttdcongll.ffi'lerate

l2] Heta-al"kose

r=J Sl ote

:·J ()u<1 r tz ite

~ Calc- s111cate. ""'rbl e

[=2d Pe l fte, s.emi:,el itc

~ : ron fo rrna tton, Iron rit.:h r,e li te

• Possib l P. mctavolcc1nic roch

~ HOl?XIIJl:!nP.OU S qrani te qn P. 1 SS

:8 Hybrid 9rantte gneiss

l'Zj '1<!togabbro

Fig. 4 -

References

~ Ci.ran; to1d gne iss. . Peter t at::e Domain

Schematic stratigraphic successi ons f rom selected areas in the Cree Lake Zone, with l ocat ions of uran i um mineral iza t ion.

Be ck, L. S. (1969): Uranium deposits of the Athabasca ~ion Saskatchewan; Sask . Cept. Min. Resoorces Rept . 126 .

D:>stal, J . and G:lpedri, S. (1978) : Uranium in Metamorphi c Roe.ks : Cbntrib. Mineral . Petrol . 66, 409-414.

Ermolaev, N.P. (1970): Progressive M;!tarnorphi sm and Ultramet amorphisrn of Ibcks as Probable sources of Substance in Epigeneti c Processes; in, Problerrs of Hydrothermal Ore Ceposition, editors z. Pouba and M. Stenprok, Stuttgart ,

Frondel, c. (1958) : Systematic Mineralogy of uranium and Thorium; Geol ogi cal Survey Bulletin 1964, United St ates Printing Office, Washington, D. C.

Heier, K.S. (1973) : Geochemi s t ry of gr anul ite facies roe.ks and probl em; of their origin; Phil Trans R. Soc. London . A, 273 , 429-442.

Heier, K.S . and J\darns, J.A.S. (1965) : Concentration of radioactive elements in deep crus t al material; Geoc.himica et Cognochima kta 29 , 53-61 .

Lewry, J .F. and Sibbal d , T. I.I . (1977): Variations i n l ithology and tectonometamorphic relationships in the Precarrbrian basement of oorthern Saskatchewan; Canadian Journal of Ear th Sciences, 14 No. 6, 1453- 1 467.

Lewry, J . F. and Sibbald, T. I .I. (1978) : A Review of Pre-Athabasca Basement Geol ogy in Northern Saskatchewan; i n UranilJll Explor ation Technieq.ies ed . G.R. Parsl ow, Saska tchewan Geological SOciety Special Publicat ion Number 4.

Maw:lsley, J .B . (1950): '!he Geology of the Olarlebois Lake Area; Saskat chewan Dept . Minera l Resources ~pt . No. 5.

Mehnert , K.R. (1968) : Mignatites and the Origin of Granitic Rocks; Elsevi er Amsterdam, 403 pp .

1'brra, F . (1976): Geology and Uranium Ceposits of the Olarlebois Higginson Lake area, north Saskatchewan; unpublished M.Sc . thes is , University of Alberta, Fdnonton.

St reckheisen, H. (1976) : 'lb each plutonic rock its proper name; Earth-Science Revi ews Vol. (1976) 1- 33 .

'Ihanas, D.J. ( 1978): Uranium Metallogenic Studies : Olarlebois Lake and CUp Lake; In: Sllllfl\ary of Investigati ons 1978; Christopher , J . E. and Macdonald , R. (&ls . ) ; Sask . Geol. Surv. pp 66- 73 .

'Itlomas , D,J , (1979) : uranii;rn Met allogenic Studies, " Pegmatit e " Prospect Geology; In: SUrrrnary of Investi gations 1979; <llristopher, J . E. and Macdonald, R. (Pds.); Sask . Geol. Surv. pp. 86- 95 .