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Uranium Metallogenetic Studies: Rabbit Lake, Mineralogy and Geochemistry

by J. Hoeve *

During development drilling by Gulf Minerals (Canada) Ltd. in the wint e r of

1977-1978 , it appeared that the Rabb it Lake deposit , like other unconformity- typ e

deposits in the Athabasca Basin, is associa ted with graphite-bearing basement rocks

(B . J ones , pers. comm.). Field work in the summer of 1978, conducted under the DMR/

SRC program of uranium metallogenetic studies, revea l ed that the graphite is con­

tained in beds of dolomitic marble and feldspathic rock that are intercalated with

the "plagioclasite" lithological unit (Sibbald, this volume) .

The graphitic r ocks are absent from the thoroughly altered upper portions of

the ore zone, wher eas they occur in the intermediate and lower portions . At the

present level of min ing , relict blocks of partially altered graphitic rocks may be

observed "floating" in a matrix of intensively brecciated and chloritized rock which

has l ost a ll recognizable trace of original metamorphic texture. Such relationships

exist on all scales r anging from cent imetres to tens of metres .

Pale green chlorit i c a lterat ion occurs along fractures in the graphitic rocks.

Veinlets of fine-grained, somewhat pinkish coloured quartz are surrounded by a

bleached choritized halo totally devoid of graphite . Microscopic examination shows

that graphite f lakes in the rock outs ide the bleached halos have frayed margins,

sugges t ive of corrosion .

These relationships provide evidence of graphite consumpt ion during alteration.

The graphite f ree upper part of the ore zone may result, therefore, from destruction

of graphite in a layer originally continuous throughout the zone. This interpre­

tation accords with the geometry of lithological s ubdivisions of the Rabbit Lake

pit proposed by Sibbald (1977, this volume).

Genesis of the Deposit

The field evi dence that graphite is consumed in the process of host rock alter­

ation l ends support to the diagenetic-hydrothermal model for the metallogenesis of

unconformity-type uranium deposits i n the Athabasca Basin. This model, which is

only briefl y out lined below, having been described in detail elsewhere (Hoeve and

Sibbald, in press), relates uranium mineralization to diagenetic processes i n the

* Saskat chewan Research Council

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Athabasca Forma tion. The general association of deposits with the sub-Athabasca

unconformity is attributed to interaction be t ween oxidizing wat e r s of an Athabasca

aquifer and r educing minerals of t he basement. Because the basement consists of

high-grade unalte red metamorphic r ocks , it was probabl y largely impe rmeable , except

in f rac ture and breccia zones. In ter act ion was limited to the vicinity of t he

unconformity.

Oxidizing gr ound waters entering the Athabasca For mat ion were heated geotherm­

ally as they migrated downwards , attaining temp er a tures of around 200°F a t the f loor

of the bas in (Pagel, 1975a and b). Diagenetic changes induced incl ude s ilicification,

intrastratal weathering and leaching of ore constituents released by ox idation of

ma fic and heavy minera l s . La rge-scal e percolation of oxidiz i ng diagenetic sol utions

may have been triggered by steepen ing of the geothermal gradient associated with the

Gr enville age mafic hypabyssal magma t ism and associated tectonism in the At habasca

Basin (Sibbald et al., 1976).

Where the oxidizing, hea ted diagenetic solutions entered the basin floor along

fracture and breccia zones and reac t ed with graphitic rocks , r educ ing so lu tions con­

t a ining carbon dioxide a nd methane were produced. Since the r educing solutions

pr obably formed part of a percolating system , t hey may have migrated upwards a long

other portions of the same fracture sys tem, event ually discha r ging into the over­

lying oxi dizing Athabasca a quifer.

Mineralization r esulted where oxidizing diagenetic sol utions and the newly

generated r educing solut ions interacted and , hence , was subj ect to hydrodynamic

controls. If this process continued for a long time exceptionally h igh grades of

mi neralization, as encountered in many unconformity- type deposits accumul ated due

t o the c ontinuous supply of ore cons t ituent s carr ied by the ox idiz i ng solutions and

to the continuous gener ation of me thane reductant.

In s uch a situation, repeated oscillat i ons between oxi d izing and r educing con­

ditions as established at Rabbit Lake (Hoeve and Sibbald, 1977, in press; Hoeve ,

1977) ar e to be expected when intermittent brecciation causes modification of the

plumb ing system and of t he f l ow paths of the solutions.

The amoW1t of methane generat ed dep ends on the oxidation state of the system

(French, 1967). I f the amount of influent oxidiz ing sol ution is relatively small,

the nature of the reaction products will be contr oll ed by the composit i on of the

graphit ic rocks, and me thane will be gene r ated in addition t o carbon dioxide; how­

ever, i n the case of s ubstantial flow, the r ocks may be f l ushed with oxid izing

~··-- -~--..... ·--- · .... ~------

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solutions and a ll graphite will be convert ed into carbon dioxide .

Appli ed t o Rabbit Lake, the histor y of the depos i t can be wri t t en a s follows

(cf. Hoeve , 1977):

Episode 1. - Oxidizi ng, hot diagene tic solutions enter ed an ancient f racture zon e

in t he basin f l oor and r eac ted wi th graphite t o yie l d r educing solut ions conta ining

me t hane and carbon dioxide . Interact ion of the oxidiz ing and reducing solution l ed

to s tage I of mineralization (Hoeve, 1977) a nd t o formation of a hi gh-gr ad e ore body.

Host rock a lteration in the ore zone i s chara cter ized by dark green chl or i tiza tion.

Episod e 2 . - Brecciation associated wi t h ini tiat i on or r eac tiva tion of the Rabb i t

Lake fault gave rise to i ncreased permeability of the a ncien t frac t ure zone l ead ing

to f lus hing of the d eposit with ox i dizing so lut i ons .

The o re boc1v f o rmed during s tage 1 of mineraliza tion w;:,. s breccia ted and par­

tially oxidized :J,i.., uis pe rs ed . Hos t r oc k a ltera tion, a c companied by pervas i ve

hemat i t e i mpregnation, i s mar ked by chloritization , t ourmalinization , l ocal sil i ci­

fication and dolomitization r esulting from dissolution and redistribu tion o f me ta­

morphic marble horizons. Volume l oss a ccompanying the dissolut i on of dolomitic

mar bl es l eads , on a l ocal scal e , t o development of collapse ( ?) brecci as character­

iz ed by the presence of unsupported, dark green or red c ol oured breccia f r agment s

embedded in a ma trix of oft en coars e-gr a ined c hlori te.

Episode 3. - Rebrecc iation caused mod ification of the hydrologi c r egime . Flow of

oxidiz ing solutions was rather limited, r esulting i n r en ewe d gener a t i on of methane

r eductan t and to " pa l e green a l ter a tion" superimposed upon "red alteration" . For­

mation of euhedra l quartz veins is a ccompan ied by s t age 2 of mineralization .

Quan tities o f Material Involved

Uranium in supe rfici al and hydrothermal solutions is thought to be transpor ted

i n the f orm of uranyldicarbonate complexes (Naumov , 1961) . Reduc tion of these com­

plexes by methane and pr ec i pitation of pi tchblende i s expressed by the fo llowing

e quat ion:

CH4 + 2Na{uo2 (co3) 2} + ~Oz + Ca++z

2uo2+ + CaC03+ + 2H20 + 4C02 + 2Na+

I n t he reducing solutions generat ed by r eacti on wi t h graphite , t he ratio of

me thane t o car bon dioxide i s dependent on the oxidation s tate of the syst em .

Assuming a r a tio of 1: 3 , t he r e action can be expressed b y t he equation:

4C + 2H2

0 + 202

t CH4 + 3C02

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From the above equa tions it is calculated that in order to prec ipitate 20,000 t

of pitchblende, a s contained i n an ore body like Rabbit Lake , 670 t of methane would

be required, which can be gene rated by 2,000 t of graphite. If the graphit e content

in the r ocks of the present a lteration halo i s estimated a t 1 percent by weight, it

follows that only 200,000 t of graphite r ock would have been sufficient to genera te

the methane needed to precip itat e the uranium a ccumulated in the ore body .

A similar calculation can be made for the much richer deposits of t he Key Lake,

Collins Bay and Midwe st Lake t ype, which are associat ed with basal Aphebian meta­

pelites containing much h igher amounts of graphite, in the o rder of S to 10 percent

and loca lly a s high as 25 percent by weight. Assuming 10 percent of graphite, it

appears that the met hane requi red to prec ipitate 50,000 t of pit chblende could have

been generated by 50,000 t of graphitic me tapelite.

The source of the ore constituents , according to the diagenetic-hydrothermal

mode l, was t he Athabasca Formation, fr om which they were leached dur ing post ­

depositional oxidation o f ma fic and heavy mineral s . If the hemati te pigment of the

Formation devel oped this way , it is obvious tha t i n addition to iron, a s uite of

other e l ements, including uranium, mus t have been released as well.

Present uranium values for the sandstones are generally low, in the order of

1 - 2 ppm, a lthough some c l ay-rich horizons may c ontain much higher values. Assum­

ing that 1 ppm was leached from the sandstone during diagenetic alteration, leaching 3

of only 7 km of sands t one could have supplied the uranium a ccumulated i n a deposit

like Rabbit Lake.

Calcula tions such as those made above are of limited va lue , because the effi­

ciency o f the processes are unknown. They do, however, indicate the order of mag­

nitude of mater ials required which, i t appears, are surprisingly sma ll.

Ac knowledgment

The co-operation, advice and hospitality of Gulf Minerals (Canada) Ltd. is

gra tefully acknowledged; a s is their permission to undertake the present study.

The views expressed in thi s paper are those of the author and do not necessarily

reflect the opinion of Gulf Minerals (Canada) Ltd .

References

French, B. M. (1966): Some geological implications of equilibrium be t ween graphite and a C-H-0 gas phase at high temperature s and pressures; Review of Geo­physics!±_, 2, pp . 223-253.

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Hoeve, J . (1977 ) : Uranium metallogene tic stud i es , I I Rabbit Lake : miner ology and geochemis try ; In: Surrnna r y of Investigations 1977, Christ opher, J . E. and Macd onald , R. ( Eds. ) ; Sask. Geol. Surv. pp . 124-135 .

_ _ _ _ _ and Sibba ld, T. I .I. (1977): Rabbit Lake Uranium deposit ; In: Ur a nium i n Saska tchewan, Dunn, C.E. (Ed .); Sask. Geol . Soc. Spec. Puhl. 3 , pp . 331-354.

and Sibba ld, T. I.I . ( in pr ess ) : On the genes i s of Rabbi t Lake and other - ----unconformi t y type uranium de posits in Saskat c hewan , Canada ; Econ . Geol .

Naumov, G. B. (1961): Some phys icochemic al charac t er istics of the behaviour of uran i um i n hydr ot hermal so lu t i ons ; Geochemis t ry I, pp. 127-147 .

Pa gel , M. (1 975a): Cadr e geologique de gisement s d ' uranium dans l a s truct ure Car s well (Saska tchewan-Canada). "Etud e des phases flu ides" Thes e de Docteur de Specialite (3e cycl e ), L' Univer sit e de Nancy 1 .

--- - - (1 975b) : De t ermina t ions des c ond i tions phys i co- chi miques de la s i lici-fi cations d iagene tique des gres Athabasca (Canada ) an moyen des incl usions f luide s: C. R. Acad . Sci . Par i s, T. 280 , Sec. D. , 2301- 2304.

Sibbald, T.l.I . (1 97 6): Uran i um Me t a llogene t ic s tudies , I Rabbi t Lake; In: Summary of Inves t i ga t i ons 1976, Chris t opher, J .E . a nd Macdonald , R. (Eds. ) ; Sask . Geol. Surv ., pp. 115-123 .

_ ___ _ _ __ (1 977) : Ur ani um metal logenet ic s tudies , I Rabbit Lake , Geology ; I n: Sununa r y of I nvestiga t ions 1977, Chr istopher, J .E. and Macdonald, R. (Eds. ); Sas k . Geo l. Surv . pp . 111-123.

---- --- -' Munday, R.J .C . and Lewry , J. F., (1977) : The geologi cal se t ting of ura nium mine r a lization in northern Saskatchewan; In : Uranium in Saska tchewan , Dunn, C.E. ( ed .); Sas k. Geol. Soc . Spe c . Publ . 3 , pp. 331-354 .