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- 89 Uranium Metallogenic Studies:
Maurice Bay Deposit
M. Mellinger
Previous work in the Maurice Bay area and on
the Maurice Bay uranium deposit has been
surrmarized by Harper (1979) and ~llinger
(1979); the reader is referred to these
articles for a description of the regional
geology, and the i.nnediate geologic setting
of the uranium mineralizatico. Investigations
during the 1980 field season carrprised a
detailed study of the stratigraphy of the
Athabasca Group in this locality and detailed
sarcpling for lithogeochemi.cal, petrographic,
and mineralogical studies.
Stratigraphy of the Athabasca Group
A detailed stu::iy of the stratigraphy of the
Athabasca Group in the vicinity of the Maurice
Bay deposit was carried out through the logging of drill holes along three main secticos
(102 + OOE, 015 and 014 + OON, see Fig. 1);
correlations ....:ere verified using intenrediate
sectioos.
The stratigraphy oonsists of a ~ r Sequence ,
characterised by abundant mud.stone, and an
Upper Sequence of conglcrrerates and sandstones.
J:.o..Jer Sequence
a)
b)
Fanglarerate, discontinuous and occurring
in lows at the sub-Athabasca unoonformity,
is the basal unit. I t is a m.rldy conglomerate**, corrprising angular quartz pebbles
and cobbles, angular to sub-rounded clay
(yell<:M) granules and pebbl es, and hematitic
(black) granules (rarely pebbles) , in a
dark red muistone to sandy mudstone matrix.
i'bere bleaching occurs, the matrix is a
light orange . The fanglarerate is interpreted as a wadi de!X)si t of surficial llt\ld
and fragrrents.
Interlarninated sandy mudstane, muddy sandstooe and nudst<Xle overlie the fanglorcerate.
The mud/sand ratio is usually between 1: 1
and 1: 3 and several cycles, fining upward
fran sandy mudstone to mudstone, and 40 an
to 1500 an thick are carm::mly present. Mudclasts and mudcracks are camon, as are thin
* Saskatchewan Research Council. Project not
funded by the Saskatchewan Geological Society
** petrogr aphic names are given according to
Folk ' s c l assificatioo (Folk, 1974).
conglareratic beds, oontaining angular quartz
granules and pebbles and clay granules.
Colour is dark red, except where bleaching
has occurred. This tmi t is interpreted as
representing a paralic environrrent wherein
sedinentation was rrore persistent than the rrore
erratic depositional episodes of the fanglorrerates.
Upper Sequence
'lhe upper s~oe carprises muddy sandy
conglarerates and muddy sandstones with a mud/
sand ratio usually l&er than 1: 3. The
depositional envirannent is interpreted as
fluvial. Four sucressive units can be distinguished:
a) Unit 1: rrostly muddy sandy oonglarerates,
and congloneratic muddy sandstones, with local
muddy sandstones, ccmronl y as gradatiooal ,
f ining up.,,ard seqtena:s (cycles). Pebbles are
nostly quartz, but include lithic fragmants of
baserrent derivatiai;argillated granules are
abundant.
b) Unit 2: essentially muddy sandstme, locally
granular; rare pebbles (mainly quartz); sandstones range fran very ooarse to very fine;
sane mudstales.
c ) Unit 3: muddy sandy conglarerate arrl cooglorreratic muddy sandstcne, with minor muddy
sandstore. Includes fe....:er lithic and argillaa:ous fragrrents than Unit 1.
d} Unit 4: characterised by muddy sandstone
with dispersed well-rounded ("rare") quartz
pebbles every f~ netres in core.
The oontacts between the units are erosional.
'lhe gereral pattern of transportation and sedinentation seerrs t o be strcngly influenced by pree xisting tqx:,graphy .
No evidence of widespread faulting was f otmd;
bedding is generally undisturbed. local disturbana: is indicated; for exarrple, by vertical dips
over a 1.5 m interval in drill hole MB-724, and
can be rel ated to nearby clayey alteration and
mireralizaticn.
Harper (1979) suggests that post-Athabasca block
faulting is responsible for the present relief
at the sl.b-Athabasca unconfonnity. Stratigraphic
- 90 -
"015 "
,::·
104 + DON
(112+50E)
Cmodlfled of1er LEHNERT-THIEl
et
"015''
19 79 l
al. ,
MAP OF THE HELI KIAN UNCONFORMITY AND
OUTLI NE
OF
Minerl)tizctlon
T HE MAURI CE
BAY URANIUM
DEPOSIT
WITH
SATELLITE
OCCU RENCES .
J
C > cu loff }
Miriuollzollon ( < cutoll )
2:J
Heliklon untonfonnlt y
{ dt pth In meir u j
SC~LE
0
100
20 0
300
M1 tr u
/
FIGURE 1
(
S ect ions ,rudit d
I n de tcu l
} .. 11 1! H<.ilo~ t
ll[ATllERfO
I GREEN ZONE (A)
UNWEATHERED
RED ZONE\
GREEN ZONE (8)
Rock Color:
"feldspar s "
pi nkish , whi ti sh
"biot it e"
dark brown
Mi ne r alogy
cordie rit e
feldspa r s
-- -
yellow
greenish
- ...... - --
---
green
--
red
il 1 it e
---
garnet
biot it e
i 11 ite
---
c hl ori te
---
c hl or ite
---
hemH 1te
quartz (una ~fected)
gra ph it e (unaf fected )
Figure 2 • Changes in rock color a nd mineralogy i n a typical weathering profi l e (pel it ic gneiss)
- 91 -
evidence , as well as the absence of widespread faulting in the Athabasca Group near
the edges of the basenent horst, suggests
that the pr esent basen-ent topography is
essentially pre-Athabasca; however, l ocal
reactivation of pre-Athabasca fault zones
i s considered as a possibility, and may
even be a prerequisite to uranium mineralization .
clayey alteratim facies, can be traced fran
cne· drill hole to the next. The alteration
patterns are not related to specific stratigraphic units, and as a whole, are far rrore
extensive on the southern side of the basetrent
horst, where northward mi.grating fluids seem
to have been stopped against the southern f lank
of the structure.
Paleo-weather ing i n Basement Rocks
Sandstme Secondary Facies
Seccodary facies in the Athabasca Gr oup
("sandstone" here afte r) are related to diagenetic and hydrothermal processes.
Three facies in the sandstone are considered
as resulting fran general diagenetic processes
not directly related to mireralization.
Purple sandstone aoguired its rolour through
the deposition of a thin hematite coating on
quartz grains. Primary heavy mineral
cmcentrations (hematite grains) occur,
although thin section examination shows these
hematite grains t o have been overgr0vn by a
new generaticn of hematite. '!he purple facies
could correspond to an earl y pervasive oxidizi ng diagenet i c stage, as other facies always
appear to be younger than the purple facies
where interacticn occurs . Thus , redbed
characteristics might have been predcmi.nant
at an earl y stage . The orange facies in
sandstone correspcnds to a general bleadring
(reduction) of pre-existing hematitic (purpl e)
sandstone . Where diss olution of hemati te i s
not a:mplete , ghost heavy mireral streaks are
still distinguishable along bedding pl anes.
Light green sandstone i s associated with
orange sandstone, and is referred to as the
"regular green" facies, in ocntrast to another
green facies which is related to alterati01
processes (see bel ow) •
A well developed pal.ea-weathering profile can
be d::>served in baseirent rocks fran the unconformi. ty domwards. The rocks show a specific
up..rard zonation in col our, due to successive
mineral weathering reactions (Fig. 2). In
hand specirren first a "green" zooe is d::>served,
in which the rocks have a :rrediurn green to yellc:kl
col our, depending on original mineralogy, ~
a "red" zme marked by hematite on iron-bearing
minerals (micas). 'Ihe transition between the
green and red zones camonly occurs as hematitizaticn of the green alcng fractures. Similar
observations are cormonly made elsewhere in the
Athabasca Basin.
In thin section , cordierite is rare l y fresh and
was prd::>ably altered to hydromuscovite during
retrograde rretanorphism. Both potassic feldspar
.and plagioclase are altered to illite (argillation) , apparently bef ore garnet and biotite are
affected; this justifies the s ubdivisirn of the
green zcne i nto two sub-zones (A and B, see
figure 2) . Biotite is the l ast netanorphic
mineral to be altered, arxl i t is progressivel y
chloritized (sOTietines partly sericitized) with
a parallel exsolutioo of Fe-Ti oxides in cl eavage pl anes . In the red zone , the majn change
in mineralogy is the develcprrent of hematite m
chlori te, which is respcnsible for the strcng
red col ouration. Zirocn, apatite, graphite
and quartz are unaffected by weathering reactions.
I t is ilrportant to note that the ori ginal texSeveral a l teration facies are recognized,
ture of the rock i s well preserved throughout
which are related i n varying degrees to
the weathering profile . Thus, variations in
uranium mi.reralization . 'nle green alteratim
the aspect of a rock for the sane weathering
f acies occurs near strongl y altered zcnes,
stage is dependent upcn both original mineralogy
and is recognized by a rredium to dark green
and texture. Grain size is an inportant variacolouring of the rock. Where alteration
bility factor; a roarse- grained mineral
r eactions i nvol ve consurrptim of quartz grai ns , (especially feldspars and garnet) will resist
a clayey al teration facies (usually of yellow
weathering longer than its fine- grained equivato whi te colour) i s developed; this facies i s
lent, thus introducing an apparent ananaly in
indicative of very strong chemical activity .
the weathering sequenoe.
Two other alteration facies seem to grade
into each other: a ~ and a dark red, both
Al ter ation in Baserrent Rocks
related to a strong hernatitizatian of the
rock matrix. All the alteratim facies appear
t o be re l at ed t o uraniun mi.reralization; the
In plares where the uranium mineralizat im is
dark r ed (hemati tizatim) facies is the nost
cl ose t o or straddl es the unoonformity , a strong
closely rel ated as it i s frequantly accaipanied argilli c alteraticn smri.lar to that ooserved in
the Athabasca Group affects baserrent rocks
by pitchblende concentrations.
(Mellinger, 1979). Quartz solution is well
Sare of the alteratioo facies , particularl y the defined, and inte rrrediate stages of quartz
- 92 -
digesticn can be observed in thin secticn,
Clay minerals in the altered rocks are illite
(white mica) and chlorite. In one case,
anphibolite is affected (Section 106 + 25E),
and extensive carbanatization accacpanies the
develq:ment of clay minerals suggesting co2
as a prominent conponent of the alteration
and prdlably also mir.eralizing fluids.
5.
The source for uranium may be within the
Athabasca Group, as a background concentration, or the underlying unweathered or
weathered baserrent carplex, as a background
or pre-existing ore ooncentration (similar
to the A zone or to other known baserrent
occurrences) •
Acknowledgerren ts
Guidelir.es to Ore Genesis
The foll<Ming are ccnsidered guidelines to
ore genesis at Maurice Bay.
1.
2.
3.
4.
The baserrent horst structure to which
mineralizaticn is spatially related is
of pre-Athabasca age. This is inferred
from the paleo--athering profiles developed along faults (M=llinger, 1979), as
-n as from stratigraphic relationships
within the Athabasca Group;
Uranium mineralization in the A zone may
have been enplaced before the deposition
of the Athabasca Group, at a tine of major
Hudsonian fault develq:,nent in baserrent
rocks. It is thus older than that in the
Main and B zones;
'lhe Athabasca Group suffered late anchizone (transitional to netarrorphism) diagenesis (Kubler, 1968; Dt.moyer de Segonzac,
1970). ValU=s for the Kubler (crystallinity) index of illite in mudstones usually
fall between 5.0 and 6.0. In this type
of environrrent, primary porosity is destroyed and pore fluids (rrost likely brines) can
only travel in rocks where secondary f()rosity has developed through mineral dissolution and/or local fracturing (Hayes, 1979;
Sdunidt and M:::Donald, 1979);
Uranium deposited in the Main and B zones
is thought to have been transf()rted and
precipitated under late diagenetic physi(X)chemical conditions. Uranium was carried
in solution by brines flooing along paths
determined by secondary porosity reveloprrent. Potassilnn and magnesium were
present in solution, resulting in the
developnent of white mica and chlorite,
and co may have played a significant role
2
in ronplexing
u6+ (or rather uoi+). The
simultaneous def()sition of pitchblende and
hematite (a cornrcri association at Maurice
Bay) is possible under specific physicochemical =ditions if a sufficiently
high terrperature is attained (Langmuir,
1978). reposition of the Main Zone along
the southern flank of the basenent uplift
could have resulted from significant changes
in hydrogeological conditions d\E to
baserrent topography, as -11 as fran
discharge of solutions of contrasting chemistry through reactivated baserrent faults;
Uranerz Exploration and Mining Ltd., Eldorado
Nuclear Ltd., and the Saskatchewan Mining
r::evelopnent Corporation are gratefully acknowledged for their support of the present research
project and for their permission to publish
this contribution. Thanks are extended to
Uranerz Exploration and Mining Ltd. for their
co-q:,eration and hospitality during field v.0rk.
References
Dunoyer de Segonzac, G. (1970): 'lh.e transformation of clay minerals during diagenesis
and low-grade rretarrorphism: a review.
Sedinentology, 15, 281-346.
FoJk, R.L. (1974) : Petrology of Sedirrentary
Rocks. Henphill Publishing Co. , Austin,
Texas, 182 pages.
Harper, C.T. (1979): Uranium !!€tallogenic
studies: Maurice Bay area, geology and
mineralization. In: Surrrnary of Investigations, 19 79 , Sask-:-Geol. Survey Misc. Report
79-10, 96-106.
Hayes, J.B. (1979): Sandstone diagenesis, the
hole truth. In: As~cts of Diagenesis,
P.A. Schollce and P.R.S. Schulger ed., SEPM
S~cial Publ. #26, 127-139.
Kubler, B. (1968): Evaluation quantitative du
rretarrorphisne par la crystallinite de
l'illite. Etat des progres realises ces
dernieres annees. Bull. Centre Rech. Pau S.N.P.A., I, 385-397.
Langmuir, D. (1978) : Uranit.nn solution - mineral
equilibria at low temperatures with applications to sedirrentary ore deposits. Geochirn. Cosrrochirn. Acta, 42, 547-569.
~llinger, M. (1979): Uranium metallogenic
studies: Maurice Bay deposit, geology and
petrography. In: Surrmary of Investigations
1979, Sask. Geol. Survey Misc. Report 79-10,
107-llO.
Schmidt, V., McD::mald, D.A. (1979): 'lhe role
of secondary porosity in the o:,urse of
sandstone diagenesis. In: As~cts of
Diagenesis, P. A. Scho]ke and P.R,S.
Schluger ed., SEPM Special Publ. #26,
175-207.