The Origins of Anomalously Graphitic Rocks and Quartzite Ridges
in the Basement to the Southeastern Athabasca Basin
C.D. Card
Card, C.D. (2012): The origins of anomalously graphitic rocks and quartzite ridges in the basement to the southeastern
Athabasca Basin; in Summary of Investigations 2012, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of the
Economy, Misc. Rep. 2012-4.2, Paper A-6, 15p.
Abstract
Remote-predictive mapping for the basement to the Athabasca Basin in NTS area 74H was augmented by a core
mapping program using drillcores stored at Kapesin Lake, near the Key Lake mine, and in La Ronge and Regina.
The unaltered basement includes: pre-Wollaston supergroup, the Karin Lake formation of the Wollaston
supergroup, and younger intrusions of pegmatitic granite. Pre-Wollaston rocks include granite, leucotonalite, and
gneissic tonalite. Rocks of the Karin Lake formation include psammopelite, the most common unit, pelite and rare
psammite, all of which contain traces of graphite. Younger intrusions are dominantly pegmatitic granites, which in
some cases contain more than 50% normative quartz.
The majority of the rocks present in the basement sections contain hydrothermal alteration assemblages. Although
graphite in concentrations of <5% can be expected in psammopelitic rocks of the type found in the Karin Lake
formation, concentrations of >5% and locally up to 25% in altered rocks are clearly anomalous and require some
form of concentration. Many such rocks contain no ferromagnesian minerals (e.g., biotite), as might be expected if
they had been derived from psammopelitic precursors, and have undergone pervasive alteration. In addition, preWollaston supergroup orthogneisses and pegmatitic granites locally contain metasomatic graphite. Contacts
between anomalously graphitic units and non-graphitic rocks are gradational. In some instances, the graphite is
foliation parallel and appears to have pseudomorphed biotite. Elsewhere, the graphite is late and randomly
oriented. Graphite is also present in late fractures, particularly those developed in pegmatitic granites. Anhedral
pyrite in veins and as amorphous replacements of various rock-forming minerals is typically associated with the
graphite-rich rocks.
Rocks containing 80 to 100% quartz are locally encountered in basement cores. Some of these may well be
Wollaston supergroup quartzite, but others lack evidence of sedimentary features common in orthoquartzites and
range from massive, where they resemble quartz veins, to well foliated. Such quartzite rocks encountered in the
studied core are most common in gradational into pegmatitic granites, suggesting they are quartzolites. Layered
quartz-rich rocks associated with the quartzolites contain relict textures such as gneissosities and likely represent
silicified country rock.
Anomalous concentrations of graphite are thought to have been precipitated from fluids generated during the
prograde metamorphic cycle. Dehydration reactions generate free H2O that can consume some of the graphite
present in originally carbonaceous metasedimentary rocks, such as black shales. Reactions associated with biotite
melting under granulite-facies conditions can also consume graphite. Graphite likely precipitated locally from
carbon-rich fluids following peak metamorphic conditions. Multiple graphitisation events are probable. The quartzrich intersections are speculated to have formed due to immiscibility in H2O-rich, low-viscosity pegmatitic granite
melts. Fluid inclusion studies in the quartz cores of pegmatitic granites indicate that H2O-rich melt fractions
enriched with carbonate have the potential for increased silica solubility. This interpretation better fits the
characteristics of the quartzites and provides a more attractive explanation for several quartzite features that form
prominent topographic features in the basement to the Athabasca Basin. Hydrothermal solutions likely enhanced
basement permeability. Enhanced permeability has the potential to lead to more efficient mixing of fluids derived
from the Athabasca Group and the basement rocks, a process essential to creating the redox reactions necessary to
precipitate uranium. Quartzite ridges may have provided a competency contrast that focussed fault systems and, if
magmatic, might have also been a local source of uranium. High-grade deposits such as McArthur River and
Phoenix are directly adjacent to ‘quartzite ridges’.
Keywords: Wollaston supergroup, Karin Lake formation, uranium system, graphite, hydrothermal alteration, C-OH fluid, massive quartz, quartz ridge, pegmatitic granite, silicification.
Saskatchewan Geological Survey
1
Summary of Investigations 2012, Volume 2
1. Introduction
Although there is modern bedrock mapping of the basement rocks around much of the Athabasca Basin (Figure 1),
very little information is available about the basement rocks directly beneath it. A preliminary remote predictive
basement map for the western Athabasca Basin was prepared during the EXTECH IV multidisciplinary uranium
study (Card, 2006), but the only published basement maps for the eastern part of the basin are those of Gilboy
(1982a, 1982b). Since the time of Gilboy’s mapping, the database available for basement mapping in the Athabasca
region has evolved considerably. In addition to the new mapping along the flanks of the basin, there has been 30
years of new drilling, information from which is stored in the Saskatchewan Geological Survey’s Mineral
Assessment database (http://www.er.gov.sk.ca/smad), and new aeromagnetic surveys for the entire Athabasca Basin
in Saskatchewan (e.g., Buckle et al., 2010). These new aeromagnetic data are the best means of interpreting the
basement units beneath the magnetically transparent Athabasca Group in the absence of drillhole information.
Talt Taltson
son
Basement
The Athabasca uranium ore systems project (Bosman et al., 2011) is a multidisciplinary geoscience project designed
to achieve a four-dimensional understanding of the Athabasca region during the entire evolution of the multiepisode uranium system. Basement geology and later overprinting events, such as fault systems and alteration, are
key components in understanding the uranium system. Basement geology in the Athabasca Basin will be compiled
at 1:250 000 (250 k) scale and published as a series of 250 k NTS map sheets and geographic information system
products. The first NTS map area chosen for compilation is 74H (Geikie River), which encompasses both the Key
Lake and McArthur River uranium mines and numerous other uranium deposits and showings. Moreover, the
transitional boundary between
the Mudjatik and Wollaston
Nolan
60 N
Train
Dodge
domains, which is thought to be
Zemlak
RAE
favourable for uranium
Beav
erlod
Mudjatik
to
ge
exploration in the Athabasca
a
t
n
Ta
Lake
Athabasca
Basin, runs northeastward across
the 74H map area from the
Black Lake
southwest corner (Figure 2) 1.
The map area is ideal for
Athabasca
compilation as there is good
on
Carswell
t
Basin
Pasfield
s
bedrock mapping along the
la
l
o
Lake
W ake
basin’s flanks and recognisable
L
aeromagnetic features of various
Lloyd
orientations that can potentially
NTS
er
be quantified with drillhole
e
d
Cree
in
74H
information (Figure 2). In
Lake
Re
Llo
addition, depth to the
yd
ke
a
L
unconformity is relatively
HE
AR
n
shallow for most of the 74H area
a
m
NE
tha
and therefore the magnetic signal
Wa
from the basement rocks is less
muted 2. The mapping process
will include compilation of
REINDEER ZONE
drillhole data from assessment
w
ne
information, selected field
BBIW
y
e
Glennie
ss
studies to help with the
Ki NW
SASK
identification of rock units, and
IW
interpretation of geophysical
Lac
HW
La
PW
Ronge
information.
PHANEROZOIC
o
ak
rL
te
n
Pe
to
as
W
oll
e
ng
La
to
ns
te
Ki
Alberta
Saskatchewan
ss
ey
n
ew
R
ot
Ro
ne
tik
ja
ud
M
110 oW
Vi
rg
in
Ri
ve
r
Cle
Sn
arw
ow
ate
bir
r
d
e
te
ct
on
ic
zo
ne
o
102 W
0 10 30 50 km
Flin Flon
Archean Windows
NW = Nistowiak Window
Deschambault
IW = Iskwatikan Window
Amisk
Lake
Lake
HW = Hunter Bay Window Sask craton
PW = Pelican Window
BBIW = Black Bear Island Window } Hearne
2. Field Studies
}
o
54 N
Figure 1 – Current subdivision of lithostructural domains in northern Saskatchewan
and northeastern Alberta. The dashed box represents the outline of NTS area 74H
and the extent of Figure 2.
Drillcores were selected for
examination to provide insight
into various aeromagnetic
features in NTS area 74H. Cores
stored at the Kapesin Lake core
1
The Wollaston-Mudjatik transition zone (Annesley et al., 2005) is commonly referred to as favourable for uranium exploration. The zone has
loosely defined dimensions, but it is thought be broadly coincident with the transitional boundary (Figure 2) between the Wollaston and Mudjatik
domains.
2
Although it is generally non-magnetic and therefore magnetically transparent, anomalies sourced from below the thicker parts of the Athabasca
Group tend to be less clear due to absolute distance from source to collector (e.g., Pilkington, 1989).
Saskatchewan Geological Survey
2
Summary of Investigations 2012, Volume 2
cache near Key Lake, and the
Saskatchewan Geological
Survey’s facilities in La Ronge
and Regina were remapped in the
summer of 2012. These included
42 basement sections of cores
stored at Kapesin Lake, six
sections from La Ronge, and 10
stored in Regina (Table 1). In
addition, 17 full sections and one
partial section of Athabasca
Group were logged at Kapesin
Lake and La Ronge, respectively
(Bosman et al., this volume). The
newly gathered basement
information will be incorporated
into subsequent remote
predictive mapping. This report
details the mesoscopic textures
observed in very common
graphitic basement rocks in NTS
area 74H and in basement
sections mapped as
metamorphosed orthoquartzites
by previous workers.
McArthur River
914
Key Lake
0
10
20
a) Basement-driven
Exploration Criteria
40
km
Both graphitic basement rocks,
commonly interpreted as having
pelitic protoliths, and long core
intervals of interpreted
‘orthoquartzite’, commonly
referred to as quartzite 3 ridges,
are key uranium exploration
features in the eastern Athabasca Basin. Graphitic pelitic rocks are common, but not ubiquitous (Yeo and Savage,
1999) in the Karin Lake formation of the lower Daly Lake group (Yeo and Delaney, 2007), and rare in the rest of
the Wollaston supergroup. The Karin Lake formation commonly sits unconformably on Archean granitoid basement
throughout much of the western Wollaston Domain and it has been proposed that the competency contrast between
the relatively rigid rocks, in this case Archean orthogneisses and granites, and less-rigid Wollaston metasedimentary
rocks might control the location of structures that were later important in focussing uranium mineralisation (e.g.,
Jefferson et al., 2007). As a result, anomalously graphitic units are commonly targeted in the search for
unconformity-related uranium deposits, and patterns indicate that drilling commonly follows regional-scale
graphitic horizons that are imaged through a variety of electromagnetic techniques. At McArthur River (Figure 2),
conductive graphitic pelite is situated adjacent to a ridge of quartzite, which theoretically provides the competency
contrast necessary to localise strain along the P2 fault system (McGill et al., 1993). As a result of the anomalous
size and grade of the McArthur River deposit (e.g., Saskatchewan Ministry of Energy and Resources, 2011),
exploration targets adjacent to quartz-rich units are considered desirable and have led to other discoveries, e.g.,
Phoenix deposit (Gamelin et al., 2010).
Figure 2 – Total magnetic intensity for NTS area 74H. Diamond drillholes mapped
in 2012 are denoted by white dots. The black dotted line represents the interpreted
transitional boundary between the Wollaston (to the southeast) and Mudjatik
domains and the white line denotes the southeastern extent of the Athabasca Basin.
Note the location of the deposits mined at Key Lake and McArthur River (grey
squares), and of provincial Highway 914 (solid black line).
Two subjects related to exploration targets in the basement to the Athabasca Group will be detailed and discussed
below: 1) the origin of anomalously graphitic units near Key Lake and McArthur River; and 2) the origin of the
quartzite units. Evidence will be based on mesoscopic investigation of drillcore and will focus on mineral textures in
a variety of basement units.
3
Quartzite refers to a “well crystallized quartz dominated rock” (Shelley, 1983, p77). The term orthoquartzite implies a metamorphosed quartz
arenite and quartzolite, a quartz-dominated igneous rock.
Saskatchewan Geological Survey
3
Summary of Investigations 2012, Volume 2
Table 1 – Location and orientation information for basement cores mapped in the summer of 2012. Inclination is calculated
by subtracting the plunge of the drillhole from horizontal (i.e., 0°-90° = -90°, which is a vertical drillhole). Depth to
unconformity (UC) refers to metres of core drilled to reach the unconformity and has not been corrected for true depth. The
depth to the unconformity has been taken from the author’s log and is rounded to the nearest metre. Note all UTM
coordinates in this document are in NAD 83, zone 13.
Drillhole
NTS Area
(50k)
UTM-E
UTM-N
4557-1-82
4560-1-82
69-1 (P1-H1)
69-3 (P1-H3)
AH-004
AH-007
AH-009
AL-015
AL-017
AL-018
AL-020
BD-005
BD-006
BD-007
CB95-057A
EL-071
EL-074
EL-078
GRL-131
GRL-133A
GTB-82-33
KAP-005
KLI-006
LE-061
LE-064
LE-065
MAC-059
MAC-167
MAC-177
MK-020
MK-028
MK-032
MK-038
ML-008
ML-014
ML-015
ML-018
ML-059
ML-063
MW-007
MW-010
P-051
P-052
PP-001
RL-042
RL-047
RL-79-15
SP-001
SP-004
TUE-06-06
WC-79-1
WL-001-Dejour-Wapata
WL-002
WL-004
WL-021
WL-022
WR-008
ZQ-010
74H-06
74H-06
74H-16
74H-16
74H-05
74H-06
74H-04
74H-04
74H-04
74H-04
74H-04
74G-08
74G-08
74G-08
74H-15
74H-05
74H-05
74H-05
74H-05
74H-05
74H-05
74H-04
74H-04
74G-07
74G-07
74G-07
74H-10
74H-10
74H-11
74H-04
74H-04
74H-04
74H-04
74H-04
74H-04
74H-04
74H-04
74H-05
74H-06
74H-04
74H-05
74H-04
74H-04
74H-04
74H-14
74H-11
74H-06
74H-07
74H-10
74K-05
73 I-5
74 I-13
74H-06
74H-06
74H-05
74H-05
74H-09
74H-06
490012
501112
532104
530116
469490
471570
469273
460977
462791
461024
462807
420412
420403
421356
506109
458639
459517
460642
466581
466638
462092
465496
462801
393381
394164
394488
508285
500075
493201
460610
460477
460255
461827
465591
463521
463012
463004
468841
471178
461992
461124
465199
465146
467401
491667
488386
471918
520399
519513
232295
466838
457736
469960
469893
466878
465084
530941
472889
6358685
6369952
6427685
6417084
6345632
6346731
6345234
6343744
6344416
6342451
6342847
6354081
6354938
6355268
6411063
6345609
6346842
6346865
6357978
6357908
6353171
6338411
6341759
6358906
6360389
6361604
6382629
6395193
6398103
6341520
6340764
6341009
6341561
6345328
6345075
6345019
6345103
6346817
6348064
6340398
6346283
6340544
6341105
6336658
6401844
6393624
6355296
6372999
6373729
6469491
6484374
6525629
6347141
6347205
6349333
6347545
6382055
6370510
Saskatchewan Geological Survey
Inclination Azimuth Depth to UC
(°)
(°)
(m)
-90
-90
-90
-90
-60
-90
-60
-90
-90
-60
-57
-88
-90
-90
-90
-60
-60
-70
-80
-79
-90
-60
-90
-58
-90
-90
-60
-90
-90
-90
-90
-90
-90
-90
-90
-90
-90
-62
-62
-90
-58
-65
-90
-90
-60
-60
-90
-60
-62
-90
-90
-90
-90
-90
-60
-60
-74
-90
4
138
318
138
138
318
318
318
318
275
320
138
318
165
21
133
313
220
220
318
138
90
-
185
195
330
318
112
128
96.6
89
99
72
97.4
201
191
206
103
174
132
327
219
258
78
73
139
152
168
202
420
584
73
61
55
48
96
119
106
114
123
214
65
113
92
112
48
636
708
173
58
48
1212
939
768
131
161
217
158
58
256
Storage
Location
Assessment File
Regina
Regina
Regina
Regina
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Regina
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
La Ronge
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
La Ronge
La Ronge
La Ronge
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Regina
Regina
La Ronge
Kapesin Lake
Kapesin Lake
Regina
Regina
Regina
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
Kapesin Lake
La Ronge
74H06-0060
74H06-0060
74H15-0002
74H15-0002
74H-0045
74H-0047
74H-0047
74H04-NE-0077
74H04-NE-0077
74H-04-NE-0097
74H-04-NE-0097
74H04-NE-0041
74H04-NE-0041
74H04-NE-0041
74H15-SW-0049
74H-04-NE-0093
74H04-NE-0087
74H04-NE-0087
74H05-173
74H05-173
74H05-0093
74H-04-NE-0094
74H-04-NE-0092
74G07-0044
74G07-0044
74G07-0044
74H10-0052
74H-0040
74H-0040
74H04-NE-0070
74H04-NE-0070
74H04-NE-0070
74H04-NE-0070
74H04-NE-0074
74H05-SE-0075
74H05-SE-0075
74H05-SE-0075
74H-04-NE-0091
74H-04-NE-0091
74H-04-NE-0091
74H-04-NE-0092
74H04-NE-0100
74H04-NE-0100
74H04-NE-0072
74H14-0019
74H14-0019
74H06-0047
74H10-0019
74H10-0019
74K03-0019
74I-0012
74I12-0002
74H05-SE-0027
74H05-SE-0027
74H04-NE-0088
74H04-NE-0088
74H10-0019
74H06-NW-0080
Summary of Investigations 2012, Volume 2
3. Lithostratigraphy
The rocks described below, with the exception of the younger intrusions, are well foliated to gneissic and have been
metamorphosed beyond the limit of minimum melting, implying metamorphic conditions of at least upper
amphibolite facies. Younger pegmatitic granitoid rocks (<1.815 Ga; Annesley et al., 2005) are typically massive,
but well foliated examples are not uncommon, indicating a late foliation-forming deformational event in the region.
a) Pre-Wollaston Basement
Granite, leucotonalite, and gneissic tonalite are uncommon constituents in the drillcores observed in 2012 and
represent the only pre-Wollaston supergroup rocks that were encountered. Fresh granite is pink, has a 1 to 3 mm
grain size and is well foliated, homogeneous and contains 5 to 10% biotite. It is typically associated with 20%
younger pegmatitic granites. Fresh leucotonalite is grey, medium grained (1 to 2 mm), homogeneous, well foliated
and contains 15% biotite (Figure 3A). Pink and white altered varieties were also present in drillcore MW-007
(Table 1). Gneissic leucotonalite (Figure 3B) is the most common of the pre-Wollaston units. It typically comprises
a 1 to 2 mm paleosome containing 5 to 10% biotite, 30% or less quartz, plagioclase and local magnetite. Layers of
injected leucosome and in situ neosome development have imparted a migmatitic gneissic banding (Figure 3B).
Some of the gneissic tonalite displays compositional layering in addition to that defined by neosome (Figure 3C).
Such rocks may have metasedimentary protoliths; however, the difference in the alteration of feldspars in adjacent
layers suggested that it is a product of metasomatism.
b) Karin Lake Formation (Wollaston supergroup)
The most common protoliths for unaltered rock types in drillcore from NTS area 74H are metasedimentary rocks of
the Karin Lake formation, lower Daly Lake group (Yeo and Delaney, 2007). Psammopelite is the most common
rock type. It contains 1 to 2% garnet that is commonly elongate (Figure 4A), implying later deformation, as well as
10 to 20% biotite and trace to 1% graphite. Sillimanite is rare. Although cordierite is common elsewhere in the
Karin Lake formation (e.g., Highrock Lake; Yeo and Savage, 1999), no porphyroblasts were observed in any of the
A
B
Figure 3 – A) Well-foliated leucotonalite (MW-007,
105.8 m). B) Gneissic leucotonalite. Layering is due to both
injected leucosome and in situ neosome (WL-022, 252 m).
C) Hydrothermally altered gneissic leucotonalite. Banding in
this rock was imparted by injected leucosome, in situ partial
melting, and alteration. Pegmatitic granite is also present (P052, 193 to 195 m).
C
Saskatchewan Geological Survey
5
Summary of Investigations 2012, Volume 2
A
B
Grt
Grt
M
Figure 4 – A) Psammopelite with elongate garnet (Grt) porphyroblasts. White leucosome bands make up less than 10% of this
example, with thin bands of melanosome (M) at the margins of the leucosome (RL-047, 769.4 m). B) Pelite with about 30%
white, dismembered melt leucosome. A ragged garnet (Grt) porphyroblast is preserved near the bottom right of the image (AL017, 140.7 m).
cores examined. The psammopelite is commonly migmatitic, containing millimetre- to centimetre-scale layers of
tonalitic leucosome and narrow bands of biotite-rich melanosome (Figure 4A). Pelite is less common. It contains 2
to 3% elongate garnet porphyroblasts and 25% biotite (Figure 4B). No cordierite was documented. Graphite is
absent or present in trace amounts. The pelite is also migmatitic but, relative to the psammopelite, contains a higher
proportion of tonalitic melt leucosome, which forms centimetre-scale bands that are rarely dismembered (Figure
4B). Psammite is uncommon and typically interlayered with the other metasedimentary gneisses. It contains 1 to
2% garnet grains, typically no larger than 1 mm in size, in a fine-grained quartzofeldspathic groundmass that
contains 5 to 10% biotite.
c) Younger Intrusions
Pegmatitic granite is present in most drillholes, regardless of host-rock type. The granites are typically twofeldspar syenogranites (Figure 5A) containing up to 5% biotite and rare pyrite, garnet and tourmaline. Plagioclase is
typically metasomatised and replaced by epidote (Figure 5A). Prismatic, radiating epidote crystals (Figure 5B) are
also presumed to be the product of plagioclase alteration. Many of the granites are silica rich with over 50%
normative quartz (Figure 5C). The silica-rich varieties are commonly gradational into nearly massive quartz (Figure
5D), which typically cores the pegmatitic intersection.
d) Graphitic Rocks
The general interpretation for graphite-bearing units of the dominantly psammopelitic to pelitic Karin Lake
formation (Yeo and Delaney, 2007) in the basement to the Athabasca Basin is that the protoliths were carbon
bearing prior to prograde metamorphism. Organic-rich mudstones and black shales (e.g., Madore and Annesley,
1997) are the most commonly proposed protoliths. Other metasedimentary units that would not classify as pelites,
i.e., containing >25% aluminosilicate minerals (e.g., biotite, garnet, sillimanite, cordierite in the classification of
Maxeiner et al., 1999), are present and contain 5% or less graphite. Extremely graphite-rich units (25% or greater)
are sporadically present. In many cases concentrations of graphite are thought to have formed via pressure solution,
with the inferred removal of quartz and feldspar occurring during deformational events (e.g., Thomas et al., 2000).
This mode of graphite concentration would be favoured where the rocks show signs of intense deformation.
Rocks with greater than 5% (volume) graphite are common in drillholes along the major uranium exploration trend
(Figure 2) in NTS area 74H and many of these are correctly interpreted as metamorphosed carbonaceous sediments.
However, rocks containing metasomatic graphite are also present. Rocks from pre-Wollaston basement, the Karin
Lake formation and younger intrusions are either locally or commonly graphitic. For example, a wide intersection of
metasomatised gneissic leucotonalite in drillcore P-052, with a similar texture and appearance to unaltered gneissic
tonalite in drillcore WL-022 (Figure 3B), is graphitic between 119.5 and 129.5 m. In contrast to the weakly
graphitic, weakly altered psammopelite and pelite described above, strongly graphitic varieties are pervasively
metasomatised, and minerals such as garnet (replaced by chlorite), and fresh biotite are absent (Figure 6A). Up to
25% graphite is present and it appears to take the place of matrix biotite. In both the tonalite and the
metasedimentary rocks, contacts between anomalously graphitic and non-graphitic rocks are diffuse or gradational.
In most cases this transition is from rocks with preserved, albeit commonly altered, mica (likely originally biotite,
but typically colourless), into rocks with a mix of matrix graphite and mica, and ultimately into graphite-rich rocks
Saskatchewan Geological Survey
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Summary of Investigations 2012, Volume 2
A
B
Qtz
C
D
Figure 5 – A) Metasomatised pegmatitic granite with plagioclase replaced by epidote. Vein quartz (Qtz) is also present in this
section of core (GRL-133A, 308.6 m). B) Radiating, prismatic epidote crystals in pegmatitic granite (GRL-133A, 365 m).
C) Metasomatised, silica-rich pegmatitic granite containing over 50% normative quartz (MW-007, 195.2 m). D) Massive
quartz from a pegmatitic granite intersection with a centimetre-scale, green, crystal of altered plagioclase(?) and a smaller
crystal of less-altered K-feldspar(?) (GRL-133A, 285 m).
without preserved matrix mica implying that mica provides a nucleus for graphite growth. Biotite and/or chlorite are
only preserved in pseudomorphed porphyroblasts, presumed to have originally been garnet. Altered mica that
appears dusted by fine-grained graphite is commonly noted. In drillholes GRL-133A and AL-015, late randomly
oriented or radiating graphite is preserved in rocks of unknown protolith (Figures 6B, 6C, and 6D). Graphite is also
common in late, pegmatitic segregations, particularly along crystal boundaries and in other planes of weakness.
Graphite also replaces biotite in some metasomatised pegmatitic samples. In drillhole Al-015, an entire domain of
original biotite at least 2 cm in long dimension appears pseudomorphed by graphite and the accompanying feldspars
are strongly altered (Figure 6D). In other cases, graphite fills or partly fills late fracture sets or forms radiating
patterns in the pegmatitic rocks (Figure 6E). The graphite in fractures, in particular, implies mobilisation of carbon
well after the emplacement of the pegmatitic granites, although more than one emplacement event remains a
possibility (Annesley and Millar, 2011).
e) Silica-rich Rocks
Thick intersections of quartz-rich rock in the basement to the Athabasca Basin are commonly referred to as quartzite
in publications (e.g., Jefferson et al., 2007) and company reports and are typically interpreted as orthoquartzites.
Orthoquartzites are bedded and commonly contain minor amounts of metamorphic minerals indicative of their
original composition, for example muscovite and/or sillimanite in the case of aluminous varieties, and diopside in
calcareous varieties. Feldspar may be recrystallized, but is still represented in metamorphosed feldspathic quartz
arenites and heavy mineral laminae may be preserved as transposed layers rich in magnetite and/or other accessory
minerals (Figure 7; Card et al., 2008). Recrystallisation accompanying metamorphism generally modifies the
original clastic texture and grain size, but both the quartz and the associated mineral impurities tend to have grain
sizes ranging from <1 to 2 mm (Figure 7). Although quartzite is present in the Wollaston supergroup, it is rare and
Saskatchewan Geological Survey
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Summary of Investigations 2012, Volume 2
B
A
Py
Py
D
C
Bt
Gr
Gr
Gr
Figure 6 – A) Metasomatised graphitic pelite. No
ferromagnesian minerals (e.g., biotite, garnet) were
identified in this specimen. The grey, metallic mineral is
graphite, which is estimated to form 15 to 20% of the rock
volume. The yellow mineral is altered feldspar (AL-018,
83.4 m). B) Randomly oriented graphite (silver) and 0.5 mm
scale spots of fine-grained pyrite (Py) developed in a
metasomatised granitoid(?) rock (AL-015, 130.3 m).
C) Randomly oriented, millimetre-scale graphite (Gr) flakes
(dark grey) in a metasomatised migmatitic rock of unknown
protolith (GRL-133A, 318.9 m). D) Top core is an unaltered
section of pegmatitic granite with centimetre-scale domains
of biotite (Bt); the bottom is a metasomatised pegmatitic
granite (Gr) where books of biotite were at least partly
replaced by a centimetre-scale mass of near-monomineralic
graphite and plagioclase is epidotised and chloritised(?).
Core diameter is about 4.75 cm (top core is from AL-017,
depth unknown; bottom core from AL-015, 130.8 m).
E) Radiating graphite (Gr) associated with pyrite (Py)
overgrowing pegmatitic granite. The country rock is also
anomalously graphitic (GRL-133A, 326 m).
E
Gr
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Summary of Investigations 2012, Volume 2
the stratigraphic units likely to contain it, the Hidden
Bay assemblage and Souter Lake group (Yeo and
Delaney, 2007), are preserved only sporadically in the
region. Orthoquartzite is also locally preserved to the
west in the Mudjatik Domain (Figure 1; Card and
Bosman, 2007).
Quartzite intersections in many drillcores are
essentially massive quartz that appears coarse grained
(Figure 8A) and are typically associated with quartzrich pegmatitic granites of varying thickness (Figure 8B
and 8C). Intrusive contacts are not developed between
the quartzite intervals and the pegmatitic granites
(Figure 8D), implying consanguinity. In other cases,
quartzite contains a planar fabric that ranges from a
spaced cleavage (Figure 8B) to a well-developed
(shear?) foliation (Figure 8C). The massive quartzite
locally grades into siliceous, layered rocks (Figures 8E
Figure 7 – Well-bedded sedimentary-derived quartzite
and 8F). These represent silicified country rocks that
crosscut by pegmatitic granite. Note the relatively small
grain size. The pen magnet indicates the presence of
have maintained their originally gneissic banding. In
magnetite-rich beds that likely originated as heavy mineral
drillhole WR-185 (Figure 8E), these layered rocks can
laminae on foresets (image from central Black Birch Lake,
be traced into their relatively unaltered, pelitic
UTM 470194 m E, 6366982 m N).
equivalents. In other cases, such as in MAC-167,
unaltered examples were not intersected and, therefore,
the layered rocks are more difficult to interpret. Given the massive nature of the quartz in this drillcore, however,
the layered rocks likely represent inclusions of silicified country rock.
4. Discussion
a) Origin of Graphitic Rocks
Given that unaltered metasedimentary rocks of the region typically contain only 0 to 2% graphite, whereas rocks
containing up to 25% graphite typically display evidence of intense post-peak metamorphic hydrothermal alteration,
particularly those of metasedimentary origin, it seems logical that the extra graphite in the altered rocks is also
hydrothermal. The carbon-bearing aqueous solutions necessary to facilitate this process are likely the product of
metamorphic processes occurring at depth (Pattison, 2006). Most prograde metamorphic reactions are dehydration
reactions, such as:
Ms + Qtz  Als + Kfs + H2O 4,
which typically take place under upper amphibolite facies conditions at moderate pressures (Yardley, 1989). The
aqueous fluid generated during the prograde process has the potential to consume graphite via the following
reaction:
2C + 2H2O  CO2 + CH4,
although it is clear that some metamorphic graphite is conserved in pelitic rocks throughout the regional
metamorphic cycle if there was originally greater than 0.2% volume (Pattison, 2006). Further graphite is consumed
at granulite facies via biotite melting by reactions such as:
Bt + Sil + Qtz ± Pl  melt + Crd + Kfs.
The consumption of biotite generates elemental Fe that must be accommodated in the crystal lattices of the product
minerals – cordierite in the example above. Biotite contains a much larger proportion of ferric iron (Fe3+) than
cordierite and therefore reduction to the ferrous species (Fe2+) is necessary for the bulk of the iron to be consumed
(Cesare et al., 2005). Graphite can help to facilitate this process during the redox reaction:
2Fe2O3 + C  4FeO + CO2 (ibid.).
4
Abbreviations in the equations: Ms = muscovite; Qtz = quartz; Als = one of the three aluminosilicate polymorphs (sillimanite, kyanite or
andalusite); Kfs = K-feldspar; H2O = water; C = carbon; CO2 = carbon dioxide; CH4 = methane; Bt = biotite; Sil = sillimanite; Pl = plagioclase;
Crd = cordierite; Fe2O3 = iron (3+) oxide; and FeO = iron (2+) oxide.
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Summary of Investigations 2012, Volume 2
B
A
Cl
Qtz
Peg
C
D
E
F
Qtz
Figure 8 – A) Background image: section of massive to foliated quartz (Qtz) that grades into a narrow interval of
metasomatised pegmatitic (Peg) granite; Inset: close-up of quartz-rich and metasomatised pegmatitic granite. The feldsparrich part of the intersection is locally interrupted by domains of massive quartz (WR-185, UTM 334554 m E, 6307584 m N, no
depth information available, image courtesy of Sean Bosman). B) Metasomatised pegmatitic granite cored by massive quartz.
Note that contacts are gradational rather than intrusive and that within the black oval outline the quartz core is continuous
with quartz in the granite. A spaced fracture cleavage (Cl) is developed in the upper row of core parallel to the dashed black
line, (MAC-167, 427.8 m). C) Well-foliated and metasomatised, siliceous pegmatitic granite. The cross-cutting pink vein is
quartz dominated and contains fragments of the host rock (MW-007, 94.5 m). D) A metasomatised, feldspar-rich section of
pegmatitic granite that changes abruptly into massive quartz, although quartz is continuous across the boundary (MAC-167,
480.2 m). E) Massive quartz (Qtz) that grades into siliceous, layered rock (arrow) near the bottom of the image. These layered
rocks likely represent silicified country rock of unknown protolith (WR-185, UTM 334554 m E, 6307584 m N, no depth
information available, image courtesy of Sean Bosman). F) Silicified gneiss(?) in a quartz-rich intersection (MAC-167,
433.8 m).
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The foregoing reactions indicate that, as a result of the prograde metamorphism 5 of carbonaceous pelitic rocks,
carbon-bearing aqueous fluids (C-O-H fluid) are likely products, in addition to graphite-bearing pelitic gneisses.
The Karin Lake formation is apparently a suitable source environment to derive carbon, as graphitic pelites are a
common constituent (Yeo and Delaney, 2007). In order to re-precipitate this carbon, the fluid would later need to
become supersaturated during cooling. There are two scenarios under which this could occur: 1) a closed system,
where there is a high fluid-to-rock ratio; or 2) an open system with a low fluid-to-rock ratio (Huizenga, 2011). In the
first scenario, a high fluid-to-rock ratio is achieved by transporting fluid in fault or shear systems that can
accommodate high volumes, and in which oxygen fugacity (fO2) is moderated by changes to the fluid rather than
being influenced by the wall rocks. In an open fluid system, the fO2 of the fluid is controlled by a mineral buffering
system, i.e., fluid chemistry is influenced by reactions with the rock it is invading, and consequently changes in fO2
cause variability in the ratio O/(O+H) (Huizenga, 2011). The typical hydrothermal graphite observed in this study is
relatively coarse and crystalline. This type of graphite is typically formed at high temperatures, e.g., upper
amphibolite or granulite-facies conditions (Pasteris, 1999; Foustoukos, 2012); however, highly crystalline graphite
can also grow from lower temperature fluids, such as at the Borrowdale graphite deposit in the United Kingdom
(Luque et al., 2009). In the latter case, the hydrothermal graphite must nucleate on a pre-existing mineral phase,
such as metamorphic graphite (Pasteris, 1999), poorly ordered hydrothermal graphite or organic material
(Foustoukos, 2012), or silicate minerals (Luque et al., 2009). It is likely that graphite was deposited over a range of
fluid temperatures during the unroofing of the Wollaston supergroup after the peak of orogenesis in the TransHudson orogeny (Rantitsch et al., 2004). Annesley and Millar (2011) proposed four to five major carbon-bearing
fluid events over a span of ca. 50 million years in the region. Graphitised exploration drillcore indicate that these
hydrothermal fluids were focussed near the Karin Lake formation/Archean unconformity in the southeast Athabasca
Basin and therefore graphitisation was not restricted to rocks of the Wollaston supergroup.
Graphitisation via high-temperature metamorphic fluids likely began relatively close to the fluid source Karin Lake
formation rocks (Henne and Craw, 2012; Huizenga and Touret, 2012) before the fluids began migrating through the
crust and cooling during the late stages of metamorphism 6, likely aided by shear/fault systems. Closed fluid system
graphitisation was possible in these conduits. Graphitisation in situ or in the wall rocks of faults or in situ during
exhumation following the metamorphic peak occurred in an open fluid system with high initial O/(O+H). Graphite
precipitation in an open fluid system is suggested as the origin for the Borrowdale graphite deposit in the United
Kingdom (Ortega et al., 2010) and is the most likely process for forming apparent replacement textures such as
those in Figure 6.
b) Origin of Silica-rich Rocks
None of the quartz-rich intersections observed in this study 7 are best interpreted as orthoquartzites. Most have one,
but typically more, of the following features indicating late-stage quartz growth: 1) massive, non-foliated quartzite;
2) massive non-foliated quartzite that grades into quartz-rich pegmatitic granite; 3) foliated quartzite that grades into
foliated, quartz-rich pegmatitic granite; and/or 4) silicified country rock adjacent to massive non-foliated or foliated
quartzite (Figure 8). One option for the origin of the quartzite is a magmatic system related to the pegmatitic
granites. In order to create the quartz-rich zones in igneous granites, low-viscosity, high-H2O melts must be
generated during partial melting during regional metamorphism (Thomas and Davidson, 2012 and references
therein). Such melts are not stable with declining temperature and pressure resulting in immiscibility and the
production of two distinct melt fractions in addition to an aqueous liquid (ibid.). In the latter case, one of the
resulting melt fractions is H2O-rich (>10%) and has a low viscosity (Thomas et al., 2000). Such melts tend to
contain other volatiles, such as Li2O, B2O3 and P2O5, Fl, and Cl, and have the potential to act as solvents and
consume trace elements as they migrate (ibid.) and may be elevated in carbonate (CO3-2) (Thomas and Davidson,
2012). Thomas and Davidson (2012) state that fluid inclusions found in the quartz cores of pegmatitic granites
suggest that carbonate-rich melts are a likely key in concentrating silica, with silica solubility increasing as a
function of temperature. This is a potential source of the SiO2-rich melt necessary to produce the observed massive
quartz cores in pegmatitic granites (Figure 8D).
An alternative fluid source might be the fluids responsible for the graphitisation described above. One of the
consequences of the destruction of silicate minerals, such as biotite and feldspar during the deposition of
metasomatic graphite, is a net loss of SiO2 in the altered rock (Galbreath et al., 1998). That SiO2 is presumably
taken up by the hydrothermal fluids and could be re-precipitated due to changes in fluid properties leading to
silicification of nearby rocks. One argument against this process is the relative immobility of Al2O3 in most
geochemical systems, as it would be a necessary constituent to produce the feldspar components observed in most
of the quartz-rich intersections (Figure 8).
5
Although it is acknowledged that most exposures of Wollaston supergroup contain upper amphibolite facies assemblages, granulite-facies
conditions are likely to have persisted deeper in the crust.
6
The final significant metamorphic episode and associated pegmatitic granite emplacement in the eastern Hearne Province is ca. 1.815 Ga (e.g.,
Annesley et al., 2005).
7
Quartzite are found in drillcores 4560-1-82, GTB-82-33, MAC-167, MW-007, MW-010, P-052, and ZQ-010.
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Summary of Investigations 2012, Volume 2
c) Synthesis
It appears that fluids and magmas produced during metamorphic processes in the hinterland to the Trans-Hudson
Orogen had a profound influence on the basement rocks of the southeastern Athabasca Basin. Therefore, basement
rocks associated with unconformity-related uranium deposits must be classified not only in terms of protolith, but
also the alteration assemblages overprinting them. The relative timing between the various alteration events, which
include epidotisation of plagioclase, graphite/sulphide precipitation, and the development of quartz-rich rocks
remains unresolved. Although Harvey and Bethune (2007) mentioned the possibility of hydrothermal graphite in
rocks in the Deilmann pit at the Key Lake mine, Annesley and Wheatley (2011) were the first to identify it in faultzone rocks, although they suggested that the graphite postdated the Athabasca Group. Annesley and Millar (2011),
however, hinted at the complexity of pre-Athabasca Group alteration in the eastern Athabasca Basin. They
suggested that there were four to five episodes of carbon-sulphur-boron (CSB) geochemical cycling in the basement
and that this cycling could be related to both pre- and post-Athabasca Group uranium deposition. In addition to the
CSB fluid cycling necessary to produce the graphitisation and associated metasomatism observed in drill cores, it is
suggested here that the quartz-rich rocks observed in the core are the products of relatively late H2O-enriched melts
and/or SiO2-rich aqueous fluids.
It is also proposed that multiple pulses of CSB aqueous fluids are likely responsible for generation of the
graphitised/sulphidised rocks. Although tourmaline was not observed in association with the rocks described here,
late-stage tourmaline is common across the region. The B necessary for the CSB fluids could be derived from
recycling of tourmaline-bearing pelites common in the Karin Lake formation (Annesley et al., 2005). Alternatively,
the B may be introduced through mixing with magmatic fluids. A mixing scenario is suggested by Galbreath et al.
(1988) for an association of hydrothermal graphite and tourmaline in the Black Hills, although no mechanism was
offered.
With respect to the origin of the quartz ridges, traditional models invoke a clastic sedimentary origin with
subsequent transformation into ridges by pop-up structures relating to outwardly diverging faults (Kyser and Cuney,
2009). The textures observed during this study clearly preclude a sedimentary origin and instead support a relatively
late-stage magmatic or aqueous fluid origin, which is not a mode of origin that has been specifically discussed in the
region to the author’s knowledge.
The specific role played by pre-Athabasca Group hydrothermal graphite and massive quartz deposits in terms of
contribution to the overall ore system is equivocal. Furthermore, the role played by graphite in unconformity-related
uranium deposition is speculative, although all of the largest deposits are directly associated with concentrations of
it (Thomas et al., 2000). The scenario presented above provides no insight into the direct influence of graphite on
mineralisation; however, it implies altered basement rocks are likely in the direct vicinity of the uranium
mineralisation and that graphitised/suphidised alteration zones are being targeted during exploration. This suggests
that there are corridors of alteration that are loosely associated with the Karin Lake formation/Archean
unconformity that have the potential for enhanced permeability as well as the narrower planar faults normally
assumed. Such corridors could provide access for oxidising fluids originating in the Athabasca Group to infiltrate a
much larger volume of the basement complex, perhaps increasing the prospectivity for basement-hosted deposits.
Alternatively, the permeability corridors could provide a volumetrically more significant pathway for reduced,
basement-derived fluid that is necessary for the redox-related precipitation of uranium.
Quartzite ridges are typically thought to provide a competency contrast that localises faulting. Although this may be
true, it seems a coincidence that high-grade uranium deposits (e.g., McArthur River, Phoenix, and the BJ zone), are
closely associated with what might be quartzolites generated during pegmatitic granite emplacement. The most
obvious benefit of a pegmatitic granite precursor is the potential for a local source of uranium. Together, a
permeable graphitic fluid corridor and quartzolite could potentially become predictive for mineral exploration.
5. Conclusions
A hydrothermal origin for both anomalously graphitic rocks and quartz ridges of the southeastern basement to the
Athabasca Basin has been implied after mesoscopic mapping of basement drillcores from NTS area 74H. Rocks that
contain anomalous concentrations (>5%) of graphite and do not always exhibit clear evidence of having
sedimentary precursors should be carefully scrutinised with respect to their degree of alteration and their
ferromagnesian contents. Quartzite drillcores should be assessed for features that might indicate that they had quartz
arenitic protoliths. Massive quartz associated with pegmatitic granite is better interpreted as having a hydrothermal/
magmatic origin.
Saskatchewan Geological Survey
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In order to understand the true extent of pre-Athabasca alteration in the basement to the southeastern Athabasca
Basin the following exercises are necessary.
1) Mapping graphitic alteration systems through the reassessment of historic drillcore in order to understand the
extent of hydrothermal graphite in the eastern Athabasca Basin.
2) Reconsideration of the origin of quartzite ridges, including an assessment of the volume of actual quartzite
versus the massive quartzolite suggested in this report.
A better understanding of these alteration systems will allow researchers to determine what role they might have
played in formation of the younger, post-Athabasca Group unconformity-related deposits.
6. Acknowledgements
Cameco Exploration is thanked for hosting us at the Key Lake mine and for providing us the logistical support
necessary to access the core at Kapesin Lake. Scott MacKnight provided able and cheerful assistance in the field
and Scott and Stephanie Boulanger are thanked for their assistance at the subsurface laboratory in Regina.
7. References
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Annesley, I.R. and Millar, R. (2011): Tourmaline- and sulfide-bearing, graphitic pelitic gneisses of the
Paleoproterozoic Wollaston Group, northern Saskatchewan: new insights into understanding the carbon-sulfurboron-uranium geochemical system with implications for U/C-type uranium deposits; in Final Programme and
Abstracts, 25th International Applied Geochemistry Symposium, August 22 to 26, Rovaniemi, p111.
Annesley, I.R. and Wheatley, K. (2011): Insights into understanding the carbon-uranium (± sulfur and boron)
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highly crystalline graphite from moderate temperature fluids; Geol., v37, p275-278.
Madore, C. and Annesley, I.R. (1997): Graphitic pelitic gneisses of the Paleoproterozoic Wollaston Group, Hearne
Province, Saskatchewan; in Papunen, H. (ed.), Mineral Deposits: Research and Exploration – Where do They
Meet?, Balkema, Rotterdam, p79-82.
Maxeiner, R.O., Gilboy, C.F., and Yeo, G.M. (1999): Classification of metamorphosed clastic sedimentary rocks: a
proposal; in Summary of Investigations 1999, Volume 1, Saskatchewan Geological Survey, Sask. Energy
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