Download A Field, Petrographic, and Geochemical Study of Gabbros and

A Field, Petrographic, and Geochemical Study of Gabbros and
Related Rocks from the Sandy Islands Gabbro Complex,
Wollaston Domain 1
C. Madore 2 and I.R. Annesley 2
Madore, C. and Annesley, I.A. (1994): A field, petrographic, and geochemical study of gabbros and related rocks from the Sandy
Islands Gabbro Complex, Wollaston Domain: in Summary of Investigations 1994, Saskatchewan Geological Survey, Sask. Energy
Mines, Misc. Rep. 94-4.
The Sandy Islands Gabbro Complex is well exposed on
a series of northeast-trending islands in the west-central
part of Wollaston lake near the eastern edge of the
Athabasca Basin (Figure 1 ). It comprises an intermediate to mafic southeastern component and a felsic to intermediate northwestern component (shown as an
approximate contact on Figure 2, after Chandler, 1978).
Preliminary mapping and sampling was undertaken in
1993 for petrological and geochronological purposes as
part of an industry-funded geological investigation. Geochemical results indicated possible potential for Au and
Cu mineralization. Further work was undertaken in 1994
to investigate the primary (igneous) and secondary
(metamorphic) characteristics, the petrochemistry, the
tectonic environment, and the economic potential of the
Sandy Islands Gabbro Complex.
Early Proterozoic(?) age). The main belt of Wollaston
Group metasediments is composed of graphitic pelitic
gneiss, metamorphosed iron formation, pelitic gneiss,
calc-silicate gneiss, psammopelitic gneiss, psammitic
gneiss, metaquartzite, and amphibolites. The Wollaston
Group rocks are complexly deformed, polymetamorphosed, and rest unconformably (and tectonically) on reworked, antiformal Archean granitoid gneisses. The
Wollaston Group metasediments are intruded by metagabbros, porphyritic granites (1815 Ma), and pegmatites
(Madore and Annesley, 1993). The precise age of the
1. Geological Setting
The major elements of the Sandy Islands Gabbro Complex were documented by Chandler (1978) and Ray
(1978). Chandler (1978) noted that the complex corresponds to an "egg-shaped" aeromagnetic high, which is
consistent with the abundance of magnetite in the various lithological phases of the complex. A monzogabbro
phase (sample A93-026a) of the Sandy Islands Gabbro
Complex has been dated at 1828 ±3 Ma by U-Pb zircon
geochronology (unpubl. data; Annesley, Madore, and
Krogh). This age is identical, within analytical error, to a
U-Pb zircon age of 1828 ±3 Ma (refined to 1829 ±1 Ma)
for a coronitic, hornblende-phyric monzogabbro in the
Karpinka Lake area (Annesley et al., 1993).
The Sandy Islands Gabbro Complex is found within the
Wollaston Domain, one of the major subdivisions of the
Cree Lake Zone (Lewry et al., 1985; Gilboy, 1983). The
Wollaston Domain is a northeast-trending, orogenic fold
and thrust belt that is fault-bounded (i.e. Needle Falls
Shear Zone) to the east with the Peter Lake Domain
and the Rottenstone Domain, and to the west borders
the eastern margin of the Hearne Province hinterland. It
comprises Archean continental crust and continental
margin supracrustal rocks (i.e. the Wollaston Group of
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Figure 1 · Map of the Precambrian Shield of northern Sask·
atchewan showing the location of the Sandy Islands Gabbro
Complex.
(1) Funding by Cameco, Cogema Resources Inc., PNC Exploration (Canada) Co. ltd. , Uraner.z Exploration and Mining ltd., and the Saskatchewan
Research Council.
(2) Saskatchewan Research Council, 15 Innovation Blvd.. Saskatoon, Saskatchewan, S7N 2X8.
148
Summary of Investigations 1994
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Figure 2 - Map showing sample locations.
Wollaston Group is not known; however, an Aphebian
age is assigned to the Wollaston Group on the basis of
its apparent unconformable relationship with the underlying Archean granitoid gneisses and its subsequent
metamorphism during the Hudsonian Orogeny. Lewry
and Sibbald (1977, 1980) and Sibbald (1983, 1985)
have documented a broadly defined stratigraphic sequence, comprising four main lithological units for the
Wollaston Group. More recently, Annesley and Madore
(1991) have introduced a relatively simple stratigraphic
sequence for the Wollaston Group, which is similar to
Early Proterozoic supracrustal sequences (e.g. the
Chantrey Group and the Ramah Group) of the Rae
Province (Hoffman, 1988 and 1989).
2. Field Relations
The Sandy Islands Gabbro Complex is well exposed
and consists of discontinuous rounded outcrops. The
best exposures occur along the wave-washed island
shorelines. Heavy lichen covers the outcrops farther inland. The gabbros are medium grey to greenish brown
mottled black on the fresh surface, and weather dull
dark grey to rusty brown to dull medium brown mottled
black. They are holocrystalline, essentially equigranular
to inequigranular-porphyritic, overall medium grained,
hypidiomorphic, ophitic to subophitic, massive, fresh to
retrograded, invariably magnetic, and dense (Figures 3
and 4). The gabbros are composed mainly of varying
proportions of plagioclase, pyroxene, hornblende and biotite, subordinate K-feldspar, opaque minerals, titanite
and apatite, and minor quartz. The gabbro phases ap-
Saskatchewan Geological Survey
pear to be layered locally, and possibly rhythmic layered but the latter is difficult to discern because of a
well-developed vein network. The vein network consists
of cross-cutting granitic veins {Figure 5). In places, the
gabbros are metasomatized with complete replacement
of primary mineralogy and textures (Figure 6).
At some localities (A93-026, M94-004, Figure 2), gabbroic phases are intruded by granodiorites (to quartz
diorites), which are light to dark flesh pink mottled black
on the fresh surface and dull flesh pink to dull light grey
mottled flesh pink on the weathered surface. They are
holocrystalline, inequigranular-porphyritic, fine to coarse
grained, hypidiomorphic-granular, massive to moderately foliated, fresh to altered, magnetic, and relatively
dense. The granodiorites are composed of quartz, Kfeldspar, plagioclase, and biotite with subordinate titanite, hornblende, and opaque minerals (magnetite and
sulphides). K-feldspar phenocrysts are up to 3.0 cm in
length. Locally, biotite flakes are partially altered to chlorite. At locality A93-026, the gabbro phase is more deformed (i.e. in part sheared) than the intrusive
granodiorite, which suggests possible emplacement of
the granodiorite during shearing. Late pegmatite dykes
and veins crosscut the gabbro complex.
3. Preliminary Petrographic Observations
Ten samples were collected from the Sandy Islands
Gabbro Complex. They represent three low-Ti02 gabbros (M94-001a, -001b, and -007a), four high-Ti02 gabbros (M94-002, -003, -004a1. and -004a2). a quartz
149
Figure 3 - Sandy Islands Gabbro illustrating a fine· to mediumgrained subophitic texture. The rock is composed of plagioc/ase, augite, biotite, hornblende, and titanomagneffte (Location
M94-001).
Figure 6 - Metasomatized gabbro is holocrystalline, equigranular to inequigranular, very fine grained, massive to weakly foliated, granoblastic in texture, and moderately altered. The mineral assemblage consists of diopside, plagioclase, quartz, hornblende, titanite, and magnetite. The primary mineralogy and texture is completely obliterated by the pervasive metasomatism
(Location M94-00S).
diorite (M94-004b), and two calc-silicate gneisses (M94004c and -005).
Figure 4 - Sandy Islands Gabbro displaying well-preserved
subophitic texture, consisting of plagioctase, augite, hornblende, biotite, titanomagnetite, pyrite, and quartz (Location
M94·007).
Figure 5 - Sandy Islands Gabbro showing a network of narrow
veins and fractures with metasomatic reaction rims (Location
M94-004).
150
The low-Ti02 gabbros are holocrystalline, fine to medium grained, inequigranular, sub-ophitic, and essentially unaltered. They are composed of plagioclase,
augite, biotite, titanomagnetite, minor amounts of quartz
and pyrite, and accessory apatite. Plagioclase laths
range from 0.85 to 6 mm in length and display albite
and Carlsbad twinning. The plagioclase grains are randomly oriented and are partly enclosed within augite
grains (Figure 7) which range from 0.6 to 7 mm in diameter, and have a green to salmon pink pleochroism.
Rutile exsolution lamellae occur along the augite cleavage planes. Hornblende partly replaces augite and biotite along their grain boundaries. Biotite flakes, from
0.65 to 4.5 mm in length, form intricate intergrowths
with titanomagnetite grains, 0.5 to 4.5 mm in size, that
are composed of a magnetite host with ilmenite lamel·
lae (Figure 8).
The high-Ti02 gabbros are characterized by a holocrystalline, inequigranular, fine- to medium-grained, hypidiomorphic to subophitic texture. They are partly
recrystallized, massive, and weakly to moderately altered. The high-Ti02 gabbros differ from the low-Ti02
gabbros by the occurrence of titanite (primary and metamorphic), and the lack of augite. The igneous mineral
assemblage is composed mainly of plagioclase, hornblende, biotite, quartz, magnetite, ilmenite, titanite, and
pyrite. Accessory minerals include apatite and chalcopyrite. The primary assemblage is partly recrystallized in
places and displays a granoblastic texture. The metamorphic minerals consist of hornblende, titanite,
scapolite, and tourmaline. Hornblende grains partly replace biotite flakes, whereas titanite grains form a reaction rim around magnetite grains (Figure 9). Plagioclase
grains are randomly oriented, range from 0.6 to 4.5 mm
in length, and show albite and Carlsbad twinning. Despite partial static recrystallization, the primary grain outline of plagioclase is still well preserved. The grains are
Summary of Investigations 1994
weakly altered to saussurite and partly replaced by
scapolite, locally. Hornblende grains are both primary
and metamorphic, range from 0.35 to 5 mm in size, and
show intricate intergrowths with adjacent plagioclase, biotite, and magnetite.
The quartz diorite is holocrystalline, equigranular to inequigranular, very fine to fine grained, strongly recrystallized, massive, granoblastic, and weakly to moderately
altered. The hypdiomorphic texture of the quartz diorite
is well preserved, despite the strong static recrystallization. The primary mineral assemblage comprises plagioclase, quartz, hornblende, biotite, magnetite, and
apatite; metamorphic minerals include titanite and epidote. Plagioclase and quartz grains form an equigranular, granoblastic texture, and their grain size ranges
from 0.2 to 0.35 mm in diameter. Poikiloblastic hornblende, ranging from 0.35 to 0.75 mm in size, forms in-
tricate intergrowths with biotite flakes and magnetite
grains.
Calc-silfcate gneisses {or metasomatized gabbro) are
holocrystalline, equigranular to inequigranular, very fine
to fine grained, recrystallized, massive to weakly foliated, and weakly altered. The primary mineralogy and
texture of the rock is completely obliterated by the pervasive metasomatism. A granoblastic texture is well developed (Figure 10), and the rocks now consist of
plagioclase, quartz, hornblende, diopside, biotite, titanite, magnetite, and pyrite.
O .SO .t\M
Figure 7 - Drawing from a photomicrograph of gabbro (sample
M94·001a) which illustrates the characteristic subophitic tex·
ture of the gabbro . The primary mineral assemblage consists
of plagioclase (Pl), augite (Aug), hornblende (Hbl), and magnetite (not indicated), with minor quar1z (Qtz).
Figure 8 - Drawing from a photomicrograph of gabbro (sample
M94-007a) showing primary magnetite (Mag) with ilmenite la·
me/lae (/Im). The magnetite grains display intricate intergrowths
with adjacent biotite flakes (Bf) and plagioclase (Pl) grains.
Saskatche wan Geological Survey
Figure 9 - Drawing from a photomicrograph of gabbro (sample
M94·002) that is holocrystalline, fine to medium grained, hypdiomorphic, and massive in texture. The igneous mineral assemblage consists of plagioclase (Pl), quartz (Qtz), biotite (Bt),
hornblende (Hbl), magnetite (black), and titanite (Ttn). Titanite
grains are late-magmatic and fonn a reaction rim around the
magnetite grains.
Figure 10 - Drawing from a photomicrograph of calc-silicate
gneiss (metasomatlzed gabbro, sample M94-004c) illustrating
holocrystalline, essentially equigranular, very fine- to finegrained, granoblastic texture. The mineral assemblage is composed of plagioclase (Pl), quartz (Qtz), hornblende (Hbl), diopside (Di), magnetite (black), and titanite (Ttn).
151
4. Geochemistry
Table 1 presents the major and trace element geochemistry for nine of the gabbros and related rock samples
from the eastern part of the Sandy Islands Gabbro. The
gabbros (M94-001a, -001b, -002, ·003, -004a1, ·004a2,
and -007a) show a relatively large degree of compositional variability. For example, T i0 2 varies from 1.01 to
3.10 wt percent and MgO from 3.39 to 5.26 wt percent.
Minimum and m aximum values for the other oxides
also differ to the same degree. In general, the S andy Is·
lands gabbros show a broad continuum from compositions with relatively high to relatively low MgO and CaO
contents. In addition, there exists an inverse correlation
between Al203 and Ti02, Fe203 (total Fe as Fe203),
and P20s. Similarly, there exists a positive correlation
between Al20a with most of the compatible elements
(e.g. Cr).
The above mentioned and other geochemical charac·
teristics of the Sandy Islands gabbros, combined with
petrographic observations, indicate that the gabbros
can be divided into geochemical g roups on the basis of
Ti02, Al203, Fe203, and P20 s contents. This approach
has been used by Bellini et al. ( 1986), Fodor ( 1987) ,
and others, in distinguishing two types of basalts (a lowTi02-P20s type and a high-Ti02-P20s type) in the
Parami flood basa lt province of Brazil.
The low-Ti02 group of Sandy Islands gabbros represents the most primitive magma compositions, as
shown by its high MgO, Ni, and Cr values, and its low
values of alkali, incompatible, partially compatible, and
rare earth elements.
5. Economic Potential
Some of the Sandy Islands gabbros contain elevated,
ano malous values of Au, as high as 1110 ppb (Table
1). They are generally depleted in Pt and Pd (1 O ppb o r
less, not presented in Table 1) and are a lso depleted in
Cu, Co, Cr, and Ni. These res ults are consistent with
the calc-alkaline nature of the gabbros.
Table 1 • Geochemical data of gabbro and related rocks from the Sandy Islands Gabbro Complex. Major oxides in
trace elements in ppm, and Au in ppb.
Sample
M94·001a M94-001b
G
G
M94-002
G
M94·003 M94·004a 1 M94-004a2 M94·004b
G
G
OD
G
wt percent,
M94·005
cs
M94-007a
G
P20s
LOI
Total
56.20
1.10
16.47
8.42
0.12
4.93
6.34
3.46
1.40
044
0.70
99.58
56.10
1.01
17.06
8.71
0.12
5.26
6.69
3.28
1.50
0.49
0.70
100.92
52.30
3.10
15.24
11 .65
0.13
3.49
6.45
2.30
2.50
1.24
1.30
99.70
52.30
2.16
14.97
13.06
0.14
3.39
6.94
3.20
2.20
1.48
1.00
100.84
49.20
3.07
12.19
15.48
0.18
4.53
8. 13
2.46
1.30
1.49
1.10
99. 13
52.30
2.80
14.62
12.09
0.1 5
4.10
7.14
2.77
1.80
1.10
1.30
100.17
65.20
0.70
14.59
6.1 4
0.06
2.97
3.21
2.29
4.20
0.19
1.30
100.85
67.80
0.43
12.71
2.97
0.04
3.49
6.04
5.36
1.00
0.14
1.00
100.98
56.40
1.38
15.82
9.07
0.1 4
5.23
6.57
3.26
1.70
0.82
0.50
100.89
C(%)
s (%)
0.09
0.09
0.05
0.06
0.10
0.13
0.07
0.17
0.17
0.36
0.1 5
0.19
0.06
0.02
0.07
0.01
0.09
0.11
SiO:i
Ti02
Al20:i
Fe~3 (total)
MnO
MgO
CaO
Na20
K20
Ba (ppm)
Be
Co
Cr
Cu
Ga
La
Mo
Nb
Ni
Pb
Rb
Sc
Sr
Th
u
v
y
Zn
Zr
Au (ppb)
850
1.0
30
130
17
24
34
5
13
82
37
35
13
852
3
3.1
116
18
109
160
14
786
1.0
31
130
18
22
31
5
9
90
3
31
14
870
3
3.5
123
17
114
100
6
603
1.8
32
10
36
25
60
5
35
31
23
103
14
379
11
5.9
280
38
85
190
111 0
990
1.4
30
10
17
28
80
5
22
9
3
56
17
729
4
5.6
134
31
156
210
12
687
1.4
47
40
75
20
53
7
23
54
2
49
23
622
3
3.5
359
26
154
120
36
1141
2.0
35
50
37
21
52
5
25
48
2
69
20
727
3
5.9
246
25
121
220
40
1239
1.6
12
130
7
22
32
5
10
40
2
166
13
104
5
3.6
101
23
37
150
140
211
1.6
8
100
1
13
31
8
11
24
2
28
8
47
25
2.9
72
32
16
120
24
872
0.8
33
120
19
24
42
5
13
88
2
40
15
883
5
3.4
148
24
117
140
50
Note: G=gabbro, OD=quartz diorite, and CS=calc-silicate.
152
Summary of Investigations 1994
6. References
Annesley, I.R. and.Madore. C. (1991): The Wollaston Group
and its underlying Archean basement: Final Report; Sask.
Resear. Coun., Publ. R-1230+C-91, 140p, (confidential).
Annesley, I.R., Madore, C., and Krogh, T.E. (1993): U·Pb
zircon, titanite, and monazite geochronology from the Wollaston-Mudjatik Domain Boundary, Karpinka Lake area,
northern Saskatchewan; Geol. Assoc. Can./Miner. Assoc.
Can., Jt. Annu. Meet., May 1993, Waterloo, Prog. Abstr.
Bellini, G., Comin-Chiaramonti, P., Marques, L.S., Melfi, A.J.,
Nardy, A.J.R., Papatrechas, C., Piccirillo, E.M., Roisenberg, A., and Stolfa, D. (1986): Petrogenetic aspects of
acid and basaltic lavas from the Parana Plateau (Brazil):
Geological, mineralogical, and petrochemical relationships;
J. Petrol., v27, p915-944.
Chandler, F.W. (1978): Geology of part of the Wollaston Lake
Fold Belt, north Wollaston Lake, Saskatchewan; Geol.
Surv. Can., Bull. 277, 25p.
Fodor, R.V. (1987): Low· and high-Ti02 flood basalts of southern Brazil: Origin from picritic parentage and a common
mantle source; Earth Planet. Sci. Lett., v84, p423-430.
Gilboy, C.F. (1983): Sub-Athabasca basement geology project;
in Summary of Investigations 1983, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 83-4, p28-31.
Hoffman, P.F. (1988): United plates of America, the birth of a
craton: Early Proterozoic assembly and growth of Laurentia; Ann. Rev. Earth Planet. Sci., v16, p543·603.
_ _ _ _ (1989): Precambrian geology and tectonic history
of North America; in Bally, AW., and Palmer, A.A. (eds.),
The Geology of North America-An Overview, Geol. Soc.
Arner., vA, p447·512.
Saskatchewan Geological Survey
Lewry, J. and Sibbald, _T.1.1. (19n): Variation in lithology and
tectonometamorph1c relationships in the Precambrian basement of northern Saskatchewan; Can. J. Earth Sci. v14
p1453·1467.
'
'
_ _ _ _ (1980): Therrnotectonic evolution of the Churchill
Province in northern Saskatchewan; Tectonophysics v68
p45-82.
'
'
Lewry,~-~-· ~ibbald, T.1.1., and Schledewitz, D.C.P. (1985):
Varrat1on 1n character of Archean rocks in the western
Churchill Province and its significance; in Ayres, L.D., Thurston, P.C., Card, K.D., and Weber, W. (eds.), Evolution of
Archean Supracrustal Sequences, Geol. Assoc. Can.,
Spec. Pap. 28, p239·261.
Madore, C. and Annesley, I.A. (1993): Metamorphic pressuretemperature conditions of the basal Aphebian Wollaston
Group, Hearne Province, northern Saskatchewan; Geol.
Assoc. CanJMiner. Assoc. Can., Jt. Annu. Meet.. May
1993, Water1oo, Prog. Abstr., pA-65.
Ray, G.E. (1978): Reconnaissance geology: Wollaston Lake
(west) area (part of NTS area 64L); in Summary of lnvesti·
gations 1978, Saskatchewan Geological Survey, Sask.
Dep. Miner. Resour., Misc. Rep. 78-10, p25-34.
Sibbald, T.J.I. (1983): Geology of the crystalline basement,
NEA/IAEA Athabasca test area; in Cameron, E.M. (ed.),
Uranium Exploration in Athabasca Basin, Geol. Surv. Can.,
Pap. 82·11, p1-14.
_ _ _ _ (1985): Geology and genesis of the Athabasca
Basin uranium deposits; in Summary of Investigations
1985, Saskatchewan Geological Survey, Sask. Energy
Mines, Misc. Rep. 85·4, p133-156.
153