Download Petrography and geochemistry of the Archean/Paleoproterozoic

Petrography and geochemistry of the Archean/Paleoproterozoic crystalline rocks
in the Highrock Lake area, Eastern Wollaston Domain, Saskatchewan
Rupan Shi* (Saskatchewan Research Council)
15 Innovation Blvd, Saskatoon, SK S7N 2X8, [email protected]
The Highrock Lake area (NTS 74H-3) is located at a NE-trending magnetic high within the west part of
the eastern Wollaston Domain in northern Saskatchewan. Three major NE-plunging F2 folds, informally
named the ‘Highrock Lake antiform’, the ‘Highrock Lake synform’, and the ‘Wilson Bay antiform’, and
NE-trending reverse faults outline the structural pattern in the study area (Shi 1999).
The rocks exposed in the area consist mainly of pelitic/psammopelitic gneisses, banded
quartzofeldspathic gneisses, and quartzofeldspathic gneisses. Minor lithologies include Archean granitic
gneisses, grey gneisses, laminated biotite gneisses, calc-silicate rocks, and late granitoids/pegmatites.
The Archean granitic gneisses are structurally situated at the core of the ‘Highrock Lake antiform’.
However, the gneisses are the major rock unit forming the basement inliers in the region (Ray, 1977). The
rocks are heterogeneously ductilely deformed. The less strained horizons show a gneissic foliation, while
the more strongly strained equivalents are characterized by flattened recrystallized quartz-feldspar
aggregates rimmed by thin biotite layers. The Archean granitic gneisses contain mainly quartz (15-30%),
K-feldspar (30-50%), plagioclase (15-25%), and biotite (1<0%). Magnetite grains are scattered in trace
amounts.
The pelitic/ psammopelitic gneisses are one of the major rock units exposed in the study area. In the
western study area, pelitic/psammopelitic gneisses rim the Archean core of the ‘Highrock Lake antiform’.
Pink lit-par-lit granitic layers along the foliation are characteristic of the pelitic/ psammopelitic gneisses
in the western study area. In the eastern study area, the metasedimentary gneisses are typified by variable
amounts (5-35%) of lensoid of quartz-feldspar aggregates and feldspar porphyroblasts (Figure 1a).
Figure 1: Pelitic gneiss. (a) Typical quartz-feldspar lenses and feldspar porphyroblasts, oriented parallel
to oblique to the foliation. (b) Photomicrograph showing the late sillimanite overgrows cordierite (crosspolarized light). The cordierite shows polysynthetic twins.
The pelitic/psammopelitic gneisses comprise mainly quartz (5 to 15%), plagioclase (20-40%), K-feldspar
(15-40%), biotite (15-40 %), cordierite (0-15 %); and sillimanite (0-3%). Cordierite and sillimanite are
more common in the leucosomes. Amphibole is present in the gneisses in a few localities. Magnetite, and
ilmenite are common (up to 2 to 5%) in the rocks. Chlorite, epidote, and carbonate occur as secondary
Petrography and geochemistry of the Archean/Paleoproterozoic crystalline rocks
phases replacing earlier minerals. Accessory minerals include apatite, titanite, monazite, and zircon. Two
generations of sillimanite are found. The early sillimanite occurs as fibrous and needle-like crystals lying
within cordierite. It is aligned, parallel to the external foliation. The late sillimanite overgrows the
cordierite (Figure 1b). Muscovite replaces or overgrows earlier minerals, such as cordierite and biotite.
The muscovite is also distributed in fractures cutting cordierite. Chlorite and epidote are present along the
cleavage planes of biotite. The epidote occasionally replaces plagioclase. Carbonate is associated with
muscovite.
The grey gneisses are locally form a thin mappable unit. In the west part of the study area, the grey
gneisses flank the Archean core (after pelitic gneisses) of the ‘Highrock Lake antiform’. The rocks are
typified by a light grey to pinkish light grey weathered color and light to medium grey fresh color. Some
equivalents show a rusty color due to oxidization of magnetite in the rocks. The grey gneisses are
heterogeneously ductilely deformed, showing a layered structure defined by grain size. Attenuated to less
deformed quartzofeldspathic leucosomal layers and granitic to pegmatitic veins are common along the
foliation of the grey gneisses. In places, isolated and attenuated feldspar lenses and quartz-feldspar
aggregates are observed in the rocks. The grey gneisses consist mainly of quartz (20 to 30%), plagioclase
(30-50%), K-feldspar (10-30%), and biotite (10-15%). Coarser-grained magnetite, pyrrhotite, and
ilmenite are less than 2 to 3 percent.
The laminated biotite gneisses are mainly associated with pelitic /psammopelitic gneisses. The rocks are
characterized by lamination structure. In places, the rocks are strongly oxidized, showing a brick red to
rust color. Leucosomal layers or sweats rimmed by coarser-grained biotite melanosomes are present.
Boudinaged pegmatite veins, 0.2 to 2.0 cm wide, are common along the lamination of the rocks.
Elongated to lensoid feldspar porphyroblasts (0.1 to 1.0 cm long) are locally present. The rocks show the
same mineralogy as the pelitic gneisses described above and are interpreted as highly strained equivalents
of the pelitic/psammopelitic gneisses.
The banded quartzofeldspathic gneisses are typified by alternated bands defined by weathered color, grain
size, and composition. Light to medium green calc-silicate bands, 3.0 to 30.0 cm wide, are common in the
rocks. Hornblende/actinolite-rich bands, ranging from 0.5 to 17.0 cm wide, are present (Figure 2a).
Figure 2: Banded quartzofeldspathic gneiss. (a) Hornblende/actinolite-rich bands. Coarser-grained
hornblende and actinolite are rimmed with quartz-feldspar aggregates. The axes of the core-rim
structures are oriented oblique to the bands and the foliation of the rocks (b) Photomicrograph showing
Poikiloblastic hornblende overgrowths (cross-polarized light).
Petrography and geochemistry of the Archean/Paleoproterozoic crystalline rocks
Coarser-grained hornblende/actinolite (up to 2.0 cm across) occur as overgrowths. They are rimmed with
quartz-feldspar aggregates. The axes of the core-rim structures are oriented oblique to the bands and
foliation of the rocks (Figure 2a). Relict cross bedding is present in the rocks. Oriented, sporadically
distributed, coarser-grained biotite and hornblende/actinolite are scattered within the rocks, defining a
typical gneissic structure. The amphibole grains overgrow the finer-grained matrix, forming a
poikiloblastic texture (Figure 2b).
The banded quartzofeldspathic gneisses are composed mainly of quartz (20-30%), plagioclase (20-35%),
K-feldspar (20-30%), biotite (5-10%), and hornblende/actinolite (1-10%). Muscovite, commonly in
association with carbonate, replaces plagioclase. It also partly replaces some biotite flakes. Chlorite
occurs also as a secondary phase, partly to completely replacing biotite and hornblende/actinolite. Ti-rich
magnetite and ilmenite are locally up to 5%. Titanite is locally up to 1%. Accessory minerals include
apatite, monazite, and zircon.
The quartzofeldspathic gneisses are another main unit in the study area. They are consistent
mineralogically with the banded quartzofeldspathic gneisses described above. The quartzofeldspathic
gneisses contain mainly quartz (30-40%), plagioclase (35-45%), and K-feldspar (5-15%). Biotite (0-3%)
and hornblende/actinolite (1-5%) are present in minor amounts. Muscovite is present either as scattered
flakes in the mineral assemblage or partly replacing biotite and plagioclase. Chlorite, epidote, zoisite, and
carbonate are present as secondary phases, mainly replacing in part amphibole. Titanite is common in the
quartzofeldspathic gneisses. Accessory minerals include zircon, monazite, and apatite.
The calc-silicate rocks occur as a mappable unit only in the eastern study area. In most cases, the rocks
are present as thin bands mainly in the banded quartzofeldspathic gneisses. They are also present in the
quartzofeldspathic gneisses. The calc-silicate rocks are characterized by a creamy green-grey color on the
weathered surfaces and grey on the fresh surfaces. The rocks are quartz-rich (40 to 45%) and contain
abundant scapolites (15 to 30%), garnet, pyroxene, amphibole, and plagioclase. Carbonate is also
common in the rocks. Titanite is up to 1 to 2%. Quartz is moderately recrystallized. Garnet is partly
replaced by epidote and carbonate, and amphibole partly by chlorite. A few pyroxene grains appear to be
partly replaced by amphibole.
Many outcrops contain variable amounts of late granitoids. Pegmatite is commonly associated. The late
granitoids are locally mappable. They contain mainly quartz (30-40%), plagioclase (20-30%), K-feldspar
(30-40%), and biotite (<10%). Magnetite and ilmenite are present in trace amounts.
The pelitic/psammopelitic gneisses contain relatively high TiO2, Al2O3, Fe2O3 (total), MgO, MnO, P2O5,
Co, Cr, Ga, Ni, Nb, Sc, and V, and low SiO2. The laminated biotite gneisses show the same geochemical
features as the pelitic/psammopelitic gneisses. There are strong geochemical similarities between the
banded quartzofeldspathic gneisses and the quartzofeldspathic gneisses. The geochemical and
mineralogical consistency of these two units disagrees with the concept of the depositional break
(unconformity) (Yeo and Savage, 1999) between these two units. In general, the rocks contain high SiO2
and low TiO2, Al2O3, Fe2O3 (total) MgO, P2O5 and Ga, Ni, Sc, Sr, and V contents. The grey gneisses
contain lower TiO2, Fe2O3 (total), P2O5 Cr, Ni, Sc, V and Hf. Sr in the rocks is variable. Sample S98-025
shows an anomalous high value of Sr (387 ppm).
Selected plots using essentially immobile major and trace elements are presented below (Figure 3). As
shown, the pelitic/psammopelitic gneisses and laminated biotite gneisses show cluster or linear features.
The banded quartzofeldspathic gneisses and the quartzofeldspathic gneisses plot within tight clusters. The
grey gneisses neighbor the banded quartzofeldspathic gneisses and quartzofeldspathic gneisses.
Petrography and geochemistry of the Archean/Paleoproterozoic crystalline rocks
8
20
Grey Gneiss
Pelitic/Psammopelitic Gneiss
7
18
Laminated Biotite Gneiss
Banded Qf Gneiss
16
Qf Gneiss
Al2O3 wt%
Fe2O3* wt%
6
5
4
14
3
12
2
10
1
100
200
300
400
500
600
8
0.1
700
Niggli si
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
TiO2 wt%
30
20
18
25
16
Th ppm
Ga ppm
14
20
15
12
10
8
6
10
4
2
5
10
15
20
25
Niggli fm
30
35
40
0
10
15
20
25
30
35
40
Niggli fm
Figure 3: Plots of Niggli si vs Fe2O3, TiO2 vs Al2O3, and Niggli fm vs Ga, and Th for the selected rocks
from the Highrock Lake area. (Qf = quartzofeldspathic).
Reference:
Ray, G.E. (1977): The geology of the Highrock Lake - Key Lake vicinity, Saskatchewan; Sask. Dep.
Miner. Resour., Rep. 197, 36 p.
Shi, R. (1999): Structural geology of the Highrock Lake area, Wollaston Domain, Saskatchewan; Geol.
Assoc. Can./Miner. Assoc. Can., Prog. Abstr., v. 24, p.A-115.
Yeo, G.M. and Savage, D.A. (1999): Geology of the Highrock Lake area, Wollaston Domain (NTS 74H-3
and 4); in Summary of Investigations 1999, Saskatchewan Geological Survey, Sask. Energy Mines, Misc.
Rep. 99-4, p. 38-54.
Biographical Note:
Rupan Shi graduated from Hebei College of Geology, China in 1982 with a B.Sc. in Geology. He
received a M.Sc. in geology from the University of Regina in 1995. He has been working as a research
geologist with Saskatchewan Research Council since 1994, mainly conducting meso- and microstructural
studies locally and along fault/shear zones for uranium and gold mining companies.