full text - NWT Protected Area Strategy

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NWT Open File 2008-05
Non-renewable Resource Assessment (Phase 1),
Acasta Gneiss Candidate Protected Area,
Northwest Territories, Canada
Carolyn Relf and John Ketchum
Recommended Citation: Relf, C. and Ketchum, J., 2008. Non-renewable Resource Assessment (Phase 1),
Acasta Gneiss Candidate Protected Area, Northwest Territories, Canada; Northwest Territories
Geoscience Office, NWT Open File 2008-05, 26 p.
>>> NORTHWEST TERRITORIES GEOSCIENCE OFFICE
PO Box 1500, 4601-B 52 Avenue, Yellowknife, NT
X1A 2R3
Northwest Territories Geoscience Office
4601-B 52 Avenue
P.O. Box 1500
Yellowknife, NT, X1A 2R3 Canada
867-669-2636
www.nwtgeoscience.ca
This publication may be obtained from the Northwest Territories Geoscience Office (see address,
phone number, and website above).
© Copyright 2008
All Rights Reserved
TABLE OF CONTENTS
NON-TECHNICAL SUMMARY .................................................................................................. 2
INTRODUCTION .......................................................................................................................... 3
Background ................................................................................................................................. 3
Terms of Reference ..................................................................................................................... 3
REGIONAL GEOLOGICAL SETTING ........................................................................................ 4
GEOLOGY OF THE ACASTA GNEISS ...................................................................................... 7
MINERAL POTENTIAL OF THE ACASTA GNEISS SITE ..................................................... 11
Volcanogenic Massive Sulphide ............................................................................................... 13
Vein-Hosted Lode Gold ............................................................................................................ 13
Banded Iron Formation Gold .................................................................................................... 14
Magmatic Sulphides.................................................................................................................. 15
Kimberlite-Hosted Diamonds ................................................................................................... 16
Quarrystone For Lapidary Purposes ......................................................................................... 16
Summary Of Mineral Potential ................................................................................................. 17
SCIENTIFIC VALUE OF ACASTA GNEISS ............................................................................ 17
RECOMMENDATIONS FOR PHASE 2 NRA ACTIVITIES AND DELIVERABLES ........... 19
ACKNOWLEDGEMENTS .......................................................................................................... 21
REFERENCES ............................................................................................................................. 22
Non-renewable Resource Assessment (Phase 1) – Acasta Gneiss
1
NON-TECHNICAL SUMMARY
Earth’s oldest rocks, the Acasta gneisses, are exposed along the Acasta River about 150 km
northeast of the community of Gamèti in Wek’eezhii (Tlicho Settlement Area), Northwest
Territories. The site is recognized as a unique window into Earth’s early history as rocks at this
locality have yielded ages of greater than 4 billion years old.
The Acasta gneisses are a finite resource with significant scientific and cultural value. For this
reason, the area is being considered for protection under the NWT Protected Areas Strategy. As
part of the process for protection, a candidate protected area’s values (minerals, energy, socioeconomic, etc.) are assessed. This report presents the results of a Phase 1 Non-renewable
Resource Assessment of the Acasta gneisses. The information presented is based on existing
data, and the scope of the report includes an evaluation of the site’s mineral potential, its value to
the public, and its scientific value.
Based on available data, the potential for the site to host significant mineral deposits ranges from
Very Low to Moderate. The two highest-ranked deposit types are kimberlite-hosted diamonds
(assigned a ranking of Moderate) and copper-nickel-platinum group metals hosted in mafic (ironand magnesium-rich) rocks (assigned a ranking of Low to Moderate). In both cases, our
confidence for these rankings is very low, as little information is publicly available to inform our
assessment. The report recommends some specific low-impact work that could be undertaken to
increase the level of certainty for the mineral potential rankings of these two deposit types.
While its value to the public is not easily quantified, we note that the NWT Geoscience Office
receives numerous requests each year to provide samples of Acasta gneiss to schools, museums,
and individuals from around the world. In addition, media interest over the years attests to the
public’s ongoing interest in these rocks.
Geologists consider the scientific value of the Acasta gneisses to be very high. This is based both
on the rarity of rocks of this age, and the potential for research to provide insights on planetary
evolution and the earliest tectonic processes on Earth. New analytical methods could one day
allow the Acasta gneisses to contribute to major advances in understanding early Earth
processes.
In conclusion, it is felt that the scientific value of the Acasta gneisses warrants continued work
toward protection of the site. It is recommended that further work be undertaken to delineate a
footprint that will preserve the scientific integrity of the site and allow for rock sampling in a
manner that does not adversely impact the landscape. The footprint could be fairly small and
would not significantly impact regional land access for other purposes such as mineral
exploration.
2
INTRODUCTION
Background
The ca. 4.0 Ga Acasta gneisses, located on a series of islands and adjacent mainland along the
Acasta River on NTS map sheet 86G/04 (approximate coordinates 65° 10’N, 115° 34’ W),
represent the oldest known intact fragment of continental crust on Earth (Bowring et al., 1989a;
Bowring and Housh, 1995; Bleeker and Stern, 1997; Iizuka et al., 2006, 2007). Attention was
first drawn to these rocks when Bowring et al. (1989a) published their landmark paper on the ca.
3.96 Ga U-Pb age of zircons from the site. Since then, a number of research teams have visited
and sampled the rocks and refined the radiometric age data. Recently, Iizuka et al. (2006)
published the first, albeit indirect, evidence for even older, ca. 4.2 Ga crust in this region. The
Acasta gneiss site represents a unique and finite scientific resource, and also highlights the
geology of the NWT, and specifically Wek'eezhii (Tlicho Settlement Area) from a tourist and
cultural perspective. In addition, public interest in owning a piece of the Earth’s oldest rocks has
sustained modest businesses selling pieces for souvenirs (e.g., http://rockofagesnwt.com/).
The NWT Geoscience Office (NTGO) has proposed to the NWT Protected Areas Strategy (PAS)
Secretariat that the site be considered for protection under the PAS, in order to preserve the rocks
for research and cultural interests. While a specific mechanism for protection (e.g. territorial
park, national historic site, UNESCO Geo-Park) has not been identified, some progress has been
made. Currently the site is designated as a Scientific Reserve. This status does not encroach on
mineral claim rights, nor does NTGO have any enforceable authority for limiting access to the
site.
The PAS Secretariat has included the Acasta gneiss site on its list of candidate protected areas.
As part of the PAS process, a candidate protected area’s mineral, energy, ecological, cultural,
and socio-economic values are assessed in order to support an informed decision regarding land
use priorities. This report presents findings from the first of two phases of a Non-renewable
Resources Assessment (NRA) of the Acasta gneiss site. The report describes regional and local
geology, and undertakes an evaluation of both the mineral potential and the potential scientific
value of the rocks.
Terms of Reference
Phase 1 NRAs are typically a qualitative assessment of an area’s mineral and hydrocarbon
potential, and are based on existing data. They include an analysis of knowledge gaps and
recommendations for further work to address these gaps. Phase 2 assessments are based on the
follow-up field and laboratory work defined in the Phase 1 assessment. While the assessment of
resource potential may still not be entirely quantitative, the level of confidence that the
assessment is accurate is higher for a Phase 2 NRA as a result of the targeted assessment work
undertaken. Notwithstanding the greater certainty, the conclusions drawn in a Phase 2 NRA can
3
change if further studies highlight new mineral deposit types or geologic features not previously
considered. To illustrate with an example, an assessment of the mineral potential of the central
Slave Province carried out 20 years ago would likely have deemed the area to have low
economic mineral potential, as diamonds would not have been considered in the study. The
discovery of diamonds in this area in 1991 would have rendered such an assessment incorrect.
Currently, all areas underlain by the Slave craton are considered potentially prospective for
diamonds.
In the case of the Acasta NRA, hydrocarbon potential has not been considered, as the study area
lies entirely within the Canadian Shield, and therefore oil and gas potential is held to be very
low. The scope of the mineral resource assessment was expanded to consider not only the
potential value of metallic and gem minerals associated with rocks in this setting, but also the
potential value of the rocks themselves as a scientific resource. The ranking system follows that
used for other NRAs undertaken in support of the NWT Protected Areas Strategy. Rankings are
presented for both mineral (rock) potential and level of confidence. Potential Rankings comprise
8 values; these are A through H, corresponding to very high to not assessed. Confident Rankings
have 4 values; 1 through 4, corresponding to highest to lowest level of confidence.
REGIONAL GEOLOGICAL SETTING
The Slave Geological Province (also termed Slave craton) is an Archean granite-greenstoneturbidite terrane in the northwestern Canadian Shield (Figure 1). The region has been the subject
of numerous geological mapping campaigns and more detailed field- and laboratory-based
studies mainly since the 1940s, but despite this effort, parts of the province remain poorly
understood in terms of geological history and mineral potential. The description that follows is
drawn largely from this previous work.
Supracrustal rocks are relatively abundant in the Slave Province and consist predominantly of ca.
2.72 to 2.66 Ga metavolcanic rocks, interfingered with and overlain by much larger volumes of
metaturbidite. A younger (<ca. 2.59 Ga) sequence of clastic sedimentary rocks, comprising
mainly conglomerates and arenites, occurs locally within narrow, isolated fault-bounded basins.
Collectively the supracrustal rocks have been designated the Yellowknife Supergroup
(Henderson, 1970). Plutonic rocks include syn-volcanic mafic to felsic dykes, sills and small
plutons spatially associated with the volcanic belts; a ca. 2.63-2.61 tonalite-trondhjemitegranodiorite suite; and widespread, ca. 2.60-2.58 Ga peraluminous granites. The tonalitetrondhjemite-granodiorite suite was emplaced during shortening and crustal thickening, which
culminated in regional high-temperature/low pressure metamorphism. Geochronologic data
coupled with metamorphic pressure-temperature studies mainly record peak metamorphism
between ca. 2.59 and 2.56 Ga at upper to mid-crustal levels.
4
Figure 1. Simplified geological map of the Slave Province, NWT. The Acasta gneisses are found
within a larger gneiss complex located near the western edge of the province. Figure modified
from Bleeker et al. (1999b).
5
Pre-Yellowknife Supergroup (> 2.85 Ga) rocks of the Slave Province consist mainly of foliated
to gneissic granodiorite and tonalite, and are exposed in several places in the western part of the
province. Pb and Nd isotopic data (Thorpe et al., 1992, and Davis and Hegner, 1992,
respectively) suggest that these old rocks underlie most of the central and western part of the
craton (Figure 2). Locally, occurrences of fuchsitic quartzite, iron formation, and minor rhyolite
were recognized adjacent to these ‘basement’ gneisses by early mappers (e.g. Easton et al.,
1982), although few appeared on regional geological maps as the units are generally too thin to
be represented at this scale. Kusky (1989) referred to the area underlain by these old rocks as the
Anton terrane and suggested that they represent the remnants of an ancient micro-continent
against which the Yellowknife Supergroup was accreted. Bleeker et al. (1999a, b) studied the
basement gneisses in more detail, demonstrating that they could be traced continuously through a
series of anti- and synclinoria that underlie the Yellowknife Supergroup in the central and
western parts of the Slave craton. In addition, they delineated the quartzite-bearing supracrustal
package as a semi-continuous cover sequence sitting unconformably on these gneisses. Most
exposures of the basement-cover contact were subsequently deformed during later tectonism,
obscuring the original unconformable relationship. The quartzite-bearing cover package was
considered by Bleeker et al. (1999a) as the lowest unit of the Yellowknife Supergroup. Bleeker
et al. (1999a) proposed the terms Central Slave Basement Complex for the basement gneisses,
and Central Slave Cover Group for its associated supracrustal package.
Rocks of the Central Slave Basement Complex represent a protracted record of Earth history.
They range in age from 4.03 Ga to 2.85 Ga, and limited petrologic and geochronologic studies
suggest that their formation was punctuated by a number of regionally-extensive but poorlyunderstood thermal/magmatic events (Davis and Bleeker, 1999). The oldest known phases of the
basement complex, located along the Acasta River in the westernmost Slave Province, have
garnered the most attention as they represent the oldest rocks currently known on Earth.
The margins of the Slave Province are marked by younger orogenic belts and sedimentary
basins. To the west is the 1.94-1.86 Ga Wopmay orogen (Bowring and van Schmus, 1984;
Hoffman, 1989), formed during the collision and accretion of Paleoproterozoic arc rocks to the
western rifted margin of the Slave craton. Slave basement and cover rocks are known to occur
beneath Proterozoic cover as far west as the Wopmay fault zone (Bowring and Podosek, 1989;
St-Onge et al., 1991; Bleeker et al., 2000). The eastern border of the Slave craton is marked by
the 2.02- 1.91 Ma Thelon Tectonic Zone, a deformed plutonic and metamorphic complex that
developed during collision with the adjacent Churchill Province (Hoffman, 1989). This collision
resulted in flexure of the Slave Province lithosphere, forming the Kilohigok Basin in the northern
part of the Slave Province and the Athapuscow Basin in the east arm of Great Slave Lake
(Grotzinger and McCormick, 1988; Hoffman, 1989). To the south, the Slave Province is
truncated by the 1.98-1.92 Ga Great Slave Lake shear zone. The northern and southern
extensions of the craton are covered by Mesoproterozoic and Phanerozoic rocks, respectively.
6
GEOLOGY OF THE ACASTA GNEISS
There are surprisingly few published descriptions of the detailed geology of the Acasta River
area despite almost two decades of knowledge that Earth’s oldest rocks occur in this region.
Emphasis has been placed instead on isotopic analysis of whole rock samples and selected
minerals (e.g., zircon, apatite, feldspar) extracted from these samples. Most workers describe the
Acasta River area (Fig. 2) as being underlain by a heterogeneous assemblage of foliated and
gneissic tonalite, granodiorite, and amphibolite. Lesser volumes of gabbro, diorite, and
ultramafic rocks commonly occur as xenoliths within the granitoid gneisses (e.g., King, 1986;
Bowring et al., 1989b; Stern and Bleeker, 1998; Iizuka et al., 2007). These rocks are cut by
several generations of younger granitoid veins and dykes that range from foliated to massive.
Post-Archean lithologies in the area include small sedimentary outliers of the Paleoproterozoic
Epworth Group, and Mesoproterozoic diabase dykes of the 1.27 Ga Mackenzie swarm (e.g.,
Bowring and Housh, 1995).
The Acasta gneisses are part of a much larger basement exposure termed the Acasta Gneiss
Complex (AGC; King, 1985). In detail, this complex consists of a series of north-trending
antiformal culminations separated by narrow synformal saddles. Bowring and Housh (1995)
described the eastern portion of the complex as being dominated by pink, foliated to massive
granite with xenoliths of amphibolitic to granitic gneiss. The number and size of xenoliths
increases noticeably toward the west. On the western side of the complex, the Archean rocks are
predominantly banded gneisses of tonalitic to granitic composition that are intruded by sheets of
foliated granite (e.g., Iizuka et al., 2006, 2007). Immediately overlying the AGC to the west and
south is a thin, discontinuous package of quartzite, quartz pebble conglomerate, and minor
banded iron formation, which is in turn overlain by deformed mafic volcanic rocks. King (1986)
and St-Onge et al. (1991) assigned these supracrustal rocks to the Paleoproterozoic Epworth
Group, a claim which Bleeker et al. (2000) have disputed. The latter authors instead assign these
supracrustal rocks to the Yellowknife Supergroup, with the quartzite-banded iron formation
package forming the westernmost occurrence of the Central Slave Cover Group. This claim is
supported by the apparent absence of Proterozoic detrital zircons in a quartzite sample from the
western edge of the basement domain (Bleeker et al., 2000; Sircombe et al., 2001), and an
unpublished, ca. 2.7 Ga zircon age from the overlying mafic volcanic package (Davis and
Bleeker, pers. comm., 2005). An important implication of this finding is that turbiditic
metasedimentary rocks immediately south and west of the quartzite-mafic volcanic packages
may also be Archean. Bleeker et al. (2000) suggested that Archean rocks may extend as far west
as the Wopmay fault zone in this region, a conclusion that was also suggested in part by St-Onge
et al. (1991). An additional implication is that the north-trending folds of the AGC may also be
Archean (Bleeker et al., 2000), in contrast to earlier suggestions that these folds were generated
during Calderian (Wopmay) orogenesis (King, 1986; St-Onge et al., 1991).
7
Figure 2. Simplified geology map of the Acasta gneiss region, modified from Iizuka et al.
(2006). Black dots indicate location of some U-Pb geochronology sample sites, with numbers
keyed to published U-Pb data summarized in Table 1. “Discovery Island” is located at 65° 10’N,
115° 34’ W.
8
The most detailed published map of the area of 4.0 Ga Acasta gneisses is that of Iizuka et al.
(2007). These workers portray the area in terms of two principal rock associations – ‘layered
gneiss’ cut by foliated granite sheets in the west, and ‘tonalitic-granitic gneiss’ with large
enclaves or intrusive bodies of dioritic gneiss in the east (Fig. 2). A two- or three-part lithological
subdivision of the map area is also supported by Bowring and Housh (1995) and Bowring and
Williams (1999), who published less detailed geological maps.
U-Pb geochronological data from the AGC have been obtained by thermal ionization mass
spectrometry (TIMS), sensitive high resolution ion microprobe (SHRIMP), and laser ablation –
inductively coupled plasma mass spectrometry (LA-ICPMS) methods. Published ages based on
zircon and apatite dating are presented in Table 1 with some sample locations shown in Figure 2.
Apart from detailed work in the area of the original discovery site, there has been relatively little
work to determine the regional extent of ~4.0 Ga crust. A geochronological investigation by
Ketchum and Bleeker (unpublished data) of Slave basement rocks located 40 km southeast of the
AGC in the Grant Lake area yielded U-Pb zircon ages no greater than ca. 3.3 Ga. Similar results
were obtained by these workers from a detailed study of gneissic basement rocks exposed both at
and southwest of Point Lake, 80-100 km east of the AGC. Hence, it would be desirable to
investigate the age of similar gneissic rocks at locations between these study areas, preferably by
working outward from the region of known 4.0 Ga occurrences.
Recent work by Iizuka et al. (2006) has pushed back the age of the oldest zircon contained in the
Acasta gneisses to ca. 4.2 Ga, which represents a significant and exciting new finding. The 4.2
Ga portion of the dated zircon is interpreted as a xenocryst, meaning that a younger magma
entrained this zircon from an older source rock. We note however, based on our own field
observations and subsequent correspondence with Iizuka, that this age comes from a sampled
boulder along the shore of the Acasta River which cannot be linked to a nearby outcrop source.
Despite this, the boulder more than likely represents an Acasta gneiss sample, and therefore the
scientific finding is not invalid. It remains possible that with additional U-Pb dating, the older
source rock could one day be discovered at the surface in the Acasta region.
The Acasta area was mildly to moderately overprinted during thermal and tectonic events
associated with the Paleoproterozoic Wopmay orogen. Some brittle structures and shear zones in
the region are likely associated with Paleoproterozoic compressional tectonism, and both
published and unpublished data from zircon, apatite, titanite, and whole rock samples indicate
that some new mineral growth and/or isotopic resetting also occurred at this time (see Table 1).
9
Table 1. U-Pb age data for the Acasta gneiss region.
notes:
1. Detrital ages are for grains <±0.5% discordant (conventional) or >95 to <105% concordant (SHRIMP)
2. Age (mineral): z - zircon; a - apatite
3. Type: C - conventional TIMS analysis; S - SHRIMP analysis; LA - laser ablation ICPMS analysis
No.
Sample
Locality
Age (mineral)
Type Significance
Reference
1
tonalite gneiss
AGC
4189±46 (z)
S
inherited core (1 anal.)
Iizuka et al. 2006
2
leucocratic quartzofeldspathic gneiss
AGC
4114±25 (z)
S
protolith age
Sano et al. 1999
3
tonalite gneiss
AGC
4065±8 (z)
S
age of inherited component
Bowring & Williams 1999
4
granodiorite gneiss
AGC
4031±3 (z)
S
protolith age
5
tonalite gneiss
AGC
4025±18 (z)
S
protolith age
Stern & Bleeker 1998
6
tonalite gneiss
AGC
4012±6 (z)
S
protolith age
7
tonalite gneiss
AGC
4002±4 (z)
S
protolith age
8
tonalite gneiss
AGC
3974±26 (z)
LA
protolith age
Iizuka et al. (2007)
9
amphibolitic-tonalitic gneiss
AGC
3964±4 (z)
S
protolith age
Bowring et al. 1989a
10
leucocratic granitic gneiss
AGC
3958±8 (z)
S
protolith age
11
tonalite gneiss
AGC
3958±40 (z)
LA
protolith age
12
granite gneiss
AGC
3941±43 (z)
LA
protolith age
13
quartz diorite gneiss
AGC
3931±34 (z)
LA
minimum protolith age
14
CSCG quartzite
western margin AGC
3775±40 (z)
S
oldest detrital zircon age (1 anal.)
Sircombe et al. 2001
15
tonalite gneiss
AGC
3758±5 (z)
S
zircon recrystallization
16
tonalite gneiss
AGC
ca. 3750 (z)
S
Pb loss from zircon
17
granodiorite gneiss
AGC
3744±41 (z)
LA
protolith age
18
tonalite gneiss
AGC
ca. 3730 (z)
S
metamorphism
19
granite gneiss
AGC
3728±40 (z)
LA
protolith age
20
quartz diorite gneiss
AGC
3691±36 (z)
LA
metamorphism, anatexis
21
granite gneiss
AGC
3689±61 (z)
LA
22
granodiorite gneiss
AGC
3680±31 (z)
S
zircon recrystallization or overgrowth age
23
tonalite gneiss
AGC
3663±42 (z)
LA
24
tonalite gneiss
AGC
3661±47 (z)
LA
minimum protolith age
25
amphibolitic-tonalitic gneiss
AGC
3621±6 (z)
S
zircon recrystallization or growth age
26
tonalite dyke
AGC
3618±21 (z)
S
protolith age
Bleeker & Stern 1997
27
tonalite gneiss
AGC
3611±11 (z)
S
metamorphic zircon overgrowth age
28
pegmatite
AGC
3589±30 (z)
LA
protolith age
29
granite gneiss
AGC
3586±26 (z)
LA
protolith age
30
granite gneiss
AGC
3585±70 (z)
LA
protolith age
31
foliated granite
AGC
3584±26 (z)
LA
protolith age
32
foliated granite
AGC
3546±60 (z)
LA
protolith age
33
dioritic banded gneiss
AGC
3509±19 (z)
S
metamorphism
34
tonalitic gneiss
AGC
3480±8 (z)
C
?
Bowring et al. 1989b
35
CSCG quartzite
western margin AGC
3410±6 (z)
S
detrital zircon model age (7 anal.)
36
CSCG quartzite
western margin AGC
3401±13 (z)
S
37
CSCG quartzite
western margin AGC
3393±9 (z)
S
38
dioritic banded gneiss
AGC
3382±8 (z)
S
leucosome development
39
CSCG quartzite
western margin AGC
3379±10 (z)
S
40
tonalite gneiss
AGC
3360 (z)
S
41
tonalite gneiss
AGC
3356±14 (z)
S
metamorphism/leucosome growth
Stern & Bleeker 1998
42
granite sheet
AGC
3348+43/-34 (z)
S
protolith age
43
CSCG quartzite
western margin AGC
2898±7 (z)
S
44
granite sheet
AGC
2877±3 (z)
S
protolith age
Bleeker & Stern 1997, unpubl.
45
CSCG quartzite
western margin AGC
2875±4 (z)
S
46
CSCG quartzite
western margin AGC
2823±11 (z)
S
detrital zircon age (1 anal.); max. deposition age
47
AGC
1967±93 (z)
S
Pb loss from zircon
Sano et al. 1999
48
AGC
1936±28 (a)
S
apatite growth or cooling age
Sano et al. 1999
49
tonalite gneiss
AGC
ca. 1700 (z)
S
Pb loss from zircon
50
tonalite gneiss
AGC
ca. 1000 (z)
S
Pb loss from zircon
10
MINERAL POTENTIAL OF THE ACASTA GNEISS SITE
Following NTGO’s Non-renewable Resource Assessment methodology, mineral potential is
assigned one of seven rankings (Rank A through G), corresponding to very high through very
low probability of undiscovered deposits, respectively. An eighth ranking (Rank H) corresponds
to mineral potential that is unknown or beyond the scope of the assessment. Each mineral
potential ranking is also assigned a confidence ranking (Rank 1 through 4) which represents the
level of confidence of the assessment by linking it to the amount and quality of reliable
information upon which the potential ranking was based. An NRA value of A1 represents the
highest probability that undiscovered mineral deposits exist in the study area and has the highest
level of certainty based on abundant, reliable information. A value of F3 indicates that while the
probably of undiscovered deposits is considered low, the assessment is based on limited
information, and therefore may be incorrect. It is unlikely that further work would change the
ranking in the former example, whereas additional data would increase the confidence ranking,
and possibly change the potential ranking, for the latter example.
Little is known about the metallic and non-metallic mineral potential of the Acasta gneisses as
research has mainly focused on petrology and isotopic data collection, and little exploration work
has been reported for this area. In this section, we compare the general geologic setting of the
Acasta gneisses with the settings of mineral deposit types commonly associated with Archean
terranes, focusing primarily on deposit types found in the Slave Province. The mineral deposit
types examined are:
•
volcanogenic massive sulphide (VMS);
•
vein-hosted lode gold;
•
banded iron formation-hosted gold (BIF gold);
•
magmatic sulphides (nickel-copper-platinum group elements); and
•
kimberlite-hosted diamonds.
In addition, because the Acasta gneisses are highly unique and therefore possesses an intrinsic
value both in raw and processed forms, we also discuss its potential as a quarrystone for (mainly)
lapidary purposes.
Descriptions of the mineral potential and confidence rankings for each of the deposit types above
are presented below and summarized in Table 2.
11
Table 2. Summary of Mineral Potential Rankings.
CONFIDENCE RANKING
POTENTIAL
RANKING
Rank 1 (abundant
reliable information)
Rank 2 (moderate
amount of information)
Rank 3 (some
information)
Rank 4 (little and/or
unreliable
information
Rank A: Very High
Quarrystone for lapidary
purposes
Rank B: High
Rank C: Moderate
to High
Rank D: Moderate
Diamonds
Rank E: Low to
Moderate
Magmatic sulphides
Rank F: Low
Rank G: Very Low
Vein-hosted gold
VMS, BIF gold
Rank H: Not
Assessed
Description of Potential Rankings:
Rank A: Geologic environment is favourable; significant deposits/accumulations are known; presence of
undiscovered deposits/accumulations is very likely
Rank B: Geologic environment is favourable; occurrences are present but significant deposits/accumulations may
not be known; presence of undiscovered deposits/accumulations is likely
Rank C: Intermediate between moderate (Rank D) and high (Rank B) potential
Rank D: Geological environment is favourable; occurrences may or may not be known; presence of undiscovered
deposits/accumulations is possible
Rank E: Intermediate between low (Rank F) and moderate (Rank D) potential
Rank F: Some of the aspects of the geologic environment may be favourable but are limited in extent; few if any
occurrences are known; low probability that undiscovered deposits/accumulations are present
Rank G: Geologic environment is unfavourable; no occurrences are known; very low probability that undiscovered
deposits/accumulations are present
Rank H: Deposit types unknown, overlooked, beyond the scope of the assessment, or not worth mentioning at the
time the assessment was done
12
Volcanogenic Massive Sulphide
Volcanogenic Massive Sulphide (VMS) base metal deposits comprise massive to disseminated
sulphides hosted within subaqueous to shallow marine volcanic sequences, and include a number
of world-class deposits of Archean age. Examples include the Kidd Creek Deposit in the Abitibi
Greenstone Belt of Ontario, and Hackett River in the Slave Province, Nunavut. Models for the
formation of this deposit type propose a sub-volcanic intrusion as the thermal driver for fluid
circulation (Franklin, 1996; Hannington et al., 1999), and require interaction between magmatic
fluids and sea water to dissolve, transport and precipitate metal-bearing sulphides in the
overlying volcanic pile (e.g. Whalen et al., 2004).
No volcanic rocks have been reported from the Acasta gneiss site. Further, there is no evidence
to suggest that any phases of the gneiss represent the plutonic root of an eroded Mesoarchean
volcanic carapace. Even if such evidence existed, in the absence of the overlying volcanic
section, the potential for VMS-type mineralization is ranked as very low. This ranking is
assigned a high level of confidence.
Vein-hosted Lode Gold
A number of settings for vein-hosted lode gold deposits have been documented in the Slave
Province, including shear zone-hosted quartz-carbonate veins in mafic volcanic rocks, saddle
reef-type sediment-hosted quartz veins, and quartz veins in sheared granitoid rocks. All of these
fall into the larger orogenic gold deposit model. The first setting includes the Con and Giant
deposits near Yellowknife, and the Boston, Doris, and Madrid deposits in the Hope Bay
Greenstone Belt in the northeastern (Nunavut) part of the Slave craton. Con and Giant mines
collectively yielded over 13 million ounces of gold over 60+ years of production, contributing
over $4 billion to the NWT’s gross domestic product (Bullen and Robb, 2006). Statistics on
Giant Mine record nearly 30,000 person-years of direct and indirect employment between 1948
and 1998. The resource contained in the Hope Bay Belt is approximately 9 million ounces of
gold (http://www.miramarmining.com/s/HopeBayProject.asp), and the property is currently in
the early development stage.
Within the Slave Province, gold-bearing vein systems hosted in metasedimentary rocks include
the past-producing Discovery Mine and numerous showings in the Yellowknife basin near
Gordon Lake. Discovery Mine produced 1 million ounces of gold between 1949 and 1969
(Bullen and Robb, 2006), and recent exploration and feasibility work by Tyhee Development
Corporation has defined a further resource of over 1 million ounces measured and indicated
(combined Ormsby and Nicholas Lake deposits; http://www.tyhee.com/projects/resources.php).
A third setting for vein-hosted gold in the Slave Province is in sheared plutonic and metaigneous rocks such as the Arcadia Bay gold deposit within the Anialik River Igneous Complex in
the northwestern Slave Province (Abraham, 1989). Exploration reports (e.g. Jones et al., 1989)
13
suggest the bulk of the gold there is contained in northeast-striking quartz veins that are
associated with a similarly-oriented shear zone. Abraham et al. (1994) carried out geochemical
and isotopic studies of the shear zone and concluded that shearing and gold deposition occurred
at or before ca. 2656 Ma. Based on textural evidence, Relf (1995) speculated that the shear zone
may have seen some Proterozoic reactivation and late quartz vein emplacement. Relf et al.
(1999) presented evidence for fluid movement along the nearby Tokhokatak shear zone, which
may be linked to the Arcadia Bay shear zone (Abraham et al., 1994), as late as ca. 1.89 Ga. The
Arcadia Bay deposit, while never developed, underwent extensive exploration in the late 1980s,
and contains an estimated resource of 860,000 tonnes grading 7 g/tonne gold (Northern Miner,
Mar. 28, 1988).
Collectively, these three sub-types of vein-hosted gold (i.e., those occurring in volcanic,
sedimentary, and plutonic protoliths) make virtually all rocks of the Slave Province potential
exploration targets. At the Acasta gneiss site, no rocks of the Yellowknife Supergroup are
present, making the potential for volcanic- and sediment-hosted gold deposits nil. However, the
potential for pluton-hosted gold-bearing quartz veins is higher. Although the possibility of vein
gold hosted in meta-igneous rocks cannot be ruled out at the Acasta gneiss site, we also note that
no significant zones of shearing or quartz veins of significant size or continuity have been
reported by previous workers. However, very little gold prospecting has been undertaken in the
area in spite of the fact that mineral rights have been held for the area by prospectors since 1990.
Overall, this suggests that gold potential is low, although the ranking is based primarily on a lack
of geologic information rather than any data collected in the search for vein-hosted gold. The
confidence for this ranking is therefore moderate to low.
Banded Iron Formation Gold
Banded iron formation (BIF)-hosted gold deposits are an important mineral deposit type in the
Slave Province. Examples include the Lupin Mine, which produced over 3 million oz of gold
between 1982 and 2004, and the Damoti gold deposit, estimated to contain 34,200 ounces
measured and indicated (http://www.anacondamining.com/damotiprofile.html). In both deposits,
the gold occurs in layered, sulphide-rich, silicate-amphibole-chlorite BIF surrounded by
metaturbidites of the Yellowknife Supergroup. Gold occurs within the BIF associated with
sulphides and chlorite, and is also associated with quartz veins that cut the iron formation
(NORMIN Mineral Showings database).
No iron formation has been identified at the Acasta gneiss site, and although minor BIF is locally
associated with the Central Slave Basement Complex (BIF comprises part of the Central Slave
Cover Group), it is the younger BIF of the overlying Yellowknife Supergroup that is the target of
gold exploration in the Slave Province. The potential for BIF-hosted gold at the Acasta site is
therefore designated as very low, with high confidence.
14
Magmatic Sulphides
Magmatic sulphides hosted in differentiated mafic to ultramafic plutonic and volcanic rocks can
contain significant amounts of nickel, copper, and platinum group elements (Ni-Cu-PGE), and as
such are a valuable mineral deposit type. Two characteristics common to magmatic sulphide
deposits are: mafic to ultramafic host rocks, and; the segregation of ore minerals (pyrrhotite +
pentlandite + chalcopyrite ± pyrite ± magnetite) near the base of the host magmatic body.
Magmatic sulphide deposits include four main sub-types:
1. rift-associated mafic intrusions associated with flood basalts, such as the Noril’sk
deposits of Siberia (Naldrett, 1989) and the Duluth complex in the central United States
(Ripley, 1986).
2. komatiite-hosted deposits such as the Archean Abitibi belt of Quebec (Corfu, 1993) and
the Proterozoic Thompson Nickel Belt of Manitoba (Peredery, 1982).
3. mafic igneous complex-hosted deposits associated with the Sudbury meteorite impact
structure, Ontario (Naldrett et al., 1984).
4. tholeiite intrusion-hosted deposits such as at Voisey’s Bay, Labrador (Ryan, 1997).
Only two and perhaps three of these sub-types have the potential to occur in the Slave Province:
komatiite- and tholeiite-hosted deposits, ± impact-generated deposits. Komatiites are very rare
and only one example of mineralized komatiites has been reported to date in the province
(http://www.ggldiamond.ca/index.php?m=projectcat&cat=nickel). Impact-generated mafic
intrusive complexes have not yet been reported. In contrast, tholeiitic gabbros are fairly common
in the Slave Province, and include small plutons and sills associated with ca. 2.7 Ga tholeiitic
volcanic belts, and gabbro dykes and sills of Paleoproterozoic age. A number of gabbro-hosted
sulphide showings have been reported in rocks of both Archean (e.g. McConnell and Knutsen,
1967, Rebic and Woollett, 1985) and Paleoproterozoic (e.g. Nickerson, 1970) age.
At the Acasta gneiss site, mafic gneiss comprises one of the principal rock types. In spite of the
significant interest by researchers in the Acasta gneisses, few detailed description of this mafic
unit’s mineralogy, texture, or compositional range could be found in the published literature (see
Iizuka et al. (2007) for a recent description). On the Iizuka et al. (2006) map, the mafic gneiss
(termed ‘dioritic gneiss’) appears to be an early phase entrained as xenoliths and large screens
within the tonalite gneiss. The largest contiguous segment is approximately ~300 m wide and ~2
km long in map view. To our knowledge, it has never been sampled for assaying. While no
significant sulphide content has been reported in the field observations of previous workers, its
mineral potential nevertheless remains largely unknown. Based on limited knowledge of the
unit, we rank its potential to host metamorphosed magmatic sulphides as low-moderate. This
potential ranking has a low confidence ranking, which could be raised with further data
collection.
15
Kimberlite-hosted Diamonds
Limited information on kimberlite exploration in the Acasta gneiss area is publicly available.
Within the last two decades, a number of mineral claims have been held in the area surrounding
the gneisses, and exploration work on these claims included till sampling and airborne
geophysics. While the results reported in the assessment files do not reveal much about the
area’s potential to host kimberlite, the fact that the claims were dropped suggests that efforts to
find kimberlites in this area were unsuccessful.
Notwithstanding the above, kimberlite pipes and dykes can be difficult to discover and
commonly require the integration of multiple exploration tools. In the Slave craton, kimberlites
rarely occur in outcrop, and so can only be definitively identified through drilling. The main
techniques used to identify drill targets are geophysical (primarily magnetic and electromagnetic,
and more recently, gravity) surveys, and till sampling surveys. The former delineate anomalies
in the physical character of the rocks relative to the surrounding rocks, and can be very subtle.
The latter involves the separation and recovery of minerals associated with kimberlites from till
samples; these “picking” results are integrated with glacial transport direction data to identify
potential source areas of the indicator minerals. Even if a kimberlite body is discovered, unless it
has encountered, entrained, and preserved material from the lithospheric mantle from within the
diamond stability field, it will not contain diamonds. Data filed in exploration assessment
reports reveal the presence of some kimberlite indicator minerals from tills in the Acasta area
(e.g. Gordanier, 1995; Blusson, 1995; McCorquodate, 1995); however, limited exploration has
been carried out in this region, and no reports of exploration results could be found for the
Acasta gneiss site itself.
Based on publicly-available data, we cannot draw any conclusions about the potential for
kimberlite-hosted diamonds within the Acasta gneiss area of interest. A moderate ranking is
assigned based on the presence elsewhere in the Slave Province of economic kimberlite pipes.
The level of confidence of this assessment is low, and given the value of diamond deposits
relative to other commodities, some further investigation is warranted.
Quarrystone for Lapidary Purposes
While granitoid gneisses are not generally considered to have significant economic value,
gneisses from the Acasta site are unique in that they represent the oldest known intact continental
crust on Earth, and therefore they are of significant interest to the general public and the media.
The NWT Geoscience Office receives numerous requests each year for hand samples of the
gneiss from schools, museums, and individuals, and at least two museums have collected large
samples for public display. In 1989, a local prospector staked the mineral rights to the discovery
site (Humphries, 1992) and established marketing rights to sell pieces of the oldest rock in the
world. The mineral claim is still in good standing at this time, although the claim is owned by a
16
different individual who has investigated the lapidary potential of the gneiss, in particular its use
in funeral urns.
Despite these activities and ongoing public interest, we suggest that the economic value of the
Acasta gneisses for use as carving stone or lapidary products (e.g. clock faces, coasters, table
tops, etc.) remains largely untested. To our knowledge, most efforts to date have gone into
selling individual small rock samples (with limited success) and there has been no longer-term
testing, production, and marketing of value-added lapidary products that would appeal to a wider
range of buyers. Just how strong and widespread that appeal might be however is not fully
known. One potentially important factor is that finished product costs would likely be relatively
high given that the Acasta area is mainly accessed by expensive air transport. However, some
form of winter road access could considerably change this situation if it was determined to be a
viable option.
While we are not qualified to assess the tourist/art market, such lapidary businesses are
successful elsewhere in North America. Furthermore, our experience suggests that the novelty
of the gneisses has significant public appeal, just as samples from the Cretaceous-Tertiary
boundary (corresponding to the extinction of dinosaurs) do. Finally, although it is arguably
intangible, the value of public education is real.
Overall, we would assess the lapidary potential of the Acasta gneisses as high for a small
business venture. The remoteness of the Acasta area and other factors would likely limit a larger
operation from developing. The level of confidence of this assessment is moderate based on the
currently known and unknown factors described above.
Summary of Mineral Potential
A summary of the mineral potential and confidence rankings for the six deposit types discussed
above is presented in Table 2. The highest-ranked mineral deposit type is kimberlite-hosted
diamonds, and two other types (magmatic sulphides and diamonds) have low confidence
rankings. Further examination of the diamond and magmatic sulphide potential is proposed
among the recommendations near the end of this report. In terms of quarrystone, the Acasta
gneisses rank highly in terms of lapidary potential due to the fact that they can be uniquely
marketed as the oldest rocks on Earth. However we can only assign a moderate confidence in
this ranking at present because the overall market appeal of Acasta lapidary products has not
been thoroughly assessed.
SCIENTIFIC VALUE OF ACASTA GNEISS
The Acasta gneisses are considered here to represent a significant scientific resource. This
assessment is based on the following facts and observations:
17
•
early Earth materials (those formed within approximately the first 500 m.y. of Earth
history) are extremely rare due to the recycling and obscuring effects of younger tectonic,
plutonic, metamorphic, and erosional activity. Although the AGC has been affected by a
number of post-4.0 Ga geological events, the rocks nevertheless survive and represent a
valuable laboratory for studying early Earth materials, in particular mineral species such
as zircon that are robust enough to retain original isotopic and chemical information;
•
links between early Earth materials and the study of planetary and solar system evolution
are strong. Researchers have long asked the question of why the Earth evolved as a
unique planet within our solar system. Such questions can only be answered with any
certainty by studying what remains of the rock, mineral, and chemical/isotopic record of
the early Earth;
•
the preservation of the Acasta gneisses itself represents an interesting scientific question
that has not been addressed in detail. How did these rocks survive, what was their
original extent, and what is it that is unique about the Slave craton that has yielded this
preservation? These questions are fundamental in determining how continental crust
formed and was subsequently stabilized during the Paleoarchean, and what this tells us
about lithospheric growth and the activity or inactivity of plate tectonic processes during
this era;
•
elsewhere, researchers have expended considerable time and effort in studying early
Earth materials such as ancient detrital zircons (e.g., Jack Hills region of Western
Australia; Wilde et al., 2001). It is expected that with the recent publication of the 4.2 Ga
Acasta zircon age (Iizuka et al., 2006), this region could potentially experience a similar
intensive research effort. We are aware of at least one major university-based study
currently underway (Munster University, Germany), and a Japanese research group has
recently completed a mapping and sampling program in the area (Iizuka et al., 2006,
2007). NTGO is supporting the work of a post-doctoral fellow at Memorial University
who is studying the age of detrital zircons that are likely sourced from eroded Acasta
crust. This study targets sedimentary units of the adjacent Wopmay orogen, and as such
will not occur within the Acasta region itself; and
•
new analytical tools are continually being developed in the Earth Sciences. For example,
the advent of ion microprobe, and more recently laser ablation microsampling methods,
has advanced the science considerably in the past two decades, and has contributed
greatly to what we currently know about the Acasta gneisses. Future analytical tools
could potentially offer similar advancements of understanding. In other words, the Acasta
area may hold future scientific potential that we cannot presently comprehend. This is a
significant fact in considering the benefits of protecting the Acasta gneiss area in
perpetuity.
18
As noted previously, Acasta field-based research to date has been somewhat limited considering
the unique nature and antiquity of these rocks. However this can be attributed mainly to
logistically difficult and expensive access to the region, and also the presence of slightly younger
rocks elsewhere on Earth that are much less deformed and metamorphosed (e.g., Isua greenstone
belt, Greenland). Such areas tend to be studied extensively, after which interest dies off for a
period until new methods of examining the rocks are developed. It can perhaps be argued that the
Acasta region has yet to reach its full research potential due to remoteness and the relatively
strong deformation and metamorphism of the 4.0 Ga rocks. However, it is likely that this region,
host to the oldest-known rocks on the planet, will continue to stimulate the interest of both
researchers and the public for some time to come, particularly if the search for old crust on Earth
does not yield anything older.
RECOMMENDATIONS FOR PHASE 2 NRA ACTIVITIES AND DELIVERABLES
This Phase 1 NRA report examines the value of the Acasta gneisses from two main perspectives:
mineral potential and scientific value. A summary of these values and recommended additional
activities are described below. Due to their highly unique nature, we also briefly discuss the
value of the Acasta gneisses as a potential tourist attraction. A proper assessment of this
potential would more fittingly be included in a socio-economic study.
Based on the geology of the site, the potential for significant mineral resources is deemed to be
low. The highest-ranked mineral deposit types are kimberlite-hosted diamonds (ranked as
moderate), and magmatic-hosted sulphides (ranked as low-to-moderate). Both rankings are
associated with high uncertainty, and it is recommended that further work be undertaken to
increase the level of confidence of these rankings and refine the mineral potential rankings.
Similarly, the high potential of Acasta gneiss as a lapidary stone should be further investigated.
However, much of the required assessment (product development, market assessment,
advertising strategies, etc.) would fall outside the scope of a non-renewable resource assessment.
In contrast to relatively low mineral potential (albeit with significant uncertainty), the scientific
value of the site is considered to be high, particularly given recent advances in the field of
isotopic analysis. While a significant body of knowledge has already been published on the
Acasta gneisses, there are some analytical techniques (e.g., in situ laser-ablation multi-collector
ICP-MS) and isotopic systems (e.g., Hf) that could be applied, or applied more extensively, to
learn more about the timing and nature of the formation of this complex segment of continental
crust.
It should be noted that the tourist value of the site is potentially significant due to its geological
novelty and other natural attributes. The site is located amongst rolling hills at the tree line along
a scenic river that offers a number of attractive beach camp sites. Given its isolation, it is
unlikely that large numbers of people would visit the area. However, those that do would mainly
19
be drawn by the uniqueness and significance of the bedrock, which is well exposed in a number
of easily-accessed locations. If the site were given some public profile, it is likely that a local
business could sell at least a limited number of educational/sightseeing/rock collecting trips to
more wealthy tourists. An assessment report filed by the prospector who originally staked the
discovery site concludes “the economic potential … is in the eye of the entrepreneur and the rock
collectors of the world” (Humphries, 1992). We would suggest that the potential remains largely
untested, but is very real.
Proposed Phase 2 NRA activities include work to increase the confidence level of the mineral
potential rankings for diamonds and magmatic sulphides, and to further evaluate the scientific
value of the rocks. The marketing potential of the rocks is already established, and as mineral
rights at the discovery site are owned by a private investor, further proof of their value lies
outside the scope of a Phase 2 resource assessment.
Regarding mineral potential, the following activities are proposed:
• a modest till sampling program of 20-30 samples ‘down-ice’ from the Acasta area (i.e.,
immediately west of the area shown in Figure 2);
•
detailed mapping and sampling of mafic gneisses to determine the extent of primary
differentiation within these bodies;
•
sampling of the mafic gneisses for assay analysis.
Further evaluate the scientific value of the gneisses by:
• generating a detailed (1:5,000) geology map of the discovery site and surrounding area
that could serve both as an inventory of lithologies for the purposes of scientific
sampling, and as a general guide to the various rock types that comprise the Acasta suite
of gneisses;
•
collecting and examining samples of various phases of the gneiss for the purpose of
undertaking analyses of both whole-rock samples and selected minerals (e.g. zircon,
monazite, titanite, etc.). Selection of samples and definition of analytical work will
depend on results of field mapping;
•
identifying an area to be proposed for the final footprint of the protected area, based on
outcrop quality, quantity, and accessibility;
•
identifying a type section of gneisses and/or a field trip for the purpose of public outreach
and education;
•
identifying a location from which samples can be collected for tourists without removing
material of high scientific value or degrading particularly illustrative or scenic outcrops.
20
ACKNOWLEDGEMENTS
We wish to thank Luke Ootes (NWT Geoscience Office) and an anonymous external reviewer
for timely and informative reviews of this manuscript. Karen MacFarlane and Scott Cairns are
also acknowledged for their efforts in editing and publishing this report.
21
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