geology of the meadowbank iron formation-hosted gold...
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GEOLOGY of the MEADOWBANKIRON FORMATION-HOSTED GOLD DEPOSITS
GEOLOGY of the MEADOWBANKIRON FORMATION-HOSTED GOLD DEPOSITS
GEOLOGY of the MEADOWBANKIRON FORMATION-HOSTED GOLD DEPOSITS
1 2 2 2 3Ross Sherlock , Brian Alexander , Roger March , Jeff Kellner , Bill Barclay1. Canada-Nunavut Geoscience Office, Iqaluit, Nunavut; 2. Cumberland Resources Ltd., Vancouver B.C.; 3. W.A. Barclay Exploration Services Ltd. Toronto Ont.
The geology of northwest Hudson Bay consists of Archean and Proterozoic supracrustal sequences and plutonic rocks. The Archean supracrustal sequences (Woodburn Lake and Prince Albert groups) are intruded by granites which are inturn overlain by pre-Hudsonian basins (Amer and Hurwitz grous). Post-Hudsonian basins (Baker Lake and Thelon basins) unconformably overly both sequences. The Woodburn Lake group is Archean in age but has been overprinted by the Paleoproterozoic Husonian Orogeny (Henderson et al., 1991). The extent of the proterozoic reworking and the implications for gold mineralization in the district is uncertain.
Daly BayComplex
MGb
Rg
Kgb
WLg
KRg
PAg
PAg
Pg
Hg
Hg
Ag
TH
BL
r S.Z.emA
Wager S.Z.
enoZ cinotceT dribwonS
Pyke Fault
Cha
nter
y S.Z
.
62° 62°
68°
92°
CommitteeBay
Hudson Bay
Post-Hudsonian Proterozoic Basins
Pre-Hudsonian Proterozoic Basins
BL - Baker LakeTH - Thelon
Pg - Penrhyn GpAg - Amer GpHg - Hurwitz Gp
Archean Supracrustal RocksPAg - Prince Albert GpWLg - Woodburn GpKRg - Ketyet River GpMGb - MacQuoid-Gibson beltRg - Rankin Inlet GpKgb - Kaminak greenstone belt
Rankin Inlet
Baker Lake
Arctic Circle
Meliadine Area
Meadowbank Area
unconformity
unconformity
0 100 200 km
39
60
33
28
Banded iron formation, magnetite-quartz-amphibole ± pyrrhoitite, pyrite and gold
Ultramafic assemblage, talc-amphibolite, locally spinifex textured
Quartzite, quartz-pebble conglomerate
Intermediate volcaniclastic rocks, dacitic-rhyodacitic in composition,locally showing well defined bedforms and blue quartz phenocrysts
Intermediate-felsic volcanic rock, dacite-rhyodacite in composition, massive unitlikely flow or shallow intrusions, locally abundant blue quartz phenocrysts
Massive plagioclase-amphibolite intrusive, gabbroic in composition
Granite, dated at 2612 Ma Apparent dip of S /S composite fabric, from drill sections1 0
S /S composite fabric, dipping & verticle1 0
F /F fold axis, folding S , and possible earlier foliations1 2 0
S fabric, crenulating and deforming ealier S /S fabrics4 1 0
dipping & verticle, axial planar to F4
F fold axis, that deform earlier S /S composite fabrics4 1 0
45
640000m. E.639000m. E.638000m. E.
7212000m. N.
7213000m. N.
7214000m. N.
7215000m. N.
7212000m. N.
7213000m. N.
7214000m. N.
7215000m. N.
7216000m. N.7216000m. N.
638000m. E. 640000m. E.639000m. E.
18
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46
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48
39
44
30
12
18
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3830
20
32
35
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35
12
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60
58
55
25
52
22
5820
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38 45
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6048
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38 80
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70
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5
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70
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70
570
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55
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65
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20
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4540
70
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30
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2555
75
45
40
4860
35 88
75
18
10
33
4755
4530
3030
3878
60
80
50
50
6042
30 20
2570
2245
22
2575
2520
30
3540
6035
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50
28
3040
4550
30
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35
50
35
45
20
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55 45
15
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6525
35
45
45
75
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70
45
30
30
45
Third P
ort
age &
Bay
Zone D
eposi
ts
Nort
h P
ort
age D
eposi
t
Goose
Isl
and D
eposi
t
North Portage Section
Third Portage Section
Goose Island Section
scale 1:10,000
Scale 1:2,500
Goose Island
Scale 1:2,500
Third Portage
Scale 1:2,500
North Portage
Total
Class Tonnes Grade (g/t) OuncesMeasured 1,191,000 6.58 252,100Indicated 6,584,000 5.65 1,196,200Inferred 3,467,000 5.60 623,800
Total 11,242,000 5.73 2,072,100
Third Portage
Class Tonnes Grade (g/t) OuncesMeasured 1,031,000 6.40 212,000Indicated 4,914,000 5.31 839,700Inferred 1,427,000 5.66 2,59,700
Total 7,372,000 5.53 1,311,400
Goose Island
Class Tonnes Grade (g/t) OuncesMeasured 75,000 10.63 25,700Indicated 636,000 9.80 200,400Inferred 456,000 9.50 139,400
Total 1,167,000 9.74 365,500
Bay Zone
Class Tonnes Grade (g/t) OuncesMeasured 51,000 5.38 8,800Indicated 394,000 4.67 49,100Inferred 239,000 4.64 35,500
Total 684,000 4.71 103,400
North Portage
Class Tonnes Grade (g/t) OuncesMeasured 34,000 5.10 5,600Indicated 640,000 4.71 97,000Inferred 1,345,000 4.38 189,200
Total 2,019,000 4.49 291,800
Open Pit Mining Reserve
Tonnes Grade (g/t) Ounces*P/P reserve 5,502,000 5.44 962,400
Resource-Reserve TableCumberland Resource Ltd. press releases
March 29, 2000; April 27, 2000
All deposits except Goose Island are reported at a 2.5 g/t gold cutoff. Goose Island is reported at a 6 g/t gold cutoff. Bay Zone and Third Potage deposits assays were capped at 45 g/t gold; Goose Island assays were capped at 35 g/t gold; no assay capping was applied to North Portage. Resource and reserve classifications are in accordance with the Canadian Institute of Mining, Metallurgy and Petroleum guidelines.
2612 ± 4 Ma2612 ± 4 Ma
00'65° 00'65°
05'96°
05'96°
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Meadowbank Area
Iron Formation, banded quartzmagnetite-amphibole, local pyrrhotitepyrite
Quartzite, with local quartz pebbleconglomerate
Gabbroic intrusions
Felsic Intrusions 2600-2620 Ma
Ultramafic schists, characterized bytalc-tremolite-serpentinite, probablekomatiitic basalt protolith
Volcanic wacke, slates and phyllites,minor ferruginous sediments
Felsic-intermediate volcanic sedimentsminor massive felsic flows/intrusives, minortalc schists and ferruginous sediments,interbedded with iron formation2710 +3.5/-2.1 Ma
0 0.5 1.0 km
Simplified geological map of the Woodburn Lake group in the Meadowbank area (Zaleski et al., 1997a; 1999a).
Armitage, A.E., James, R.S., and Goff, S.P. 1996: Gold mineralization in Archean banded iron formation, Third Portage Lake area, Northwest Territories, Canada. Exploration and Mining Geology, v. 5, p.1-15.
Henderson, J.R., and Henderson, M.N. 1994: Geology of the Whitehills-Tehek Lakes area, District of Keewatin, Northwest Territories (parts of 56D, 56E, 66A and 66H); Geological Survey of Canada, Open File 2923, scale 1:100 000.
Kerswill, J.A., Goff, S.P., Wilkinson, L., Jenner, G.A., Kjarsgaard, B.A., Bretzlaff, R., and Samaras, C. 1998: An update on the metallogeny of the Woodburn Lake group, western Churchill Province, Northwest Territories; in Current Research 1998-C, Geological Survey of Canada, p. 29-41.
Zaleski, E., Corrigan, D., Kjarsgaard, B.A., Kerwill, J.A., Jenner, G.A., and Henderson, J.R. 1997b: Preliminary results of mapping and structural interpretation from the Woodburn project, western Churchill Province, Northwest Territories; in Current Research 1997-C; Geological Survey of Canada, p. 91-100.
Zaleski, E., Duke, N.L., L'Heureux, R., and Wilkinson, L. 1999a: Geology, Woodburn Lake group, Amarulik Lake to Tehek Lake, Kivalliq Region, Nunavut; Geological Survey of Canada Open File 3743, scale 1:50 000.
Zaleski, E., L'Heureux, R., Duke, N., Wilkinson, L., and Davis, W.J. 1999: Komatiitic and felsic volcanic rocks overlain by quartzite, Woodburn Lake group, Meadowbank River Area, Western Churchill Province, Northwest Territories (Nunavut); in Current Research 1999-C; Geological Survey of Canada.
Kerswill, J.A., Goff, S.P., Kjarsgaard, B.A., Jenner, G.A. and Wilkinson, L. 2000: Highlights of recent metallogenic investigations in western Churchill Province, Nunavut, Canada: implications for mineral exploration in Archean greenstone belts; [extended abstract] in GeoCanada 2000- The Millennium Geoscience Summit CD-ROM, Calgary, Abstract 736.
Pehrsson, S., Wilkinson, L., Zaleski, E., Kerswill, J., and Alexander, B. 2000: Structural geometry of the Meadowbank deposit area, Woodburn Lake group implications for a major gold deposit in the Western Churchill Province; [extended abstract] in GeoCanada 2000, Calgary Alberta.
Zaleski, E., Corrigan, D., Kjarsgaard, B.A., Kerswill, J.A., Jenner, G.A. and Henderson, J.R. 1997a: Geology, Woodburn Lake group, Meadowbank River to Tehek lake (66H/1, 56E/4), District of Keewatin (Nunavut), Northwest Territories; Geological Survey of Canada, Open File 3461, scale 1:50 000.
NEqual Area(Schmidt)
Axial N = 99
Poles to S1/Socomposite fabric
NEqual Area(Schmidt)
Axial N = 19
F1 fold axes
A B
C
D
Stereonet plot of poles to the composite S /S fabric (A) and measured F and F axes (B). For both plots 0 1 1 2
circles represent data from the North Portage area and diamonds are from the Third Portage area. (C) Photograph of F folds and axial planar cleavage from Trench D at the Third Portage deposit. (D) 2
Photograph of F fold in drill core with a strong axial planar S cleavage from the Third Portage deposit 2 2
(sample TP98 291, 38.5 m).
NEqual Area(Schmidt)
Axial N = 14
Poles to S4 fabric
NEqual Area(Schmidt)
Axial N = 52
F4 fold axes
A B
CStereonet plot of poles to S fabric (A) 4
and measured F axes (B). For both plots 4
circles represent data from the North Portage deposit and diamonds from the Third Portage deposit. (C) Photograph of F folds from the North Portage area.4
The stratigraphy, that hosts the deposits, consists of felsic to intermediate volcaniclastic rocks and interbedded iron formation. The stratigraphic sequence is polydeformed with four phases of deformation recognized regionally; two of which have substantially modified the geometry of the strata. The initial phase of deformation was a progressive transposition of the stratigraphy with tight to isoclinal folding which culminated in low angle high strain zones. This progressive deformation event appears to have controlled the distribution of gold mineralization. Overprinting, NE-SW shallow plunging folds further modified the geometry of the mineralized body.
Structural Geology
Sulphide bearing iron formation. Sulphides are dominately pyrrhotite and lesser pyrite. Arsenopyrite is rare to absent Auriferous narrow blue quartz veins, in D /D fabric and cut by D , hosted by felsic 1 2 4
volcaniclastic rocks Coarse grained ampbiboles in iron formation, often where mineralized these are overprinted and deformed by strong D fabrics2
The distribution of gold in the Third Portage deposit is well defined by closely spaced drilling (~25 m) and surface trenching, and is similar in style and geologic setting to the adjacent North Portage, Bay Zone and Goose Island deposits. Mineralization, at the Third Portage deposit, is best developed at the lithologic contact between an ultramafic body and the volcanosedimentary package. The ultramafic body may represent the core to an east verging antiform or alternatively may be a boudin within the transposed/flattened stratigraphy. Mineralization is almost entirely restricted to the structural footwall or the hinge area of the ultramafic body; occurring only sporadically and weakly in the structural hanging wall of the ultramafic unit. The hanging wall is low-strain with planar lithologic contacts and S /S fabrics, whereas the footwall is generally higher strain with contorted F folds and strong S 0 1 2 2
fabrics.
Gold is contained within sulphide-bearing iron formation (Armitage et al., 1996; Kerswill et al., 1998), characterized by disseminated to semimassive pyrrhotite-pyrite which preferentially replaces magnetite (Armitage et al., 1996). Mineralization is also developed in the felsic-intermediate volcanic package as disseminated pyrrhotite and pyrite along with narrow grey quartz veins also containing pyrrhotite and pyrite.
Sulphides show a variety of relationships with deformation fabrics. Pyrrhotite, pyrite and sulphide-bearing quartz veins can be aligned along S /S fabrics as well as within S foliations overprinting S /S fabrics (Kerswill et al., 0 1 2 0 1
2000; Pehrsson et al. 2000). Spatially, at the Third Portage deposit, the bulk of the mineralization is concentrated at the structural footwall or hinge area of ultramafic units within, and adjacent to, high strain zones defined by penetrative S fabrics. Suggesting either different generations of sulphides or that the sulphides are related to the progressive D -D deformation event and concentrated in areas of higher strain.2 1 2
Mineralization
The degree of transposition of the host stratigraphy makes it difficult to trace individual units within the felsic volcaniclastic and iron formation package. Even in the Third Portage area, where trenching and close spaced diamond drilling provides exceptional data density, the surface trace of the individual units is uncertain. The gold mineralization however is very continuous along strike and down dip. Mineralized envelopes, of greater than 1.0 g/t Au show good continuity and can be traced for 100's of meters. It is this feature of the mineralization which has enabled Cumberland to produce a proven-probable mining reserve. Mineralization occurs in iron formation and volcaniclastic rocks and is more continuous than any individual stratigraphic interval.
Armitage et al. (1996), based on whole rock and mineral chemistry, has shown that auriferous iron formations are characterized by a mineral assemblage of cummingtonite and biotite with pyrrhotite-pyrite-magnetite and quartz. Barren, but sulphidic, iron formations are characterized by an assemblage of grunerite ± hornblende + stilpnomelane with pyrrhotite, pyrite, quartz and magnetite. Whereas barren, non sulfide-bearing, iron formations are characterized by quartz and magnetite. Based on these mineral assemblages and their relationship to gold mineralization, Armitage et al. (1996) have suggested that mineralization is related to metasomatic fluids that have altered the iron formation with the introduction of Mg-K-Ca-S-As-Cu and Au during D -D deformation.1 2
Although speculative; it is possible that metasomatic fluids, introduced into dilational sites during D -D deformation, altered the iron formation and locally the volcaniclastic rocks. The mechanical 1 2
contrast between the ultramafic and volcaniclastic rocks partitioned strain into the volcaniclastics and iron formation, which preferentially localized dilational settings. This allowed fluid influx and the resulting alteration assemblages, described by Armitage et al. (1996), to be concentrated in the higher strain corridor adjacent to the ultramafic rocks. The progressive nature of D -D formed the 1 2
various relationships of sulphide minerals and deformation fabrics.
Summary
Geochemistry
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
0.0 50.0 100.0 150.0 200.0
Y (
ppm
)
Zr (ppm)
Zr/Y
= 5
Zr/Y = 10
Zr/Y ratios for the felsic volcanic rocks range between 7 and 12, suggesting acalc-alkaline geochemical affinity
<d/l
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
<d/l 0.200 0.400 0.600 0.800
Series1
Series2
Iron Formation
Intermediate Volcanics
AlO
(%
)2
3
TiO (%)2
Immobile element ratios for the iron formations, form a straight line, consistent withthe ratios from the felsic volcanic rocks. This suggests that a detrital felsic volcaniccomponent was added to the iron formations during deposition.
sedi
men
tary
inpu
t of f
elsic
volcan
ic d
etrit
us
into
iron
form
atio
n
.01 .1 1 10.001
.01
.1
1
Zr/TiO2
Nb/Y
SubAlkaline Basalt
Andesite/Basalt
Andesite
Rhyodacite/Dacite
Rhyolite
Alk-Bas
TrachyAnd
Trace element data for the flesic volcanic rocks, showing that they are rhyodacitic to daciticin composition