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Musselwhite |
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Ontario, Canada |
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Au
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Super Porphyry Cu and Au
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The Musselwhite gold deposit is located ~500 km north of Thunder Bay, Ontario, Canada, and occurs as two main ore zones below Opapimiskan Lake (#Location: 52° 36' 47"N, 90° 21' 58"W).
The Musselwhite deposit is hosted in polydeformed, Mesoarchean amphibolite facies banded iron formation (BIF) of the North Caribou greenstone belt, in the northwestern Superior Province. The deposit comprises narrow, sub-vertical, ore zones that are the result of the intersection of second-order, Au-bearing structures and strongly reactive, silicate-rich BIF horizons (Oswald et al., 2015).
The North Caribou greenstone belt is composed of 3.05 to 2.87 Ga meta-volcanic dominated lithostratigraphic assemblages and meta-sedimentary dominated suites that are younger than 2.89 Ga. It is surrounded by the tonalite-trondhjemite-granodiorite (TTG) type batholiths of the 2.87 to 2.85 Ga North Caribou pluton and the 2.86 to 2.84 Ga Schade Lake gneissic complex, both comprising several intrusive phases, as well as the smaller, ~2.73 to 2.72 Ga composite Southern batholith (Biczok et al., 2012; Kalbfleisch 2012).
Three major phases of deformation have been differentiated in the belt (Piroshco and Shields 1985; Breaks et al. 2001). The structural pattern is dominated by NW-trending D2 structures with a strong SW-NE strain gradient, whilst the regional metamorphism ranges from mid-greenschist to mid-amphibolite facies (Breaks et al., 1985; Otto 2002) that was imposed during the late stages of D2 (Hall and Rigg 1986).
The encompassing succession at the Musselwhite deposit comprises the South Rim volcanic assemblage and structurally underlying Opapimiskan-Markop assemblages, which, in turn, is structurally underlain by the Zeemel-Heaton succession.
The Opapimiskan-Markop assemblage comprises two main BIF sequences (Moran 2008), the Northern and Southern iron formations, both of which are intercalated with volcanic rocks that progressively range, from structural bottom to top, from i). tholeiitic, komatiitic basalts and ultramafic volcanic rocks, to ii). tholeiitic, mafic volcanic and subvolcanic rocks, to iii). calc-alkaline, felsic to intermediate volcanic rocks (Oswald et al., 2015). The Northern iron formation hosts the bulk of the economic Au mineralisation (Oswald et al., 2015). The Zeemel-Heaton succession includes polymictic conglomerates, other clastic sediments and felsic volcanic rocks. The entire package is folded by a NW-trending, F2 synform-antiform pair, the East Bay Synform and West Anticline
Geochronological, geological and structural interpretations suggest a D1 deformation produced a previously unrecognised, kilometre-scale syncline which has an axis to the south of the deposit, and that this fold was associated with a major thrust fault, whilst polymictic conglomerates mapped along this boundary implies it may represent a reactivated unconformity.
The felsic rocks at the core of the F2 East Bay Synform in the mine area have been dated at ~2978 Ma (U-Pb dating), whilst underlying garnet-boiotite schist of the Northern iron formation has been shown to be younger than 2967 Ma, taken to imply the mine sequence is inverted (McNicoll et al., 2013). The Zeemel-Heaton meta-sedimentary rocks to the east of the deposit, have also dated as younger than ~2850 Ma. This suggests a 60 to 110 Ma time gap between the Opapimiskan-Markop and Zeemel-Heaton assemblages (Oswald et al., 2014; McNicoll et al., 2016). The D2 West Anticline, which also contains an auriferous zone, is located ~1 km west of the main Musselwhite deposit (Hall and Rigg 1986).
The bulk of the ore in the Musselwhite deposit is hosted by silicate-rich BIF, occurring as stratabound pyrrhotite replacements with associated silica flooding, and local discordant quartz-pyrrhotite veins. The individual ore zones are concentrated in shallowly NW plunging F2 fold hinges, and are commonly associated with axial planar high-strain zones with strong lateral deformation gradients, as well as along strongly attenuated fold limbs (Oswald et al., 2015; after Biczok et al., 2012).
Oswald et al. (2015) describe high grade Au ore as typically having mm- to cm-scale metamorphic and/or metasomatic layering composed of abundant coarse-grained, red, almandine garnet porphyroblasts, intergrown with fine to medium grained, bladed grunerite, green hornblende/ferro-tschermakite, red brown biotite, iron carbonates and locally hedenbergite. Abundant transposed pyrrhotite veinlets and disseminations that fill fractures in the garnet porphyroblasts, as well as silica-flooding are also described as typical of ore zones.
There is a contrast in intensity of pyrrhotite replacement and/or quartz flooding between the preferred garnet-grunerite facies host and the essentially barren chert-magnetite facies of the iron formation, taken to illustrate a lithological control on sulphide precipitation and associated Au deposition (Oswald et al., 2015). In addition, the Northern and Southern iron formations have contrasting gold endowments, with the latter being significantly thinner and lacking a reactive silicate facies.
The thickness of the BIF sequence and the anisotropic layering induced by its presence within the enclosing mafic and ultramafic volcanic rocks has influenced the rheological response to deformation on all scales, and hence is regarded as having played a significant role in fluid flow, gold deposition and distribution (Oswald et al., 2015).
Cross-cutting relationships show gold mineralisation is syn- to late-D2 deformation and broadly synchronous with metamorphism. The timing of mineralisation has been constrained by 2690 ±9 Ma Sm-Nd age dating of garnet interpreted to be hydrothermal in origin (Biczok et al., 2012). A U-Pb age of ~2666 Ma (Oswald et al., 2015) on late-M2 monazite gives a minimum age constraint for the regional D2 metamorphic/deformation event to which most of the Au mineralisation at Musselwhite is associated. D3 deformation produced minor moderately SW plunging Z-folds, local kink bands, and conjugate set of S3 crenulation cleavages overprinting the composite S1- 2. (Oswald et al., 2014).
Ore Reserves and Mineral Resources as at 31 December, 2022 were (Newmont Reserves and Resources report, 2023):
Ore Reserves
Proved Reserves - 3.400 Mt @ 5.48 g/t Au,
Probable Reserves - 7.000 Mt @ 5.89 g/t Au,
Proved + Probable Reserves - 10.400 Mt @ 5.76 g/t Au for 59.8 tonnes of gold.
Mineral Resources
Measured Resource - 1.300 Mt @ 3.92 g/t Au,
Indicated Resources - 2.600 Mt @ 3.93 g/t Au,
Measured + Indicated Resources - 3.900 Mt @ 3.93 g/t Au for 15.24 tonnes of gold.
Inferred Resources - 3.000 Mt @ 4.15 g/t Au for 12.5 tonnes of gold.
NOTE: Resources are additional to reserves.
The most recent source geological information used to prepare this decription was dated: 2015.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Musselwhite
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Selected References:
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Oswald, W., Malo, M., Castonguay, S., Dube, B., Mercier-Langevin, P., McNicoll, V. and Biczok, J., 2015 - Geological Setting of the World-Class Musselwhite BIF Hosted Gold Deposit, Ontario, Canada: in Andre-Mayer, A.S., Cathelineau, M., Muchez, Ph., Pirard, E., Sindern, S., (Eds), 2015 Mineral Resources in a sustainable world - Iron ores including IOCG Session, 13th Biennial SGA Meeting, 24-27 August 2015, Nancy, France, Proceedings Volume 3, pp. 1115-1118.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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