Summary:
The mineralization at Musselwhite Mine can be classified as an Iron Formation-hosted gold deposit. Typically, gold in these deposits occurs in cross-cutting quartz veins and veinlets or as fine disseminations associated with pyrite, pyrrhotite, and arsenopyrite hosted in iron formations and adjacent rocks within volcanic or sedimentary sequences (McMillan, 1996).
The Musselwhite Mine deposit exhibits many features common with this deposit type: Gold occurring as free (native) gold in quartz veining, sulfides and metamorphic minerals; Quartz veining (but not predominantly cross cutting); Predominantly stratabound mineralization, and; Gold mineralization associated with shear zones.
Mineralization
Mineralization at Musselwhite is predominantly found in sub-vertical high strain zones in the favourable silicate facies of the Northern Iron Formation, and to a lesser extent the oxide facies in both the Northern and Southern Iron Formation. Significant mineralization is also locally hosted in mafic volcanics and garnet-biotite schists in the West Limb deposits. In addition to the main hosts of mineralization, anomalous gold concentrations occur across the property and within all the major lithologies. A positive correlation exists between gold and pyrrhotite mineralization in the Northern Iron Formation silicate facies. In general terms, this translates to 1 g/t Au for each percentage increase in pyrrhotite, up to approximately 15% pyrrhotite. This correlation between gold and pyrrhotite does not apply to mineralization in the Southern Iron Formation or the West Limb.
Gold mineralization is interpreted to have formed as the result of sulfide replacement of iron formation with quartz-pyrrhotite flooding and veining. Mineralization is best developed where structural permeability has been increased, either by folding, brittle or ductile deformation or in combination. Mineralization is thought to have been emplaced during D2 deformation and peak metamorphism (Oswald, 2018).
Quartz-pyrrhotite veins/flooding are composed of massive, glassy blue to grey quartz with up to 20% fine to medium-grained pyrrhotite locally and occur as anastomosing networks of multiple veinlets that pinch and swell along strike as well as up and down dip. Accessory minerals include albite, almandine garnet and calcite, minor arsenopyrite, pyrite, chalcopyrite, and native gold. Sulphide mineralization in the veins is strongly structurally controlled, occurring within small-scale boudins, along the margins of the veins and as fine stringers within the vein itself. Sulphide replacement style mineralization is characterized by 2% to 15% fine-grained disseminated pyrrhotite, trace to 2% arsenopyrite, and trace to 2% pyrite. Gangue minerals consist of almandine garnet, quartz and or chert, grunerite, actinolite, biotite, magnetite, and calcite with accessory epidote and zircon.
Visible native gold, usually the size of a pin tip, is commonly observed as isolated specks within quartz. The majority of the gold occurs within pyrrhotite micro-fractures within garnet-rich silicate domains.
As with any long-running active operation, the focus on key mineralized areas and related ore zones has shifted over the years at Musselwhite with continued exploitation, new discoveries/extensions and/or evolved interpretations as new data becomes available. Following are summaries of both recent and historical key mineralized areas within the mine.
Recent Key Mineralized Areas
PQ Deeps and Lynx
The PQ Deeps (C-Block) and Lynx occur in the East Limb of the deposit and host the vast majority of remaining gold resources and reserves at Musselwhite based on current data and scheduling forecasts (i.e., Life of Mine planning; see Table 16.7). Mineralization is hosted in a garnet-grunerite schist (4EA) in the hinge of a synclinal fold. The orientation of mineralization is parallel to the plunge of the fold axis and is known to have a continuity of at least 5 km along plunge, and is still open at depth. Grade is moderately continuous in a vertical direction (up to 125 m) and least continuous in the east-west direction (15-15 m). Grade and size of the PQ Deeps and Lynx zones are believed to influenced by the intersection of sub-vertical shear zones with the favourable host rock. The basis for future exploration is to target the down-plunge extension of 4EA unit.
Esker Zone
Located within the PQ limb, in the vicinity of the esker dividing Opapimiskan Lake, three separately correlated gold zones named the Esker, Root, and Core zones have been identified. These zones have been traced 900 m along strike, from section 11,700N to 12,600N in a dynamic fold system characterized by a north-westerly plunge of 5 m to 10 m. Structurally controlled, gold-bearing axial planar quartz-pyrrhotite veins result in stratabound gold zones found primarily within a garnet-actinolite-chert-grunerite (4EA) host. Additional work will be required to resolve the economic potential within this extremely complex geological setting.
Redwings
The Redwings zone is hosted in chert-magnetite BIF of the Southern Iron Formation. Mineralization occurs as pyrrhotite cement and stringers in breccias hosted in a parasitic fold. The best gold grades occur in the antiform hinge and short limb of the parasitic fold and are most continuous down-plunge of the fold. Continuity in the main reserves and resource is 2 km, and mineralization is open down plunge. Exploratory drilling indicates the same fold is still present 1 km down plunge and is the focus of exploration targeting.
West Limb
Mineralization in the West Limb is predominantly hosted in rock types that have traditionally been considered waste elsewhere in the mine. Current reserves and resources are hosted in mafic volcanic greenschist/amphibolites and garnet-biotite schists. Gold is associated with wispy pyrrhotite disseminations aligned parallel to the dominant foliation. The best grades are associated with a gentle parasitic fold and occur in the hanging wall and foot wall rocks to an ultramafic sill that cuts the contact between the mafic volcanics and schists. Exploration is focused on following this zone of structural complexity up and down-plunge and identifying other potential areas of increased permeability.
Deposits along the T-Antiform
The T-Antiform structure was historically known to host the largest gold concentrations on the Musselwhite Mine property.
The T-Antiform is an asymmetrical fold with the right limb being stretched and almost detached from the left limb. This division was used historically (ca. 1989) to divide the antiform into two (2) deposits: the “T” deposit and the “S” deposit.
The T deposit can be further subdivided into three separate zones. From west to east, they are known as the “WA”, “T” and “C” zones. Each zone is dominated by a second-order antiform. Areas containing weak mineralization between these zones are dominated by synforms. All three zones in the T deposit trend at 317°, are near-vertical dipping, and plunge 16° to 18° to the north-west. Based on gold grade distribution in diamond drill holes, there appears to be an echelon movement from west to east going from the south to the north in the better-mineralized structure (i.e., the WA and C zones are better mineralized in the southwest and northeast portions of the T deposit, respectively).
he S deposit is located on the attenuated, and partly detached, east limb of the T-Antiform structure. This fold structure starts to the south of 9,150N and extends north of 11,200N. Mineralization along this limb has since been subdivided into 3 separate (S1, S2 and S3) zones. The plunge of the S zones is consistently shallower than that of the C, T, and WA zones. The S zones plunge on the order of 10° to 12°, while the zones on the west limb of the T-Antiform average approximately 15° to 18°.