Margaret Laverge, MSc
M. Laverge1, B. Lafrance1, J. Simmons1, S. Brueckner1, W. Boheme1, D. Tinkham1, J.C. Ordóñez Calderón2
1Mineral Exploration Research Centre, Harquail School of Earth Sciences, Laurentian University, Sudbury, ON, Canada,
2Kinross Gold Corporation, Toronto, ON, Canada
The Great Bear property, located 25 km south of the productive Red Lake gold camp (>29 Moz gold) in the NW Superior craton, hosts a new world-class, structurally-controlled gold deposit with a combined resource estimate of 6.6 Moz Au. The property is bisected by the SE-striking LP fault, which straddles the contact between a mafic domain to the south and a felsic domain to the north. Mineralization is hosted within three distinct zones: the LP Zone, Hinge Zone, and Limb Zone. The LP Zone, which hosts the bulk of the known gold mineralization, sits within the felsic hanging wall of the LP fault, while the Hinge and Limb zones sit in the mafic footwall. Mineralization is characterized by gold-bearing deformed quartz veins oriented parallel and oblique to the foliation but also occurs disseminated in altered felsic volcanics.
The LP Fault Zone is expressed as a ~500m wide high-strain zone characterised by a penetrative, NW-striking, steeply NE-dipping foliation and transposed isoclinal folds. A mineral stretching lineation plunges steeply (~79°) to the N along the foliation. Later Z-shaped folds strike E-W and are associated with steeply dipping dextral shear bands. Several lines of evidence suggest that the development of these structures occurred during a single long-lived dextral transpressional event. First, deformed quartz phenocrysts with dextral asymmetrical strain shadows on horizontal outcrop surfaces are stretched vertically on vertical outcrop surfaces. Second, the fold axes of both early isoclinal and later Z-shaped folds and the mineral lineations are rotated into parallelism, likely during concurrent dextral shearing and oblique to near-vertical extension. Finally, the mineral stretching lineation is consistently steeper than the calculated slip direction.
Structures along the LP Fault Zone formed after 2712.3 ± 2.2 Ma, which is the age of the mylonitized and mineralized quartz-feldspar porphyry host rocks, and locally outlasted 2692.4 ± 4.5 Ma, which is the age of a weakly-deformed granitic intrusion. This indicates that deformation on the Great Bear Property is the local expression of the D3 deformation in the Red Lake Greenstone Belt, which resulted during the last increment of regional shortening associated with the Uchian Phase of the Kenoran Orogeny.
Gold-bearing quartz veins were deformed by both early and late-stage transpression, indicating pre- to early syn-deformation emplacement of mineralization, followed by subsequent reorientation during D3 deformation. This suggests that gold mineralization occurred during a pre-existing deformation event that has been overprinted by D3 deformation.