DEFORMATIONAL PROCESSES IN THE FORMATION OF THE BELLECHASSE-TIMMINS GOLD DEPOSIT, SOUTHERN QUÉBEC APPALACHIANS, CANADA

The Bellechasse-Timmins (BT) gold deposit is located 110Km south of Québec City. It is hosted by the Ordovician Magog Group, a synorogenic forearc basin sequence in the Dunnage Zone of the Québec Appalachians.

Structurally, BT occurs within the St-Victor synclinorium, a major regional fold between the Baie Verte-Brompton line to the NW and the La Guadeloupe fault to the SE. It consists of Au-rich quartz veins hosted by diorite sills (and dykes) crosscutting the Middle-to-Upper Ordovician Etchemin Formation, which has been deformed and metamorphosed during the Acadian orogeny. Petrographic study of diorite shows a metamorphic mineral assemblage typical of greenschist facies. The Au mineralization is found within quartz veins developed in the diorite intrusions, as the result of fracturing and variations in hydrothermal fluid pressure during regional folding. Our structural analysis shows that both the intrusions and hosting sedimentary rocks are crosscut by a steeply-dipping, NE-trending schistosity (S₁). The diorite bodies are crosscutted by brittle-ductile, anastomosing, 50cm- to 2m-wide shear zones subparallel to S₁. These shear zone host steeply-plunging slickenlines and lineations and preserved structural evidence for reverse faulting. Subhorizontal lineations/fault striae are also locally found, suggesting late-stage strike-slip motion. Structural relationships between bedding and S₁ indicate that sedimentary strata hosting the diorite intrusions are tightly folded, with fold axes plunging moderately to steeply (60°) towards both the SW and the NE. Similar folds characterize the sedimentary rocks whereas the intrusions show evidence of concentric folding, as a result of rheological contrast between these rock units.

There are 3 main orientations of veins. The overall geometry of the ore zones suggests that mineralization has been controlled by and coeval with the regional folding event, and basically represents the product of hydrothermal circulation and fracturing related to folding of the diorite dykes/sills. Fluids migrated along faults/shear zones during regional folding and precipitated in favourable low-pressure dilation zones (e.g. faults, bedding planes, fold axes) created when the hydraulic pressure of trapped fluids exceeded lithostatic pressure.