Deformational and Metamorphic Processes in the Formation of the Bellechasse-Timmins Orogenic Gold Deposit, Southern Québec Appalachians, Canada

English Abstract:

The Bellechasse-Timmins (BT) gold deposit is located 110 Km 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, which has been deformed and metamorphosed at greenschist facies during the Acadian orogeny.

The Au mineralization is developed in diorite sills (and dykes) crosscutting the Upper Ordovician (Caradocian) Etchemin Formation, as the result of fracturing and variations in hydrothermal fluid pressure during regional metamorphism and deformation. The BT deposit consists of Au-rich quartz veins and stockwork hosted by faults and anastomosing shear zones formed at or above the brittle-ductile transition. Our structural analysis shows that both the intrusions and hosting sedimentary rocks are crosscut by a steeply-dipping, NE-trending axial-planar schistosity (S₁) and by 50 cm- to 2 metres-wide shear zones subparallel to that S₁schistosity. The shear zones host steeply-plunging slickenlines/lineations and preserved structural evidence for reverse faulting, mainly due to flattening and “locking up” of fold hinges, which have been sheared out by high-angle reverse faults. Subhorizontal lineations/fault striae are also locally found, suggesting late-stage strike-slip motion.

There are 3 main orientations of auriferous veins, almost perpendicular to each other, suggesting that the overall geometry of the ore zones is mainly controlled by and coeval with the regional folding event. It basically represents the product of hydrothermal circulation and fracturing related to regional metamorphism and folding affecting the diorite dykes/sills and the formation of saddle-reef and related structures. Fluids migrated along faults/shear zones and other weakness planes during 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 the lithostatic pressure.