TECTONIC EVOLUTION OF THE MARBANITE-NORBENITE DEFORMATION ZONE, VAL-D'OR, QUEBEC: IMPLICATIONS FOR GOLD MINERALIZATION IN THE MALARTIC GROUP

The structural evolution of the Marbanite-Norbénite deformation zone (MNDZ) has long been considered to consist of two distinct episodes of shearing, the first a N-S oriented regional compression under ductile conditions (D1), followed by regional dextral shearing (D2) under brittle-to-ductile conditions. Recent field mapping and vorticity studies suggest, however, that a significant period of dextral transpressive deformation occurred following the N-S shortening event. Analysis of shear band cleavages and related structures in the high-strain portion of the MNDZ indicates the existence of a dextral transpressive deformational event, characterized by strong spatial partitioning of strain, which overprints earlier down-dip stretching lineations associated with the N-S contraction. Au-bearing quartz ± carbonate ± tourmaline veins are interpreted to be early in the tectonic evolution of the MNDZ as they exhibit post-emplacement deformation structures consistent with a dextral transpressive regime under ductile conditions, such as the concurrent development and interference of shallowly plunging isoclinal folds and steeply plunging Z-folds. It is therefore suggested that Au-bearing hydrothermal systems were mainly active during D1 and possibly early D2 deformation. A three-part tectonic and mineralization evolution is suggested: (1) N-S compression (D1) concurrent with an active hydrothermal system leading to the formation of an important volume of Au-bearing quartz veins, followed by (2) a dextral transpressive regime (D2) associated with the folding and boudinage of existing mineralized veins as well as possible mechanical remobilization of Au and finally (3) localized orogen-parallel dextral shearing (D3).