Structural evolution of the Grevet Zn-Cu massive sulfide deposit, Lebel-sur-Quevillon area, Abitibi Subprovince, Quebec

Abstract

The Grevet Zn-Cu massive sulfide deposit is located within the major southeast-trending Cameron deformation zone in the central part of the Abitibi subprovince. This 5-km-wide deformation zone crosscuts the fabrics related to the earlier regional deformation and is characterized by a dextral horizontal component of movement. The Grevet deposit is composed of several sheetlike sulfide lenses hosted by a sequence of alternating units of mafic and felsic lava and volcaniclastic rocks, all of which are transected by several generations of mafic to felsic dikes.The Grevet area is characterized by: (1) mylonitic fabric axial planar to isoclinal folds with subhorizontal plunge, (2) subhorizontal stretching lineations, (3) boudinage with subvertical boudin axes, (4) transposition of earlier structures with Z-asymmetrical folds affecting originally oblique mafic dikes, (5) dextral shear-sense indicators, (6) late east-west crenulation cleavage associated with minor Z-asymmetrical folds, and (7) late shear fractures and kink bands. These features are all part of an evolutionary deformational sequence characterized by several increments of shearing and shortening components within the Cameron deformation zone.The 5-cm- to 12-m-thick sulfide lenses are parallel to the mylonitic foliation with lengths exceeding 100 m and occur at progressively greater depths from southwest to northeast. The major axes of the orebodies coincide well with the attitudes of fold axes and the stretching lineation. Three types of sulfide assemblages are distinguished in the mineralized lenses: (1) massive sphalerite and pyrite, (2) stringer and breccia sphalerite and pyrite, and (3) disseminated pyrite. These assemblages are irregularly distributed within each sulfide lens. The sulfide lenses have been folded, and the brecciated and disseminated sulfide assemblages can be interpreted in part as a result of tectonic dismemberment, especially in the hinges of folds. Local remobilization of sulfides has been observed.This study indicates that the Grevet deposit is probably a volcanogenic massive sulfide deposit modified during the progressive evolution of a dextral transpressive deformation zone.