Kinematics Of Shear Zones Research Articles

The influence of dikes on auriferous shear zone development within granitoid intrusions: the Bourlamaque pluton, Val-d'Or district, Abitibi greenstone belt

Shear-zone-related gold–quartz veins in granitoid intrusions are commonly intimately associated with mafic dikes, which may have a profound influence on the localization, orientation, and kinematics of auriferous shear zones. The Bourlamaque pluton of the Val-d'Or district contains several economic auriferous shear zones, most of which follow and overprint diorite dikes. Mineralization in all deposits consists of quartz–tourmaline–pyrite veins in reverse- oblique orientation with a significant range of strike, dip, and slip direction. The geometry and kinematics of shear zone and vein array within the pluton is more complex than the simple conjugate pattern predicted for a deforming homogeneous intrusion. The stress tensor determined from the auriferous shear zones within the pluton indicates the same northerly-directed compression recorded by similar shear zones outside the pluton. This indicates that the complex shear zone and vein pattern within the pluton reflects the influence of diorite dikes, which acted as weak layers that were activated during subsequent deformation, showing the importance of layer anisotropy in auriferous shear zone development.The plunges of orebodies bear simple geometric relationships to the slip direction along a host shear zone: these are generally perpendicular to, or in some cases parallel to, the slip direction. Knowledge of the slip directions along activated dikes would therefore allow prediction of the possible plunge(s) of orebodies at early stages of exploration programs. Slip direction along an activated layer is controlled by the orientation of the layer with respect to the stress field and by the relative magnitudes of the three principal stresses. Using techniques developed for analysis of fault slip data, both parameters can be determined, provided there is a sufficient database, and slip direction can be predicted for activated layers of any orientations.

The geometrical relationship between the stretching lineation and the movement direction of shear zones: Discussion

IN n recent paper, Lin & Williams (1992) emphasized the difference between the plunge of stretching lineations on S-surfaces and that of the movement direction in shear zones, and discussed its implications for interpretation of shear zone kinematics. They further showed that, knowing the strike of a shear zone, the orientation of the S-surface and of the stretching lineation, it is possible to determine the dip of the shear zone and the movement direction along it. They applied this technique to the D1 deformation increment documented by Robert (1989) along the eastern segment of the Cadillac tectonic zone (CTZ) and they proposed an alternative interpretation. This major fault zone extends over 200 km of strike length in an E-W direction, parallel to contacts between major lithologic units. The D1 increment of deformation along the CTZ was interpreted by Robert (1989) as transpressional, involving a significant component of shortening across the CTZ, accompanied by localized dip-slip movements, and a minor component of dextral transcurrent shear. This interpretation was proposed to reconcile the obliquity of $1 to the shear zone boundaries and other asymmetric features, observed in plan but not in cross-section, and the contained steeply E-plunging elongation lineation. Lin & Williams (1992), considering that the obliquity of foliation to shear zones boundaries observed in plan view is apparent, used the orientation of $1 and the stretching lineation to infer a southern dip of 60 °. With a note of caution, they proposed that D~ deformation along the CTZ recorded reverse oblique movements (south over north) with a minor dextral component. Alternative interpretations of the deformational history of the CTZ are welcome because the internal structure of the CTZ is very complex and challenges conventional shear zone analysis. However, as will be discussed below, the interpretation proposed by Lin & Williams (1992) is not consistent with the available data. I also take this opportunity to present additional observations that bear on the interpretation of the CTZ. BOUNDARIES OF THE CTZ AND OBLIQUITY OF $1