THE INFLUENCE OF DIKES ON AURIFEROUS SHEAR ZONE DEVELOPMENT WITHIN GRANITOID INTRUSIONS - THE BOURLAMAQUE PLUTON, VAL-DOR DISTRICT, ABITIBIGREENSTONE-BELT

Shear-zone-related gold - quartz veins in granitoid intrusions are com monly intimately associated with mafic dikes, which may have a profoun d influence on the localization, orientation, and kinematics of aurife rous shear zones. The Bourlamaque pluton of the Val-d'Or district cont ains several economic auriferous shear zones, most of which follow and overprint diorite dikes. Mineralization in all deposits consists of q uartz - 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 ho mogeneous intrusion. The stress tensor determined from the auriferous shear zones within the pluton indicates the same northerly-directed co mpression recorded by similar shear zones outside the pluton. This ind icates that the complex shear zone and vein pattern within the pluton reflects the influence of diorite dikes, which acted as weak layers th at were activated during subsequent deformation, showing the importanc e of layer anisotropy in auriferous shear zone development. The plunge s of orebodies bear simple geometric relationships to the slip directi on along a host shear zone: these are generally perpendicular to, or i n some cases parallel to, the slip direction. Knowledge of the slip di rections along activated dikes would therefore allow prediction of the possible plunge(s) of orebodies at early stages of exploration progra ms. Slip direction along an activated layer is controlled by the orien tation of the layer with respect to the stress field and by the relati ve magnitudes of the three principal stresses. Using techniques develo ped for analysis of fault slip data, both parameters can be determined , provided there is a sufficient database, and slip direction can be p redicted for activated layers of any orientations.