METALLOGENIC AND STRUCTURAL CHARACTERIZATION OF THE DUNRAINE SHEAR ZONE, VAL D'or, QuÉbec

The Dunraine shear zone is a sub-vertical, ENE-WSW trending deformation corridor crosscutting the Val d'Or Fm (VDF). The VDF is interpreted as an Archean island arc massif. The VDF is characterised by andesitic to rhyolitic lavas and volcaniclastic rocks and hosts several VMS. In the study area, the bedrock mainly consists of massive feldspar-phyric, intermediate lavas (andesite-dacite) and intermediate/felsic lapilli tuffs to tuff breccias with minor felsic lava flows. To the south, this volcanic sequence is intruded by a syn-volcanic diorite (e.g. Dunraine sill). The regional schistosity (S1) varies between N245º and N260º and dips 75º to 90º northwestward. One crenulation and one lineation are hosted by this schistosity. L1 is a down-dip stretching lineation suggesting n N-S oriented compression deformation event. The sub-vertical S1 is associated with a conjugate set of kink bands representing a pure shear deformation. The subvertical kink bands highlight the regional sub-horizontal NNW-SSE oriented compresional axis (σ1). L2 is a sub-horizontal crenulation lineation plunging 10º-40º West, which appears to be associated with «late» dextral shearing. Dragging structures affecting S1 are consistent with that dextral movement. Cm- to m-wide, flat-lying and gently folded extensional veins are present. Their orientation varies from N165º to N200º and dip values are 20º to 30º westward. ENE-WSW boudinaged vertical veins are also present. The northward-directed stratigraphic way-up is indicated by load cast and channelling structures in the volcaniclastic sequence exposed on the DUN-3 stripping. The sulphide mineralisation is marked by a narrow (1-10 m) but laterally extensive (> 2 km) chalcopyrite- and pyrite-rich stringers horizon hosted by dacitic lapilli tuffs and/or lavas. The stringers are 1 mm to 3 dm-wide with lengths varying from 10 cm to several metres-long. The occurrence of chalcopyrite and pyrite is commonly accompanied by an intense silicification and chloritisation of host rocks. The sulphide stringers are affected by S1 and commonly transposed subparallel to the schistosity. Evidence for crystal deformation of the sulphides suggests that mineralisation occurred before regional deformation and is therefore likely of syn-volcanic origin.