FIELD-BASED EVIDENCE FOR DILATIONAL BRECCIATION PRIOR TO DUCTILE LOCALIZATION AND FAULT-VALVE BEHAVIOR ALONG A GOLD-BEARING SPLAY OF THE ARCHEAN LARDER LAKE-CADILLAC DEFORMATION ZONE, ONTARIO, CANADA

Field-based investigations constrain the early fault behavior and brittle-ductile structural development of the Kirana deformation zone (Kdz), a gold-bearing splay of the crustal-scale Larder Lake-Cadillac deformation zone (LLCdz) in Ontario, Canada. The Kdz preserves fault breccia, mylonite, and several generations of fault-related veins. Based on cross-cutting relationships, the earliest structure is fault breccia. Exposures preserve a >35 m damage zone containing variably developed crackle to chaotic breccia. The breccia bodies display high matrix to clast ratios, angular clasts, and ‘exploded’ jigsaw patterns. The breccia matrix consists of a black, quartz-rich material that contains abundant micron-scale vugs, chlorite, and rutile. To quantify the breccia textures, we analyzed their particle size distributions, which yielded D-values of 0.93-1.71. These results are low relative to attrition products, but are similar to breccia produced by a dilational mechanism, consistent with the field observations. Dilational breccia, can form at step-overs or asperities along fault segments, where off-fault dynamic rupture results in the formation of tensile fracture networks in damage zones. Dilational breccia is crosscut by mylonite and quartz-carbonate veins. Both are texturally associated with greenschist facies metamorphic assemblages, sericite, carbonate, or sulfide alteration, and, locally, gold. Mylonite zones are defined by a NE-trending, steeply dipping fabric with dextral shear indicators and a steeply NE-plunging lineation. The shear zones are interpreted to relate to a transpressional strain with a steeply inclined extension direction. Mylonite zones overprint composite veins, but are also cross-cut by successive sets of fault veins, indicating that vein emplacement was periodic due to fault-valve behavior. Analysis of fault veins and slickenside striation orientations indicates vein emplacement during NNW-SSE shortening, which is similar to the ductile shortening direction. Thus, shear and vein emplacement are interpreted to have occurred during oblique shortening. In summary, the structural development of the Kdz is characterized by dilational brecciation along fault segments, followed by localized ductile shear, intermittent fault-valve behavior, and related gold.