Characteristics and distribution of brittle-ductile structures associated with dextral transpression along the Larder Lake-Cadillac deformation zone in the southern Abitibi subprovince, Canada
Ben Frieman, Colorado School of Mines
Yvette Kuiper, Colorado School of Mines
Thomas Monecke, Colorado School of Mines
Nigel Kelly, University of Colorado at Boulder
This study investigates the structural characteristics and extent of strain associated with the Larder Lake-Cadillac deformation zone (LLCdz) in the southern Abitibi subprovince in Ontario, Canada. The structural history along the regional scale LLCdz and its immediate splays is well-constrained, because orogenic gold deposits are spatially associated with these deformation zones. However, the distribution of strain away from the immediate deformation zones is poorly constrained. New structural data is used here to investigate structural controls on orogenic gold mineralization and to better establish the role of strain localization mechanisms in the development of these zones.
We present new structural data from a ~7x10 km area around the LLCdz in Kirkland Lake, Ontario. The dominant structural fabric is a steeply NNW- or SSE-dipping penetrative foliation (SD) and an associated moderately to steeply NE-plunging mineral/elongation lineation (LD). SD/LD is best developed in ENE-trending high-strain zones (HSZs) that are <1m to >100m in width, spaced at ~500 m, and occur up to 15 km from the LLCdz. Within the HSZs, SD is associated with dextral shear sense indicators such as Z-folds and sigma clasts and is commonly defined by sericite- and/or iron-carbonate mineral-rich folia. We interpret that LD developed during dextral shear and is indicative of a subvertical extension component. This is consistent with earlier interpretations of dextral transpression along the LLCdz. Brittle deformation features within the HSZs include discrete fault planes with reverse shear sense indicators, fault gouge, cataclasite, and pseudotachylite-like structures. Furthermore, the HSZs commonly contain shallowly dipping vein sets, pervasive Fe-carbonate metasomatism, and sulfide mineralization. Kinematic analysis of the fault and slickenside lineation data indicates that they developed during NNW-SSE shortening. In general, both brittle and ductile structures in the HSZs are marked by pervasive Fe-carbonate metasomatism and both display complex mutually cross-cutting relationships. The major HSZs in the study area are spatially coincident with structures interpreted to have originated as brittle fault zones. Thus, it appears as if the early development of brittle damage zones, including cataclasites and pseudotachylite-like textures, produced favorable zones for progressive ductile shear localization. The association between alteration and deformation in the transpressional HSZs as far as 10 km from the LLCdz suggests that relatively distal HSZs may be prospective for orogenic gold in the Kirkland Lake area. Overall, the Kirkland Lake area provides a unique location to investigate how strain localization processes control fluid flow and orogenic gold mineralization along regionally extensive deformation zones.