Leah Clapp HBSc thesis abstract

Thesis Title: 
Microstructural Analysis of the Main Break in Kirkland Lake, ON

Microstructural analysis provides evidence of significant ductile deformation concentrated along the Main Break, in Kirkland Lake, ON.  The Main Break is an east-west striking mineralized fault system that has sustained multiple gold mines since its discovery in 1911.  It is found in the southern Abitibi gold belt on Kirkland Lake Gold Inc.’s Lakeshore Mine property, which is a structurally controlled orogenic gold deposit.

Oriented samples for microstructural analysis were collected from two new 1-meter long channels across the Main Break spaced eight meters apart. 
Samples were examined in transmitted and reflected light microscopy.  Evidence of ductile deformation by dislocation creep to produce mylonite, inclues porphyroclasts of potassium feldspar in an extremely fine-grained matrix, mineral alignment, and undulatory extinction and subgrains in potassium feldspar; as well as lenticular aggregates of feldspar and muscovite showing dextral shear sense.  Dislocation creep occurs in feldspar during ductile deformation at temperatures in the amphibolite facies of metamorphism or higher, which puts deformation of the Main Break at temperatures above 400

Sericite aggregates replacing potassium feldspar likely enhanced grain softening during this ductile deformation process.  Sericite as well as evidence of pressure solution along grain boundaries indicate the presence of a hydrous fluid during deformation.

Ductile deformation will produce porphyroclasts with penetrative foliations wrapping around them. 
Such porphyroclasts would normally be explained by a difference in rheology, however, even in monomineralic rocks it is possible to have adjacent grains deforming by different mechanisms.  These different responses could be controlled by variations in crystallographic orientation of the grains where the slip planes are favorably oriented for slip in one grain but in a hard orientation for surrounding grains or simply different sized grains of the same mineral accommodating deformation by different mechanisms.  The most significant evidence of simultaneous brittle-ductile deformation is a potassium feldspar porphyroclast with strong undulatory extinction and subgrains in one half of the grain and microfractures in the other half.

It is concluded that the Main Break is a narrow ductile shear zone with minor brittle deformation.